Nonaqueous electrolyte storage element and method for manufacturing the same

Abstract

To provide a nonaqueous electrolyte power storage device which can reduce an initial DC resistance at a low temperature, and suppress the worsening of a capacity-keeping rate after having gone through a charge-discharge cycle.SOLUTION: A nonaqueous electrolyte power storage device according to an aspect of the present invention comprises: a positive electrode; a negative electrode; a separator interposed between the positive and negative electrodes; and a nonaqueous electrolyte. The separator has a porosity of 44 vol.% or more. The nonaqueous electrolyte contains a compound represented by the formula (1). In the formula (1), R1 and R2 independently represent a hydrogen atom or a fluorine atom.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a nonaqueous electrolyte electricity storage element and a method for manufacturing a nonaqueous electrolyte electricity storage element. [Background technology]

[0002] 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, automobiles, etc. The non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. Furthermore, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte energy storage elements other than non-aqueous electrolyte secondary batteries.

[0003] In general, the non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt that dissolves in the non-aqueous solvent, and other components are 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). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-289414 Summary of the Invention [Problem to be solved by the invention]

[0005] The use of a nonaqueous electrolyte containing a fluorinated carboxylic acid ester or the like tends to improve charge-discharge cycle performance, but the use of a nonaqueous electrolyte containing a fluorinated carboxylic acid ester or the like may result in high initial DC resistance at low temperatures in nonaqueous electrolyte secondary batteries and other nonaqueous electrolyte storage elements.

[0006] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a nonaqueous electrolyte energy storage element that can reduce the initial DC resistance at low temperatures and suppress a decrease in the capacity retention rate after charge-discharge cycling. [Means for solving the problem]

[0007] A non-aqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the separator has a porosity of 44% by volume or more, and the non-aqueous electrolyte contains a compound represented by the following formula (1):

[0008] [ka] In formula (1), R 1 and R 2 are each independently a hydrogen atom or a fluorine atom.

[0009] A method for producing 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):

[0010] [ka] In formula (1), R 1 and R 2 are each independently a hydrogen atom or a fluorine atom. [Effects of the Invention]

[0011] A nonaqueous electrolyte storage element according to one aspect of the present invention can reduce the initial DC resistance at low temperatures and suppress a decrease in the capacity retention rate after charge-discharge cycling. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element. [Figure 2] 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. DETAILED DESCRIPTION OF THE INVENTION

[0013] First, an outline of the nonaqueous electrolyte electricity storage element disclosed in this specification will be described.

[0014] A non-aqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the separator has a porosity of 44% by volume or more, and the non-aqueous electrolyte contains a compound represented by the following formula (1):

[0015] [ka] In formula (1), R 1 and R 2 are each independently a hydrogen atom or a fluorine atom.

[0016] 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 reason is presumed, for example: One factor that reduces the battery life performance is that decomposition products of the nonaqueous electrolyte decomposed on the negative electrode reach the positive electrode, 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 storage element is thought to have a particularly enhanced charge-discharge cycle performance due to a synergistic effect, such as the fact that the charge-discharge reactions at the positive and negative electrodes proceed more deeply due to the 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 may facilitate the transport of 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 the charge-discharge cycle performance is not impaired even when combined with a separator with a high porosity. Therefore, the nonaqueous electrolyte storage element can reduce initial DC resistance at low temperatures and suppress a decrease in capacity retention after charge-discharge cycles.

[0017] The content of the compound in the non-aqueous electrolyte is preferably 0.7% by mass or more and 5.0% by mass or less, which can suppress an increase in initial resistance.

[0018] A method for producing 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):

[0019] [ka] In formula (1), R 1 and R 2 are each independently a hydrogen atom or a fluorine atom.

[0020] According to this method for producing a nonaqueous electrolyte storage element, it is possible to produce a nonaqueous electrolyte storage element that can reduce the initial DC resistance at low temperatures and suppress a decrease in the capacity retention rate after charge / discharge cycling.

[0021] The configuration of a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention, the configuration of an electricity storage device, and a method for manufacturing a nonaqueous electrolyte electricity storage element, as well as other embodiments, will be described in detail below. Note that the names of the components used in each embodiment may differ from the names of the components used in the background art.

[0022] <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.

[0023] (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.

[0024] The positive electrode substrate is electrically conductive. Whether or not it has "electrical conductivity" is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 7 The threshold value is Ω·cm. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy of these. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. 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).

[0025] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the secondary battery.

[0026] 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 configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0027] The positive electrode active material layer contains a positive electrode active material and, if necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0028] The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the positive electrode active material for a lithium ion secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 crystal structure, lithium transition metal composite oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having an α-NaFeO2 crystal structure include Li[Li x Ni (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co (1-x-γ-β) ]O2(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) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Lithium transition metal composite oxides with spinel crystal structure include Li x Mn2O4, Li x Ni γ Mn (2-γ)Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. 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 may be used in combination.

[0029] The positive electrode active material is usually 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. By setting the average particle size of the positive electrode active material to the above lower limit or more, the positive electrode active material becomes easier to manufacture and handle. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that 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. The "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).

[0030] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0031] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting 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.

[0032] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. 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 CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0033] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent within this range, the energy density of the secondary battery can be increased.

[0034] 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.

[0035] The binder content in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the active material can be stably held.

[0036] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. When 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.

[0037] The filler is not particularly limited, and examples of the filler 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, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0038] 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.

[0039] (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.

[0040] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the viewpoint of cost. 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.

[0041] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the secondary battery.

[0042] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0043] 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.

[0044] 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 usually used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metallic Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、Examples of the material include titanium-containing oxides such as TiNbO; polyphosphate compounds; silicon carbide; and 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 used in combination.

[0045] "Graphite" refers to a carbon material having an average lattice spacing (d002) of 0.33 nm or more and less than 0.34 nm in the (002) plane as determined by X-ray diffraction before charge / discharge or in the discharged state. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of availability of a material with stable physical properties.

[0046] "Non-graphitic carbon" refers to the carbon that has an average lattice spacing (d 002 ) is 0.34 nm or more and 0.42 nm or less. Non-graphitic carbon includes 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.

[0047] Here, the "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released. For example, in a single-electrode battery using a negative electrode containing a carbonaceous material as a negative electrode active material as a working electrode and metallic Li as a counter electrode, this refers to a state in which the open circuit voltage is 0.7 V or higher.

[0048] "Non-graphitizable carbon" refers to a carbon material having the above d002 of 0.36 nm or more and 0.42 nm or less.

[0049] "Graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0050] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. 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 or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above lower limit, the negative electrode active material can be easily produced or handled. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and the powder classification method can be selected from, for example, the methods exemplified for the positive electrode. 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.

[0051] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0052] (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.

[0053] 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 a predetermined value or less 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 aluminosilicate; 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 of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0054] The lower limit of the porosity of the separator is 44% by volume, and preferably 50% by volume. When the porosity of the separator is equal to or greater than the lower limit, the initial DC resistance of the nonaqueous electrolyte storage element at low temperatures can be reduced, and the compound represented by formula (1) can be more effectively used to suppress a decrease in the 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. When the porosity of the separator is equal to or less than the upper limit, the strength of the separator can be improved.

[0055] 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 materials [g cm -3 ] t: thickness [cm]

[0056] The upper limit of the separator's air permeability is 150 seconds / 100 cm 3 is preferred, 120 seconds / 100 cm 3 By setting the separator air permeability to the above upper limit or less, the initial DC resistance of the nonaqueous electrolyte storage element at low temperatures can be reduced. On the other hand, the lower limit of the separator air permeability is set to 50 sec / 100 cm from the viewpoint of maintaining the strength of the separator. 3 is preferred, 60 seconds / 100cm 3 The air permeability of the separator is also called the Gurley value, which indicates the number of seconds it takes for a certain volume of air to pass through a separator of a certain area under a certain pressure difference, and is a value measured in accordance with JIS-P8117 (2009).

[0057] 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.

[0058] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0059] (non-aqueous electrolyte) The non-aqueous electrolyte contains a compound represented by the following formula (1). A non-aqueous electrolyte solution may be used as the non-aqueous electrolyte. 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).

[0060] 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 the non-aqueous electrolyte storage element.

[0061] [ka]

[0062] In formula (1), R 1 and R 2 are each independently a hydrogen atom or a fluorine atom. 1 and R 2 When is a fluorine atom, it is believed that the oxidation resistance of the compound represented by formula (1) can be further improved.

[0063] Examples of the compound include compounds represented by the following formulas (1-1) to (1-3).

[0064] [ka]

[0065] The lower limit of the content of the 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. By ensuring that the content of the compound is equal to or greater than the 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. Meanwhile, the upper limit of the content of the compound is preferably 5.0% by mass, more preferably 4.0% by mass. By ensuring that the content of the compound is equal to or less than the upper limit, an increase in initial resistance can be suppressed.

[0066] 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.

[0067] Examples of 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, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0068] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0069] 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 can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce 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.

[0070] The non-aqueous electrolyte may contain other additives in addition to the compound represented by formula (1), as long as the effects of the present invention are not impaired. Examples of the other additives include various additives contained in general non-aqueous electrolytes.Examples of the other 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 ... 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, etc. 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.

[0071] 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, based on the total mass of the non-aqueous electrolyte. By setting the content of other additives within the above range, it is possible to improve capacity retention or cycle performance after high-temperature storage, and further improve safety.

[0072] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0073] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalates 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.

[0074] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3 More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3 More than 1.5mol / dm 3By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0075] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0076] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc., and is solid at room temperature (for example, 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

[0077] As the sulfide solid electrolyte, for example, Li2S-P2S5, LiI-Li2S-P2S5, Li 10 Ge-P2S 12 etc.

[0078] <Configuration of the 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.

[0079] FIG. 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. The 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.

[0080] The nonaqueous electrolyte energy storage element of this embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements 1 in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, the technology of the present invention may be applied to at least one nonaqueous electrolyte energy storage element included in the energy storage unit.

[0081] 2 shows an example of an electricity storage device 30 in which electricity storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte electricity storage elements 1, are further assembled. The electricity storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte electricity storage elements 1, a bus bar (not shown) that electrically connects two or more electricity storage units 20, etc. The electricity storage unit 20 or the electricity storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte electricity storage elements.

[0082] <Method of manufacturing nonaqueous electrolyte energy storage element> The method for manufacturing a nonaqueous electrolyte storage element of this 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):

[0083] [ka] In formula (1), R 1 and R 2 are each independently a hydrogen atom or a fluorine atom.

[0084] The manufacturing method includes other steps, 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 the negative electrode with the separator interposed therebetween. Specific configurations of the separator and the non-aqueous electrolyte are as described above.

[0085] The method for placing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and then the inlet may be sealed.

[0086] According to this method for producing a nonaqueous electrolyte storage element, it is possible to produce a nonaqueous electrolyte storage element that can reduce the initial DC resistance at low temperatures and suppress a decrease in the capacity retention rate after charge / discharge cycling.

[0087] <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.

[0088] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery), but the type, shape, size, 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.

[0089] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode. [Example]

[0090] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0091] [Examples 1 to 3 and Comparative Examples 1 to 10] (Preparation of positive electrode) As the positive electrode active material, LiNi 1 / 2 Co 1 / 5 Mn 3 / 10 A lithium transition metal composite oxide represented by O2 was used.

[0092] A positive electrode paste 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 was prepared using N-methylpyrrolidone (NMP) as a dispersion medium. This positive electrode mixture paste was applied to both sides of an aluminum foil positive electrode substrate with an average thickness of 15 μm, leaving uncoated areas (areas where the positive electrode mixture layer was not formed), and then dried to prepare a positive electrode mixture layer. Then, a roll press was performed to prepare a positive electrode. The thickness of the positive electrode was 135 μm.

[0093] (Preparation of negative electrode) Graphite was used as the negative electrode active material. A negative electrode mixture paste was prepared using water as a dispersion medium, containing the above-mentioned 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. This negative electrode mixture paste was applied to both sides of a copper foil negative electrode substrate with an average thickness of 10 μm, leaving uncoated areas (areas where the negative electrode mixture layer was not formed), and then dried to prepare a negative electrode mixture layer. Subsequently, a roll press was performed to prepare a negative electrode. The thickness of the negative electrode was 148 μm.

[0094] (Preparation of non-aqueous electrolyte) A mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 was added to the mixture, and the additives listed in Tables 1 and 2 were added at a concentration of 1.0 mol / dm 3 In Comparative Examples 1 to 3, only LiPF6 was dissolved in the mixed solvent.

[0095] (separator) The separator used was a polypropylene microporous membrane with 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.

[0096] (Fabrication of non-aqueous electrolyte energy 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. After the container and a cover plate were welded, 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.

[0097] (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. Charging was terminated after 3 hours. 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 charge / discharge cycle was repeated for two cycles.

[0098] (Charge-discharge cycle test) Next, the following charge-discharge cycle test was conducted. Each nonaqueous electrolyte storage element was stored in a 45°C thermostatic chamber 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 constitutes one cycle, and this cycle was repeated 300 times. The charge, discharge, and rest periods were all performed in a 45°C thermostatic chamber.

[0099] (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 [%].

[0100] (Initial DC resistance at low temperatures) The initial DC resistance of each nonaqueous electrolyte storage element at low temperatures was evaluated. After the initial charge / discharge, each nonaqueous electrolyte storage element was charged at a constant current of 1.0 C in a thermostatic chamber at 25°C until the SOC (State of Charge) reached 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, the element was charged at a constant current of 0.05 C until the SOC reached 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 slope of the line was determined as the initial DC resistance at -10°C.

[0101] The evaluation results are shown in Tables 1 and 2 below.

[0102] [Table 1]

[0103] [Table 2]

[0104] As shown in Tables 1 and 2, in 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, the initial DC resistance at low temperatures was reduced and the decrease in capacity retention rate after charge-discharge cycling was sufficiently suppressed.

[0105] Furthermore, as shown in Table 1, in Comparative Examples 2 and 3, in which the nonaqueous electrolyte did not contain any 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, it was found that by setting the separator porosity to 44% by volume or more, the effect of suppressing the decrease in the capacity retention rate after charge-discharge cycling was enhanced. Therefore, it was found that the combination of the compound represented by formula (1) above with a separator having a porosity of 44% by volume or more can achieve the unexpected effect of suppressing the decrease in the capacity retention rate after charge-discharge cycling.

[0106] Furthermore, as shown in Table 2, in Comparative Examples 9 and 10, even if the non-aqueous electrolyte had a dimer structure similar to that of the compound represented by formula (1) above, when the non-aqueous electrolyte did not have an ethylene carbonate dimer structure like the compound represented by formula (1), 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 non-aqueous electrolyte containing vinylene carbonate 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 non-aqueous electrolyte containing the compound represented by formula (1) reduced the initial DC resistance at low temperatures and was highly effective in suppressing the decrease in capacity retention rate after charge-discharge cycling.

[0107] The above results demonstrate that the nonaqueous electrolyte storage element can reduce the initial DC resistance at low temperatures and suppress a decrease in the capacity retention rate after charge-discharge cycling. [Industrial Applicability]

[0108] The present invention is applicable to nonaqueous electrolyte electricity storage elements used as power sources for electronic devices such as personal computers and communication terminals, automobiles, and the like. [Explanation of symbols]

[0109] 1. Non-aqueous electrolyte energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage unit 30 Energy storage device

Claims

1. 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; Non-aqueous electrolyte It is equipped with 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): 【Chemical 1】 In formula (1), R １ and R ２ are each independently a hydrogen atom or a fluorine atom.

2. 2. 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.

3. Preparing a separator having a porosity of 44% by volume or more and a porous resin film substrate; preparing a non-aqueous electrolyte containing a compound represented by the following formula (1); A method for manufacturing a nonaqueous electrolyte electricity storage element comprising: 【Chemistry 2】 In formula (1), R １ and R ２ are each independently a hydrogen atom or a fluorine atom.

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