JP7700924B2 - Nonaqueous electrolyte storage element, its manufacturing method and nonaqueous electrolyte - Google Patents

Description

The present invention relates to a non-aqueous electrolyte storage element, a manufacturing method thereof, and a non-aqueous electrolyte.

Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc., due to their high energy density. The non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the two electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte storage elements. Metallic lithium is known as a negative electrode active material with high energy density used in non-aqueous electrolyte storage elements (see Patent Documents 1 and 2).

JP 2016-100065 A Japanese Patent Application Publication No. 07-245099

In non-aqueous electrolyte storage elements that use metallic lithium as the negative electrode active material, such as lithium batteries, metallic lithium may precipitate in a dendritic form on the surface of the negative electrode during charging (hereinafter, metallic lithium in a dendritic form will be referred to as "dendrites"). When these dendrites grow and penetrate the separator to come into contact with the positive electrode, they cause a micro-short circuit. For this reason, non-aqueous electrolyte storage elements that contain metallic lithium as the negative electrode active material have the disadvantage that they are prone to micro-short circuits due to repeated charging and discharging.

The present invention was made based on the above circumstances, and its purpose is to provide a nonaqueous electrolyte storage element in which the occurrence of micro-short circuits due to repeated charging and discharging is suppressed when the negative electrode contains metallic lithium, a method for manufacturing such a nonaqueous electrolyte storage element, and a nonaqueous electrolyte for lithium batteries that can suppress the occurrence of micro-short circuits.

One aspect of the present invention is a non-aqueous electrolyte storage element that includes a negative electrode containing metallic lithium and a non-aqueous electrolyte containing a polyvalent metal cation and an imide anion.

Another aspect of the present invention is a method for producing a nonaqueous electrolyte storage element, comprising preparing a negative electrode containing metallic lithium and preparing a nonaqueous electrolyte containing a polyvalent metal cation and an imide anion.

Another aspect of the present invention is a non-aqueous electrolyte for a lithium battery that contains a polyvalent metal cation and an imide anion.

According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte storage element in which the occurrence of micro-short circuits due to repeated charging and discharging is suppressed when the negative electrode contains metallic lithium, a method for manufacturing such a nonaqueous electrolyte storage element, and a nonaqueous electrolyte for a lithium battery that can suppress the occurrence of micro-short circuits.

FIG. 1 is an external perspective view showing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing an electricity storage device formed by assembling a plurality of nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention.

First, we will provide an overview of the nonaqueous electrolyte storage element, the method for manufacturing the nonaqueous electrolyte storage element, and the nonaqueous electrolyte disclosed in this specification.

A nonaqueous electrolyte storage element according to one embodiment of the present invention is a nonaqueous electrolyte storage element that includes a negative electrode containing metallic lithium and a nonaqueous electrolyte that includes a polyvalent metal cation and an imide anion.

In this nonaqueous electrolyte storage element, even though the negative electrode contains metallic lithium, the occurrence of micro-short circuits due to repeated charging and discharging is suppressed. That is, in this nonaqueous electrolyte storage element, the occurrence of micro-short circuits caused by repeated charging and discharging is delayed. The reason for this is unclear, but the following reasons are presumed. First, since the polyvalent metal cation is polyvalent, the cation and the anion are not completely dissociated in the nonaqueous electrolyte, and it is presumed that the polyvalent metal cation and the anion such as the imide anion form an ion pair or a partially dissociated multiple ion, and the ion pair or the multiple ion is in a solvated state. For this reason, when the polyvalent metal cation is reduced on the surface of the negative electrode to form a coating, the imide anion also decomposes, and the coating formed contains components derived from the imide anion together with the polyvalent metal, so that the coating has low resistance and is of good quality. Secondly, when dendrites grow on the surface of the negative electrode, the polyvalent metal cation is reduced at the dendritic tip of the dendrite, thereby suppressing the growth of the dendrite toward the tip. As a result, the shape of the dendrite changes from dendritic to clumpy, which is thought to suppress the occurrence of micro-short circuits caused by repeated charging and discharging. For this reason, it is presumed that the occurrence of micro-short circuits caused by repeated charging and discharging is suppressed in a nonaqueous electrolyte storage element according to one embodiment of the present invention, which includes a negative electrode containing metallic lithium and a nonaqueous electrolyte containing a polyvalent metal cation and an imide anion.

The polyvalent metal cation is preferably a cation of a Group 2, Group 11, or Group 12 element. By containing cations of these elements in the nonaqueous electrolyte, the occurrence of micro-short circuits due to repeated charging and discharging is further suppressed.

The polyvalent metal cation is preferably a magnesium or copper cation. When the nonaqueous electrolyte contains cations of these elements, the occurrence of micro-short circuits caused by repeated charging and discharging is further suppressed.

It is also preferable that the polyvalent metal cation is a cation of a typical element. Even when the polyvalent metal cation contained in the nonaqueous electrolyte is a cation of a typical element, the occurrence of micro-short circuits due to repeated charging and discharging can be sufficiently suppressed.

It is preferable that the non-aqueous electrolyte contains a fluorinated solvent. When the non-aqueous electrolyte contains a fluorinated solvent, the coating formed is of better quality, and the occurrence of micro-short circuits caused by repeated charging and discharging is further suppressed.

A method for producing a nonaqueous electrolyte storage element according to one embodiment of the present invention includes preparing a negative electrode containing metallic lithium, and preparing a nonaqueous electrolyte containing a polyvalent metal cation and an imide anion.

This manufacturing method makes it possible to manufacture a nonaqueous electrolyte storage element in which the occurrence of micro-short circuits caused by repeated charging and discharging is suppressed, even though the negative electrode contains metallic lithium.

The nonaqueous electrolyte according to one embodiment of the present invention is a nonaqueous electrolyte for a lithium battery that contains a polyvalent metal cation and an imide anion.

This non-aqueous electrolyte can suppress the occurrence of micro-short circuits that occur during repeated charging and discharging in lithium batteries.

The nonaqueous electrolyte storage element according to one embodiment of the present invention, the manufacturing method of the nonaqueous electrolyte storage element, and the nonaqueous electrolyte will be described below.

<Non-aqueous electrolyte electricity storage element>
The nonaqueous electrolyte storage element according to one embodiment of the present invention has a positive electrode, a negative electrode, and a nonaqueous electrolyte. Hereinafter, a nonaqueous electrolyte secondary battery (hereinafter, simply referred to as a "secondary battery") will be described as an example of a nonaqueous electrolyte storage element. The positive electrode and the negative electrode are usually stacked or wound alternately with a separator interposed therebetween to form an electrode body. This electrode body is housed in a container, and the container is filled with a nonaqueous electrolyte. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode. In addition, as the container, a known metal container, a resin container, or the like that is usually used as a container for a secondary battery can be used.

(positive electrode)
The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

The positive electrode substrate has electrical conductivity. Having "electrical conductivity" means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 ⁷ Ω·cm or less, and "non-electrically conductive" means that the volume resistivity is more than 10 ⁷ Ω·cm. As the material of the positive electrode substrate, metals such as aluminum, titanium, tantalum, stainless steel, etc., or alloys thereof are used. Among these, aluminum and aluminum alloys are preferred in terms of the balance between potential resistance, high electrical conductivity, and cost. In addition, examples of the formation form of the positive electrode substrate include foil, vapor deposition film, etc., and foil is preferred in terms of cost. In other words, aluminum foil is preferred as the positive electrode substrate. In addition, examples of aluminum or aluminum alloys include A1085P, A3003P, etc., as specified in JIS-H-4000 (2014).

The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the secondary battery. The "average thickness" refers to the value obtained by dividing the punched mass when a substrate of a given area is punched out by the true density and punched area of the substrate. The same applies to the "average thickness" of the negative electrode substrate and the negative electrode active material layer described below.

The intermediate layer is a coating layer on the surface of the positive electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The composition of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles.

The positive electrode active material layer is a layer formed from a so-called positive electrode mixture that contains a positive electrode active material. The positive electrode mixture that forms the positive electrode active material layer may contain optional components such as a conductive agent, a binder, a thickener, and a filler as necessary.

The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the positive electrode active material for a lithium secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO ₂ type crystal structure, lithium transition metal composite oxides having a spinel type crystal structure, polyanion compounds, chalcogen compounds, sulfur, and the like. Examples of lithium transition metal composite oxides having α-NaFeO ₂ type crystal structure include Li[Li _x Ni _1-x ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Co _1-x-γ ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Co _1-x ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Mn _1-x-γ ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Ni _γ Mn _β Co _1-x-γ-β ]O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li _x Ni _γ Co _β Al _1-x-γ-β ]O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), etc. Examples of lithium transition metal composite oxides having a spinel crystal structure include Li _x Mn ₂ O ₄ and Li _x Ni _γ Mn _2-γ O ₄ , etc. Examples of polyanion compounds include LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , and Li ₂ CoPO ₄ F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide, etc. Atoms or polyanions in these materials may be partially substituted with atoms or anion species consisting of other elements. The surface of these materials may be covered with another material. In the positive electrode active material layer, these materials may be used alone or in combination of two or more.

As the positive electrode active material, a lithium transition metal composite oxide is preferable, and a lithium transition metal composite oxide having an α-NaFeO ₂ type crystal structure is more preferable. The lithium transition metal composite oxide preferably contains nickel or manganese as a transition metal, and more preferably contains both nickel and manganese. The lithium transition metal composite oxide may further contain other transition metals such as cobalt. In the lithium transition metal composite oxide having an α-NaFeO ₂ type crystal structure, the molar ratio (Li/Me) of lithium (Li) to the transition metal (Me) is preferably more than 1, more preferably 1.1 or more, and even more preferably 1.2 or more, and even more preferably 1.3 or more. By using such a lithium transition metal composite oxide, the electric capacity can be increased. In addition, in the case of a nonaqueous electrolyte storage element using such a positive electrode active material, it tends to be used at a high current density, and dendrites are usually easy to grow. Therefore, in the case of a nonaqueous electrolyte storage element having a positive electrode having the above-mentioned lithium transition metal composite oxide, the effect of suppressing the occurrence of a micro-short circuit can be more significantly enjoyed. The upper limit of the molar ratio of lithium to the transition metal (Li/Me) is preferably 1.6, and more preferably 1.5.

As the lithium transition metal composite oxide having an α- _NaFeO2 type crystal structure, a compound represented by the following formula (1) is preferable.
Li _1+α Me _1-α O ₂ ...(1)
In formula (1), Me is a transition metal including Ni or Mn, and 0<α<1.

In formula (1), Me preferably contains Ni and Mn. Me preferably consists essentially of two elements, Ni and Mn, or three elements, Ni, Mn, and Co. Me may contain other transition metals.

In formula (1), the lower limit of the molar ratio of Ni to Me (Ni/Me) is preferably 0.1, and more preferably 0.2. On the other hand, the upper limit of this molar ratio (Ni/Me) is preferably 0.5, and more preferably 0.45. By setting the molar ratio (Ni/Me) within the above range, the energy density is improved.

In formula (1), the lower limit of the molar ratio of Mn to Me (Mn/Me) is preferably 0.5, more preferably 0.55, and even more preferably 0.6. On the other hand, the upper limit of this molar ratio (Mn/Me) is preferably 0.75, and more preferably 0.7. By setting the molar ratio (Mn/Me) within the above range, the energy density is improved.

In formula (1), the upper limit of the molar ratio of Co to Me (Co/Me) is preferably 0.3, more preferably 0.2, and even more preferably 0.1. This molar ratio (Co/Me) or the lower limit of this molar ratio (Co/Me) may be 0.

In formula (1), the molar ratio of Li to Me (Li/Me), i.e., (1+α)/(1-α), is preferably greater than 1.0 (α>0), more preferably 1.1 or greater, even more preferably 1.2 or greater, and even more preferably 1.3 or greater. On the other hand, the upper limit of this molar ratio (Li/Me) is preferably 1.6, and more preferably 1.5. By setting the molar ratio (Li/Me) within the above range, the electrical capacity is increased.

The lithium transition metal composite oxide having a molar ratio (Li/Me) of lithium (Li) to the transition metal (Me) of more than 1 is preferably one having no diffraction peak in the range of 20° to 22° in the X-ray diffraction diagram using CuKα radiation. The lithium transition metal composite oxide having a molar ratio (Li/Me) of lithium (Li) to the transition metal (Me) of more than 1 generally has a large electric capacity through initial charging and discharging until the positive electrode potential reaches, for example, 4.5 V vs. Li/Li ⁺ or more. In addition, due to the change in the crystal structure during such initial charging and discharging, the diffraction peak that existed in the range of 20° to 22° before the initial charging and discharging disappears. That is, a lithium transition metal composite oxide having a molar ratio (Li/Me) of lithium (Li) to the transition metal (Me) of more than 1 and having no diffraction peak in the range of 20° to 22° in the X-ray diffraction diagram has a large electric capacity.

In this specification, the composition ratio of the lithium transition metal composite oxide refers to the composition ratio when the lithium transition metal composite oxide is fully discharged by the following method. First, the nonaqueous electrolyte storage element is charged at a constant current of 0.05C until the charge end voltage during normal use is reached, and the element is fully charged. After a 30-minute pause, the element is discharged at a constant current of 0.05C to the lower limit voltage during normal use. The element is disassembled, the positive electrode is removed, and a test battery is assembled with a metallic lithium electrode as the counter electrode. A constant current discharge is performed at a current value of 10 mA per 1 g of the positive electrode mixture until the positive electrode potential reaches 2.0 V vs. Li/Li ⁺ , and the positive electrode is adjusted to a fully discharged state. For the metallic lithium electrode here, pure metallic lithium is used, not a lithium alloy. The element is disassembled again, and the positive electrode is removed. The nonaqueous electrolyte attached to the removed positive electrode is thoroughly washed using dimethyl carbonate, and the positive electrode is dried overnight at room temperature, and then the lithium transition metal composite oxide of the positive electrode active material is collected. The collected lithium transition metal composite oxide is subjected to measurement. The operations from disassembly of the nonaqueous electrolyte storage element to measurement are performed in an argon atmosphere with a dew point of −60° C. or lower. Here, normal use refers to a case where the nonaqueous electrolyte storage element is used by adopting charge/discharge conditions recommended or specified for the nonaqueous electrolyte storage element, and in the case where a charger for the nonaqueous electrolyte storage element is prepared, the charger is used to use the nonaqueous electrolyte storage element.

X-ray diffraction measurements of lithium transition metal composite oxides are performed on lithium transition metal composite oxides that have been fully discharged by the above method. Specifically, X-ray diffraction measurements are performed by powder X-ray diffraction measurements using an X-ray diffractometer (Rigaku's "MiniFlex II") with a CuKα radiation source, a tube voltage of 30 kV, and a tube current of 15 mA. At this time, the diffracted X-rays pass through a Kβ filter with a thickness of 30 μm and are detected by a high-speed one-dimensional detector (D/teX Ultra 2). The sampling width is 0.02°, the scan speed is 5°/min, the divergence slit width is 0.625°, the receiving slit width is 13 mm (OPEN), and the scattering slit width is 8 mm.

The lower limit of the content of the lithium transition metal composite oxide relative to all the positive electrode active materials is preferably 50% by mass, more preferably 80% by mass, and even more preferably 95% by mass. The content of the lithium transition metal composite oxide relative to all the positive electrode active materials may be 100% by mass.

The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above lower limit or more, the positive electrode active material can be easily manufactured or handled. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Here, "average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by a laser diffraction/scattering method for a diluted solution in which particles are diluted with a solvent in accordance with JIS-Z-8825 (2013).

In order to obtain particles of the positive electrode active material in a predetermined shape, a grinder, a classifier, etc. are used. Examples of grinding methods include methods using a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling airflow type jet mill, or a sieve. When grinding, wet grinding in the presence of water or an organic solvent such as hexane can also be used. As a classification method, a sieve or an air classifier can be used as necessary for both dry and wet methods.

The content of the positive electrode active material in the positive electrode active material layer is preferably 70% by mass or more and 98% by mass or less, more preferably 80% by mass or more and 97% by mass or less, and even more preferably 90% by mass or more and 96% by mass or less. By setting the content of the positive electrode active material within the above range, the electrical capacity of the secondary battery can be increased.

The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Among these, carbonaceous materials are preferred from the viewpoint of electrical conductivity and coatability. Of these, acetylene black and ketjen black are preferred. The conductive agent may be in the form of a powder, a sheet, or a fiber.

The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 40% by mass or less, and more preferably 2% by mass or more and 10% by mass or less. By setting the content of the conductive agent in the above range, the energy density of the secondary battery can be increased.

Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

The binder content in the positive electrode active material layer is preferably 0.5% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 6% by mass or less. By keeping the binder content within the above range, the active material can be stably maintained.

Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. In addition, when the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or the like. In one aspect of the present invention, it may be preferable that the thickener is not contained in the positive electrode active material layer.

The filler is not particularly limited. Examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, poorly soluble ion crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, mineral resource-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof. In one aspect of the present invention, it may be preferable that the filler is not contained in the positive electrode active material layer.

The positive electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metallic elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

(Negative electrode)
The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The intermediate layer of the negative electrode may have the same structure as the intermediate layer of the positive electrode.

The negative electrode substrate can be of the same construction as the positive electrode substrate, but the material used is a metal such as copper, nickel, stainless steel, or nickel-plated steel, or an alloy thereof, or a carbonaceous material, and copper or a copper alloy is preferred. In other words, copper foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

The average thickness of the negative electrode substrate is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, even more preferably 4 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. By setting the average thickness of the negative electrode substrate within the above range, it is possible to increase the strength of the negative electrode substrate while increasing the energy density per volume of the secondary battery.

The negative electrode active material layer contains metallic lithium. That is, the secondary battery of this embodiment is a lithium battery. Metallic lithium is a component that functions as a negative electrode active material. Metallic lithium may exist as pure metallic lithium consisting essentially of lithium element alone, or as a lithium alloy containing other metal elements. Examples of lithium alloys include lithium silver alloys, lithium zinc alloys, lithium calcium alloys, lithium aluminum alloys, lithium magnesium alloys, and lithium indium alloys. The lithium alloy may contain multiple metal elements other than lithium.

The negative electrode active material layer may be a layer consisting essentially of metallic lithium. The content of metallic lithium in the negative electrode active material layer may be 90% by mass or more, may be 99% by mass or more, or may be 100% by mass.

The negative electrode active material layer may be a metallic lithium foil or a lithium alloy foil. The negative electrode active material layer may be a non-porous layer (solid layer). The average thickness of the negative electrode active material layer is preferably 5 μm or more and 1,000 μm or less, more preferably 10 μm or more and 500 μm or less, and even more preferably 30 μm or more and 300 μm or less.

(Separator)
The separator can be appropriately selected from known separators. For example, a separator consisting of only a base layer, a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both sides of the base layer, etc. can be used as the separator. The base layer of the separator may contain heat-resistant particles. Examples of the material of the base layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these materials, porous resin film is preferable from the viewpoint of strength, and nonwoven fabric is preferable from the viewpoint of non-aqueous electrolyte retention. As the material of the base layer of the separator, polyolefin such as polyethylene and polypropylene is preferable from the viewpoint of shutdown function, and polyimide and aramid are preferable from the viewpoint of oxidation decomposition resistance. A material obtained by combining these resins may be used as the base layer of the separator.

The heat-resistant particles contained in the heat-resistant layer and the base material layer preferably have a mass loss of 5% or less when heated from room temperature to 500°C in air, and more preferably have a mass loss of 5% or less when heated from room temperature to 800°C in air. Examples of materials that have a mass loss of a specified amount or less when heated include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, barium titanate, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; poorly soluble ion crystals such as calcium fluoride and barium fluoride; covalent crystals such as silicon and diamond; mineral resource-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. As inorganic compounds, these substances may be used alone or in combination of two or more. Among these inorganic compounds, silicon oxide, aluminum oxide, and aluminosilicate are preferred from the viewpoint of the safety of the storage element.

The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured using a mercury porosimeter.

As the separator, a polymer gel composed of a polymer and a non-aqueous electrolyte may be used. Examples of the polymer include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polyethylene carbonate, polypropylene carbonate, polyvinyl carbonate, polyalkyl methacrylates such as polymethyl methacrylate, polyalkyl acrylates such as polymethyl acrylate, polyvinyl ethylene carbonate, polyvinyl acetate, polyvinylpyrrolidone, polymaleic acid and its derivatives, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, and copolymers of monomers constituting these polymers, as well as mixtures of these polymers. These polymers may also be combined with inorganic salts or ionic liquids. The use of a polymer gel has the effect of suppressing leakage. As the separator, a polymer gel may be used in combination with the porous resin film or nonwoven fabric described above.

(Non-aqueous electrolyte)
The non-aqueous electrolyte includes a polyvalent metal cation and an imide anion. The non-aqueous electrolyte usually further includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The polyvalent metal cation and the imide anion may be added to or contained in the non-aqueous electrolyte as an additive (polyvalent metal imide salt) composed of them. In addition, the polyvalent metal cation and the imide anion may exist in a state in which a part or all of them form an ion pair, may exist in a state of partially dissociated multiple ions, or may exist in a state of complete dissociation. The imide anion may include one derived from the electrolyte salt. The non-aqueous electrolyte may further include other additives, etc.

(polyvalent metal cation)
Examples of polyvalent metal cations include polyvalent cations of metal elements from Group 2 to Group 14. The valence of the polyvalent metal cation may be divalent, trivalent, or tetravalent or more, preferably divalent or trivalent, and more preferably divalent. One or more types of polyvalent metal cations may be contained.

Specific examples of polyvalent metal cations include polyvalent cations of Group 2 elements (Mg ²⁺ , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ , etc.), polyvalent cations of Group 3 elements (Sc ³⁺ , La ³⁺ , etc.), polyvalent cations of Group 4 elements (Ti ³⁺ , etc.), polyvalent cations of Group 5 elements (V ³⁺ , etc.), polyvalent cations of Group 6 elements (Cr ³⁺ , etc.), polyvalent cations of Group 7 elements (Mn ²⁺ , etc.), polyvalent cations of Group 8 elements (Fe ²⁺ , Fe ³⁺ , etc.), polyvalent cations of Group 9 elements (Co ²⁺ , etc.), polyvalent cations of Group 10 elements (Ni ²⁺ , etc.), polyvalent cations of Group 11 elements (Cu ²⁺ , etc.), polyvalent cations of Group 12 elements (Zn ²⁺ , Cd ²⁺ , etc.), and polyvalent cations of metal elements of Group 13 elements (Al ³⁺ , etc.), and polyvalent cations of metal elements in Group 14 elements (Sn ²⁺ , Pb ²⁺ , etc.).

Among these, polyvalent cations of Group 2, Group 3, Group 11 or Group 12 elements are preferred, polyvalent cations of Group 2, Group 11 or Group 12 elements are more preferred, polyvalent cations of Group 2 or Group 11 elements are even more preferred, and magnesium ion (Mg ²⁺ ) and copper ion (Cu ²⁺ ) are even more preferred. Polyvalent metal cations of elements of Period 3 or Period 4 are also preferred. By containing such polyvalent metal cations in the non-aqueous electrolyte, the effect of suppressing the occurrence of micro-short circuit caused by repeated charging and discharging is enhanced.

It is also preferable that the polyvalent metal cation is a cation of a typical element. A typical element refers to an element other than a transition metal element (elements of groups 3 to 11). In other words, examples of polyvalent metal cations that are cations of typical elements include polyvalent cations of elements of groups 2 and 12, and polyvalent cations of metal elements of groups 13, 14, and 15.

The content of polyvalent metal cations in the non-aqueous electrolyte is preferably 0.0001% by mass to 1% by mass, more preferably 0.001% by mass to 0.1% by mass, and even more preferably 0.002% by mass to 0.03% by mass. The content of polyvalent metal cations in the non-aqueous electrolyte is preferably 0.01 mmol/kg to 100 mmol/kg, more preferably 0.1 mmol/kg to 10 mmol/kg, and even more preferably 0.5 mmol/kg to 4 mmol/kg. By having the content of polyvalent cations equal to or greater than the lower limit, a coating of sufficient quantity and quality is formed, and the effect of suppressing the occurrence of micro-short circuits due to repeated charging and discharging is enhanced. On the other hand, by having the content of polyvalent cations equal to or less than the upper limit, the solubility and conductivity of lithium ions can be maintained in a sufficient state, and the effect of suppressing the occurrence of micro-short circuits due to repeated charging and discharging is enhanced, and the charge and discharge performance is also improved.

(imide anion)
Examples of the imide anion include symmetric sulfonylimide anions such as N(SO ₂ F) ₂ ⁻ (bis(fluorosulfonyl)imide anion: FSI ⁻ ), N(CF ₃ SO ₂ ) ₂ ⁻ (bis(trifluoromethanesulfonyl)imide anion: TFSI ⁻ ), N(C ₂ F ₅ SO ₂ ) ₂ ⁻ (bis(pentafluoroethanesulfonyl)imide anion: BETI ⁻ ), N(C ₄ F ₉ SO ₂ ) ₂ ⁻ (bis(nonafluorobutanesulfonyl)imide anion), CF ₃ —SO ₂ —N—SO ₂ —N—SO ₂ —CF ₃ ⁻ , and CF ₃ —SO _{2 —N} —SO ₂ —CF ₂ —SO ₂ —N—SO ₂ —CF ₃ ^2- ; F)(SO ₂ CF ₃ ) ^-- ((fluorosulfonyl)(trifluoromethanesulfonyl)imide anion), N(SO ₂ F)(SO ₂ C ₂ F ₅ ) ^-- ((fluorosulfonyl)(pentafluoroethanesulfonyl)imide anion), N(SO ₂ F)(SO ₂ C ₄ F ₉ ) ^-- ((fluorosulfonyl)(nonafluorobutanesulfonyl)imide anion), N(SO ₂ CF ₃ )(SO ₂ C ₄ F ₉ ) ^-- ((trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide anion), FSO ₂ --N--SO ₂ --C ₄ F ₉ ^-- , CF ₃ --SO ₂ --N--SO ₂ --C ₄ F ₉ ^-- , CF ₃ --SO ₂ --N--SO ₂ ^Examples of the imide anion include asymmetric sulfonylimide anions such as --CF2 _{-- SO32-} , CF3 _{-- SO2} -- _N --SO2-- ^{CF2--SO2--C(--SO2CF3)22-} _{, and phosphonylimide anions such} as N( _POF2 ) _2-- ⁽ bis(difluorophosphonyl)imide anion), and sulfonylimide anions (symmetric sulfonylimide anions and asymmetric sulfonylimide anions) are preferred. One or more types of imide anions can be used.

The imide anion preferably has a fluorine atom, specifically, for example, a fluorosulfonyl group, a difluorophosphonyl group, a fluoroalkyl group, or the like. As the imide anion, a sulfonylimide anion having a fluorine atom is more preferable, and bis(trifluoromethanesulfonyl)imide anion (TFSI ⁻ ) is particularly preferable. By using such an imide anion, the formed coating film has better quality, and the occurrence of micro-short circuit can be further suppressed.

The content of imide anions in the non-aqueous electrolyte is preferably 0.001% by mass or more and 50% by mass or less, more preferably 0.005% by mass or more and 10% by mass or less, even more preferably 0.01% by mass or more and 3% by mass or less, even more preferably 0.02% by mass or more and 1% by mass or less, and particularly preferably 0.03% by mass or more and 0.2% by mass or less. The content of imide anions in the non-aqueous electrolyte is preferably 0.02 mmol/kg or more and 3 mol/kg or less, more preferably 0.1 mmol/kg or more and 1 mol/kg or less, even more preferably 0.2 mmol/kg or more and 200 mmol/kg or less, even more preferably 0.5 mmol/kg or more and 20 mmol/kg or less, and particularly preferably 1 mmol/kg or more and 8 mmol/kg or less. By having the content of imide anions be equal to or more than the above lower limit, a coating of sufficient quantity and quality is formed, and the effect of suppressing the occurrence of micro-short circuits due to repeated charging and discharging is enhanced. On the other hand, by having the content of imide anions be equal to or less than the upper limit, the conductivity can be maintained in a sufficient state, the effect of suppressing the occurrence of micro-short circuits due to repeated charging and discharging is enhanced, and the charge and discharge performance is also improved. Note that, in the case where the imide anions in the non-aqueous electrolyte are, for example, those added or contained in the non-aqueous electrolyte as a polyvalent metal imide salt and those contained in the non-aqueous electrolyte as an electrolyte salt, the content of the imide anions derived from these polyvalent metal imide salts and the content of the imide anions derived from the electrolyte salt are the sum of these.

(Polyvalent metal imide salt)
As described above, the polyvalent metal cation and the imide anion may be added to or contained in the nonaqueous electrolyte as an additive composed of them, i.e., a polyvalent metal imide salt. One or more types of polyvalent metal imide salts can be used.

Examples of polyvalent metal imide salts include Mg(TFSI) ₂ , Ca(TFSI) ₂ , La(TFSI) ₃ , Ti(TFSI) ₃ , Mn(TFSI) ₂ , Cu(TFSI) ₂ , Zn(TFSI) ₂ , Al(TFSI) ₃ , Mg(FSI) ₂ , Ca(FSI) ₂ , La(FSI) ₃ , Ti(FSI) ₃ , Mn(FSI) ₂ , Cu(FSI) ₂ , Zn(FSI) ₂ , Al(FSI) ₃ , Mg(BETI) ₂ , Mg(N(POF2) ₂ ) _{2 ,} and the like.

The amount of polyvalent metal imide salt added to the non-aqueous electrolyte is preferably 0.001% by mass to 3% by mass, more preferably 0.01% by mass to 0.5% by mass, and even more preferably 0.03% by mass to 0.2% by mass. The amount of polyvalent metal imide salt added to the non-aqueous electrolyte is preferably 0.01 mmol/kg to 100 mmol/kg, more preferably 0.1 mmol/kg to 10 mmol/kg, and even more preferably 0.5 mmol/kg to 4 mmol/kg.

(Non-aqueous solvent)
The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, carboxylates, phosphates, sulfonates, ethers, amides, and nitriles. As the non-aqueous solvent, those in which some of the hydrogen atoms contained in these compounds are substituted with halogens may be used.

As the non-aqueous solvent, it is preferable to use at least one of a cyclic carbonate and a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. By using a cyclic carbonate, it is possible to promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. By using a chain carbonate, it is possible to keep the viscosity of the non-aqueous electrolyte low. When a cyclic carbonate and a chain carbonate are used in combination, it is preferable that the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is, for example, in the range of 5:95 to 50:50.

Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (including trans isomer, cis isomer, and mixtures thereof), 4-(fluoromethyl)-1,3-dioxolan-2-one, 4-(difluoromethyl)-1,3-dioxolan-2-one, trifluoropropyl carbonate, 4-(trifluoroethyl)-1,3-dioxolan-2-one, styrene carbonate, catechol carbonate, phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate. The number of carbon atoms in the cyclic carbonate may be, for example, 3 or more and 13 or less, or 7 or less, or 5 or less.

As the cyclic carbonate, from the viewpoint of oxidation resistance, etc., fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), difluoroethylene carbonate, 4-(fluoromethyl)-1,3-dioxolan-2-one, 4-(difluoromethyl)-1,3-dioxolan-2-one, trifluoropropyl carbonate, and 4-(trifluoroethyl)-1,3-dioxolan-2-one are preferred.

Examples of chain carbonates include diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, phenyl methyl carbonate, diphenyl carbonate, trifluoromethyl ethyl carbonate, 2,2,2-trifluoroethyl methyl carbonate (TFEMC), bis(trifluoromethyl)carbonate, bis(2,2,2-trifluoroethyl)carbonate, and ethyl-2,2,2-trifluoroethyl carbonate. The number of carbon atoms in the chain carbonate may be, for example, 3 or more and 13 or less, or 7 or less, or 5 or less.

As the chain carbonate, from the viewpoint of oxidation resistance, etc., fluorinated chain carbonates such as 2,2,2-trifluoroethyl methyl carbonate (TFEMC), bis(2,2,2-trifluoroethyl) carbonate, and ethyl-2,2,2-trifluoroethyl carbonate are preferred.

The carbonate content relative to the total non-aqueous solvent is preferably 50% by volume or more and 100% by volume or less, more preferably 80% by volume or more, and in some cases even more preferably 90% by volume or more, 95% by volume or more, or 99% by volume or more.

The non-aqueous solvent preferably contains a fluorinated solvent. The total content of the fluorinated solvent relative to the total non-aqueous solvent is preferably 60% by volume or more, more preferably 90% by volume or more, even more preferably 99% by volume or more, and particularly preferably 100% by volume. The fluorinated solvent refers to a solvent (non-aqueous solvent) having a fluorine atom in the molecule, such as a fluorinated carbonate (a fluorinated linear carbonate and a fluorinated cyclic carbonate), a fluorinated ether, etc. In this way, by including a fluorinated solvent, preferably including it at the above-mentioned lower limit or more, it is possible to further improve the suppression of micro-short circuits, oxidation resistance, etc.

(Electrolyte Salt)
The electrolyte salt is usually a lithium salt. Examples of the lithium salt include inorganic lithium salts such _{as LiPF6} , _{LiPO2F2 , LiBF4} , _LiClO4 , and LiN( _SO2F ) _2, and organic lithium salts such as _LiSO3CF3 , _LiN ( _SO2CF3 ) ₂ , _LiN ( _SO2C2F5 ) ₂ , LiN( _SO2CF3 ) _{( SO2C4F9 )} , LiC( _{SO2CF3 ) 3 ,} and LiC( _SO2C2F5 ) ₃ . The _inorganic lithium salt refers to a lithium salt whose anion does not contain a carbon atom, and the organic lithium salt refers to a lithium salt _whose anion contains _a carbon atom. The organic lithium salt preferably has a halogenated hydrocarbon group, more preferably has a fluorinated hydrocarbon group, and further preferably has a fluoroalkyl group. The electrolyte salt may be an imide salt or an electrolyte salt other than an imide salt. As the electrolyte salt, inorganic lithium salts or salts containing phosphorus atoms in the anion are preferred, and LiPF ₆ is more preferred. In addition, from the viewpoints of oxidation resistance and good film formation, the electrolyte salt is preferably one containing a fluorine atom.

The content of the electrolyte salt (excluding polyvalent metal imide salt) in the nonaqueous electrolyte is preferably 0.1 mol/dm ³ or more and 2.5 mol/dm ³ or less, more preferably 0.3 mol/dm ³ or more and 2.0 mol/dm ³ or less, still more preferably 0.5 mol/dm ³ or more and 1.7 mol/dm ³ or less, and particularly preferably 0.7 mol/dm ³ or more and 1.5 mol/dm ³ or less. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the nonaqueous electrolyte can be increased.

The non-aqueous electrolyte may contain other additives. Examples of other additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, dimethyl sulfate, and ethylene sulfate. Examples of such additives include ethylene, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, perfluorooctane, tristrimethylsilyl borate, tris(trimethylsilyl) phosphite, tris(trimethylsilyl) phosphate, and tetrakistrimethylsilyl titanate. These additives may be used alone or in combination of two or more.

The content of other additives (excluding polyvalent metal imide salts) contained in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass. By setting the content of the additives within the above range, it is possible to improve the capacity retention performance or charge/discharge cycle performance after high-temperature storage, and to further improve safety.

In the secondary battery (nonaqueous electrolyte storage element), the positive electrode potential at the end of charge voltage during normal use is preferably 4.30 V vs. Li/Li ⁺ or more, more preferably 4.35 V vs. Li/Li ⁺ or more, and in some cases even more preferably 4.40 V vs. Li/Li ⁺ or more. By setting the positive electrode potential at the end of charge voltage during normal use to the above lower limit or more, the discharge capacity can be increased and the energy density can be increased.

The term "normal use" refers to a case where the nonaqueous electrolyte storage element is used under the charging conditions recommended or specified for the nonaqueous electrolyte storage element. For example, if a charger is provided for the nonaqueous electrolyte storage element, it refers to a case where the nonaqueous electrolyte storage element is used using the charger.

The upper limit of the positive electrode potential at the end-of-charge voltage during normal use of the secondary battery is, for example, 5.0 V vs. Li/Li ⁺ , or may be 4.8 V vs. Li/Li ⁺ , 4.7 V vs. Li/Li ⁺ , or 4.6 V vs. Li/ ^{Li +} .

Dendrites tend to grow easily when the current density during charging is high. Therefore, the nonaqueous electrolyte storage element according to one embodiment of the present invention can be suitably used in applications where charging is performed at a high current density. Examples of such applications include power sources for automobiles such as electric vehicles (EVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs), and power sources for charging regenerative power.

The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples include cylindrical batteries, laminated film batteries, square batteries, flat batteries, coin batteries, button batteries, etc.

Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a square battery. Note that this figure is a see-through view of the inside of the container. An electrode body 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a square container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

<Configuration of Nonaqueous Electrolyte Electricity Storage Device>
The nonaqueous electrolyte storage element of the present embodiment can be mounted as an electricity storage unit (battery module) comprising a plurality of nonaqueous electrolyte storage elements assembled together in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), etc., a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, it is sufficient that the technique according to one embodiment of the present invention is applied to at least one of the nonaqueous electrolyte storage elements included in the electricity storage unit.

Figure 2 shows an example of an energy storage device 30 in which an energy storage unit 20, which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1, is further assembled. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, and the like. The energy storage unit 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte energy storage elements.

<Method of Manufacturing Nonaqueous Electrolyte Storage Element>
A method for producing a nonaqueous electrolyte storage element according to one embodiment of the present invention includes preparing a negative electrode containing metallic lithium, and preparing a nonaqueous electrolyte containing a polyvalent metal cation and an imide anion.

Preparing a negative electrode containing metallic lithium may be preparing a negative electrode containing metallic lithium. The negative electrode can be prepared by laminating a negative electrode active material layer containing metallic lithium directly or via an intermediate layer on a negative electrode substrate, and pressing or the like. The negative electrode active material layer containing metallic lithium may be metallic lithium foil or lithium alloy foil. The specific and preferred forms of the prepared negative electrode can be those described above as a negative electrode provided in a nonaqueous electrolyte storage element.

Preparing a nonaqueous electrolyte containing a polyvalent metal cation and an imide anion may mean preparing a nonaqueous electrolyte containing a polyvalent metal cation and an imide anion. The preparation of the nonaqueous electrolyte can be performed, for example, by adding components other than the nonaqueous solvent, such as a polyvalent metal imide salt composed of a polyvalent metal cation and an imide anion, an electrolyte salt, etc., to the nonaqueous solvent and mixing them. The specific form and preferred form of the prepared nonaqueous electrolyte can be the form described above as the nonaqueous electrolyte provided in the nonaqueous electrolyte storage element.

For example, a method for manufacturing the nonaqueous electrolyte storage element includes preparing or producing a positive electrode, preparing or producing a negative electrode, preparing or preparing a nonaqueous electrolyte, preparing or producing a separator, forming an electrode body in which the positive electrode and the negative electrode are alternately stacked by stacking or rolling the separator between them, housing the positive electrode and the negative electrode (electrode body) in a container, and injecting the nonaqueous electrolyte into the container. After injection, the nonaqueous electrolyte storage element can be obtained by sealing the injection port.

The manufacturing method of the nonaqueous electrolyte storage element may further include initially charging and discharging the assembled uncharged storage element. For example, when a lithium transition metal composite oxide having a molar ratio (Li/Me) of lithium (Li) to transition metal (Me) exceeding 1 is used as the positive electrode active material of the nonaqueous electrolyte storage element, the capacity increases through the initial charging and discharging. The number of charging and discharging in the initial charging and discharging may be one or two, or may be three or more. When a lithium transition metal composite oxide having a molar ratio (Li/Me) of lithium (Li) to transition metal (Me) exceeding 1 is used as the positive electrode active material of the nonaqueous electrolyte storage element, the positive electrode potential (positive electrode reaching potential) at the end-of-charge voltage in the initial charging and discharging is preferably 4.5 V vs. Li/Li ⁺ or more and 4.7 V vs. Li/Li ⁺ or less.

<Non-aqueous electrolyte>
The nonaqueous electrolyte according to one embodiment of the present invention is a nonaqueous electrolyte for a lithium battery containing a polyvalent metal cation and an imide anion. A lithium battery is a secondary battery in which metallic lithium (pure metallic lithium or a lithium alloy) is used as a negative electrode active material. That is, specific and preferred forms of the nonaqueous electrolyte include those described above as the nonaqueous electrolyte constituting the nonaqueous electrolyte storage element according to one embodiment of the present invention.

<Other embodiments>
The present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the scope of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and part of the configuration of one embodiment may be replaced with the configuration of another embodiment or a well-known technique. Furthermore, part of the configuration of one embodiment may be deleted. Also, a well-known technique may be added to the configuration of one embodiment.

In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (lithium battery), but the type, shape, size, capacity, etc. of the nonaqueous electrolyte storage element are arbitrary. The nonaqueous electrolyte storage element of the present invention can also be applied to various nonaqueous electrolyte secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[Example 1]
(Preparation of Positive Electrode)
The positive electrode active material used was a lithium transition metal composite oxide having an α-NaFeO ₂ type crystal structure and expressed as Li _1+α Me _1-α O ₂ (Me is a transition metal), where the molar ratio Li/Me of Li to Me was 1.33, and Me was composed of Ni and Mn in a molar ratio of Ni:Mn=1:2.

A positive electrode paste was prepared using N-methylpyrrolidone (NMP) as a dispersion medium, containing the above positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 94:4.5:1.5. The above positive electrode paste was applied to one side of an aluminum foil as a positive electrode substrate, and after drying, the substrate was pressed to prepare a positive electrode in which a positive electrode active material layer was disposed on one side of the positive electrode substrate.

(Preparation of negative electrode)
A metallic lithium foil (pure metallic lithium containing 100% by mass of metallic lithium) was laminated as a negative electrode active material layer on one side of a copper foil as a negative electrode substrate, and then pressed to prepare a negative electrode.
The positive and negative electrodes were designed and fabricated so that the current density at 1 C was 5.0 mA/cm ² .

(Preparation of non-aqueous electrolyte)
_LiPF6 was dissolved at a concentration of 1 mol/ ^dm3 in a mixed solvent prepared by mixing fluoroethylene carbonate (FEC) and 2,2,2-trifluoroethyl methyl carbonate (TFEMC) in a volume ratio of 30:70, and 0.1 mass% of Mg(TFSI) ₂ (Mg2 ⁺ : 0.0042 mass%, 1.7 mmol/kg, TFSI- ^: 0.096 mass%, 3.4 mmol/kg) was further added and dissolved as an additive to prepare a nonaqueous electrolyte.

(Preparation of non-aqueous electrolyte storage element)
The positive electrode and the negative electrode were laminated with a separator made of a microporous polyolefin membrane interposed therebetween to produce an electrode assembly, which was then housed in a container made of a metal resin composite film, the nonaqueous electrolyte was poured into the container, and the container was sealed by heat welding to obtain a nonaqueous electrolyte storage element (laminate-type lithium battery) of Example 1.

[Examples 2 to 5 and Comparative Examples 1 to 4]
Each of the nonaqueous electrolyte storage elements of Examples 2 to 5 and Comparative Examples 1 to 4 was obtained in the same manner as in Example 1, except that the additive listed in Table 1 was used in place of Mg(TFSI) ₂ in an amount of 0.1 mass%, or no additive was added.

(Initial charge/discharge)
The obtained nonaqueous electrolyte storage elements were initially charged and discharged under the following conditions. At 25° C., constant current and constant voltage charging was performed with a charge amount of 0.1 C and a charge end voltage of 4.6 V. The charge end condition was that the charge current was until it reached 0.05 C. Then, a rest period of 10 minutes was provided. Then, constant current discharging was performed with a discharge current of 0.1 C and a discharge end voltage of 2.0 V, and then a rest period of 10 minutes was provided. This charge and discharge cycle was performed for two cycles.

(Charge-discharge cycle test)
Next, the following charge-discharge cycle test was performed. At 25°C, constant current and constant voltage charging was performed with a charging current of 0.2C and a charging end voltage of 4.6V. The charging end condition was that the charging current was until 0.05C. Then, a rest period of 10 minutes was provided. Then, constant current discharging was performed with a discharging current of 0.1C and a discharging end voltage of 2.0V, and then a rest period of 10 minutes was provided. This charging and discharging cycle was repeated, and the number of cycles until a micro-short circuit occurred was recorded. The results (the number of cycles until a micro-short circuit occurred) are shown in Table 1.

As shown in Table 1, the nonaqueous electrolyte storage elements of Examples 1 to 5, which contained a polyvalent metal imide salt, i.e., a nonaqueous electrolyte containing a polyvalent metal cation and an imide anion, exceeded 70 cycles before a micro-short circuit occurred, confirming an excellent effect of suppressing the occurrence of micro-short circuits with repeated charging and discharging. On the other hand, the nonaqueous electrolyte storage elements of Comparative Examples 2 and 3, which contained an imide salt of a monovalent metal, and Comparative Example 4, which contained another salt, were not able to sufficiently suppress the occurrence of micro-short circuits with repeated charging and discharging.

The present invention can be applied to nonaqueous electrolyte storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

Reference Signs List 1 Non-aqueous electrolyte storage element 2 Electrode body 3 Container 4 Positive electrode terminal 41 Positive electrode lead 5 Negative electrode terminal 51 Negative electrode lead 20 Storage unit 30 Storage device

Claims (2)

A positive electrode containing a lithium transition metal composite oxide having an α-NaFeO2 _type crystal structure;
A non-aqueous electrolyte storage element comprising: a negative electrode containing metallic lithium; and a non-aqueous electrolyte containing a polyvalent metal cation and an imide anion. Providing a positive electrode containing a lithium transition metal composite oxide having an α-NaFeO2 _type crystal structure;
A method for producing a nonaqueous electrolyte storage element, comprising: preparing a negative electrode containing metallic lithium; and preparing a nonaqueous electrolyte containing a polyvalent metal cation and an imide anion.