JP7705769B2 - Non-aqueous alkali metal storage element - Google Patents

Description

The present invention relates to a non-aqueous alkali metal storage element.

In recent years, from the viewpoint of the conservation of the global environment and the effective use of energy aimed at resource conservation, attention has been focused on wind power generation power smoothing systems or nighttime power storage systems, distributed household power storage systems based on solar power generation technology, power storage systems for electric vehicles, etc. The first requirement for batteries used in these power storage systems is high energy density. As a promising candidate for a high energy density battery that can meet such requirements, the development of lithium ion secondary batteries is being actively pursued. The second requirement is high output characteristics. For example, in the combination of a high-efficiency engine and a power storage system (e.g., hybrid electric vehicles) or the combination of a fuel cell and a power storage system (e.g., fuel cell electric vehicles), a power storage system that exhibits high output discharge characteristics during acceleration is required. Currently, electric double layer capacitors, nickel-metal hydride batteries, etc. are being developed as high-output power storage devices.

Among electric double layer capacitors, those that use activated carbon for the electrodes have output characteristics of about 0.5 to 1 kW/L. This electric double layer capacitor not only has high output characteristics, but also has high durability (cycle characteristics and high-temperature storage characteristics), and has been considered to be the optimal device for the above-mentioned fields that require high output. However, since its energy density is only about 1 to 5 Wh/L, further improvement in energy density is necessary.

Nickel-metal hydride batteries, which are currently commonly used in hybrid electric vehicles, have a high output equivalent to that of electric double-layer capacitors and an energy density of about 160 Wh/L. However, vigorous research is being conducted to further increase their energy density and output characteristics, as well as to improve their durability (especially their stability at high temperatures).

Research is also being conducted on lithium-ion secondary batteries to increase their output. For example, lithium-ion secondary batteries have been developed that can obtain high output exceeding 3 kW/L at a discharge depth (i.e., the ratio (%) of the discharge amount to the discharge capacity of the storage element) of 50%. However, their energy density is 100 Wh/L or less, and the design deliberately suppresses the high energy density that is the greatest feature of lithium-ion secondary batteries. In addition, their durability (cycle characteristics and high-temperature storage characteristics) is inferior to that of electric double-layer capacitors, so such lithium-ion secondary batteries are used in a discharge depth range narrower than 0 to 100% in order to provide practical durability. The capacity of lithium-ion secondary batteries that can actually be used will be even smaller, so research is being actively conducted to further improve durability.

As described above, there is a strong demand for practical use of storage elements that combine high energy density, high output characteristics, and high durability. However, since the above-mentioned existing storage elements each have their own advantages and disadvantages, a new storage element that meets these technical requirements is required. As a promising candidate, a storage element called a lithium-ion capacitor has attracted attention and is being actively developed. A lithium-ion capacitor is a type of storage element that uses a non-aqueous electrolyte containing lithium salt (hereinafter also referred to as a "non-aqueous alkali metal storage element"). At the positive electrode, a non-Faraday reaction occurs due to the adsorption and desorption of anions at approximately 3 V or more, similar to an electric double layer capacitor, and at the negative electrode, a Faraday reaction occurs due to the absorption and release of lithium ions, similar to a lithium ion secondary battery. This storage element charges and discharges.

To summarize the electrode materials commonly used in the above energy storage elements and their characteristics, generally, when materials such as activated carbon are used for the electrodes and charging and discharging is performed by adsorption and desorption of ions on the surface of the activated carbon (non-Faraday reaction), high output and high durability are obtained, but the energy density is low (for example, 1x). On the other hand, when oxide or carbon materials are used for the electrodes and charging and discharging is performed by the Faradaic reaction, the energy density is high (for example, 10x that of a non-Faraday reaction using activated carbon), but there are issues with durability and output characteristics.

As a combination of these electrode materials, electric double layer capacitors use activated carbon (energy density x1) for the positive and negative electrodes, and are characterized by charging and discharging through non-Faradic reactions at both the positive and negative electrodes. As such, they have high output and high durability, but low energy density (positive electrode x1 x negative electrode = 1).

Lithium-ion secondary batteries use lithium transition metal oxides (10 times the energy density) for the positive electrode and carbon materials (10 times the energy density) for the negative electrode, and are characterized by charging and discharging both the positive and negative electrodes through the Faraday reaction. As such, they have a high energy density (10 times the positive electrode x 10 times the negative electrode = 100), but they have issues with output characteristics and durability. Furthermore, to meet the high durability required for hybrid electric vehicles and the like, the depth of discharge must be limited, and with lithium-ion secondary batteries, only 10 to 50% of the energy can be used.

A lithium ion capacitor is characterized by using activated carbon (energy density x1) for the positive electrode and a carbon material (energy density x10) for the negative electrode, and charging and discharging is performed by a non-Faradaic reaction at the positive electrode and a Faradaic reaction at the negative electrode, and is therefore an asymmetric capacitor that combines the characteristics of an electric double layer capacitor and a lithium ion secondary battery. Lithium ion capacitors are characterized by having high output and durability, as well as a high energy density (1x positive electrode x 10x negative electrode = 10), and no need to limit the depth of discharge like lithium ion secondary batteries.

Various studies have been conducted to further improve the reliability and high-temperature durability of the lithium-ion capacitor (Patent Documents 1 to 4). Patent Documents 1 and 2 disclose a lithium-ion secondary battery in which an insulating tape is attached to prevent short circuits between the positive and negative electrodes (Patent Document 1) and a coated insulating layer is provided (Patent Document 2). Patent Document 3 discloses a technology in which a protective layer containing _AlF3 is provided on the surface of an aluminum current collector to suppress corrosion of the current collector. Patent Document 4 discloses a non-aqueous lithium storage element that suppresses corrosion of aluminum foil in a high-voltage environment and has high durability when stored in a high-temperature environment of 85°C or higher.

JP 2006-19199 A JP 2016-186945 A Special table 2017-528880 publication WO 2019/156090

E. P. Barrett, L. G. Joyner and P. Halenda, J. Am. Chem. Soc. , 73, 373 (1951) B. C. Lippens, J. H. de Boer, J. Catalysis, 4319 (1965) R. S. Michael, S. Brunauer, E. E. Bodor, J. Colloid Interface Sci. , 26, 45 (1968)

However, none of the documents take into consideration corrosion of the positive electrode current collector caused by in-plane potential variations in the electrode during high-temperature, high-voltage cycles, or the occurrence of side reactions with excess electrolyte in the cell at the end of the negative electrode. Furthermore, no consideration is given to preventing both the deactivation of alkali metal ions doped in the negative electrode in a high-temperature environment and the associated decrease in capacity retention.

In view of the above-mentioned current situation, the problem that the present invention aims to solve is to provide a non-aqueous alkali metal storage element that suppresses resistance increase during high-temperature, high-voltage cycles and has a high capacity retention rate.

The above problems can be solved by the following technical means. That is, the present invention is as follows.
[1]
a positive electrode including a positive electrode active material layer disposed on a positive electrode current collector, a negative electrode including a negative electrode active material layer disposed on a negative electrode current collector, and a separator; and a nonaqueous electrolyte solution including alkali metal ions, wherein the positive electrode active material layer includes a positive electrode active material including a carbon material, and an alkali metal carbonate, and the nonaqueous electrolyte solution has a concentration of hexafluorophosphate ions of 0.00 mol/L or more and 0.05 mol/L or less, the positive electrode has a protective layer and a non-protective layer on the positive electrode current collector, and the protective layer and a negative electrode end portion overlap on a projected plane in a stacking direction of the electrode assembly, and the alkali metal ion concentration per unit area of the outer periphery of the negative electrode is A (mAh/g) and the alkali metal ion concentration per unit area of the central portion is B (mAh/g), where A is the alkali metal ion concentration per unit area of the outer periphery of the negative electrode, and B is the alkali metal ion concentration per unit area of the central portion, the relationship is 1.02≦B/A≦1.45.
[2]
2. The nonaqueous alkali metal storage element according to item 1, wherein the protective layer is present on both sides of the positive electrode.
[3]
3. The nonaqueous alkali metal storage element according to item 1 or 2, wherein the negative electrode has the negative electrode active material layer on both sides of the negative electrode current collector, an outermost layer of the electrode body is the negative electrode, and when an alkali metal ion concentration per unit area in a central part of the negative electrode active material layer not facing the positive electrode is E (mAh/g), B>E.
[4]
4. The nonaqueous alkali metal storage element according to any one of items 1 to 3, wherein the protective layer is a protective tape, and the base material of the protective tape is at least one material selected from the group consisting of polyether ketone, polyphenylene sulfide, polyethylene terephthalate, polyethylene naphthalate, polyamide, polyimide, aramid, acetate cloth, and polytetrafluoroethylene tape.
[5]
5. The nonaqueous alkali metal storage element according to any one of items 1 to 4, wherein the protective layer contains at least one carbon material selected from the group consisting of carbon black, graphite, graphene, and carbon nanotubes.
[6]
6. The nonaqueous alkali metal storage element according to any one of items 1 to 5, wherein the protective layer contains inorganic fine particles.
[7]
7. The nonaqueous alkali metal storage element according to any one of items 1 to 6, wherein the positive electrode and/or the negative electrode contain carbon nanotubes.
[8]
8. An electricity storage module comprising the nonaqueous alkali metal electricity storage element according to any one of items 1 to 7.
[9]
Item 9. The power storage module according to item 8, wherein the power storage module is incorporated into at least one system selected from the group consisting of a power regeneration assist system, a power load leveling system, an uninterruptible power supply system, a contactless power supply system, an energy harvesting system, a power storage system, a solar power generation storage system, an electric power steering system, an emergency power supply system, an in-wheel motor system, an idling stop system, an electric vehicle, a plug-in hybrid vehicle, a hybrid vehicle, an electric motorcycle, a quick charging system, and a smart grid system.
[10]
The nonaqueous alkali metal storage element according to any one of items 1 to 7,
An energy storage system that connects lead batteries, nickel-metal hydride batteries, lithium-ion secondary batteries, or fuel cells in series or parallel.

The present invention provides a non-aqueous alkali metal storage element that has a small resistance increase rate by suppressing corrosion of the positive electrode current collector and side reactions at the negative electrode end during high-temperature, high-voltage cycles, and has a high capacity retention rate by suppressing deactivation of alkali metal ions in the negative electrode.

FIG. 1 is a projection view of the electrode assembly in the lamination direction. FIG. 2 is a cross-sectional view of the electrode body. FIG. 3 is an image diagram showing the outer periphery and center of the negative electrode, showing a schematic top view of the negative electrode (a) and a schematic diagram for explaining the outer periphery, center, and end portions of the negative electrode (b).

The following describes an embodiment of the present invention (hereinafter, referred to as "the present embodiment") in detail, but the present invention is not limited to this embodiment. The upper and lower limits of each numerical range in this embodiment can be arbitrarily combined to form any numerical range.

<<Non-aqueous alkali metal storage element>>
The nonaqueous alkali metal storage element of the present embodiment mainly comprises a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution, which is a solution in which alkali metal ions are dissolved in an organic solvent.

<Negative electrode>
The negative electrode in this embodiment has a negative electrode current collector and a negative electrode active material layer provided on one or both sides of the negative electrode current collector.

[Negative electrode active material layer]
The negative electrode active material layer contains a negative electrode active material, and may further contain optional components such as a dispersant, a conductive filler, and a binder, as necessary.

(Negative electrode active material)
The negative electrode active material may be a material capable of absorbing and releasing alkali metal ions.Specific examples include carbon materials, titanium oxides, silicon, silicon oxides, silicon alloys, silicon compounds, tin and tin compounds.The content of the negative electrode active material in the negative electrode active material layer of the negative electrode precursor is preferably 70% by mass or more, more preferably 80% by mass or more, based on the total mass of the negative electrode active material layer.The negative electrode active material is preferably particulate.

The negative electrode active material preferably contains a carbon material. The content of the carbon material relative to the total amount of the negative electrode active material is preferably 50% by mass or more, and more preferably 70% by mass or more. The content of the carbon material can be 100% by mass, but from the viewpoint of obtaining a good effect by using other materials in combination, it is preferable that the content is, for example, 95% by mass or less, and may be 90% by mass or less. The upper and lower limits of the range of the carbon material content can be combined arbitrarily.

Examples of carbon materials include non-graphitizable carbon materials; graphitizable carbon materials; carbon black; carbon nanoparticles; activated carbon; artificial graphite; natural graphite; graphitized mesophase carbon spheres; graphite whiskers; amorphous carbonaceous materials such as polyacene-based substances; carbonaceous materials obtained by heat treating carbon precursors such as petroleum-based pitch, coal-based pitch, mesocarbon microbeads, coke, and synthetic resins (e.g., phenolic resins); pyrolysates of furfuryl alcohol resins or novolac resins; fullerenes; carbon nanofibers; and composite carbon materials of these.

Among these, from the viewpoint of reducing the resistance of the negative electrode, a composite carbon material is preferred in which one or more graphite materials selected from artificial graphite, natural graphite, graphitized mesophase carbon spheres, graphite whiskers, and high specific surface area graphite are coexisted with one or more carbonaceous material precursors selected from petroleum-based pitch, coal-based pitch, mesocarbon microbeads, coke, and synthetic resins, such as phenolic resins, to form a composite carbon material. There are no particular limitations on the carbonaceous material precursor as long as it becomes a carbonaceous material by heat treatment, but petroleum-based pitch or coal-based pitch is particularly preferred. Before the heat treatment, the graphite material and the carbonaceous material precursor may be mixed at a temperature higher than the melting point of the carbonaceous material precursor. The heat treatment temperature may be any temperature at which the components generated by the volatilization or pyrolysis of the carbonaceous material precursor used become carbonaceous materials, but is preferably 400°C or higher and 2,500°C or lower, more preferably 500°C or higher and 2,000°C or lower, and even more preferably 550°C or higher and 1,500°C or lower. There are no particular restrictions on the atmosphere in which the heat treatment is performed, but a non-oxidizing atmosphere is preferred.

The BET specific surface area of the composite carbon material is preferably 1 m ² /g or more and 50 m ² /g or less, more preferably 1.5 m ² /g or more and 40 m ² /g or less, and even more preferably 2 m ² /g or more and 25 m ² /g or less. If the BET specific surface area of the composite carbon material is 1 m ² /g or more, a sufficient number of reaction sites with lithium ions in the non-aqueous electrolyte can be secured, and high input/output characteristics can be exhibited. If the BET specific surface area of the composite carbon material is 50 m ² /g or less, the charge/discharge efficiency of lithium ions is improved and the reductive decomposition of the non-aqueous electrolyte during charge/discharge is suppressed, and therefore high high-load charge/discharge cycle characteristics can be exhibited.

The average pore diameter of the composite carbon material is preferably 1.5 nm to 25 nm, more preferably 2 nm to 22 nm, even more preferably 3 nm to 20 nm, and particularly preferably 3.5 nm to 18 nm. If the average pore diameter of the composite carbon material is 1.5 nm or more, there are many pores larger than the size (1.2 nm or less) of the solvated alkali metal ions in the non-aqueous electrolyte, so that the diffusion of the solvated alkali metal ions in the composite carbon material is good, and the non-aqueous alkali metal storage element using this can exhibit high input/output characteristics. On the other hand, if the average pore diameter of the composite carbon material is 25 nm or less, the bulk density of the negative electrode active material layer using this can be sufficiently improved, so that a high energy density can be exhibited.

The composite carbon material may be particulate, and the average particle size is preferably 1 μm or more and 10 μm or less, more preferably 2 μm or more and 8 μm or less, and even more preferably 3 μm or more and 6 μm or less. If the average particle size of the composite carbon material is 1 μm or more, the charge/discharge efficiency of the alkali metal ions can be improved, and high high-load charge/discharge cycle characteristics can be exhibited. If the average particle size of the composite carbon material is 10 μm or less, the number of reaction sites with the alkali metal ions in the non-aqueous electrolyte increases, and high input/output characteristics can be exhibited.

In the composite carbon material, the mass ratio of the carbonaceous material to the graphite material is preferably 1% by mass or more and 20% by mass or less, more preferably 1.2% by mass or more and 15% by mass or less, even more preferably 1.5% by mass or more and 10% by mass or less, and even more preferably 2% by mass or more and 5% by mass or less. If the mass ratio of the carbonaceous material is 1% by mass or more, the carbonaceous material can sufficiently increase the reaction sites with the alkali metal ions in the non-aqueous electrolyte, and the alkali metal ions can be easily desolvated, so that high input/output characteristics can be exhibited. If the mass ratio of the carbonaceous material is 20% by mass or less, the solid diffusion of the alkali metal ions between the carbonaceous material and the graphite material can be well maintained, so that high input/output characteristics can be exhibited. In addition, the charge/discharge efficiency of the alkali metal ions can be improved, so that high high-load charge/discharge cycle characteristics can be exhibited.

The doping amount of alkali metal ions per unit mass of the composite carbon material is preferably 50 mAh/g or more and 700 mAh/g or less, more preferably 70 mAh/g or more and 650 mAh/g or less, even more preferably 90 mAh/g or more and 600 mAh/g or less, and even more preferably 100 mAh/g or more and 550 mAh/g or less. By doping with alkali metal ions, the negative electrode potential is lowered. Therefore, when a negative electrode containing a composite carbon material doped with alkali metal ions is combined with a positive electrode, the voltage of the non-aqueous alkali metal storage element becomes higher and the usable capacity of the positive electrode becomes larger. Therefore, the capacity and energy density of the obtained non-aqueous alkali metal storage element become higher. If the doping amount of alkali metal ions per unit mass of the composite carbon material is 50 mAh/g or more, the alkali metal ions are well doped even in irreversible sites in the composite carbon material where the alkali metal ions cannot be removed once they are inserted, so that a high energy density can be obtained. The greater the doping amount, the lower the negative electrode potential, and the better the input/output characteristics, energy density, and durability. If the doping amount of alkali metal ions per unit mass of the composite carbon material is 700 mAh/g or less, side effects such as precipitation of alkali metals such as lithium metal are less likely to occur.

The BET specific surface area of the graphite material used in the composite carbon material is preferably 0.5 ^m2 /g to 80 ^m2 /g, more preferably 1 ^m2 /g to 70 ^m2 /g, and even more preferably 1.5 ^m2 /g to 60 ^m2 /g. If the BET specific surface area of the graphite material used in the composite carbon material is within the above range, the BET specific surface area of the composite carbon material can be adjusted to the above range.

The graphite material used in the composite carbon material may be particulate, and the average particle size is preferably 1 μm or more and 10 μm or less, more preferably 2 μm or more and 8 μm or less. If the average particle size of the graphite material used in the composite carbon material is in the range of 1 μm or more and 10 μm or less, the average particle size of the composite carbon material can be adjusted to the above range.

The carbonaceous material precursor used as the raw material for the composite carbon material is a solid, liquid, or solvent-soluble organic material that can be composited with a graphitic material by heat treatment. Examples of the carbonaceous material precursor include pitch, mesocarbon microbeads, coke, and synthetic resins such as phenolic resins. Among these carbonaceous material precursors, it is preferable to use pitch, which is inexpensive, in terms of production costs. Pitch is broadly divided into petroleum-based pitch and coal-based pitch. Examples of petroleum-based pitch include crude oil distillation residue, fluid catalytic cracking residue (decant oil, etc.), bottom oil derived from thermal crackers, and ethylene tar obtained during naphtha cracking.

(Other Components of the Negative Electrode Active Material Layer)
The negative electrode active material layer in this embodiment may contain optional components such as a conductive filler, a dispersant, and a binder, in addition to the negative electrode active material, as necessary.

The conductive filler is preferably made of a conductive carbonaceous material having a higher conductivity than the negative electrode active material. Such a conductive filler is preferably one or more selected from, for example, carbon black, carbon nanotubes, graphene, etc., and mixtures thereof. Examples of carbon black include ketjen black and acetylene black. As the conductive filler, it is preferable to use carbon nanotubes due to their high electronic conductivity.

(Carbon Nanotubes)
The negative electrode active material layer preferably contains carbon nanotubes. As the carbon nanotubes, single-walled carbon nanotubes and multi-walled carbon nanotubes are preferably used, and single-walled carbon nanotubes are particularly preferably used. The average fiber diameter of the carbon nanotubes is preferably 1 nm or more and less than 20 nm, more preferably 1.5 nm or more and 15 nm or less. If the average fiber diameter is 1 nm or more, the dispersibility of the carbon nanotubes is improved. If the average fiber diameter is less than 20 nm, a nonaqueous alkali metal storage element having even higher input/output characteristics can be provided.

The single-walled carbon nanotube preferably has a structure in which single-walled carbon nanotubes are bundled (hereinafter referred to as "bundled" single-walled carbon nanotubes). The fiber length of a single single-walled carbon nanotube is about several microns, but the bundled single-walled carbon nanotube is presumed to have a fiber length of about several microns to several tens of microns, with multiple single-walled carbon nanotubes linked or twisted together. The carbon nanotubes preferably cover the surface of the active material in a mesh-like manner, and it is also preferable that the carbon nanotubes bridge the active material. Here, "mesh-like" means that multiple carbon nanotubes and/or bundled single-walled carbon nanotubes cross each other in various directions on the surface of the active material to form a mesh-like structure consisting of parts covered with carbon nanotubes and parts where the active material is exposed. The average fiber diameter of the bundled single-walled carbon nanotubes is preferably 5 nm or more and 20 nm or less, more preferably 6 nm or more and 17 nm or less, and even more preferably 7 nm or more and 14 nm or less. Bundled single-walled carbon nanotubes tend to form agglomerates like thread balls, but if the average fiber diameter of the bundled single-walled carbon nanotubes is 5 nm or more, dispersibility is improved and the formation of agglomerates like thread balls tends to be suppressed, and if it is 20 nm or less, the binding between active material particles and electronic conductivity can be improved. Carbon nanotubes can be synthesized by any suitable method, such as chemical vapor deposition, arc discharge, or laser evaporation.

The carbon nanotube content in the negative electrode active material layer is preferably 0.003% by mass or more and 0.120% by mass or less, when the total mass of the negative electrode active material layer is taken as 100% by mass. If it is 0.003% by mass or more, the adhesion between the negative electrode active material particles and the electronic conductivity can be improved, and the increase in resistance in a high-temperature environment of 80°C or more can be suppressed. If it is 0.120% by mass or less, the negative electrode active material particles are not excessively covered with carbon nanotubes, and the amount of SEI (solid electrolyte interface) formed on the carbon nanotubes in the entire negative electrode active material layer is reduced, so that the ion diffusion resistance can be reduced, and the amount of pre-doped alkali metal ions to the negative electrode can be increased by reducing the irreversible capacity derived from the carbon nanotubes.

It is preferable that the carbon nanotubes are uniformly dispersed on the surface and between the particles of the negative electrode active material. By uniformly dispersing the carbon nanotubes on the surface and between the particles of the negative electrode active material, the electronic conductivity and binding between the particles of the negative electrode active material can be increased, and it becomes possible to reduce the content of the binder. Since the binder gradually decomposes in a high-temperature environment of 80°C or higher, by reducing the content of the binder, durability in a high-temperature environment of 80°C or higher can be imparted.

The dispersant is not particularly limited, but one or more selected from, for example, carboxymethylcellulose, polycarboxylic acid, polycarboxylate, polyvinylpyrrolidone, polyvinyl alcohol, and surfactants are preferably used. In particular, by using two or more of the above dispersants, it is possible to achieve both the dispersibility of the carbon nanotubes and the stability of the coating liquid. It is particularly preferable that the dispersant contains, for example, carboxymethylcellulose and one or more selected from polyvinylpyrrolidone and polyvinyl alcohol. The content of the dispersion stabilizer is preferably 0.1% by mass or more and 7.0% by mass or less with respect to 100% by mass of the solid content in the negative electrode active material layer. If the amount of the dispersion stabilizer is 7.0% by mass or less, the ingress and egress of ions into and the diffusion of ions into the negative electrode active material are not hindered, and high input/output characteristics are expressed.

Examples of binders that can be used include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), fluororubber, polyimide, latex, styrene-butadiene copolymer, and acrylic copolymer. The content of the binder in the negative electrode active material layer is preferably 0 to 20% by mass, and more preferably 0.1 to 15% by mass, relative to 100% by mass of the negative electrode active material.

[Negative electrode current collector]
The material constituting the negative electrode current collector according to this embodiment is preferably a material that has high electronic conductivity and does not dissolve in the electrolyte or deteriorate due to reaction with the electrolyte or ions, and may be, for example, a metal foil. There is no particular limitation on such metal foil, and examples thereof include aluminum foil, copper foil, nickel foil, and stainless steel foil. As the negative electrode current collector in the nonaqueous alkali metal storage element according to this embodiment, when the nonaqueous electrolyte contains lithium ions, copper foil is preferable, and when the nonaqueous electrolyte uses an electrolyte consisting of sodium ions or potassium ions, aluminum foil is preferable. The metal foil as the negative electrode current collector may be a normal metal foil that does not have unevenness or through holes, or may be a metal foil having unevenness that has been subjected to embossing, chemical etching, electrolytic deposition, blasting, or the like, or may be a metal foil having through holes such as expanded metal, punching metal, and etching foil. The thickness of the negative electrode current collector is not particularly limited as long as it can sufficiently maintain the shape and strength of the negative electrode, and is, for example, 1 to 100 μm.

The thickness of the negative electrode active material layer is preferably 10 μm to 70 μm per side of the current collector, and more preferably 15 μm to 60 μm. If the thickness is 10 μm or more, good charge/discharge capacity can be achieved. On the other hand, if the thickness is 70 μm or less, the cell volume can be reduced, and the energy density can be increased. When the negative electrode current collector has holes, the thickness of the negative electrode active material layer refers to the average thickness per side of the portion of the negative electrode current collector that does not have holes.

<Positive electrode>
The positive electrode of this embodiment has a positive electrode current collector and a positive electrode active material layer provided on one or both sides of the positive electrode current collector. The positive electrode active material layer according to this embodiment contains a positive electrode active material and an alkali metal carbonate.

As described later, in this embodiment, it is preferable to pre-dope the negative electrode with alkali metal ions during the assembly process of the storage element, and as a pre-doping method, it is preferable to assemble the storage element using a positive electrode precursor containing an alkali metal carbonate, a negative electrode, a separator, an exterior body, and a non-aqueous electrolyte, and then apply a voltage between the positive electrode precursor and the negative electrode. In this case, the alkali metal carbonate may be contained in the positive electrode precursor in any manner. For example, the alkali metal carbonate may be present between the positive electrode collector and the positive electrode active material layer, may be present on the surface of the positive electrode active material layer, or may be present in the positive electrode active material layer. The alkali metal carbonate is preferably contained in the positive electrode active material layer formed on the positive electrode collector of the positive electrode precursor. In such an embodiment, as the alkali metal ions are pre-doped into the negative electrode, vacancies are formed in the positive electrode active material layer, and the effective area of the positive electrode active material layer is increased. In this specification, the positive electrode before the alkali metal doping process is defined as the "positive electrode precursor," and the positive electrode after the alkali metal doping process is defined as the "positive electrode."

[Cathode active material layer]
The positive electrode active material layer contains a positive electrode active material and an alkali metal carbonate. In addition to these, the positive electrode active material layer may contain optional components described below, as necessary.

(Cathode active material)
The positive electrode active material preferably contains activated carbon, and may further contain graphene, a conductive polymer, a lithium transition metal oxide, and the like in addition to activated carbon.

When activated carbon is used as the positive electrode active material, there is no particular limitation on the type of activated carbon and its raw material, but in order to achieve both high input/output characteristics and high energy density, it is preferable to optimally control the pores of the activated carbon. Specifically, when the amount of mesopores derived from pores with a diameter of 20 Å to 500 Å calculated by the BJH method is V ₁ (cc/g), and the amount of micropores derived from pores with a diameter of less than 20 Å calculated by the MP method is V ₂ (cc/g),
(1) For high input/output characteristics, activated carbon that satisfies 0.3<V1 _≦ 0.8 and 0.5≦ _V2 ≦1.0 and has a specific surface area of 1,500 ^m2 /g or more and 3,000 ^m2 /g or less as measured by the BET method (hereinafter, also referred to as activated carbon 1) is preferred.
(2) In order to obtain a high energy density, activated carbon that satisfies 0.8<V1 _≦ 2.5 and 0.8< _V2 ≦3.0 and has a specific surface area measured by the BET method of 2,300 ^m2 /g or more and 4,000 ^m2 /g or less (hereinafter also referred to as activated carbon 2) is preferred.

The BET specific surface area, mesopore volume, micropore volume, and average pore diameter of the active material in this embodiment are values that are determined by the following method. The sample is vacuum dried at 200°C for 24 hours, and the adsorption and desorption isotherms are measured using nitrogen as the adsorbate. Using the adsorption side isotherms obtained here, the BET specific surface area is calculated by the BET multipoint method or BET single point method, the mesopore volume is calculated by the BJH method, and the micropore volume is calculated by the MP method. The BJH method is a calculation method generally used to analyze mesopores, and was proposed by Barrett, Joyner, Halenda, et al. (Non-Patent Document 1). The MP method refers to a method of determining the micropore volume, micropore area, and micropore distribution using the "t-plot method" (Non-Patent Document 2), and is a method devised by R. S. Mikhair, Brunauer, and Bodor (Non-Patent Document 3). The average pore size refers to the total pore volume per mass of the sample, which is obtained by measuring the equilibrium adsorption amount of nitrogen gas at each relative pressure at liquid nitrogen temperature, divided by the BET specific surface area.

(Activated carbon 1)
The mesopore volume V ₁ of the activated carbon 1 is preferably a value larger than 0.3 cc/g in terms of increasing the input/output characteristics when the positive electrode material is incorporated into an electric storage element. _{V 1} is preferably 0.8 cc/g or less in terms of suppressing a decrease in the bulk density of the positive electrode. _{V 1} is more preferably 0.35 cc/g or more and 0.7 cc/g or less, and even more preferably 0.4 cc/g or more and 0.6 cc/g or less. The micropore volume V ₂ of the activated carbon 1 is preferably 0.5 cc/g or more in order to increase the specific surface area of the activated carbon and increase the capacity. V ₂ is preferably 1.0 cc/g or less in terms of suppressing the bulk of the activated carbon, increasing the density as an electrode, and increasing the capacity per unit volume. _{V 2} is more preferably 0.6 cc/g or more and 1.0 cc/g or less, and even more preferably 0.8 cc/g or more and 1.0 cc/g or less. The ratio of mesopore volume _V1 to micropore volume _V2 ( _V1 / _V2 ) is preferably in the range of 0.3≦ _V1 / _V2 ≦0.9. In other words, from the viewpoint of increasing the ratio of mesopore volume to micropore volume to an extent that a decrease in output characteristics can be suppressed while maintaining high capacity, _V1 / _V2 is preferably 0.3 or more. On the other hand, from the viewpoint of increasing the ratio of micropore volume to mesopore volume to an extent that a decrease in capacity can be suppressed while maintaining high output characteristics, _V1 / _V2 is preferably 0.9 or less. A more preferable range for _V1 / _V2 is 0.4≦ _V1 / _V2 ≦0.7, and an even more preferable range for _V1 / _V2 is 0.55≦ _V1 / _V2 ≦0.7.

The average pore diameter of the activated carbon 1 is preferably 17 Å or more, more preferably 18 Å or more, and most preferably 20 Å or more, from the viewpoint of maximizing the output of the obtained electric storage element. Also, from the viewpoint of maximizing the capacity, the average pore diameter of the activated carbon 1 is preferably 25 Å or less. The BET specific surface area of the activated carbon 1 is preferably 1,500 m ² /g or more and 3,000 ^{m 2} /g or less, and more preferably 1,500 m ² /g or more and 2,500 m ² /g or less. When the BET specific surface area is 1,500 ^{m 2} /g or more, a good energy density is easily obtained, while when the BET specific surface area is 3,000 ^{m 2} /g or less, it is not necessary to add a large amount of binder to maintain the strength of the electrode, so the performance per electrode volume is high.

Activated carbon 1 having the above-mentioned characteristics can be obtained, for example, by using the raw materials and processing method described below. In this embodiment, the carbon source used as the raw material of activated carbon 1 is not particularly limited. For example, plant-based raw materials such as wood, wood flour, coconut shells, by-products of pulp production, bagasse, and blackstrap molasses; fossil-based raw materials such as peat, lignite, brown coal, bituminous coal, anthracite, petroleum distillation residue components, petroleum pitch, coke, and coal tar; various synthetic resins such as phenol resin, vinyl chloride resin, vinyl acetate resin, melamine resin, urea resin, resorcinol resin, celluloid, epoxy resin, polyurethane resin, polyester resin, and polyamide resin; synthetic rubbers such as polybutylene, polybutadiene, and polychloroprene; other synthetic wood, synthetic pulp, and carbonized products thereof. Among these raw materials, plant-based raw materials such as coconut shells and wood flour, and carbonized products thereof are preferred from the viewpoint of mass production and cost, and coconut shell carbonized products are particularly preferred.

As the carbonization and activation method for converting these raw materials into the activated carbon 1, for example, known methods such as a fixed bed method, a moving bed method, a fluidized bed method, a slurry method, a rotary kiln method, etc. can be adopted. As a method for carbonizing these raw materials, a method of burning them at about 400 to 700 ° C (preferably 450 to 600 ° C) for about 30 minutes to 10 hours using an inert gas such as nitrogen, carbon dioxide, helium, argon, xenon, neon, carbon monoxide, combustion exhaust gas, or a mixed gas containing these inert gases as the main component and other gases can be mentioned. As a method for activating the carbonized material obtained by the carbonization method described above, a gas activation method in which the carbonization is performed using an activation gas such as water vapor, carbon dioxide, or oxygen is preferably used. Of these, a method of using water vapor or carbon dioxide as the activation gas is preferable. In this activation method, it is preferable to activate the obtained carbonized material by heating it to 800 to 1,000°C over 3 to 12 hours (preferably 5 to 11 hours, more preferably 6 to 10 hours) while supplying the activation gas at a rate of 0.5 to 3.0 kg/h (preferably 0.7 to 2.0 kg/h). Furthermore, prior to the activation treatment of the carbonized material described above, the carbonized material may be first activated. In this first activation, a method of gas activation by firing the carbon material at a temperature of less than 900°C using an activation gas such as water vapor, carbon dioxide, or oxygen can be preferably used. By appropriately combining the firing temperature and firing time in the carbonization method described above with the activation gas supply amount, heating rate, and maximum activation temperature in the activation method, activated carbon 1 that can be used in this embodiment can be produced.

The average particle diameter of the activated carbon 1 is preferably 2 to 20 μm. If the average particle diameter is 2 μm or more, the density of the active material layer is high, so the capacity per electrode volume tends to be high. Note that a small average particle diameter may result in the drawback of low durability, but if the average particle diameter is 2 μm or more, such a drawback is unlikely to occur. On the other hand, if the average particle diameter is 20 μm or less, it tends to be more suitable for high-speed charging and discharging. The average particle diameter of the activated carbon 1 is more preferably 2 to 15 μm, and even more preferably 3 to 10 μm.

(Activated carbon 2)
The mesopore volume V ₁ of the activated carbon 2 is preferably a value greater than 0.8 cc/g from the viewpoint of increasing the output characteristics when the positive electrode material is incorporated into an electric storage element. V ₁ is preferably 2.5 cc/g or less from the viewpoint of suppressing a decrease in the capacity of the electric storage element. _{V 1} is more preferably 1.00 cc/g or more and 2.0 cc/g or less, and even more preferably 1.2 cc/g or more and 1.8 cc/g or less. The micropore volume V ₂ of the activated carbon 2 is preferably a value greater than 0.8 cc/g in order to increase the specific surface area of the activated carbon and increase the capacity. V ₂ is preferably 3.0 cc/g or less from the viewpoint of increasing the density of the activated carbon as an electrode and increasing the capacity per unit volume. _{V 2} is more preferably greater than 1.0 cc/g and 2.5 cc/g or less, and even more preferably 1.5 cc/g or more and 2.5 cc/g or less. The activated carbon 2 having the above-mentioned mesopore volume and micropore volume has a higher BET specific surface area than the activated carbon used for conventional electric double layer capacitors or lithium ion capacitors. The specific value of the BET specific surface area of the activated carbon 2 is preferably 2,300 m ² /g or more and 4,000 m ² /g or less, more preferably 3,000 m ² /g or more and 4,000 m ² /g or less, and even more preferably 3,200 m ² /g or more and 3,800 m ² /g or less. When the BET specific surface area is 2,300 m ² /g or more, a good energy density is easily obtained, and when the BET specific surface area is 4,000 m ² /g or less, it is not necessary to add a large amount of binder to maintain the strength of the electrode, so that the performance per electrode volume is high.

Activated carbon 2 having the above-mentioned characteristics can be obtained, for example, by using the raw materials and processing methods described below. The carbonaceous material used as the raw material for activated carbon 2 is not particularly limited as long as it is a carbon source that is normally used as an activated carbon raw material, and examples include plant-based raw materials such as wood, wood powder, and coconut shells; fossil-based raw materials such as petroleum pitch and coke; and various synthetic resins such as phenolic resin, furan resin, vinyl chloride resin, vinyl acetate resin, melamine resin, urea resin, and resorcinol resin. Among these raw materials, phenolic resin and furan resin are particularly preferred because they are suitable for producing activated carbon with a high specific surface area.

Methods for carbonizing these raw materials or methods for heating during activation treatment include well-known methods such as the fixed bed method, moving bed method, fluidized bed method, slurry method, and rotary kiln method. The atmosphere used during heating is an inert gas such as nitrogen, carbon dioxide, helium, argon, or a gas containing these inert gases as the main component and mixed with other gases. The carbonization temperature is generally about 400 to 700°C and the material is fired for about 0.5 to 10 hours. Methods for activating charcoal include the gas activation method, in which the material is fired using an activation gas such as water vapor, carbon dioxide, or oxygen, and the alkali metal activation method, in which the material is mixed with an alkali metal compound and then heated, but the alkali metal activation method is preferred for producing activated carbon with a high specific surface area. In this activation method, the carbide and an alkali metal compound such as KOH or NaOH are mixed in a mass ratio of 1:1 or more (the amount of the alkali metal compound is the same as or greater than the amount of the carbide), and then heated in an inert gas atmosphere at 600 to 900°C for 0.5 to 5 hours, after which the alkali metal compound is washed and removed with acid and water, and then dried. In order to increase the amount of micropores and not increase the amount of mesopores, it is recommended to mix a larger amount of carbide with KOH during activation. In order to increase both the amount of micropores and the amount of mesopores, it is recommended to use a larger amount of KOH. In addition, in order to mainly increase the amount of mesopores, it is preferable to perform steam activation after performing alkali activation treatment. The average particle size of the activated carbon 2 is preferably 2 μm to 20 μm, more preferably 3 μm to 10 μm.

(Use of activated carbon)
The activated carbons 1 and 2 may each be one type of activated carbon, or a mixture of two or more types of activated carbons, each of which exhibits the above-mentioned characteristic values as a whole mixture. The activated carbons 1 and 2 may be used by selecting one of them, or may be used by mixing the two. The positive electrode active material may contain a material other than the activated carbons 1 and 2 (for example, an activated carbon that does not have the specific _V1 and/or _V2 described above, or a material other than the activated carbon (for example, a conductive polymer, etc.)). In an exemplary embodiment, the total content of the activated carbons 1 and 2 is preferably 40.0 mass% or more and 70.0 mass% or less, more preferably 45.0 mass% or more and 65.0 mass% or less. If this value is 40.0 mass% or more, the energy density can be increased. If this value is 70.0 mass% or less, the high temperature durability and high voltage durability can be improved.

(Lithium transition metal oxides)
The positive electrode active material layer preferably further contains a lithium transition metal compound. The non-aqueous alkali metal storage element can have a high capacity by containing a lithium transition metal oxide. The lithium transition metal oxide includes a transition metal oxide capable of absorbing and releasing lithium. There is no particular limitation on the transition metal oxide used as the positive electrode active material. Examples of the transition metal oxide include oxides containing at least one element selected from the group consisting of cobalt, nickel, manganese, iron, vanadium, and chromium. Specific examples of the transition metal oxide include oxides represented by the following formula:
Li _x CoO ₂ {wherein x satisfies 0≦x≦1.}
Li _x NiO ₂ {wherein x satisfies 0≦x≦1.}
Li _x Ni _y M _(1-y) O ₂ {wherein M is at least one element selected from the group consisting of Co, Mn, Al, Fe, Mg, and Ti, x satisfies 0≦x≦1, and y satisfies 0.05<y<0.97.}
Li _x Ni _1/3 Co _1/3 Mn _1/3 O ₂ {wherein x satisfies 0≦x≦1.}
Li _x MnO ₂ {wherein x satisfies 0≦x≦1.}
α-Li _x FeO ₂ {wherein x satisfies 0≦x≦1.}
Li _x VO ₂ {wherein x satisfies 0≦x≦1.}
Li _x CrO ₂ {wherein x satisfies 0≦x≦1.}
Li _x FePO ₄ {wherein x satisfies 0≦x≦1.}
Li _x MnPO ₄ {wherein x satisfies 0≦x≦1.}
Li _z V ₂ (PO ₄ ) ₃ {wherein z satisfies 0≦z≦3.}
Li _x Mn ₂ O ₄ {wherein x satisfies 0≦x≦1.}
Li _x M _y Mn _(2-y) O ₄ {wherein M is at least one element selected from the group consisting of Co, Mn, Al, Fe, Mg, and Ti, x satisfies 0≦x≦1, and y satisfies 0.05<y<0.97.}
Li _x Ni _a Co _b Al _(1-a-b) O ₂ {wherein x satisfies 0≦x≦1, and a and b satisfy 0.02<a<0.97 and 0.02<b<0.97.}
Li _x Ni _c Co _d Mn _(1-cd) O ₂ {wherein x satisfies 0≦x≦1, and c and d satisfy 0.02<c<0.97 and 0.02<d<0.97.}
Among these, from the viewpoints of high capacity, low resistance, cycle characteristics, decomposition of alkali metal carbonate, and suppression of loss of positive electrode active material during pre-doping, the compounds represented by the above formula Li _x Ni _a Co _b Al _(1-ab) O ₂ , Li _x Ni _c Co _d Mn _(1-cd) O ₂ , Li _x CoO ₂ , Li _x Mn ₂ O ₄ , or Li _x FePO ₄ , Li _x MnPO ₄ , and Li _z V ₂ (PO ₄ ) ₃ are preferred.

In this embodiment, if the positive electrode precursor contains an alkali metal carbonate different from the positive electrode active material, the alkali metal carbonate becomes a dopant source for the alkali metal during pre-doping, and the negative electrode can be pre-doped. Therefore, even if the transition metal compound does not contain lithium ions in advance (i.e., even if x = 0 or z = 0), it can be used as a non-aqueous alkali metal storage element for electrochemical charging and discharging. The content of lithium transition metal oxide in the positive electrode active material layer of the positive electrode precursor is preferably 8.0 mass% or more and 30.0 mass% or less. If the content of lithium transition metal oxide is 8.0 mass% or more, the capacity of the non-aqueous alkali metal storage element can be increased. If the content of lithium transition metal oxide is 30.0 mass% or less, the resistance of the non-aqueous alkali metal storage element can be reduced.

(alkali metal carbonate)
In this embodiment, the positive electrode active material layer of the positive electrode precursor may contain an alkali metal carbonate other than the positive electrode active material. The alkali metal carbonate decomposes in the positive electrode precursor to release cations, and is reduced at the negative electrode, thereby pre-doping the negative electrode. As a result of the positive electrode active material layer of the positive electrode precursor containing an alkali metal carbonate, it is preferable that the positive electrode active material layer of the positive electrode obtained after pre-doping also contains an alkali metal carbonate.

Specific examples of such alkali metal carbonates include lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, and cesium carbonate. In particular, lithium carbonate, sodium carbonate, and potassium carbonate are more preferably used, and lithium carbonate is particularly preferably used from the viewpoint of high capacity per unit mass.

The alkali metal carbonate is preferably in the form of fine particles. In this embodiment, the average particle diameter of the alkali metal carbonate is preferably 0.1 μm or more and 10 μm or less. If the average particle diameter is 0.1 μm or more, the alkali metal carbonate has excellent dispersibility in the positive electrode precursor. If the average particle diameter is 10 μm or less, the surface area of the alkali metal carbonate increases, so the decomposition reaction proceeds efficiently. In addition, the average particle diameter of the alkali metal carbonate is preferably smaller than the average particle diameter of the activated carbon described above. If the average particle diameter of the alkali metal carbonate is smaller than the average particle diameter of the activated carbon, the electronic conductivity of the positive electrode active material layer increases, which can contribute to reducing the resistance of the electrode body or the storage element. There is no particular limit to the method for measuring the average particle diameter of the alkali metal carbonate in the positive electrode precursor, but it can be calculated from SEM images and SEM-EDX images of the positive electrode cross section. As for the method for forming the positive electrode cross section, BIB processing can be used, in which an Ar beam is irradiated from above the positive electrode and a smooth cross section is created along the edge of a shielding plate placed directly above the sample. Various methods can be used to finely pulverize the alkali metal carbonate. For example, grinding machines such as ball mills, bead mills, ring mills, jet mills, and rod mills can be used.

The content of the alkali metal carbonate contained in the positive electrode active material layer of the positive electrode precursor is preferably 25.0% by mass or more and 50.0% by mass or less, when the total mass of the positive electrode active material layer is taken as 100% by mass. If this value is 25.0% by mass or more, a sufficient amount of alkali metal ions can be pre-doped into the negative electrode, and the capacity of the non-aqueous alkali metal storage element can be increased. If this value is 50.0% by mass or less, the electronic conductivity in the positive electrode precursor can be increased, and the alkali metal carbonate can be decomposed efficiently. The above-mentioned alkali metal elements and alkaline earth metal elements can be quantified by ICP-AES, atomic absorption spectrometry, X-ray fluorescence spectrometry, neutron activation analysis, ICP-MS, etc.

(Other Components of Positive Electrode Active Material Layer)
In addition to the positive electrode active material and the alkali metal carbonate, the positive electrode active material layer of the positive electrode precursor in the present invention may contain other components such as a dispersant, a conductive filler, and a binder, as necessary. The dispersant, conductive filler, and binder in the positive electrode active material layer may be appropriately selected from those exemplified above as the dispersant, conductive filler, and binder in the negative electrode active material layer. The content of the dispersant and binder in the positive electrode active material layer may be within the above-mentioned range as the content of the dispersant and binder in the negative electrode active material, respectively. The content of the conductive filler in the positive electrode active material layer of the positive electrode precursor is preferably 0 to 20 mass% relative to 100 mass% of the positive electrode active material. From the viewpoint of high input, it is preferable to mix as much conductive filler as possible. However, if the mixing amount is more than 20 mass%, the content of the positive electrode active material in the positive electrode active material layer is reduced, so that the energy density per volume of the positive electrode active material layer is reduced, which is not preferable.

(Carbon Nanotubes)
The positive electrode active material layer preferably contains carbon nanotubes as a conductive filler. The carbon nanotubes contained in the positive electrode active material layer may be the same as the carbon nanotubes contained in the negative electrode active material layer. The carbon nanotubes may be contained in the positive electrode and/or the negative electrode.

The carbon nanotube content in the positive electrode active material layer is preferably 0.010% by mass or more and 0.200% by mass or less when the total mass of the positive electrode active material layer is 100% by mass. If the content is 0.010% by mass or more, the adhesion between the positive electrode active material particles and electronic conductivity can be improved, and the increase in resistance in a high-temperature environment of 80°C or more can be suppressed. At the same time, the decomposition of the electrolyte can be suppressed by promoting the decomposition of the alkali metal carbonate contained in the positive electrode active material layer, and the pre-doping amount of the alkali metal ions to the negative electrode can be increased. If the content is 0.200% by mass or less, the carbon nanotubes do not excessively cover the positive electrode active material particles, and the amount of SEI (solid electrolyte interface) formed on the carbon nanotubes in the entire positive electrode active material layer is reduced, so that the diffusion resistance of ions can be reduced.

It is preferable that the carbon nanotubes are uniformly dispersed on the surface and between the particles of the positive electrode active material. By uniformly dispersing the carbon nanotubes on the surface and between the particles of the positive electrode active material, the electronic conductivity and binding between the particles of the positive electrode active material can be increased, and it becomes possible to reduce the content of the binder. Since the binder gradually decomposes in a high-temperature environment of 80°C or higher, reducing the content of the binder can impart durability in a high-temperature environment of 80°C or higher.

By uniformly dispersing carbon nanotubes on the surface and between the particles of the positive electrode active material, it is possible to increase the electronic conductivity and binding between the particles of the positive electrode active material, and to reduce the binder content. Since the binder gradually decomposes in a high temperature environment of 80°C or higher, or in a high voltage environment of 4.1V or higher, reducing the binder content can impart high temperature durability of 80°C or higher and high voltage durability of 4.1V or higher.

[Positive electrode current collector]
The material constituting the positive electrode current collector according to this embodiment is not particularly limited as long as it has high electronic conductivity and does not deteriorate due to elution in the electrolytic solution or reaction with the electrolyte or ions, but metal foil is preferred. Aluminum foil is more preferred as the positive electrode current collector in the nonaqueous alkali metal storage element according to this embodiment. The metal foil may be a normal metal foil without unevenness or through holes, or may be a metal foil with unevenness obtained by embossing, chemical etching, electrolytic deposition, blasting, or the like, or may be a metal foil with through holes such as expanded metal, punching metal, and etching foil. From the viewpoint of the alkali metal doping process described later, non-porous aluminum foil is more preferred, and it is particularly preferred that the surface of the aluminum foil is roughened. The thickness of the positive electrode current collector is not particularly limited as long as it can sufficiently maintain the shape and strength of the positive electrode, but for example, 1 to 100 μm is preferred. It is preferable to provide an anchor layer containing a conductive material such as graphite, flake graphite, carbon nanotubes, graphene, ketjen black, acetylene black, vapor-grown carbon fiber, etc. on the surface of the metal foil. By providing the anchor layer, electrical conduction between the positive electrode current collector and the positive electrode active material layer can be improved and resistance can be reduced. The thickness of the anchor layer is preferably 0.1 μm or more and 5 μm or less per side of the positive electrode current collector.

The positive electrode current collector according to this embodiment has a protective layer and a non-protective layer. By providing a protective layer on the positive electrode current collector, it is possible to suppress corrosion of the current collector in a high-temperature, high-voltage environment. In addition, by providing a non-protective layer on the positive electrode current collector, it is possible to increase the welding strength between the positive electrode current collector and the positive electrode terminal and reduce the electron transfer resistance of the welded portion. The protective layer on the positive electrode current collector may be present on one side or both sides of the positive electrode current collector.

The material of the positive electrode protective layer is not limited, but examples thereof include protective tape, a carbon layer, and an inorganic fine particle layer. The protective tape is preferably insulating, and the base material is preferably at least one material selected from the group consisting of polyether ketone, polyphenylene sulfide, polyethylene terephthalate, polyethylene naphthalate, polyamide, polyimide, aramid, acetate cloth, and polytetrafluoroethylene tape. The carbon layer is not particularly limited, but at least one carbon material selected from the group consisting of carbon black, graphite, graphene, and carbon nanotubes is preferably used. The carbon layer can also be used as a protective layer by providing the above-mentioned anchor layer wider than the active material layer. The inorganic fine particle layer is not particularly limited, but at least one material selected from the group consisting of aluminum oxide, silicon oxide, and zirconia is preferably used as the inorganic fine particles.

The non-protective layer refers to the portion of the positive electrode that does not have a protective layer, and the material may be any. For example, the portion of the positive electrode where the positive electrode current collector is exposed may be called the non-protective layer.

The thickness of the positive electrode active material layer according to this embodiment is preferably 10 μm or more and 200 μm or less per side of the positive electrode current collector. The thickness of the positive electrode active material layer is more preferably 20 μm or more and 150 μm or less per side, and even more preferably 30 μm or more and 100 μm or less. If the thickness is 10 μm or more, sufficient charge/discharge capacity can be achieved. On the other hand, if the thickness is 200 μm or less, the ion diffusion resistance in the electrode can be kept low. Therefore, sufficient output characteristics can be obtained, and the cell volume can be reduced, and therefore the energy density can be increased. Note that the thickness of the positive electrode active material layer in the case where the positive electrode current collector has through holes or unevenness refers to the average thickness per side of the part of the positive electrode current collector that does not have through holes or unevenness.

<Separator>
The positive electrode precursor and the negative electrode are laminated through a separator, or laminated and wound to form an electrode laminate or an electrode wound body having the positive electrode precursor, the negative electrode, and the separator. As the separator, a polyethylene microporous film or a polypropylene microporous film used in a lithium ion secondary battery, or a cellulose nonwoven paper used in an electric double layer capacitor, or the like, can be used. A film made of organic or inorganic fine particles may be laminated on one or both sides of these separators. In addition, organic or inorganic fine particles may be contained inside the separator. The thickness of the separator is preferably 5 μm or more and 35 μm or less. By making the thickness 5 μm or more, it is preferable because self-discharge due to internal micro-short circuits tends to be small. On the other hand, by making the thickness 35 μm or less, it is preferable because the output characteristics of the storage element tend to be high. In addition, the thickness of the film made of organic or inorganic fine particles is preferably 1 μm or more and 10 μm or less. By making this film 1 μm or more thick, it is preferable because self-discharge due to internal micro-short circuits tends to be small. On the other hand, by making the thickness of this film 10 μm or less, the output characteristics of the electricity storage element tend to be improved, which is preferable.

<Exterior body>
As the exterior body, a metal can, a laminate film, etc. can be used. As the metal can, one made of aluminum is preferable. The metal can may be, for example, rectangular, round, cylindrical, etc. As the laminate film, a film in which a metal foil and a resin film are laminated is preferable, and a three-layer structure consisting of an outer layer resin film/metal foil/interior resin film is exemplified. The outer layer resin film is for preventing the metal foil from being damaged by contact, etc., and resins such as nylon or polyester can be suitably used. The metal foil is for preventing the permeation of moisture and gas, and foils such as copper, aluminum, and stainless steel can be suitably used. In addition, the interior resin film is for protecting the metal foil from the electrolyte stored inside and for melt-sealing the opening when the exterior body is heat-sealed, and polyolefin, acid-modified polyolefin, etc. can be suitably used.

<Electrolyte>
The electrolytic solution in this embodiment is a non-aqueous electrolytic solution. That is, this electrolytic solution contains a non-aqueous solvent. The non-aqueous electrolytic solution contains 0.5 mol/L or more of an alkali metal salt based on the total amount of the non-aqueous electrolytic solution. That is, the non-aqueous electrolytic solution contains an alkali metal salt as an electrolyte. Examples of the non-aqueous solvent contained in the non-aqueous electrolytic solution include cyclic carbonates such as ethylene carbonate and propylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

Examples of electrolyte salts containing alkali metal ions that dissolve in non-aqueous solvents as described above include MFSI, _MBF4 , _MPF6 , _MClO4 , LiB( _C2O4 ) ₂ , and _LiBF2 ( _C2O4 ), where M is Li, Na, _K , _Rb , or Cs.

The electrolyte salt concentration in the electrolyte solution is preferably in the range of 0.5 to 2.0 mol/L. At an electrolyte salt concentration of 0.5 mol/L or more, anions are sufficiently present, and the capacity of the nonaqueous alkali metal storage element is maintained. On the other hand, at an electrolyte salt concentration of 2.0 mol/L or less, the salt is sufficiently dissolved in the electrolyte solution, and the appropriate viscosity and conductivity of the electrolyte solution are maintained. The electrolyte salt concentration of MPF ₆ contained in the nonaqueous electrolyte solution in this embodiment is preferably in the range of 0.00 mol/L or more and 0.05 mol/L or less. If the concentration is 0.05 mol/L or less, decomposition of hexafluorophosphate ions in a high-temperature environment can be suppressed, and an increase in resistance in a high-temperature high-voltage cycle can be suppressed. From the same viewpoint, in the nonaqueous alkali metal storage element according to this embodiment, the concentration of hexafluorophosphate ions (PF ₆ ⁻ ) contained in the nonaqueous electrolyte solution is preferably 0.00 mol/L or more and 0.05 mol/L or less.

The amount of moisture contained in the non-aqueous electrolyte is preferably 0 ppm or more and 500 ppm or less. If the amount of moisture is 0 ppm or more, the alkali metal carbonate in the positive electrode precursor can be slightly dissolved, and pre-doping can be performed under mild conditions, resulting in high capacity and low resistance. If the amount of moisture is 500 ppm or less, decomposition of the electrolyte is suppressed, improving high-temperature storage characteristics. The amount of moisture in the electrolyte can be measured by the Karl Fischer method described above.

<<Method for producing a non-aqueous alkali metal storage element>>
[Production of negative electrode]
The negative electrode has a negative electrode active material layer on one or both sides of the negative electrode current collector. In a typical embodiment, the negative electrode active material layer is fixed to the negative electrode current collector. The negative electrode can be manufactured by known manufacturing techniques for electrodes in lithium ion secondary batteries, electric double layer capacitors, etc. For example, various materials including the negative electrode active material are dispersed or dissolved in water or an organic solvent to prepare a slurry-like coating liquid, and the coating liquid is applied to one or both sides of the negative electrode current collector to form a coating film, which is then dried to obtain a negative electrode. The obtained negative electrode may be pressed to adjust the film thickness or bulk density of the negative electrode active material layer.

The method for preparing the coating liquid is not particularly limited, but can be suitably carried out using a dispersing machine such as a homodisper, a multi-axis dispersing machine, a planetary mixer, or a thin film rotating high-speed mixer. To obtain a coating liquid in a well-dispersed state, it is preferable to disperse the coating liquid at a peripheral speed of 1 m/s or more and 50 m/s or less. A peripheral speed of 1 m/s or more is preferable because various materials are well dissolved or dispersed. A peripheral speed of 50 m/s or less is preferable because various materials are not destroyed by heat or shear force due to dispersion, and re-agglomeration is suppressed.

The degree of dispersion of the coating liquid is preferably 0.1 μm or more and 100 μm or less in particle size measured with a particle gauge. The upper limit of the degree of dispersion is more preferably 80 μm or less, and even more preferably 50 μm or less, in particle size measured with a particle gauge. If the particle size is within this range, the material is not crushed when preparing the coating liquid, and nozzle clogging during coating and the occurrence of streaks in the coating film are suppressed, allowing for stable coating.

The viscosity (ηb) of the coating liquid is preferably 100 mPa·s or more and 10,000 mPa·s or less, more preferably 300 mPa·s or more and 5,000 mPa·s or less, and even more preferably 500 mPa·s or more and 3,000 mPa·s or less. If the viscosity (ηb) is 100 mPa·s or more, dripping during coating film formation is suppressed, and the coating film width and thickness can be well controlled. Furthermore, if the viscosity is 10,000 mPa·s or less, there is little pressure loss in the flow path of the coating liquid when using a coating machine, stable coating can be performed, and the coating film thickness can be easily controlled.

The TI value (thixotropy index value) of the coating liquid is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.5 or more. If the TI value is 1.1 or more, the coating width and thickness can be well controlled.

The coating film of the negative electrode active material layer is not particularly limited, but a coating machine such as a die coater, a comma coater, a knife coater, or a gravure coater can be preferably used. The coating film may be formed by single-layer coating, or may be formed by multi-layer coating. In the case of multi-layer coating, the coating liquid composition may be adjusted so that the content of the components in each layer of the coating film is different. When applying the coating film to the negative electrode current collector, it may be multi-line coating, intermittent coating, or multi-line intermittent coating. When forming the negative electrode active material layer on both sides of the negative electrode current collector, sequential coating may be performed by coating one side of the negative electrode current collector and drying, and then coating the other side and drying, or simultaneous double-sided coating may be performed by simultaneously coating both sides of the negative electrode current collector with the coating liquid and drying. In this case, the difference in thickness between the negative electrode active material layers on the front and back sides of the negative electrode current collector is preferably 10% or less of the average thickness of both. The closer the mass ratio and film thickness ratio of the negative electrode active material layer on the front and back surfaces are to 1.0, the less the charge/discharge load is concentrated on one surface, improving the high-load charge/discharge cycle characteristics.

After forming a coating film of the negative electrode active material layer on the negative electrode current collector, the coating film is dried. The coating film of the negative electrode precursor is dried by an appropriate drying method such as hot air drying or infrared (IR) drying, preferably far infrared rays, near infrared rays, or hot air. The coating film may be dried at a single temperature, or may be dried by changing the temperature in multiple stages. A combination of multiple drying methods may be used. The drying temperature is preferably 25°C or higher and 200°C or lower, more preferably 40°C or higher and 180°C or lower, and even more preferably 50°C or higher and 160°C or lower. If the drying temperature is 25°C or higher, the solvent in the coating film can be sufficiently volatilized. On the other hand, if the drying temperature is 200°C or lower, the coating film can be cracked due to sudden solvent evaporation, uneven distribution of the binder due to migration, and oxidation of the negative electrode current collector or the negative electrode active material layer can be suppressed.

The amount of moisture contained in the negative electrode active material layer after drying is preferably 0.1% by mass or more and 10% by mass or less, when the total mass of the negative electrode active material layer is 100% by mass. If the amount of moisture is 0.1% by mass or more, deterioration of the binder due to excessive drying can be suppressed, and resistance can be reduced. If the amount of moisture is 10% by mass or less, deactivation of alkali metal ions can be suppressed, and capacity can be increased. When N-methyl-2-pyrrolidone (NMP) is used to adjust the coating liquid, the content of NMP in the negative electrode active material layer after drying is preferably 0.1% by mass or more and 10% by mass or less, when the total mass of the negative electrode active material layer is 100%. The amount of moisture contained in the negative electrode active material layer can be measured, for example, by Karl Fischer titration (JIS 0068 (2001) "Method for measuring moisture in chemical products"). The amount of NMP contained in the negative electrode active material layer can be determined by immersing the negative electrode active material layer in ethanol with a mass that is 50 to 100 times the mass of the negative electrode active material layer for 24 hours in a 25°C environment to extract the NMP, and then measuring with GC/MS and quantifying it based on a previously prepared calibration curve.

For pressing the negative electrode active material layer, an appropriate press machine such as a hydraulic press machine, a vacuum press machine, or a roll press machine can be preferably used. The thickness, bulk density, and electrode strength of the negative electrode active material layer can be adjusted by the press pressure, the gap between the press rolls, and the surface temperature of the press section, which will be described later. The press pressure is preferably 0.5 kN/cm or more and 20 kN/cm or less, more preferably 1 kN/cm or more and 10 kN/cm or less, and even more preferably 2 kN/cm or more and 7 kN/cm or less. If the press pressure is 0.5 kN/cm or more, the electrode strength can be sufficiently high. On the other hand, if the press pressure is 20 kN/cm or less, the negative electrode does not bend or wrinkle, and the negative electrode active material layer can be adjusted to a desired thickness or bulk density. When a roll press machine is used for pressing, the gap between the press rolls can be set to an appropriate value so that the negative electrode active material layer has the desired thickness and bulk density. The press speed can be set to an appropriate speed at which the negative electrode does not bend or wrinkle.

The surface temperature of the press part may be room temperature, or may be heated if necessary. The lower limit of the surface temperature of the press part when heated is preferably 60°C minus the melting point of the binder used, more preferably 45°C minus the melting point of the binder, and even more preferably 30°C minus the melting point of the binder. On the other hand, the upper limit of the surface temperature of the press part when heated is preferably 50°C plus the melting point of the binder used, more preferably 30°C plus the melting point of the binder, and even more preferably 20°C plus the melting point of the binder. For example, when polyvinylidene fluoride (melting point 150°C) is used as the binder, it is preferable to heat the press part to 90°C to 200°C, more preferably 105°C to 180°C, and even more preferably 120°C to 170°C. In addition, when a styrene-butadiene copolymer (melting point 100°C) is used as the binder, it is preferable to heat the press part to 40°C or more and 150°C or less, more preferably 55°C or more and 130°C or less, and even more preferably 70°C or more and 120°C or less. The melting point of the binder can be determined from the endothermic peak position of DSC (Differential Scanning Calorimetry). For example, using a PerkinElmer differential scanning calorimeter "DSC7", 10 mg of sample resin is placed in the measurement cell, and the temperature is raised from 30°C to 250°C at a heating rate of 10°C/min in a nitrogen gas atmosphere. The endothermic peak temperature during the heating process is the melting point.

Pressing may be performed multiple times while changing the conditions of press pressure, gap, speed, and surface temperature of the press part. When the negative electrode active material layer is applied in multiple stripes, it is preferable to slit it before pressing. If the negative electrode is pressed without slitting the multiple stripe-applied negative electrode active material layer, excessive stress may be applied to the negative electrode current collector part not coated with the negative electrode active material layer, which may cause wrinkles. After pressing, the negative electrode active material layer may be slit again.

[Production of positive electrode precursor]
In this embodiment, the positive electrode precursor which becomes the positive electrode of the non-aqueous alkali metal storage element can be manufactured by a known manufacturing technique for electrodes in lithium ion secondary batteries, electric double layer capacitors, etc. For example, a positive electrode active material, an alkali metal carbonate, and other optional components used as necessary are dispersed or dissolved in water or an organic solvent to prepare a slurry-like coating liquid, and this coating liquid is applied to one or both sides of a positive electrode current collector to form a coating film, which is then dried to obtain a positive electrode precursor. Furthermore, the obtained positive electrode precursor may be pressed to adjust the film thickness or bulk density of the positive electrode active material layer. The preparation of the coating liquid for forming the positive electrode active material layer, the application of the coating liquid to the positive electrode current collector, the drying of the coating film, and the pressing can each be performed in accordance with the method described above for the manufacture of the negative electrode.

[Assembly process]
In an assembly process of one embodiment, for example, a positive electrode terminal and a negative electrode terminal are connected to a laminate formed by stacking a positive electrode precursor and a negative electrode cut into a sheet shape with a separator interposed therebetween to prepare an electrode laminate. In another embodiment, a positive electrode terminal and a negative electrode terminal may be connected to a wound body formed by stacking and winding a positive electrode precursor and a negative electrode with a separator interposed therebetween to prepare an electrode wound body. The shape of the electrode wound body may be cylindrical or flat. The method of connecting the positive electrode terminal and the negative electrode terminal is not particularly limited, but methods such as resistance welding and ultrasonic welding can be used. It is preferable to dry the electrode body (electrode laminate or electrode wound body) to which the terminals are connected to remove the remaining solvent. The drying method is not limited, but it can be dried by vacuum drying or the like. The remaining solvent is preferably 1.5 mass% or less per total mass of the positive electrode active material layer or the negative electrode active material layer. If the remaining solvent is more than 1.5 mass%, the solvent remains in the system, which is not preferable because it deteriorates the self-discharge characteristics. The dried electrode body is preferably stored in an exterior body such as a metal can or a laminate film in a dry environment with a dew point of -40°C or less, and is preferably sealed except for one opening for injecting a non-aqueous electrolyte. If the dew point is higher than -40°C, moisture will adhere to the electrode body, water will remain in the system, and the self-discharge characteristics will deteriorate, which is not preferable. The method for sealing the exterior body is not particularly limited, but methods such as heat sealing and impulse sealing can be used.

The non-aqueous alkali metal storage element according to this embodiment is characterized in that the protective layer of the positive electrode collector described above and the negative electrode end overlap on the projection surface in the stacking direction of the electrode body. In the charge and discharge of the non-aqueous alkali metal storage element, if there is a positive electrode collector that does not face the negative electrode active material layer, or a negative electrode collector that does not face the positive electrode active material layer, the potential fluctuation at such a collector portion becomes abrupt. In particular, when the amount of hexafluorophosphate ion (PF ₆ ⁻ ) present in the non-aqueous electrolyte is 0.05 mol/L or less, a stable passive film is not formed on the positive electrode collector, and the corrosion of the collector progresses in the high-temperature high-voltage cycle described later, resulting in an increase in resistance. Therefore, by providing the positive electrode protective layer on the positive electrode collector facing the negative electrode active material layer, the corrosion of the positive electrode collector can be suppressed.

Figure 1 is a schematic projection view of the stacking direction of an electrode body according to one embodiment of the present invention. Figure 2 is a schematic cross-sectional view of an electrode body according to one embodiment of the present invention. By referring to Figures 1 and 2, it is possible to position the negative electrode (1), negative electrode end (e), negative electrode current collector (2), negative electrode active material layer (3), separator (4), positive electrode (5), positive electrode current collector (6), positive electrode active material layer (7), protective layer (8), non-protective layer (9), etc. in the electrode body (E).

[Injection, impregnation, and sealing processes]
After the assembly process, the non-aqueous electrolyte is injected into the electrode body housed in the exterior body. After the injection, it is desirable to further impregnate the positive electrode, the negative electrode, and the separator so that they are sufficiently soaked with the non-aqueous electrolyte. In a state where the electrolyte is not soaked in at least a part of the positive electrode, the negative electrode, and the separator, the alkali metal doping proceeds unevenly in the alkali metal doping process described later, so that the resistance of the obtained non-aqueous alkali metal storage element increases or the durability decreases. The impregnation method is not particularly limited, but for example, the electrode body after the injection is placed in a reduced pressure chamber with the exterior body open, and the chamber is reduced pressure using a vacuum pump, and the pressure is returned to atmospheric pressure again. After the impregnation, the electrode body with the exterior body open can be sealed while reducing the pressure.

[Alkali metal doping process]
In the alkali metal doping step, it is preferable to apply a voltage between the positive electrode precursor and the negative electrode, decompose the alkali metal carbonate preferably contained in the positive electrode precursor to release the alkali metal ion, and reduce the alkali metal ion at the negative electrode, thereby pre-doping the alkali metal ion into the negative electrode active material layer. In the alkali metal doping step, gas such as CO ₂ is generated due to the oxidative decomposition of the alkali metal carbonate in the positive electrode precursor. Therefore, when applying a voltage, it is preferable to take a means for releasing the generated gas to the outside of the exterior body. Examples of this means include a method of applying a voltage with a part of the exterior body opened; a method of applying a voltage with a suitable gas release means such as a gas vent valve or a gas permeable film installed in advance on a part of the exterior body; and the like. In addition, in the alkali metal doping step, it is preferable to pressurize both sides of the center of the electrode body sealed in the exterior body by sandwiching them with a buffer material, and it is more preferable to pressurize both sides of the center of the electrode body by sandwiching them with a buffer material from the outside of the exterior body. The pressure during pressing is preferably 10 kPa or more and 1000 kPa or less. If the pressure during pressurization is 10 kPa or more, the gas generated during the doping process is discharged from inside the electrode body, and the decomposition of the alkali metal carbonate is promoted. If the pressure during pressurization is 1000 kPa or less, the non-aqueous electrolyte is sufficiently retained inside the electrode body, and the decomposition of the alkali metal carbonate is promoted. Examples of the buffer material include silicone sponge, silicone rubber, chloroprene rubber, butyl rubber, urethane rubber, styrene butadiene rubber, and acrylonitrile rubber.

[Aging process]
After the alkali metal doping step, it is preferable to perform aging on the electrode body. In the aging step, the solvent in the electrolyte is decomposed at the negative electrode, and a solid polymer coating permeable to alkali metal ions is formed on the surface of the negative electrode. The aging method is not particularly limited, but for example, a method of reacting the solvent in the electrolyte under a high temperature environment can be used.

[Gas removal process]
After the aging process, it is preferable to perform degassing to reliably remove gas remaining in the electrolyte, the positive electrode, and the negative electrode. When gas remains in at least a part of the electrolyte, the positive electrode, and the negative electrode, ion conduction is inhibited, and the resistance of the obtained nonaqueous alkali metal storage element increases. The method of degassing is not particularly limited, but for example, a method of placing the electrode body in a reduced pressure chamber with the exterior body open and reducing the pressure in the chamber using a vacuum pump can be used. After degassing, the exterior body is sealed to close the exterior body, and a nonaqueous alkali metal storage element can be produced.

[High temperature and high voltage cycle characteristics]
In a high temperature environment of 85°C or higher and a high voltage environment of 4.0V or higher, the decomposition reaction of the electrolyte is easily promoted, and in the negative electrode, the reduction reaction of the electrolyte solvent proceeds on the negative electrode where the potential is lowered due to the doping of alkali metal ions. In particular, since there is a large amount of excess electrolyte around the electrode end, this side reaction is likely to proceed. Furthermore, in a high temperature environment, the deactivation of the alkali metal ions doped in the negative electrode active material is promoted, so the potential is likely to increase, and as a result, the capacity is likely to decrease. In other words, by suppressing the side reaction at the negative electrode end and suppressing the deactivation of the alkali metal ions doped in the negative electrode, it is possible to suppress the increase in resistance in the high temperature high voltage cycle and improve the capacity retention rate.

The side reaction at the negative electrode end is correlated with the concentration of alkali metal ions present at the negative electrode end, that is, the smaller the amount of alkali metal ions per unit area of the outer periphery of the negative electrode, the lower the reactivity. On the other hand, by increasing the amount of alkali metal ions at the center of the negative electrode, the increase in potential can be suppressed even if the alkali metal ions doped in the negative electrode are somewhat deactivated. In other words, when the alkali metal ion concentration per unit area of the outer periphery of the negative electrode is A (mAh/g) and the alkali metal ion concentration per unit area of the center is B (mAh/g), if 1.02≦B/A≦1.45, the resistance increase in the high-temperature high-voltage cycle can be suppressed and the capacity retention rate can be improved. If B/A is 1.02 or more, the resistance increase in the high-temperature high-voltage cycle can be suppressed by suppressing the side reaction at the negative electrode end. If B/A is 1.45 or less, the excessive potential increase at the positive electrode end facing the negative electrode end can be suppressed, and the resistance increase can be suppressed by suppressing the corrosion of the positive electrode current collector. From this viewpoint, 1.03≦B/A≦1.44 is preferable. FIG. 3 is an image diagram showing the outer periphery and center of the negative electrode, showing a schematic top view (a) of the negative electrode and a schematic view (b) for explaining the outer periphery, center, and end of the negative electrode. By referring to FIG. 3, the outer periphery (o) and center (c) of the negative electrode (1) to be measured can be defined. In the negative electrode (1), the negative electrode active material layer (3) is disposed on the negative electrode current collector (2), and the center (c) is the area surrounded by the dotted line located 5% in length from the electrode end (e) shown by the thick line toward the center, and the outer periphery (o) is the area surrounded by the dotted line and the thick line.

The method for making B/A 1.02 or more and 1.45 or less is not particularly limited, but examples include a method of increasing the concentration of alkali metal carbonate in the center of the positive electrode precursor facing the negative electrode, a method of locally heating the center of the electrode body during alkali metal doping to promote the alkali metal doping reaction in the center, a method of locally pressurizing the center of the electrode body during alkali metal doping to promote the alkali metal doping reaction in the center, and a method of assembling an electrode body using a negative electrode whose center has already been doped with an alkali metal.

[Calculation method of A and B]
The alkali metal ion concentration A per unit area of the outer periphery and the alkali metal ion concentration B per unit area of the central portion can be determined by the following method.

First, the non-aqueous alkali metal storage element is adjusted to a voltage of 3.8 V in a 25° C. environment, and then disassembled in an argon atmosphere with a dew point of −60° C. or less, and the negative electrode is taken out. The obtained negative electrode is washed twice with dimethyl carbonate, air-dried, and then cut out in a region including a distance of 5% or less from the electrode end to obtain a sample of the outer periphery. In addition, the position with the longest distance from the electrode end is set as the center point, and the electrode is cut out in a region including this center point to obtain a sample of the center. There is no particular specification for the area of the electrode to be cut out, but it is preferable to cut out the electrode so that the area of the negative electrode including the active material layer is 5% or more and 20% or less when the electrode area of the negative electrode including the active material layer is 100%. The obtained sample is used as the working electrode, and the separator and metallic lithium are used as the counter electrode and reference electrode, and the above-mentioned non-aqueous electrolyte is poured to prepare a negative electrode half cell. For the working electrode sample, if active material layers are present on both sides of the current collector, the active material layer on one side is peeled off using a spatula or a brush. For the non-aqueous electrolyte solution used, it is preferable to use an electrolyte salt containing the same cation as the alkali metal ion species doped in the negative electrode, and to use a mixed solvent of a chain carbonate and a cyclic carbonate (for example, a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2). The obtained negative electrode half cell is charged at a constant current of 0.1 mA/cm ² in a 25° C. environment until the potential of the working electrode reaches 2.5 V. For the charge capacity at this time, for example, with reference to FIG. 3, the evaluation result of the outer peripheral sample is A1 (mAh), the evaluation result of the central sample is B1 (mAh), and A and B can be calculated by dividing A1 and B1 by the weight of the active material layer of the working electrode sample.

[In the case of a negative electrode in which the outermost layer has active material layers on both sides]
In the case where the outermost layer of the electrode laminate or electrode wound body is a negative electrode and the negative electrode active material layer is provided on both sides of the negative electrode current collector of the negative electrode, when the alkali metal ion concentration per unit area of the central part of the negative electrode active material layer not facing the positive electrode is E (mAh/g), it is preferable that B>E. By making B>E, it is possible to suppress side reactions on the negative electrode of the outermost layer. E can be calculated in the same manner as the alkali metal ion concentration B described above.

<<Uses of non-aqueous alkali metal storage elements>>
A storage module can be fabricated by connecting a plurality of nonaqueous alkali metal storage elements according to this embodiment in series or in parallel. The nonaqueous alkali metal storage element and storage module according to this embodiment can achieve both high input/output characteristics and safety at high temperatures. Therefore, they can be used in power regeneration assist systems, power load leveling systems, uninterruptible power supply systems, non-contact power supply systems, energy harvesting systems, storage systems, solar power generation storage systems, electric power steering systems, emergency power supply systems, in-wheel motor systems, idling stop systems, quick charging systems, smart grid systems, and the like. The storage system is preferably used for natural power generation such as solar power generation or wind power generation, the power load leveling system is preferably used for microgrids, and the uninterruptible power supply system is preferably used for factory production facilities, and the like. In contactless power supply systems, non-aqueous alkali metal storage elements are preferably used for leveling out voltage fluctuations such as those caused by microwave transmission or electric field resonance, and for storing energy; in energy harvesting systems, non-aqueous alkali metal storage elements are preferably used for using power generated by vibration power generation, etc.

In the energy storage system, a plurality of nonaqueous alkaline metal storage elements are connected in series or parallel to form a cell stack, or a nonaqueous alkaline metal storage element is connected in series or parallel to a lead battery, a nickel-metal hydride battery, a lithium-ion secondary battery, or a fuel cell. In addition, the nonaqueous alkaline metal storage element according to this embodiment can achieve both high input/output characteristics and safety at high temperatures, and can therefore be mounted on vehicles such as electric vehicles, plug-in hybrid vehicles, hybrid vehicles, and electric motorcycles. The above-described power regeneration assist system, electric power steering system, emergency power supply system, in-wheel motor system, idling stop system, or a combination thereof, is preferably mounted on the vehicle.

Below, the embodiments of the present invention will be specifically explained by showing examples and comparative examples. However, the present invention is not limited in any way by the following examples and comparative examples. In the following explanation, the abbreviations CMC1, CMC2, and CMC3 are used, but CMC1 to CMC3 are displayed for the purpose of calculating the blending ratio, and are the same carboxymethyl cellulose.

<Production Example of Negative Electrode, Positive Electrode Precursor, and Electrolyte>
[Preparation of Carbon Nanotube Dispersion]
A carbon nanotube dispersion was prepared by mixing 0.400 mass% of commercially available single-walled carbon nanotubes (CNT), 0.600 mass% of carboxymethyl cellulose (CMC1) as a dispersant, and 99.000 mass% of distilled water, and dispersing the mixture at a peripheral speed of 17 m/s using a thin-film swirling high-speed mixer, Filmix, manufactured by PRIMIX Corporation.

[Production of negative electrode 1]
94.800% by mass of artificial graphite with an average particle size of 4.5 μm, 3.000% by mass of carbon black, 2.000% by mass of carboxymethylcellulose (CMC2) as a dispersant, and distilled water were mixed to obtain a mixture with a mass ratio of solids of 28.0% by mass. The obtained mixture was dispersed for 10 minutes at a rotation speed of 2,000 rpm using a revolution mixer "Awatori Rentaro (registered trademark)" manufactured by Thinky Corporation to obtain a mixture. The above-prepared mixture was mixed with a carbon nanotube dispersion liquid so that carbon nanotubes (CNT) were 0.080% by mass and CMC1 were 0.120% by mass, and dispersed for 10 minutes at a rotation speed of 2,000 rpm to prepare a negative electrode coating liquid 1. The viscosity (ηb) and TI value of the obtained negative electrode coating liquid 1 were measured using an E-type viscometer TVE-35H manufactured by Toki Sangyo Co., Ltd. As a result, the viscosity (ηb) was 1,560 mPa·s and the TI value was 3.7. A negative electrode coating solution 1 was applied to one side of a 10 μm thick electrolytic copper foil using a doctor blade and dried for 10 minutes on a hot plate heated to 50 ° C. Then, a negative electrode 1 was produced by pressing using a roll press under conditions of a pressure of 5 kN / cm and a surface temperature of the press part of 25 ° C. The negative electrode 1 was cut into a size of 3 cm x 3 cm, and the weight of the negative electrode active material layer was divided by the negative electrode area to calculate the weight of the negative electrode active material layer, which was 44.0 g / m ² .

[Manufacture of negative electrodes 2 to 4]
Negative electrodes 2 to 4 were produced in the same manner as in [Production of Negative Electrode 1], except that the weight per unit area of the negative electrode active material layer was adjusted as shown in Table 1.

[Preparation of Positive Electrode Active Material]
The crushed coconut shell carbonized material was placed in a small carbonization furnace and carbonized at 500°C for 3 hours under a nitrogen atmosphere to obtain a carbonized material. The obtained carbonized material was placed in an activation furnace, and steam heated in a preheating furnace was introduced into the activation furnace at 1 kg/h, and the temperature was raised to 900°C over 8 hours to activate the material. The activated carbon was removed and cooled under a nitrogen atmosphere to obtain activated activated carbon. The obtained activated activated carbon was washed with water for 10 hours, drained, dried for 10 hours in an electric dryer maintained at 115°C, and then pulverized in a ball mill for 1 hour to obtain activated carbon 1. The average particle size of activated carbon 1 was measured using a laser diffraction particle size distribution analyzer (SALD-2000J) manufactured by Shimadzu Corporation, and was found to be 5.5 μm. The pore size distribution of the activated carbon 1 was measured using a pore size distribution measuring device (AUTOSORB-1 AS-1-MP) manufactured by Yuasa Ionics Co., Ltd. As a result, the BET specific surface area was 2360 ^{m 2} /g, the mesopore volume (V ₁ ) was 0.52 cc/g, the micropore volume (V ₂ ) was 0.88 cc/g, and V ₁ /V ₂ = 0.59.

[Production of positive electrode precursor 1]
Activated carbon 1 was 46.200% by mass, carboxymethyl cellulose (CMC3) was 2.000% by mass, lithium carbonate was 40.000% by mass, carbon black was 5.000% by mass, and acrylic latex (LTX) was 4.500% by mass, PVP (polyvinylpyrrolidone) was 2.000% by mass, and distilled water was mixed so that the mass ratio of the solid content was 34.1% by mass, and this was dispersed for 20 minutes at a rotation speed of 2000 rpm using a revolution mixer "Awatori Rentaro (registered trademark)" manufactured by Thinky Corporation to obtain a mixture. The obtained mixture was mixed with a carbon nanotube dispersion liquid so that carbon nanotubes (CNT) were 0.120% by mass and CMC1 were 0.180% by mass, and dispersed for 10 minutes at a rotation speed of 2,000 rpm to prepare a positive electrode coating liquid 1. The viscosity (ηb) and TI value of the obtained positive electrode coating liquid 1 were measured using an E-type viscometer TVE-35H manufactured by Toki Sangyo Co., Ltd. As a result, the viscosity (ηb) was 2,340 mPa·s and the TI value was 5.1. In addition, the degree of dispersion of the obtained positive electrode coating liquid 1 was measured using a particle gauge manufactured by Yoshimitsu Seiki Co., Ltd. As a result, the particle size was 33 μm. Using a doctor blade, the positive electrode coating liquid 1 was applied to one side of an aluminum foil having a thickness of 15 μm, and then dried for 10 minutes on a hot plate heated to 50 ° C., and then pressed using a roll press machine under conditions of a pressure of 6 kN / cm and a surface temperature of the press part of 25 ° C. to obtain a positive electrode precursor 1. When the basis weight of the positive electrode precursor 1 was measured by the same method as above, it was 48.0 g / m ² .

[Production Examples of Positive Electrode Precursors 2 to 4]
Positive electrode precursors 2 to 4 were produced in the same manner as in [Production of positive electrode precursor 1], except that the amounts of each component used were adjusted as shown in Table 2.

[Preparation of electrolyte solution 1]
A mixed solvent of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 33:67 (volume ratio) was used as the organic solvent, and LiFSI was used as the electrolyte. An electrolyte salt was dissolved therein to a concentration of 1.0 mol/L, thereby obtaining a non-aqueous electrolyte solution 1.

[Preparation of Electrolyte Solutions 2 to 5]
Non-aqueous electrolyte solutions 2 to 5 were produced in the same manner as in [Preparation of electrolyte solution 1], except that the amounts of each component used were adjusted as shown in Table 3.

Example 1
<Production of non-aqueous alkali metal electricity storage element>
[Assembly process]
The obtained positive electrode precursor 1 was cut out into one piece so that the positive electrode active material layer was 4.4 cm x 9.4 cm, and the positive electrode current collector portion without the positive electrode active material layer was 1.5 cm x 2.5 cm in size, and a polyimide tape having a size of 0.5 cm x 2.5 cm and a thickness of 35 μm was attached from the end of the positive electrode active material layer to the positive electrode current collector without the positive electrode active material layer. Next, the negative electrode 1 was cut out into one piece so that the negative electrode active material layer was 4.5 cm x 9.5 cm, and the current collector portion without the negative electrode active material layer was 1.4 cm x 2.5 cm in size. In addition, one polyethylene separator (manufactured by Asahi Kasei Corporation, thickness 15 μm) of 4.7 cm x 9.8 cm was prepared. Using these, the positive electrode precursor 1, the separator, and the negative electrode 1 were laminated in this order, with the separator sandwiched between them, so that the center of the positive electrode active material layer overlapped with the center of the negative electrode active material layer, and the negative electrode end overlapped with the protective layer of the positive electrode current collector, to obtain an electrode laminate. The positive electrode terminal and the negative electrode terminal were ultrasonically welded to the obtained electrode body, which was then placed in an exterior body formed of an aluminum laminate packaging material, and the three sides including the electrode terminal portion were sealed by heat sealing.

[Injection, impregnation, and sealing processes]
About 2.5 g of non-aqueous electrolyte 1 was injected into the exterior housing containing the electrode laminate under a dry air environment at atmospheric pressure, temperature 25 ° C., and dew point of -40 ° C. or less. Next, the exterior housing containing the electrode laminate and the non-aqueous electrolyte was placed in a vacuum chamber, depressurized from atmospheric pressure to -87 kPa, then returned to atmospheric pressure, and left to stand for 5 minutes. Then, the process of depressurizing the exterior housing in the chamber from atmospheric pressure to -87 kPa and then returning to atmospheric pressure was repeated four times, and then left to stand for 15 minutes under atmospheric pressure. Through the above process, the non-aqueous electrolyte 1 was impregnated into the electrode laminate. Then, the electrode laminate impregnated with the non-aqueous electrolyte 1 was placed in a vacuum sealer, and the exterior housing was sealed by sealing at a pressure of 0.1 MPa for 10 seconds at 180 ° C. in a state where the pressure was reduced to -95 kPa.

[Alkali metal doping process]
Both sides of the central portion of the sealed electrode laminate were sandwiched between 3.0 cm × 7.0 cm silicone sponge cushioning materials and pressed with a pressure of 30 kPa. This was charged at a constant current of 10 mA at a temperature of 45° C. until the voltage reached 4.5 V, and then initially charged at a constant voltage of 4.5 V for 2 hours, thereby doping the negative electrode with an alkali metal.

[Aging process]
The electrode laminate after doping with the alkali metal was taken out of the dry box, and constant current discharge was performed at 50 mA in a 25° C. environment until the voltage reached 2.0 V, and then constant current discharge at 2.0 V was performed for 1 hour to adjust the voltage to 2.0 V. Subsequently, the electrode laminate sealed in an exterior body was stored in a thermostatic chamber at 85° C. for 12 hours.

[Gas removal process]
After aging, a part of the exterior body was opened in a dry air environment at a temperature of 25° C. and a dew point of −40° C., and the electrode laminate was taken out. The electrode laminate taken out was placed in a vacuum chamber, and a process of reducing the pressure from atmospheric pressure to −80 kPa over 3 minutes using a diaphragm pump, and then returning to atmospheric pressure over 3 minutes was repeated three times in total. Thereafter, the electrode laminate was placed back into the exterior body, and the pressure was reduced to −90 kPa using a vacuum sealer, and the exterior body was sealed at 200° C. for 10 seconds at a pressure of 0.1 MPa to produce a nonaqueous alkali metal storage element.
Furthermore, for the nonaqueous alkali metal electricity storage element, each value was measured or calculated as described in the above section [Calculation method of A and B], and is shown in Table 4.

Evaluation of non-aqueous alkali metal storage element
[Measurement of internal resistance Ra]
The internal resistance Ra (mΩ) is a value obtained by the following method. First, the non-aqueous alkaline metal storage element is charged at a constant current of 20 C in a thermostatic chamber set at 25 ° C. until it reaches 4.0 V, and then a constant voltage charge of 4.0 V is applied for a total of 30 minutes. Next, a sampling interval is set to 0.05 seconds, and a constant current discharge is performed at a current value of 20 C to 2.0 V to obtain a discharge curve (time-voltage). In this discharge curve, when the voltage at the discharge time = 0 seconds obtained by extrapolating the voltage values at the time points of 1 second and 2 seconds of discharge time by linear approximation is Eo, the value is calculated as Ra = ΔE / (current value of 20 C) from the drop voltage ΔE = 4.0 - Eo. The internal resistance Ra of one of the obtained nonaqueous alkali metal storage elements was measured by the above-mentioned method in a thermostatic chamber set at 25° C. using a charge/discharge device (5 V, 10 A) manufactured by Asuka Electronics Co., Ltd. The Ra was 80.5 mΩ. This value is shown in Table 5 as the initial internal resistance Ra.

[Measurement of Capacity Qa]
The capacity Qa (mAh) is a value obtained by the following method. First, the non-aqueous alkali metal storage element is charged at a constant current of 20C in a thermostatic chamber set at 25°C until it reaches 4.0V, and then a constant voltage charge of 4.0V is applied for a total of 30 minutes. Next, the discharge capacity when the sampling interval is set to 1.0 second and the constant current discharge is performed at a current value of 1C to 2.0V is defined as the capacity Qa. The capacity Ra of the non-aqueous alkali metal storage element was measured by the above-mentioned method using a charge/discharge device (5V, 10A) manufactured by Asuka Electronics Co., Ltd. in a thermostatic chamber set at 25°C, and the Qa was 6.58mAh. This value is shown in Table 5 as the initial capacity Qa.

[High temperature and high voltage cycle test]
The resistance and capacity change rate after the high-temperature, high-voltage cycle test are values measured by the following method. First, in a thermostatic chamber set at 85° C., the nonaqueous alkali metal storage element and the corresponding cell are charged at a constant current of 50 C until 4.2 V is reached, and then discharged at a constant current of 50 C until 3.0 V is reached. After repeating this charge and discharge 1000 times, the internal resistance Rb after the high-temperature, high-voltage cycle test is measured according to the above-mentioned internal resistance measurement method, and the capacity Qb after the high-temperature, high-voltage cycle test is measured according to the above-mentioned capacity measurement method. Rb/Ra is the resistance change rate after the high-temperature, high-voltage cycle test, and Qb/Qa is the capacity change rate after the high-temperature, high-voltage cycle test. These results are shown in Table 5.

Examples 2 to 11 and Comparative Examples 1 to 7
Non-aqueous alkali metal storage elements were produced and evaluated in the same manner as in Example 1, except that the negative electrode, positive electrode precursor, and non-aqueous electrolyte solution shown in Tables 1, 2, and 3 were used under the production conditions shown in Table 4, and no polyimide tape was attached to the positive electrode current collector in Comparative Examples 1 to 4. The evaluation results are shown in Tables 4 and 5.

Comparing the Examples and Comparative Examples, it appears that by applying pressure to the center of the non-aqueous alkali metal storage element during alkali metal doping, the decomposition reaction of the alkali metal carbonate in the center is promoted, making it possible to change the alkali metal ion concentration in the outer periphery and center of the negative electrode, thereby suppressing the increase in resistance during high-temperature, high-voltage cycles and improving the capacity retention rate.

Examples 12 to 17
A nonaqueous alkali metal electricity storage element was produced and evaluated in the same manner as in Example 1, except that the positive electrode protective layer was as shown in Table 6.

In Examples 14 and 15, in which a carbon material was used as the positive electrode protective layer, an anchor foil was used as the positive electrode current collector, in which an anchor layer was applied to a width of 7.0 cm and a thickness of 1 μm on an aluminum foil having a width of 15.0 cm and a thickness of 15 μm, and a positive electrode precursor was produced by applying a positive electrode active material layer to a width of 6.0 cm, leaving a 0.5 cm wide exposed portion of the anchor layer. Next, a nonaqueous alkali metal storage element was produced by cutting out the positive electrode precursor so that the anchor layer became a protective layer measuring 0.5 cm x 2.5 cm.

In Examples 16 and 17, in which inorganic fine particles were used as the positive electrode protective layer, an aqueous dispersion of 80 parts by weight of inorganic particles, 5 parts by weight of carboxymethyl cellulose, and 15 parts by weight of acrylic latex was applied to a width of 0.5 cm from the end of the positive electrode active material layer of the positive electrode precursor 1 and then dried.

Example 18
Negative electrode coating solution 1 was applied to one side of an electrolytic copper foil having a thickness of 10 μm using a doctor blade, and dried for 10 minutes on a hot plate heated to 50° C. Next, negative electrode coating solution 1 was applied to the back side of the electrolytic copper foil, and dried in the same manner as above. Next, a roll press was used to press the foil under conditions of a pressure of 5 kN/cm and a surface temperature of the press part of 25° C., thereby preparing a negative electrode 5.

Using a doctor blade, positive electrode coating solution 1 was applied to one side of a 15 μm thick aluminum foil, and then dried for 10 minutes on a hot plate heated to 50°C. Next, positive electrode coating solution 1 was applied to the back side of the aluminum foil, and then dried in the same manner as above. Positive electrode precursor 5 was obtained by pressing using a roll press under conditions of a pressure of 6 kN/cm and a surface temperature of the pressing part of 25°C.

A non-aqueous alkali metal storage element was produced in the same manner as in Example 1, except that 0.5 cm x 2.5 cm polyimide tape was attached to both sides of the current collector of the positive electrode precursor 5, and the negative electrode 5, the same separator as that used in Example 1, the positive electrode precursor 5, the separator, and the negative electrode 5 were laminated in this order, with the positive electrode active material layer and the negative electrode active material layer facing each other with the separator in between, to obtain an electrode laminate.

Comparative Example 8
A negative electrode 6 was produced in the same manner as in Example 18, except that an etched copper foil having a thickness of 12 μm and an aperture ratio of 20% was used. A positive electrode precursor 6 was produced in the same manner as in Example 18, except that an etched aluminum foil having a thickness of 20 μm and an aperture ratio of 20% was used. A nonaqueous alkali metal storage element was produced in the same manner as in Comparative Example 1, except that the negative electrode 6, the separator, the positive electrode precursor 6, the separator, and the negative electrode 6 were laminated in this order so that the positive electrode active material layer and the negative electrode active material layer faced each other with the separator sandwiched therebetween to obtain an electrode laminate.

The evaluation results for Example 18 and Comparative Example 8 are shown in Table 7.

In Comparative Example 8, the use of an etched foil with openings as the current collector foil allowed the alkali metal ions to be uniformly doped into the outermost negative electrode active material layer. As a result, the potential of the outermost negative electrode active material layer decreased, making it easier for a reaction to occur with excess nonaqueous electrolyte outside the electrode body, which is thought to have led to an increase in resistance and a decrease in capacity retention rate during high-temperature, high-voltage cycles.

The nonaqueous alkali metal storage element of the present invention can be suitably used as a nonaqueous alkali metal storage element for use in power regeneration systems for hybrid drive systems of automobiles that require high durability in high-temperature, high-voltage environments, power load leveling systems for natural power generation such as solar power generation and wind power generation and microgrids, uninterruptible power supply systems for factory production facilities, non-contact power supply systems for the purpose of leveling voltage fluctuations such as microwave transmission and electrolytic resonance and storing energy, and energy harvesting systems for the purpose of utilizing power generated by vibration power generation, etc. The nonaqueous alkali metal storage element can be used to create a storage module, for example, by connecting multiple nonaqueous alkali metal storage elements in series or in parallel. The nonaqueous alkali metal storage element of the present invention is preferable because the effects of the present invention are maximized when it is applied as a lithium ion capacitor.

E Electrode body 1 Negative electrode 2 Negative electrode current collector 3 Negative electrode active material layer 4 Separator 5 Positive electrode 6 Positive electrode current collector 7 Positive electrode active material layer 8 Protective layer 9 Non-protective layer c Central portion of negative electrode e End portion of negative electrode o Outer periphery of negative electrode

Claims (10)

A non-aqueous alkali metal storage element includes an electrode assembly including a positive electrode including a positive electrode active material layer disposed on a positive electrode current collector, a negative electrode including a negative electrode active material layer disposed on a negative electrode current collector, and a separator, and a non-aqueous electrolyte solution including alkali metal ions, wherein the positive electrode active material layer includes a positive electrode active material including a carbon material, and an alkali metal carbonate, and the non-aqueous electrolyte solution includes a bis(fluorosulfonyl)imide ion concentration of 0.97 mol/L or more and a hexafluorosulfonyl ion concentration of 1.0 mol/L or more. a nonaqueous alkali metal storage element having a phosphate ion concentration of 0.00 mol/L or more and 0.05 mol/L or less, the positive electrode having a protective layer and a non-protective layer on a positive electrode current collector, the protective layer and a negative electrode end portion overlap on a projected surface in a stacking direction of the electrode body, and wherein, when an alkali metal ion concentration per unit area in an outer periphery of the negative electrode is A (mAh/g) and an alkali metal ion concentration per unit area in a central portion is B (mAh/g), 1.02≦B/A≦1.45. The nonaqueous alkali metal storage element according to claim 1, wherein the protective layer is present on both sides of the positive electrode. The negative electrode has the negative electrode active material layer on both sides of the negative electrode current collector, the outermost layer of the electrode body is the negative electrode, and when the alkali metal ion concentration per unit area of the central part of the negative electrode active material layer that does not face the positive electrode is E (mAh/g), B>E. The nonaqueous alkali metal storage element according to claim 1 or 2. The non-aqueous alkali metal storage element according to any one of claims 1 to 3, wherein the protective layer is a protective tape, and the base material of the protective tape is at least one material selected from the group consisting of polyether ketone, polyphenylene sulfide, polyethylene terephthalate, polyethylene naphthalate, polyamide, polyimide, aramid, acetate cloth, and polytetrafluoroethylene tape. The non-aqueous alkali metal storage element according to any one of claims 1 to 4, wherein the protective layer contains at least one carbon material selected from the group consisting of carbon black, graphite, graphene, and carbon nanotubes. The non-aqueous alkali metal storage element according to any one of claims 1 to 5, wherein the protective layer contains inorganic fine particles. The nonaqueous alkali metal storage element according to any one of claims 1 to 6, wherein the positive electrode and/or the negative electrode contain carbon nanotubes. A storage module comprising the non-aqueous alkali metal storage element according to any one of claims 1 to 7. The energy storage module according to claim 8, wherein the energy storage module is incorporated into at least one system selected from the group consisting of a power regeneration assist system, a power load leveling system, an uninterruptible power supply system, a contactless power supply system, an energy harvesting system, an energy storage system, a solar power generation and storage system, an electric power steering system, an emergency power supply system, an in-wheel motor system, an idling stop system, an electric vehicle, a plug-in hybrid vehicle, a hybrid vehicle, an electric motorcycle, a quick charging system, and a smart grid system. The nonaqueous alkali metal storage element according to any one of claims 1 to 7,
An energy storage system that connects lead batteries, nickel-metal hydride batteries, lithium-ion secondary batteries, or fuel cells in series or parallel.

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