JP7725830B2 - Nonaqueous electrolyte storage element and method for manufacturing the same - Google Patents

Description

The present invention relates to a nonaqueous electrolyte storage element and a method for manufacturing a nonaqueous electrolyte storage element.

Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as in automobiles. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte sandwiched between the electrodes, and are configured to charge and discharge by transferring ions between the 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 energy storage elements.

Generally, the non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt that dissolves in the non-aqueous solvent, with other components added as needed. For example, a non-aqueous electrolyte solution for secondary batteries has been proposed in which a fluorinated carboxylic acid ester and a specific film-forming compound are contained in a non-aqueous solvent (see Patent Document 1).

JP 2009-289414 A

The use of non-aqueous electrolytes containing fluorinated carboxylic acid esters and the like tends to improve charge-discharge cycle performance. However, the use of non-aqueous electrolytes containing fluorinated carboxylic acid esters and the like can result in high initial DC resistance at low temperatures in non-aqueous electrolyte secondary batteries and other non-aqueous electrolyte storage elements.

The present invention was made based on the above circumstances, and its purpose is to provide a nonaqueous electrolyte energy storage element that reduces initial DC resistance at low temperatures and suppresses a decrease in capacity retention rate after charge-discharge cycling.

A nonaqueous electrolyte storage element according to one aspect of the present invention comprises a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte, wherein the separator has a porosity of 44% by volume or more, and the nonaqueous electrolyte contains a compound represented by the following formula (1):

In formula (1), R ¹ and R ² each independently represent a hydrogen atom or a fluorine atom.

A method for manufacturing a nonaqueous electrolyte storage element according to another aspect of the present invention includes preparing a separator having a porosity of 44% by volume or more, and preparing a nonaqueous electrolyte containing a compound represented by the following formula (1):

In formula (1), R ¹ and R ² each independently represent a hydrogen atom or a fluorine atom.

A nonaqueous electrolyte storage element according to one aspect of the present invention can reduce initial DC resistance at low temperatures and suppress a decrease in capacity retention rate after charge-discharge cycling.

FIG. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element. FIG. 2 is a schematic diagram showing one embodiment of an electricity storage device configured by assembling a plurality of nonaqueous electrolyte electricity storage elements.

First, we will provide an overview of the nonaqueous electrolyte energy storage element disclosed in this specification.

A nonaqueous electrolyte storage element according to one aspect of the present invention comprises a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte, wherein the separator has a porosity of 44% by volume or more, and the nonaqueous electrolyte contains a compound represented by the following formula (1):

In formula (1), R ¹ and R ² each independently represent a hydrogen atom or a fluorine atom.

In this nonaqueous electrolyte storage element, the porosity of the separator is 44% by volume or more, and the nonaqueous electrolyte contains a compound represented by formula (1) above. This reduces the initial DC resistance at low temperatures and suppresses a decrease in capacity retention after charge-discharge cycling. While the reason for this is unclear, the following is presumed, for example: One factor that reduces the battery's life performance is the decomposition products of the nonaqueous electrolyte decomposed on the negative electrode reaching the positive electrode and causing the positive electrode to deteriorate. However, the compound represented by formula (1) is reductively decomposed to form a high-quality coating on the negative electrode, thereby protecting the negative electrode and suppressing decomposition of the nonaqueous electrolyte, thereby improving the capacity retention. Furthermore, in this nonaqueous electrolyte storage element, the porosity of the separator is 44% by volume or more, thereby reducing the initial DC resistance of the nonaqueous electrolyte storage element at low temperatures and further improving the effect of the compound represented by formula (1) in suppressing a decrease in capacity retention after charge-discharge cycling. In addition to the improved charge-discharge cycle performance of the compound, the nonaqueous electrolyte energy storage element is thought to have a particularly significant effect on improving charge-discharge cycle performance due to a synergistic effect: the charge-discharge reactions at the positive and negative electrodes proceed more deeply due to a small increase in resistance, thereby accelerating the reductive decomposition reaction of the compound. On the other hand, adding a large amount of additive to the nonaqueous electrolyte tends to increase the initial resistance increase and reduce charge-discharge cycle performance due to oxidative or thermal decomposition of the remaining additive. Furthermore, a high porosity of the separator can easily transport the remaining additive or reductive decomposition products of the nonaqueous electrolyte from the negative electrode to the positive electrode, potentially reducing charge-discharge cycle performance. In contrast, the compound reductively decomposes to form a high-quality coating on the negative electrode, adequately protecting the negative electrode without adversely affecting the positive electrode. Therefore, it is thought that charge-discharge cycle performance will not be reduced even when combined with a separator with a high porosity. Therefore, the nonaqueous electrolyte energy storage element can reduce initial DC resistance at low temperatures and suppress a decrease in capacity retention after charge-discharge cycling.

The content of the above compound in the non-aqueous electrolyte is preferably 0.7% by mass or more and 5.0% by mass or less. By having the content of the above compound be 0.7% by mass or more and 5.0% by mass or less, an increase in initial resistance can be suppressed.

A method for manufacturing a nonaqueous electrolyte storage element according to another aspect of the present invention includes preparing a separator having a porosity of 44% by volume or more, and preparing a nonaqueous electrolyte containing a compound represented by the following formula (1):

In formula (1), R ¹ and R ² each independently represent a hydrogen atom or a fluorine atom.

This method for manufacturing a nonaqueous electrolyte storage element makes it possible to manufacture a nonaqueous electrolyte storage element that reduces initial DC resistance at low temperatures and suppresses a decrease in capacity retention rate after charge/discharge cycling.

The configuration of a nonaqueous electrolyte energy storage element according to one embodiment of the present invention, the configuration of an energy storage device, and a method for manufacturing a nonaqueous electrolyte energy storage element, as well as other embodiments, will be described in detail. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

<Non-aqueous electrolyte energy storage element>
A nonaqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, a nonaqueous electrolyte, and a container that accommodates the electrode assembly and the nonaqueous electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a stack of positive electrodes and negative electrodes with separators interposed therebetween is wound. The nonaqueous electrolyte is present in a state contained in the positive electrode, the negative electrode, and the separator. As an example of a nonaqueous electrolyte storage element, a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

(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 is conductive. Whether or not a material is "conductive" is determined by a volume resistivity of 10 ⁷ Ω·cm measured in accordance with JIS-H-0505 (1975). Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30 as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

The average thickness of the positive electrode substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 40 μm or less, even more preferably 8 μm or more and 30 μm or less, and particularly preferably 10 μm or more and 25 μm or less. By keeping the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while also increasing the energy density per volume of the secondary battery.

The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent 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 may include, for example, a binder and a conductive agent.

The positive electrode active material layer contains a positive electrode active material. If necessary, the positive electrode active material layer may also contain optional components such as a conductive agent, a binder, a thickener, and a filler.

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 typically used as the positive electrode active material for lithium ion secondary batteries. 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, and sulfur. 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 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.} 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.} Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. The atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more of them may be used in combination.

The positive electrode active material is typically in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. Setting the average particle size of the positive electrode active material above the lower limit facilitates the production and handling of the positive electrode active material. Setting the average particle size of the positive electrode active material below the upper limit improves the electronic conductivity of the positive electrode active material layer. When a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. "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 laser diffraction/scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

To obtain powders of a specified particle size, mills and classifiers are used. Examples of milling methods include those using a mortar, ball mill, sand mill, vibrating ball mill, planetary ball mill, jet mill, counter jet mill, swirling air jet mill, or sieves. Wet milling, in which water or an organic solvent such as hexane is present, can also be used. Sieves and air classifiers can be used as classification methods, both dry and wet, as needed.

The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 80% by mass or more and 95% by mass or less. By keeping the content of the positive electrode active material within this range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

The conductive agent is not particularly limited as long as it is a material that is conductive. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Non-graphitic carbon includes carbon nanofiber, pitch-based carbon fiber, and carbon black. Carbon black includes furnace black, acetylene black, and ketjen black. Graphene-based carbon includes graphene, carbon nanotubes (CNT), and fullerene. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNT may be used. Among these, carbon black is preferred from the standpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the content of the conductive agent within this 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, polyacrylic, 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 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the binder content within this range, the active material can be stably maintained.

Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. If the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene; inorganic oxides such as silicon dioxide, alumina, 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; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate; nitrides such as aluminum nitride and silicon nitride; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; and synthetic versions of these.

The positive electrode active material layer may contain typical non-metallic 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 metal 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 configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

The negative electrode substrate is electrically conductive. Materials used for the negative electrode substrate include metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys of these metals, and carbonaceous materials. Among these, copper or copper alloys are preferred. Examples of negative electrode substrates include foil, vapor-deposited film, mesh, and porous materials, with foil being preferred from a cost perspective. Therefore, copper foil or copper alloy 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 keeping the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while also increasing the energy density per volume of the secondary battery.

The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer also contains optional components such as a conductive agent, binder, thickener, and filler as needed. The optional components such as a conductive agent, binder, thickener, and filler can be selected from the materials exemplified for the positive electrode above.

The negative electrode active material layer may contain typical non-metallic 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 metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is typically used as the negative electrode active material for lithium ion secondary batteries. Examples of negative electrode active materials include metal Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; titanium-containing oxides such as Li ₄ Ti ₅ O ₁₂ , LiTiO _{2 , and} TiNb ₂ O ₇ ; polyphosphate compounds; silicon carbide; carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be mixed and used.

"Graphite" refers to a carbon material in which the average lattice spacing (d002) of the (002) plane, as determined by X-ray diffraction, is 0.33 nm or more and less than 0.34 nm before charge/discharge or in a discharged state. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

"Non-graphitic carbon" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitic carbon include non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

Here, "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, has been discharged so that lithium ions capable of being absorbed and released during charging and discharging are sufficiently released. For example, in a single-electrode battery using a negative electrode containing a carbonaceous material as the negative electrode active material as the working electrode and metallic Li as the counter electrode, this is a state in which the open circuit voltage is 0.7 V or higher.

"Non-graphitizable carbon" refers to a carbon material with the above d002 of 0.36 nm or more and 0.42 nm or less.

The term "easily graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.34 nm or more and less than 0.36 nm.

The negative electrode active material is typically in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm to 100 μm. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm to 100 μm. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm to 1 μm. Setting the average particle size of the negative electrode active material to be above the lower limit facilitates the production and handling of the negative electrode active material. Setting the average particle size of the negative electrode active material to be below the upper limit improves the electronic conductivity of the active material layer. To obtain powder with a specified particle size, a pulverizer, a classifier, or the like is used. The pulverization method and powder classification method can be selected from, for example, the methods exemplified for the positive electrode above. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material may be in the form of a foil.

The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By keeping the content of the negative electrode active material within this range, it is possible to achieve both high energy density and manufacturability of the negative electrode active material layer.

(separator)
The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the form of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these forms, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of less than the specified value include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more may be mixed and used. Of these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate are preferred from the perspective of the safety of the energy storage element.

The lower limit of the separator's porosity is 44% by volume, and 50% by volume is preferred. Having a separator porosity at or above this lower limit reduces the initial DC resistance of the nonaqueous electrolyte storage element at low temperatures, and improves the effect of the compound represented by formula (1) above in suppressing a decrease in capacity retention rate after charge-discharge cycling. On the other hand, the upper limit of the porosity is preferably 70% by volume, and more preferably 60% by volume. Having a separator porosity at or below this upper limit improves the strength of the separator.

The porosity [%] of the separator is calculated using the following formula.
Porosity (%) = 100-(W/(ρ×t)×100)
W: mass per unit area [g·cm ⁻² ]
ρ: true density of constituent material [g·cm ⁻³ ]
t: thickness [cm]

The upper limit of the separator's air permeability is preferably 150 sec/100 ^cm3 , and more preferably 120 sec/100 ^cm3 . Having a separator air permeability equal to or less than the upper limit allows for a reduction in the initial DC resistance of the nonaqueous electrolyte storage element at low temperatures. On the other hand, the lower limit of the separator's air permeability is preferably 50 sec/100 ^cm3 , and more preferably 60 sec/100 ^cm3 , from the viewpoint of maintaining the separator's strength. The separator's air permeability, also known as the Gurley value, indicates the number of seconds it takes for a given volume of air to pass through a separator of a given area under a given pressure difference, and is a value measured in accordance with JIS-P8117 (2009).

The lower limit of the average thickness of the separator is preferably 14 μm, more preferably 16 μm. The upper limit of the average thickness is preferably 25 μm, more preferably 22 μm. By making the average thickness of the separator equal to or greater than the lower limit, it is possible to prevent a decrease in the safety of the nonaqueous electrolyte storage element. Furthermore, by making the average thickness of the separator equal to or less than the upper limit, it is possible to reduce the initial DC resistance at low temperatures.

A polymer gel composed of a polymer and a non-aqueous electrolyte may be used as the separator. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. Using a polymer gel has the effect of suppressing leakage. The separator may also be a combination of a porous resin film or nonwoven fabric, such as those described above, and a polymer gel.

(Non-aqueous electrolyte)
The non-aqueous electrolyte contains a compound represented by the following formula (1). The non-aqueous electrolyte may be a non-aqueous electrolyte solution. The non-aqueous electrolyte solution contains a non-aqueous solvent, an electrolyte salt dissolved in the non-aqueous solvent, and the compound represented by the following formula (1).

The non-aqueous electrolyte contains a compound represented by the following formula (1) as an additive. When the non-aqueous electrolyte contains the compound represented by formula (1), the compound is reductively decomposed to form a high-quality coating on the negative electrode. As a result, it is believed that the negative electrode is protected and the capacity retention rate is improved in this non-aqueous electrolyte storage element.

In formula (1), ^R1 and ^R2 are each independently a hydrogen atom or a fluorine atom. When ^R1 and ^R2 are each a fluorine atom, it is believed that the oxidation resistance of the compound represented by formula (1) can be further improved.

Examples of the above compounds include compounds represented by the following formulas (1-1) to (1-3).

The lower limit of the content of the above compound in the non-aqueous electrolyte is preferably 0.7% by mass, more preferably 0.8% by mass, and even more preferably 1.0% by mass. When the content of the above compound is at or above this lower limit, the initial DC resistance at low temperatures can be reduced and a decrease in capacity retention rate after charge-discharge cycling can be suppressed. On the other hand, the upper limit of the content of the above compound is preferably 5.0% by mass, more preferably 4.0% by mass. When the content of the above compound is at or below this upper limit, an increase in initial resistance can be suppressed.

The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms in these compounds have been substituted with halogens may also be used.

Cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate. Of these, EC is preferred.

Examples of chain carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, and bis(trifluoroethyl) carbonate. Of these, EMC is preferred.

It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate promotes dissociation of the electrolyte salt, improving the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate reduces the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

The non-aqueous electrolyte may contain additives other than the compound represented by formula (1) above, as long as they do not impair the effects of the present invention. Examples of such additives include various additives commonly contained in non-aqueous electrolytes. Examples of such additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexyl terphenyl, and the like. Aromatic compounds such as hexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, acid, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, 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, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. These additives may be used alone or in combination of two or more.

The content of other additives contained in the non-aqueous electrolyte is preferably 0.2% by mass or more and 3.0% by mass or less, and more preferably 0.5% by mass or more and 2.0% by mass or less, relative to the total mass of the non-aqueous electrolyte. By keeping the content of other additives within the above range, it is possible to improve capacity retention or cycle performance after high-temperature storage, and to further improve safety.

The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

Examples of the lithium salt include inorganic lithium _salts such as _LiPF6 , _LiPO2F2 , _LiBF4 , _LiClO4 , and LiN( _SO2F ) ₂ ; lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis _{( oxalate} )difluorophosphate (LiFOP); _and lithium salts having _{a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3 ) 2 , LiN} ( _SO2C2F5 ) ₂ , LiN _{( SO2CF3 )} ( _{SO2C4F9 ) ,} LiC( _{SO2CF3 ) 3} , and LiC(SO2C2F5) _{3 .} Among these, inorganic lithium salts are preferred, and _LiPF6 is more preferred.

The content of the electrolyte salt in the non-aqueous electrolyte solution at 20°C and 1 atmosphere is preferably 0.1 mol/dm3 or ^more and 2.5 mol/dm3 or ^less , more preferably 0.3 mol/dm3 or ^more and 2.0 mol/dm3 or ^less , even ^more preferably 0.5 mol/dm3 or more and 1.7 mol/dm3 or ^less , and particularly preferably 0.7 mol/ ^dm3 or ^more and 1.5 mol/dm3 or less. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte solution can be increased.

A solid electrolyte may be used as the non-aqueous electrolyte, or a non-aqueous electrolyte solution and a solid electrolyte may be used in combination.

The solid electrolyte can be selected from any material that has ionic conductivity, such as lithium, sodium, or calcium, and is solid at room temperature (e.g., 15°C to 25°C). Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

In the case of a lithium ion secondary battery, examples of sulfide solid electrolytes include Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP ₂ S ₅ , and Li ₁₀ Ge-P ₂ S ₁₂ .

<Configuration of Power Storage Device>
The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. Note that this figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic 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.

The nonaqueous electrolyte storage element of this embodiment can be mounted as an energy storage unit (battery module) comprising a collection of multiple nonaqueous electrolyte storage elements 1 in automotive power sources such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power sources for electronic devices such as personal computers and communication terminals, or power storage power sources. In this case, the technology of the present invention needs to be applied to at least one nonaqueous electrolyte storage element included in the energy storage unit.

Figure 2 shows an example of an energy storage device 30 that further assembles energy storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1. 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 also 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 energy storage element>
The method for manufacturing a nonaqueous electrolyte storage element of the present embodiment includes preparing a separator having a porosity of 44% by volume or more, and preparing a nonaqueous electrolyte containing a compound represented by the following formula (1):

In formula (1), R ¹ and R ² each independently represent a hydrogen atom or a fluorine atom.

Other steps of the manufacturing method include, for example, preparing an electrode assembly and housing the electrode assembly and the non-aqueous electrolyte in a container. Preparing the electrode assembly includes preparing a positive electrode and a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and negative electrode with the separator interposed therebetween. The specific configurations of the separator and the non-aqueous electrolyte are as described above.

The non-aqueous electrolyte can be placed in the container by any known method. For example, if a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution can be poured into an inlet formed in the container, and then the inlet can be sealed.

This method for manufacturing a nonaqueous electrolyte storage element makes it possible to manufacture a nonaqueous electrolyte storage element that reduces initial DC resistance at low temperatures and suppresses a decrease in capacity retention rate after charge/discharge cycling.

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

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

In the above embodiment, an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly does not need to include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other, with a non-conductive layer formed on the active material layer of the positive electrode or negative electrode.

The present invention will be explained in more detail below using examples. The present invention is not limited to the following examples.

[Examples 1 to 3 and Comparative Examples 1 to 10]
(Preparation of Positive Electrode)
As the positive electrode active material, a lithium transition metal composite oxide represented by LiNi _1/2 Co _1/5 Mn _3/10 O ₂ was used.

A positive electrode paste was prepared using N-methylpyrrolidone (NMP) as a dispersion medium, containing the above positive electrode active material, acetylene black as a conductive agent, and PVdF as a binder in a mass ratio of 93:4:3. This positive electrode mixture paste was applied to both sides of aluminum foil with an average thickness of 15 μm, which served as a positive electrode substrate, leaving uncoated areas (areas where the positive electrode mixture layer was not formed), and then dried to form a positive electrode mixture layer. The positive electrode was then prepared by roll pressing. The thickness of the positive electrode was 135 μm.

(Preparation of negative electrode)
Graphite was used as the negative electrode active material. A negative electrode mixture paste containing the above negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener in a mass ratio of 96:2:2 was prepared using water as a dispersion medium. This negative electrode mixture paste was applied to both sides of a copper foil having an average thickness of 10 μm as a negative electrode substrate, leaving uncoated areas (areas where the negative electrode mixture layer was not formed), and then dried to prepare a negative electrode mixture layer. Thereafter, roll pressing was performed to prepare a negative electrode. The thickness of the negative electrode was 148 μm.

(Preparation of non-aqueous electrolyte)
A nonaqueous electrolyte was prepared by dissolving the additives shown in Tables 1 and 2 and _LiPF6 at a concentration of 1.0 mol/ ^dm3 in a mixed solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70. In Comparative Examples 1 to 3, only _LiPF6 was dissolved in the mixed solvent.

(separator)
The separator used was a microporous polypropylene membrane having an average thickness of 20 μm. Tables 1 and 2 show the porosity and air permeability of the separators of Examples 1 to 3 and Comparative Examples 1 to 10.

(Fabrication of non-aqueous electrolyte storage element)
The positive electrode and the negative electrode were stacked and wound with the separator interposed therebetween, and then the positive electrode mixture layer non-forming region of the positive electrode and the negative electrode mixture layer non-forming region of the negative electrode were welded to a positive electrode lead and a negative electrode lead, respectively, and the resulting container was sealed. The container and a cover plate were welded together, and then the nonaqueous electrolyte was poured into the container and the container was sealed. Thus, nonaqueous electrolyte storage elements of Examples 1 to 3 and Comparative Examples 1 to 10 were obtained.

(Initial charge/discharge)
Each of the obtained nonaqueous electrolyte storage elements was initially charged and discharged under the following conditions. Constant-current, constant-voltage charging was performed at 25°C with a charging current of 1.0 C and a cut-off voltage of 4.25 V. The charging was terminated after 3 hours of charging. A 10-minute rest period was then provided. Subsequently, constant-current discharging was performed with a discharging current of 1.0 C and a cut-off voltage of 2.75 V, followed by a 10-minute rest period. This charging and discharging cycle was repeated for two cycles.

(Charge-discharge cycle test)
Next, the following charge-discharge cycle test was conducted. Each nonaqueous electrolyte storage element was stored in a thermostatic chamber at 45°C for 3 hours, then charged at a constant current of 1 C to 4.25 V, and then charged at a constant voltage of 4.25 V until the charging time reached 3 hours. Subsequently, the element was discharged at a constant current of 1.0 C to 2.75 V, followed by a 10-minute rest period. This charge and discharge process constituted one cycle, and this cycle was repeated 300 times. The charge, discharge, and rest periods were all performed in a thermostatic chamber at 45°C.

(Capacity retention rate after charge/discharge cycle test)
For each nonaqueous electrolyte storage element after the charge-discharge cycle test, the discharge capacity at 45° C. at the 300th cycle relative to the discharge capacity at 45° C. at the 1st cycle was calculated as the capacity retention rate [%].

(Initial DC resistance at low temperatures)
The initial DC resistance of each nonaqueous electrolyte storage element at low temperature was evaluated. After the initial charge and discharge, each nonaqueous electrolyte storage element was charged at a constant current of 1.0 C in a thermostatic chamber at 25 ° C, and the SOC (State of Charge) was adjusted to 50%. Each nonaqueous electrolyte storage element was stored in a thermostatic chamber at -10 ° C for 4 hours, and then discharged at currents of 0.1 C, 0.2 C, and 0.3 C for 30 seconds. After each discharge, each element was charged at a constant current of 0.05 C, and the SOC was adjusted to 50%. The voltage 10 seconds after the start of discharge was plotted on the vertical axis and the discharge current on the horizontal axis, and the value corresponding to the gradient of the straight line was determined and used as the initial DC resistance at -10 ° C.

The evaluation results are shown in Tables 1 and 2 below.

As shown in Tables 1 and 2, the nonaqueous electrolyte storage elements of Examples 1 to 3, in which the separator had a porosity of 44% by volume or more and the nonaqueous electrolyte contained the compound represented by formula (1) above, reduced initial DC resistance at low temperatures and sufficiently suppressed a decrease in capacity retention rate after charge-discharge cycling.

Furthermore, as shown in Table 1, in Comparative Examples 2 and 3, in which the nonaqueous electrolyte contained no additive, and in Comparative Example 5, in which the nonaqueous electrolyte contained vinylene carbonate, when the separator porosity was 44% by volume or more, the capacity retention rate after charge-discharge cycling actually decreased. On the other hand, in Examples 1 and 2, in which the nonaqueous electrolyte contained the compound represented by formula (1) above, setting the separator porosity to 44% by volume or more was found to be more effective in suppressing the decrease in capacity retention rate after charge-discharge cycling. Therefore, it can be seen that combining the compound represented by formula (1) above with a separator having a porosity of 44% by volume or more provides the unexpected effect of suppressing the decrease in capacity retention rate after charge-discharge cycling.

Furthermore, as shown in Table 2, in Comparative Examples 9 and 10, even if the compounds had a dimer structure similar to that of the compound represented by formula (1) above, when the compound did not have an ethylene carbonate dimer structure like the compound represented by formula (1) above, the capacity retention rate after charge-discharge cycling was lower than that of Comparative Example 7, which contained no additive, even when the separator porosity was 44 vol% or higher. Furthermore, in Comparative Example 7, the inclusion of vinylene carbonate in the non-aqueous electrolyte increased the initial DC resistance at low temperatures compared to Comparative Example 7, which contained no additive. On the other hand, as shown in Example 3, the inclusion of the compound represented by formula (1) in the non-aqueous electrolyte not only reduced the initial DC resistance at low temperatures, but also significantly suppressed the decrease in capacity retention rate after charge-discharge cycling.

These results demonstrate that the nonaqueous electrolyte storage element can reduce initial DC resistance at low temperatures and suppress a decrease in capacity retention rate after charge-discharge cycling.

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, as well as automobiles.

REFERENCE SIGNS LIST 1 nonaqueous 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 (3)

A positive electrode and
a negative electrode;
a separator interposed between the positive electrode and the negative electrode and having a porous resin film substrate ;
a non-aqueous electrolyte;
The porosity of the separator is 44% by volume or more,
The nonaqueous electrolyte storage element includes a compound represented by the following formula (1):
In formula (1), R ¹ and R ² each independently represent a hydrogen atom or a fluorine atom. The nonaqueous electrolyte storage element according to claim 1, wherein the content of the compound in the nonaqueous electrolyte is 0.7% by mass or more and 5.0% by mass or less. Preparing a separator having a porosity of 44% by volume or more and a porous resin film substrate ;
A method for manufacturing a nonaqueous electrolyte storage element, comprising: preparing a nonaqueous electrolyte containing a compound represented by the following formula (1):
In formula (1), R ¹ and R ² each independently represent a hydrogen atom or a fluorine atom.