JP7753882B2 - Nonaqueous electrolyte storage element and method for manufacturing same - Google Patents

Description

The present invention relates to a non-aqueous electrolyte storage element and a method for manufacturing the same.

Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium-ion non-aqueous electrolyte secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as automobiles. Non-aqueous electrolyte secondary batteries generally comprise an electrode assembly having a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes. They 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 energy storage elements.

Lithium-ion non-aqueous electrolyte secondary batteries with rapid charging capabilities are required as energy sources for the above-mentioned automobiles and other vehicles. For example, Patent Document 1 proposes a technology that enables rapid charging by using a positive electrode active material that contains a manganese-containing oxide with a specific composition and a spinel structure, and a nickel-containing oxide with a specific composition and a layered structure.

JP 2011-076997 A

However, in recent years, demand for lithium-ion secondary batteries as an energy source has rapidly increased, calling for further improvements in rapid charging performance.

The present invention was made based on the above circumstances, and aims to provide a non-aqueous electrolyte storage element with excellent rapid charging characteristics and a method for manufacturing the same.

A nonaqueous electrolyte storage element according to one aspect of the present invention comprises a negative electrode, a positive electrode, and a nonaqueous electrolyte solution, wherein the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin, and the nonaqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate.

A method for manufacturing a nonaqueous electrolyte storage element according to one aspect of the present invention includes housing a negative electrode, a positive electrode, and a nonaqueous electrolyte solution in a case, wherein the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin, and the nonaqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate.

The nonaqueous electrolyte storage element according to one aspect of the present invention has excellent rapid charging characteristics.

A method for manufacturing a nonaqueous electrolyte storage element according to one aspect of the present invention can produce a nonaqueous electrolyte storage element with excellent rapid charging characteristics.

FIG. 1 is a perspective view showing the appearance of 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 constructed by assembling a plurality of nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention.

A nonaqueous electrolyte storage element according to one aspect of the present invention comprises a negative electrode, a positive electrode, and a nonaqueous electrolyte solution, wherein the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin, and the nonaqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate.

Graphite, which has a high charge/discharge capacity per unit mass at a base potential close to that of lithium, is widely used as the negative electrode material for nonaqueous electrolyte storage elements. Styrene-butadiene rubber, which can be used in relatively small amounts, is widely used as the negative electrode binder. However, the inventors have discovered that when the negative electrode active material layer of a nonaqueous electrolyte storage element contains graphite as the negative electrode active material and an acrylic resin as the binder, and the nonaqueous electrolyte contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate, the nonaqueous electrolyte exhibits excellent rapid charging performance. While the reason for this is unclear, it is thought to be as follows: When styrene-butadiene rubber is used as the negative electrode binder, it is distributed relatively largely on the edge surfaces of the graphite, which is the negative electrode active material. In contrast, when acrylic resin is used as the negative electrode binder, it is distributed uniformly around the graphite. Furthermore, when the non-aqueous electrolyte contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate, a high-quality coating of lithium difluorooxalatoborate or lithium difluorophosphate is formed on the edge surfaces of the graphite, which is believed to result in improved rapid charging performance of the non-aqueous electrolyte storage element.

The content of lithium difluorooxalatoborate or lithium difluorophosphate in the nonaqueous electrolyte solution is preferably 0.2% by mass or more and 2.0% by mass or less. By ensuring that the content of lithium difluorooxalatoborate or lithium difluorophosphate is within the above range, the rapid charging performance of the nonaqueous electrolyte storage element can be further improved.

It is preferable that the positive electrode contains a positive electrode active material containing nickel, cobalt, and manganese. By containing a positive electrode active material containing nickel, cobalt, and manganese, the energy density of the nonaqueous electrolyte storage element can be improved.

A nonaqueous electrolyte electricity storage element according to one embodiment of the present invention will be described in detail below.
<Non-aqueous electrolyte energy storage element>
A nonaqueous electrolyte storage element according to one embodiment of the present invention comprises a negative electrode, a positive electrode, and a nonaqueous electrolyte solution. Hereinafter, a nonaqueous electrolyte secondary battery will be described as an example of a nonaqueous electrolyte storage element. The positive electrode and negative electrode are usually stacked or wound alternately with a separator interposed therebetween to form an electrode assembly. This electrode assembly is housed in a case, and the case is filled with a nonaqueous electrolyte. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode. The case may be a known metal case, resin case, or the like, that is typically used as a case for a nonaqueous electrolyte secondary battery.

[Negative electrode]
The negative electrode includes a negative electrode substrate and a negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer is laminated along at least one surface of the negative electrode substrate directly or via an intermediate layer.

(Negative electrode substrate)
The negative electrode substrate is a substrate having electrical conductivity. Metals such as copper, nickel, stainless steel, and nickel-plated steel, or alloys thereof, are used as the material for the negative electrode substrate, with copper or a copper alloy being preferred. The negative electrode substrate may be in the form of a foil, a vapor-deposited film, or the like, with foil being preferred from the standpoint of cost. That is, copper foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil. Having "electrical conductivity" means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 1×10 ⁷ Ω·cm or less, and having "non-electrical conductivity" means that the volume resistivity is greater than 1×10 ⁷ Ω·cm.

(Negative electrode active material layer)
The negative electrode active material layer is laminated directly or via an intermediate layer along at least one surface of the negative electrode substrate. The negative electrode active material layer is formed from a so-called negative electrode mixture containing a negative electrode active material. The negative electrode active material layer contains graphite and an acrylic resin.

The negative electrode active material is typically a material capable of absorbing and releasing lithium ions. The nonaqueous electrolyte storage element contains graphite as the negative electrode active material. Examples of graphite include natural graphite and artificial graphite.

The above-mentioned negative electrode active material layer may contain other negative electrode active materials, such as other carbon materials such as non-graphitizable carbon (hard carbon) and easily graphitizable carbon (soft carbon), semi-metals such as Si, metals such as Sn, oxides of these semi-metals or metals, or composites of these semi-metals or metals with carbon materials. These materials may be used alone or in appropriate combinations of two or more. Among these, it is preferable to contain non-graphitizable carbon. By containing non-graphitizable carbon, it is possible to minimize expansion of the negative electrode during charging. Furthermore, it is possible to more stably maintain the shape of the negative electrode active material layer over a long period of time.

"Graphite" 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 0.33 nm or more and less than 0.34 nm. 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-graphitizable 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. "Non-graphitizable carbon" refers to a carbon material in which the d ₀₀₂ is 0.36 nm or more and 0.42 nm or less. "Graphitizable carbon" refers to a carbon material in which the d ₀₀₂ is 0.34 nm or more and less than 0.36 nm.

Here, "discharged state" refers to a state in which the open circuit voltage is 0.7 V or higher in a single-electrode battery using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode. Because the potential of the metallic Li counter electrode in the open circuit state is approximately equal to the redox potential of Li, the open circuit voltage in the single-electrode battery is approximately equal to the potential of the negative electrode containing the carbon material relative to the redox potential of Li. In other words, an open circuit voltage of 0.7 V or higher in the single-electrode battery means that sufficient lithium ions capable of being absorbed and released during charging and discharging have been released from the carbon material, which is the negative electrode active material.

The lower limit of the graphite content in the negative electrode active material is preferably 60% by mass, more preferably 70% by mass, and even more preferably 80% by mass. Meanwhile, the upper limit of this content is preferably 99% by mass, and more preferably 95% by mass.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but the lower limit is preferably 50% by mass, more preferably 80% by mass, and even more preferably 90% by mass. On the other hand, the upper limit of this content is preferably 99% by mass, and more preferably 98% by mass.

(binder)
The negative electrode mixture of the nonaqueous electrolyte storage element contains an acrylic resin as a binder. "Acrylic resin" refers to a resin formed from a monomer primarily composed of acrylic acid, methacrylic acid, or a derivative thereof. "Mainly composed" means that the content of structural units derived from acrylic acid, methacrylic acid, or a derivative thereof in the acrylic resin is 50% by mass or more. The lower limit of the content of structural units derived from acrylic acid, methacrylic acid, or a derivative thereof in the acrylic resin is 50% by mass, preferably 60% by mass, more preferably 70% by mass, and even more preferably 75% by mass. Examples of the acrylic resin include polyacrylic acid, methyl polyacrylate, polyacrylamide, copolymers containing acrylic acid, and alkali metal salts of polyacrylic acid. Polyacrylic acid, methyl polyacrylate, polyacrylamide, and alkali metal salts of polyacrylic acid are preferred, and polyacrylic acid is more preferred. Note that polyacrylonitrile and copolymers containing acrylonitrile are not included in the acrylic resin.

The negative electrode mixture may contain other binders, such as thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic acid, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers. From the perspective of input characteristics, the binder preferably contains a small amount of resin having structural units derived from butadiene, such as styrene-butadiene rubber (SBR), and more preferably does not contain substantially any resin having structural units derived from butadiene. Specifically, the upper limit of the mass ratio of the styrene-butadiene rubber (SBR) to the acrylic resin is preferably 2.3, more preferably 1.5, even more preferably 1.0, and particularly preferably 0.5. Similarly, in copolymers of structural units derived from acrylic acid and structural units derived from butadiene, it is preferable that the content of structural units derived from butadiene be low. Specifically, the upper limit of the content of butadiene-derived structural units in a copolymer containing acrylic acid-derived structural units and butadiene-derived structural units is, for example, preferably 50% by mass, more preferably 40% by mass, even more preferably 30% by mass, and even more preferably 25% by mass. The acrylic resin may not contain butadiene-derived structural units from the viewpoint of improving output performance, and may contain butadiene-derived structural units from the viewpoint of adhesion of the negative electrode active material layer. The lower limit of the content of butadiene-derived structural units in a copolymer containing acrylic acid-derived structural units and butadiene-derived structural units may be, for example, 1% by mass, and may be preferably 2% by mass, 5% by mass, or 10% by mass.

The content of acrylic resin in the binder is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 99% by mass or more, and may be 100% by mass.

The lower limit of the binder content in the negative electrode active material layer is preferably 0.2% by mass, more preferably 0.5% by mass, and even more preferably 1% by mass. Meanwhile, the upper limit of this content is preferably 10% by mass, and more preferably 5% by mass.

The binder content in the negative electrode active material layer is preferably 0.2% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. By keeping the binder content within the above range, the active material can be stably maintained.

(Other optional ingredients)
The negative electrode mixture may contain optional components such as a conductive agent, a thickener, and a filler as needed.

The conductive agent is not particularly limited as long as it is a conductive material. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Carbonaceous materials include graphitized carbon, non-graphitized carbon, and graphene-based carbon. Non-graphitized 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.

Examples of the thickener include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. Furthermore, if the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or other methods.

The filler is not particularly limited. Examples of the main components of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, and glass.

(middle class)
The intermediate layer is a coating layer on the surface of the negative electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the negative electrode substrate and the negative electrode active material layer. As with the positive electrode, the configuration 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.

[Positive electrode]
The positive electrode includes a positive electrode substrate and a positive electrode active material layer. The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer is laminated along at least one surface of the positive electrode substrate directly or via an intermediate layer.

(Positive electrode substrate)
The positive electrode substrate is a substrate having electrical conductivity. 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 and aluminum alloys are preferred in terms of the balance between potential resistance, high electrical conductivity, and cost. Examples of the form of the positive electrode substrate include foil and vapor-deposited film, with foil being preferred in terms of cost. That is, aluminum foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085 and A3003 as specified in JIS-H4000 (2014).

(Positive electrode active material layer)
The positive electrode active material layer is formed from a so-called positive electrode mixture containing a positive electrode active material. The positive electrode active material can be appropriately selected from, for example, 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 positive electrode active materials include lithium transition metal composite oxides having an α-NaFeO _2- type crystal structure, lithium transition metal oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having α-NaFeO type ₂ 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 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 , Li ₂ CoPO ₄ F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. The atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. Among these, the lithium transition metal composite oxides are preferred as the positive electrode active material from the viewpoint of achieving high energy density, and nickel-cobalt-manganese-containing lithium transition metal composite oxides containing nickel, cobalt, and manganese as constituent elements in addition to Li are more preferred.

The surfaces of the materials listed above as positive electrode active 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 may be mixed and used.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but the lower limit is preferably 50% by mass, more preferably 80% by mass, and even more preferably 90% by mass. On the other hand, the upper limit of this content is preferably 99% by mass, and more preferably 98% by mass.

(Other optional ingredients)
The positive electrode mixture may contain optional components such as a binder, a conductive agent, a thickener, and a filler, as required.

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

Optional components such as conductive agents, thickeners, fillers, etc. can be selected from the materials exemplified for the negative electrode above.

(middle class)
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 configuration 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.

[Non-aqueous electrolyte]
The nonaqueous electrolyte typically contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. In the nonaqueous electrolyte storage element according to this embodiment, the nonaqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate as an additive.

(Non-aqueous solvent)
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 contained in these compounds have been substituted with halogens may also be used.

Cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. 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 even 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.

(Electrolyte salt)
The electrolyte salt may be any known electrolyte salt commonly used in non-aqueous electrolytes for general energy storage elements, including lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts, with lithium salts being preferred.

Examples of the lithium salt include inorganic lithium salts such as _LiPF6 , _LiPO2F2 , _LiBF4 , _{and LiClO4} , _{and lithium salts having a hydrocarbon group in which hydrogen is substituted with fluorine, such as LiSO3CF3, LiC(SO2CF3)3, and LiC(SO2C2F5)3 . Among these , inorganic lithium salts are} preferred, and _LiPF6 is more preferred.

The lower limit of the content of the electrolyte salt in the non-aqueous solution is preferably 0.1 mol/ ^dm3 , more preferably 0.3 mol/ ^dm3 , even more preferably 0.5 mol/ ^dm3 , and particularly preferably 0.7 mol/ ^dm3 . On the other hand, the upper limit is not particularly limited, but is preferably 2.5 mol/ ^dm3 , more preferably 2 mol/ ^dm3 , and even more preferably 1.5 mol/ ^dm3 . The non-aqueous solution refers to a state in which an electrolyte salt is dissolved in a non-aqueous solvent, and refers to the state before an additive such as a boron-containing oxalate complex salt is dissolved.

(Additives)
The non-aqueous electrolyte contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate. By containing at least one of lithium difluorooxalatoborate and lithium difluorophosphate in the non-aqueous electrolyte, the non-aqueous electrolyte storage element has excellent rapid charging characteristics.

The lower limit of the content of lithium difluorooxalatoborate or lithium difluorophosphate in the nonaqueous electrolyte is preferably 0.05% by mass, more preferably 0.2% by mass, even more preferably 0.3% by mass, and even more preferably 0.5% by mass. Meanwhile, the upper limit of this content is preferably 2.0% by mass, and even more preferably 1.5% by mass. By keeping the content of lithium difluorooxalatoborate or lithium difluorophosphate within the above range, the effect of suppressing the increase in internal resistance after charge-discharge cycling can be further improved. Here, the content of lithium difluorooxalatoborate or lithium difluorophosphate refers to the mass of lithium difluorooxalatoborate or lithium difluorophosphate relative to the mass of the nonaqueous solution.

The non-aqueous electrolyte may contain both lithium difluorooxalatoborate and lithium difluorophosphate.
When the non-aqueous electrolyte contains both lithium difluorooxalatoborate and lithium difluorophosphate, the lower limit of the sum of the contents of lithium difluorooxalatoborate and lithium difluorophosphate is preferably 0.05% by mass, more preferably 0.2% by mass, even more preferably 0.3% by mass, and even more preferably 0.4% by mass. On the other hand, the upper limit of this content is preferably 4.0% by mass, more preferably 3.0% by mass, even more preferably 2.0% by mass, and even more preferably 1.5% by mass. When the sum of the contents of lithium difluorooxalatoborate and lithium difluorophosphate is within the above range, the effect of suppressing the increase in internal resistance after charge-discharge cycles can be further improved. Here, the sum of the contents of lithium difluorooxalatoborate and lithium difluorophosphate means the sum of the masses of lithium difluorooxalatoborate and lithium difluorophosphate relative to the mass of the non-aqueous solution.

The nonaqueous electrolyte may contain additives other than lithium difluorooxalatoborate and lithium difluorophosphate. Examples of such additives include lithium bis(fluorosulfonyl)imide (LiFSI), lithium fluorosulfonate, and lithium tetrafluorooxalatophosphate.

The non-aqueous electrolyte can be obtained by dissolving the electrolyte salt, lithium difluorooxalatoborate, or lithium difluorophosphate in the non-aqueous solvent.

[Separator]
The separator may be, for example, a woven fabric, a nonwoven fabric, or a porous resin film. Among these, a porous resin film is preferred from the viewpoint of strength, and a nonwoven fabric is preferred from the viewpoint of nonaqueous electrolyte retention. As the main component of the separator, a polyolefin such as polyethylene or polypropylene is preferred from the viewpoint of strength, and a polyimide or aramid is preferred from the viewpoint of resistance to oxidative decomposition. These resins may also be combined.

An inorganic layer may be disposed between the separator and the electrode (usually the positive electrode). This inorganic layer is a porous layer also known as a heat-resistant layer. A separator having an inorganic layer formed on one side of a porous resin film may also be used. The inorganic layer is typically composed of inorganic particles and a binder, and may contain other components.

[Specific Configuration of Energy Storage Element]
FIG. 1 is a schematic diagram of a rectangular nonaqueous electrolyte storage element 1 (nonaqueous electrolyte secondary battery) that is one embodiment of the nonaqueous electrolyte storage element according to the present invention. This figure is a see-through view of the inside of the case. The nonaqueous electrolyte storage element 1 shown in FIG. 1 includes an electrode assembly 2 housed in a case 3. The electrode assembly 2 is formed by winding a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer with a separator interposed therebetween. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode current collector 4′, and the negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode current collector 5′. A nonaqueous electrolyte is poured into the case 3.

The configuration of the nonaqueous electrolyte storage element of the present invention is not particularly limited, and examples include cylindrical batteries, prismatic batteries (rectangular batteries), and flat batteries.

[Method of manufacturing nonaqueous electrolyte energy storage element]
A method for manufacturing a nonaqueous electrolyte storage element according to one embodiment of the present invention includes housing the negative electrode, a positive electrode, and a nonaqueous electrolyte containing at least one of lithium difluorooxalatoborate and lithium difluorophosphate in a case. The positive electrode can be obtained by laminating the positive electrode active material layer directly onto a positive electrode substrate or via an intermediate layer. The positive electrode active material layer is laminated by applying a positive electrode mixture paste to the positive electrode substrate. Similarly to the positive electrode, the negative electrode can be obtained by laminating the negative electrode active material layer directly onto a negative electrode substrate or via an intermediate layer. The negative electrode active material layer is laminated by applying a negative electrode mixture paste containing graphite and an acrylic resin to the negative electrode substrate. The positive electrode mixture paste and the negative electrode mixture paste may contain a dispersion medium. Examples of the dispersion medium include aqueous solvents such as water and mixed solvents mainly composed of water; and organic solvents such as N-methylpyrrolidone and toluene.

In addition, the manufacturing method of the nonaqueous electrolyte storage element further includes, as another step, stacking the negative electrode and the positive electrode with a separator interposed therebetween. By stacking the negative electrode and the positive electrode with a separator interposed therebetween, an electrode body is formed.

The negative electrode, positive electrode, non-aqueous electrolyte, etc. can be housed in the case by a known method. After housing, the housing opening is sealed to obtain a non-aqueous electrolyte storage element. Details of each element that makes up the non-aqueous electrolyte storage element obtained by the above manufacturing method are as described above.

[Other embodiments]
The nonaqueous electrolyte electricity storage element of the present invention is not limited to the above-described embodiment.

In addition, while the above embodiments have been described focusing on the nonaqueous electrolyte storage element being a nonaqueous electrolyte secondary battery, other nonaqueous electrolyte storage elements may also be used. Examples of other nonaqueous electrolyte storage elements include capacitors (electric double layer capacitors, lithium ion capacitors), etc. Examples of nonaqueous electrolyte secondary batteries include lithium ion nonaqueous electrolyte secondary batteries.

In addition, while the above embodiment uses a wound electrode body, a laminated electrode body formed from a laminate of multiple sheet bodies each having a positive electrode, a negative electrode, and a separator may also be used.

The present invention can also be realized as an energy storage device comprising a plurality of the above-described nonaqueous electrolyte energy storage elements. In this case, the technology of the present invention needs to be applied to at least one of the nonaqueous electrolyte energy storage elements included in the energy storage device. Furthermore, a battery pack can be constructed using one or more nonaqueous electrolyte energy storage elements (cells) of the present invention, and this battery pack can be used to construct an energy storage device. The above-described energy storage device can be used as a power source for automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Furthermore, the above-described energy storage device can be used in various power supply devices such as engine starting power supplies, auxiliary power supplies, and uninterruptible power supplies (UPSs).

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, and a bus bar (not shown) that electrically connects two or more energy storage units 20. 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 energy storage elements.

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

[Examples 1 to 2 and Comparative Examples 1 to 8]
(Negative electrode)
A coating liquid (negative electrode mixture paste) containing graphite and non-graphitizable carbon as negative electrode active materials, a binder as shown in Table 1, and carboxymethyl cellulose (CMC) as a thickener, with water as a dispersion medium, was prepared. The mass ratio of graphite to non-graphitizable carbon in the negative electrode active material was 85:15. The mixing ratio of the negative electrode active material, binder, and thickener was 96:2:2 by mass. The coating liquid was applied to both sides of a 10 μm-thick copper foil substrate and dried to form negative electrode active material layers, thereby obtaining negative electrodes of the examples and comparative examples.
(Non-aqueous electrolyte)
A non-aqueous solution was prepared by dissolving 1.4 mol/ ^dm3 of LiPF6 in a non-aqueous solvent prepared by mixing ethylene carbonate ( _EC ) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70. The non-aqueous solution was prepared by dissolving additives in the amounts shown in Table 1 (content ratios when the non-aqueous solution is taken as 100% by mass) to obtain a non-aqueous electrolyte. The additives used were lithium difluorooxalatoborate (LiFOB), lithium difluorophosphate (LiDFP), vinylene carbonate (VC), lithium difluorobisoxalatophosphate (LiFOP), and lithium bisoxalatoborate (LiBOB). In Table 1, "-" indicates that the respective components were not used.
(positive electrode)
_A positive electrode containing NCM ( _{LiNi0.5Mn0.3Co0.2O2} ) with _an α- _NaFeO2 crystal structure as the positive _electrode active material was fabricated. The positive electrode contained the positive electrode active material, polyvinylidene fluoride (PVDF) as a binder, and acetylene black as a conductive agent. A coating liquid (positive electrode mixture paste) was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The mixing ratio of the positive electrode active material, binder, and conductive agent was 93:4:3 by mass. The coating liquid was applied to both sides of a positive electrode substrate, dried, and pressed to form a positive electrode active material layer. Aluminum foil with a thickness of 15 μm was used as the positive electrode substrate.

(Fabrication of non-aqueous electrolyte storage element)
Next, the positive electrode and the negative electrode were stacked with a separator made of a polyethylene substrate and an inorganic layer formed on the polyethylene substrate interposed therebetween to prepare an electrode assembly. This electrode assembly was then housed in an aluminum rectangular battery case, and a positive electrode terminal and a negative electrode terminal were attached. The nonaqueous electrolyte was poured into the case (rectangular battery case), which was then sealed to obtain nonaqueous electrolyte storage elements of Examples and Comparative Examples.

[evaluation]
(Measurement of initial discharge capacity)
Each of the obtained nonaqueous electrolyte storage elements was subjected to constant current/constant voltage charging at 0.18 A for 7 hours in a temperature environment of 25°C, with a charge cut-off voltage of 4.25 V. After charging, a constant current discharge was performed at a current value of 0.18 A with a discharge cut-off voltage of 2.75 V. After a 10-minute pause, a constant current/constant voltage charge was performed at 0.9 A for 3 hours in a temperature environment of 25°C, with a charge cut-off voltage of 4.25 V. After a 10-minute pause, a constant current discharge was performed at a current value of 0.9 A. This discharge capacity was designated the "initial discharge capacity."

(Quick charging performance)
After measuring the initial discharge capacity, each of the nonaqueous electrolyte storage elements was evaluated for rapid charging performance under the following conditions.
At 25°C, the battery was charged at a constant current of 2.2 A up to 4.25 V, and then charged at a constant voltage of 4.25 V. The charging termination condition was a charging time of 3 hours. A 10-minute rest period was then provided. Subsequently, a 2.2 A constant current discharge was performed up to 2.75 V, followed by a 10-minute rest period. This charge/discharge cycle was repeated 10 times.
The obtained value indicates the capacity loss after 10 cycles, and the smaller the value, the better the fast charging performance. The results are shown in Table 1 below.

As shown in Table 1, Examples 1 and 2, in which the negative electrode active material layer contained an acrylic resin as a binder and the non-aqueous electrolyte contained at least one of lithium difluorooxalatoborate and lithium difluorophosphate as an additive, had good rapid charging performance.

Comparative Examples 1 and 2, in which the nonaqueous electrolyte did not contain any additives, exhibited inferior fast-charging performance compared to the Examples, regardless of whether the binder was an acrylic resin or a styrene-butadiene copolymer. Furthermore, Comparative Examples 3 and 4, in which the nonaqueous electrolyte contained at least one of lithium difluorooxalatoborate and lithium difluorophosphate as an additive, but the binder was a styrene-butadiene copolymer, did not achieve improved fast-charging performance. Furthermore, Comparative Examples 5 to 7, in which the nonaqueous electrolyte contained vinylene carbonate (VC), lithium difluorobisoxalatophosphate (LiFOP), or lithium bisoxalatoborate (LiBOB) as an additive, did not achieve improved fast-charging performance, even though the binder was an acrylic resin.

On the other hand, the results of Comparative Examples 1 and 8 show that when the acrylic resin used as a binder contains butadiene monomer units, rapid charging performance decreases.

As described above, this non-aqueous electrolyte storage element was shown to have excellent rapid charging characteristics.

The present invention is suitable for use as a non-aqueous electrolyte storage element, including non-aqueous electrolyte secondary batteries used as power sources for electronic devices such as personal computers and communication terminals, automobiles, etc.

REFERENCE SIGNS LIST 1 nonaqueous electrolyte energy storage element 2 electrode body 3 case 4 positive electrode terminal 4' positive electrode current collector 5 negative electrode terminal 5' negative electrode current collector 20 energy storage unit 30 energy storage device

Claims (4)

A battery comprising a negative electrode, a positive electrode, and a non-aqueous electrolyte solution,
the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin,
the non-aqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate ,
The nonaqueous electrolyte storage element , wherein the acrylic resin is a resin having a content of structural units derived from acrylic acid of 75% by mass or more . The nonaqueous electrolyte storage element according to claim 1, wherein the content of lithium difluorooxalatoborate or lithium difluorophosphate in the nonaqueous electrolyte is 0.2% by mass or more and 2.0% by mass or less. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the positive electrode contains a positive electrode active material including nickel, cobalt, and manganese. a negative electrode, a positive electrode, and a non-aqueous electrolyte solution housed in a case;
the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin,
the non-aqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate,
The method for producing a nonaqueous electrolyte storage element , wherein the acrylic resin is a resin having a content of structural units derived from acrylic acid of 75 mass % or more .