Nonaqueous electrolyte solution and nonaqueous electrolyte secondary battery

Abstract

This nonaqueous electrolyte solution contains an electrolyte salt and an organic solvent, and is characterized in that: the total content of cyclic carbonates in the organic solvent is 10 vol% or less with respect to the total volume of the organic solvent; the content of a fluorine-free cyclic carbonate belonging to the cyclic carbonates is 60 vol% or less with respect to the total volume of the cyclic carbonates; the concentration of the electrolyte salt with respect to the organic solvent is 1.5 mol / L to 2.5 mol / L inclusive; the electrolyte salt contains lithium bis(fluorosulfonyl)imide; and the content of the lithium bis(fluorosulfonyl)imide is 10 mol% or more of the whole electrolyte salt.

Description

Non-aqueous electrolyte and non-aqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte solution and a non-aqueous electrolyte secondary battery.

[0002] BACKGROUND ART Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are being actively developed as secondary batteries for driving motors in, for example, electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0003] For example, Patent Document 1 proposes the use of an electrolyte solution for a nonaqueous electrolyte secondary battery, which contains an electrolyte salt and an organic solvent, the organic solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC), the weight percentage of the ethylene carbonate (EC) in the organic solvent being ≦10%, and the weight percentage of the ethyl methyl carbonate (EMC) in the organic solvent being 70% to 95%. Patent Document 1 also describes that the use of this electrolyte solution makes it possible to achieve both good high-temperature storage performance and high-temperature cycle performance.

[0004] Special Publication No. 2022-553168

[0005] However, the electrolyte solution of Patent Document 1 poses a problem of an increase in battery resistance due to high-temperature charge-discharge cycles. Suppressing the increase in battery resistance due to high-temperature charge-discharge cycles leads to an increase in battery output. Furthermore, an increase in battery output is an important characteristic required for a secondary battery for driving a motor.

[0006] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte solution that can suppress an increase in battery resistance that accompanies high-temperature charge-discharge cycles, and a nonaqueous electrolyte secondary battery that includes the nonaqueous electrolyte solution.

[0007] A nonaqueous electrolyte solution according to one aspect of the present disclosure comprises an electrolyte salt and an organic solvent, wherein the total content of cyclic carbonates in the organic solvent is 10% by volume or less relative to the total volume of the organic solvent, the content of fluorine-free cyclic carbonates belonging to the cyclic carbonates is 60% by volume or less relative to the total volume of the cyclic carbonates, the concentration of the electrolyte salt in the organic solvent is 1.5 mol / L or more and 2.5 mol / L or less, the electrolyte salt contains lithium bis(fluorosulfonyl)imide, and the content of the lithium bis(fluorosulfonyl)imide is 10 mol% or more of the total electrolyte salt.

[0008] A nonaqueous electrolyte secondary battery according to an aspect of the present disclosure includes a positive electrode having a positive electrode composite layer, a negative electrode having a negative electrode composite layer, a separator disposed between the positive electrode and the negative electrode, and the nonaqueous electrolyte solution.

[0009] According to the present disclosure, it is possible to provide a nonaqueous electrolyte solution capable of suppressing an increase in battery resistance due to high-temperature charge-discharge cycles, and a nonaqueous electrolyte secondary battery including the nonaqueous electrolyte solution.

[0010] 1 is a perspective view showing a non-aqueous electrolyte secondary battery according to an embodiment of the present invention;

[0011] The nonaqueous electrolyte solution according to one embodiment of the present disclosure comprises an electrolyte salt and an organic solvent, wherein the total content of cyclic carbonates in the organic solvent is 10% by volume or less relative to the total volume of the organic solvent, the content of fluorine-free cyclic carbonates belonging to the cyclic carbonates is 60% by volume or less relative to the total volume of the cyclic carbonates, the concentration of the electrolyte salt relative to the organic solvent is 1.5 mol / L or more and 2.5 mol / L or less, the electrolyte salt contains lithium bis(fluorosulfonyl)imide, and the content of the lithium bis(fluorosulfonyl)imide is 10 mol% or more of the total electrolyte salt. Furthermore, by using the nonaqueous electrolyte solution according to one embodiment of the present disclosure, it is possible to suppress an increase in battery resistance associated with high-temperature charge-discharge cycles. The mechanism by which the above effect is achieved is not fully understood, but the following is presumed.

[0012] In a non-aqueous electrolyte solution, the presence of free cyclic carbonate that is not solvated with ions formed by dissociating the electrolyte salt can cause side reactions at the positive electrode, leading to positive electrode degradation. The presence of free cyclic carbonate can also promote the leaching of metal elements from the positive electrode active material. Side reactions and leaching of metal elements at the positive electrode are accelerated by high-temperature charge-discharge cycles, leading to an increase in battery resistance. However, the non-aqueous electrolyte solution of the present disclosure has a high electrolyte salt concentration and a low cyclic carbonate content, resulting in a low proportion of free cyclic carbonate. This suppresses side reactions and leaching of metal elements at the positive electrode during high-temperature charge-discharge cycles, leading to a suppression of an increase in battery resistance. Furthermore, the non-aqueous electrolyte solution of the present disclosure limits the content of fluorine-free cyclic carbonate, which belongs to cyclic carbonates, and also contains a predetermined amount of lithium bis(fluorosulfonyl)imide (hereinafter, LiFSI) as an electrolyte salt. As a result, the components of the SEI (Solid Electrolyte Interphase) coating formed on the negative electrode surface are reduced in the amount of organic coating components derived from fluorine-free cyclic carbonate, and a certain amount of inorganic coating components derived from LiFSI is included. An SEI coating of such components has high shear stress and high insulating properties. Therefore, even if the electrode expands and contracts during high-temperature charge-discharge cycles, the high shear stress suppresses damage to the SEI coating, and the high insulating properties prevent metal elements eluted from the positive electrode from depositing on the SEI coating, thereby suppressing an increase in battery resistance.

[0013] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery including a nonaqueous electrolyte solution according to the present disclosure will be described with reference to the drawings.

[0014] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. The nonaqueous electrolyte secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte solution, insulating plates 18 and 19 disposed above and below the electrode assembly 14, and a battery case 15 for accommodating the above components. The battery case 15 includes a cylindrical case body 16 with a bottom and a sealing member 17 for closing the opening of the case body 16. Note that, instead of the wound electrode assembly 14, other electrode bodies may be used, such as a laminated electrode body formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include cylindrical, prismatic, coin-shaped, or button-shaped metal cases, and resin cases (so-called pouch-shaped cases) formed by laminating resin sheets.

[0015] The case body 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and supports the sealing body 17 on its upper surface.

[0016] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the nonaqueous electrolyte secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0017] 1 , a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the underside of a filter 23, which is the bottom plate of the sealing body 17, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner bottom surface of the case body 16, and the case body 16 serves as the negative electrode terminal.

[0018] The nonaqueous electrolyte solution, the positive electrode 11, the negative electrode 12, and the separator 13 that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below.

[0019] [Non-aqueous Electrolyte] The non-aqueous electrolyte contains an electrolyte salt and an organic solvent.

[0020] The organic solvent may contain a cyclic carbonate, but the total content of the cyclic carbonate in the non-aqueous electrolyte is 10% by volume or less, preferably 8% by volume or less, and more preferably 5% by volume or less, relative to the total volume of the organic solvent, in order to reduce the proportion of unsolvated free cyclic carbonate, which leads to the suppression of an increase in battery resistance during high-temperature charge-discharge cycles as described above. The total content of cyclic carbonate refers to the content of all cyclic carbonates contained in the non-aqueous electrolyte. For example, when two or more types of cyclic carbonates are contained in the non-aqueous electrolyte, it means the total content of the two or more cyclic carbonates. Furthermore, the content of fluorine-free cyclic carbonates belonging to the cyclic carbonates is 60% by volume or less, preferably 40% by volume or less, more preferably 20% by volume or less, and more preferably 0% by volume, relative to the total volume of cyclic carbonates in the organic solvent, in order to reduce the amount of components of the organic coating derived from the fluorine-free cyclic carbonate among the components of the SEI coating, which leads to the suppression of an increase in battery resistance during high-temperature charge-discharge cycles as described above.

[0021] Examples of cyclic carbonates include fluorine-free cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), and butylene carbonate (BC), and fluorine-containing cyclic carbonates such as fluoroethylene carbonate (FEC) and fluoropropylene carbonate (FPC).

[0022] Among the cyclic carbonates, it is particularly preferable to limit the content of ethylene carbonate in order to further suppress side reactions of the positive electrode 11 and elution of metal elements from the positive electrode active material, and the content of ethylene carbonate is preferably 5 vol% or less, and more preferably 0 vol%, relative to the total volume of the organic solvent.

[0023] When a cyclic carbonate is contained in the non-aqueous electrolyte, it is preferable that a fluorine-containing cyclic carbonate is contained. As a result, the SEI coating formed on the surface of the negative electrode 12 contains an inorganic coating derived from the fluorine-containing cyclic carbonate, which may impart high shear stress and high insulating properties. Among fluorine-containing cyclic carbonates, fluoroethylene carbonate is preferred. The content of fluoroethylene carbonate is preferably 2% by volume or more and 10% by volume or less, more preferably 2% by volume or more and 7% by volume or less, relative to the total volume of the organic solvent, in order to form a certain amount of an inorganic coating derived from the fluorine-containing cyclic carbonate on the negative electrode surface and to impart high shear stress and high insulating properties to the SEI coating.

[0024] The organic solvent includes solvents other than cyclic carbonates. Examples of solvents other than cyclic carbonates include chain carbonates, esters, ethers, aromatic hydrocarbons, etc. Among these, chain carbonates, esters, ethers, etc. are preferred in terms of battery capacity, charge / discharge cycle characteristics, etc., and chain carbonates are particularly preferred.

[0025] Examples of chain carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Examples of esters include cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone, and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate. Examples of ethers include diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, and dihexyl ether. These solvents may also contain halogen-substituted products in which at least a portion of the hydrogen atoms in the solvents have been substituted with halogen atoms such as fluorine.

[0026] The total content of the chain carbonates is preferably 50% by volume or more, more preferably 70% by volume or more, and even more preferably 85% by volume or more, relative to the total volume of the organic solvent, from the viewpoints of, for example, battery capacity and charge / discharge cycle characteristics. The total content of the chain carbonates refers to the content of all chain carbonates contained in the non-aqueous electrolyte. Among the chain carbonates, it is preferable to contain a certain amount of ethyl methyl carbonate (EMC), from the viewpoints of charge / discharge and cycle characteristics at low temperatures, and the content of ethyl methyl carbonate is preferably 15% by volume or more, more preferably 20% by volume or more and 40% by volume or less, relative to the total volume of the organic solvent.

[0027] The content of the electrolyte salt relative to the organic solvent is 1.5 mol / L or more and 2.5 mol / L or less, preferably 1.5 mol / L or more and 2.0 mol / L or less, more preferably 1.6 mol / L or more and 1.9 mol / L or less, and more preferably 1.7 mol / L or more and 1.85 mol / L or less, in order to reduce the proportion of unsolvated free cyclic carbonate, which leads to suppression of an increase in battery resistance due to high-temperature charge-discharge cycling, as described above. The electrolyte salt contains LiFSI, and the LiFSI content is 10 mol% or more, preferably 10 mol% or more and 80 mol% or less, and more preferably 30 mol% or more and 60 mol% or less, in order to form an SEI coating containing a certain amount of LiFSI-derived inorganic coating components, which leads to suppression of an increase in battery resistance due to high-temperature charge-discharge cycling, as described above.

[0028] The electrolyte salt may contain a fluorinated lithium imide salt other than LiFSI, such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(perfluoroethanesulfonyl)imide (LiBETI), lithium (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide, or lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (LiFTI).

[0029] The electrolyte salt may also contain a Li salt other than a fluorinated lithium imide salt such as LiFSI, for example, LiBF ４ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiAlCl ４ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , Li(P(C ２ O ４ ) F ４ ), LiPF ６－ｘ (C ｎ F ２ｎ＋１ ) ｘ (1<x<6, n is 1 or 2), LiB１０ Cl １０ , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, Li ２ B ４ O ７ , Li(B(C ２ O ４ ) F ２ Among these, LiPF is particularly preferred in terms of battery capacity, charge-discharge cycle characteristics, etc. ６ (lithium hexafluorophosphate) is preferred.

[0030] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector and a positive electrode composite layer formed on the positive electrode current collector. The positive electrode current collector can be, for example, a foil of a metal that is stable in the potential range of the positive electrode, such as aluminum or an aluminum alloy, or a film with such a metal disposed on the surface layer. The positive electrode composite layer contains, for example, a positive electrode active material, a binder, and a conductive material. The positive electrode composite layer is formed on one or both sides of the positive electrode current collector. The positive electrode 11 can be manufactured, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc., onto the positive electrode current collector, drying and rolling the coating, and forming a positive electrode composite layer on the positive electrode current collector.

[0031] The positive electrode active material includes, for example, a lithium-containing metal composite oxide. Examples of metal elements contained in the lithium-containing metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Ca, Sb, Pb, Bi, Ge, Mo, and Si.

[0032] The lithium-containing metal composite oxide preferably contains a positive electrode active material A that is a single-crystal particle and a positive electrode active material B that is a polycrystalline particle, for example, in that it can increase the capacity of the battery. The single-crystal particle includes not only particles that are completely separated into individual primary particles, but also particles that are aggregates of several to a dozen (specifically, 2 to 19) primary particles. The polycrystalline particle refers to particles (secondary particles) that are aggregates of several tens or more (specifically, 20 or more) primary particles.

[0033] The positive electrode active materials A and B are lithium-containing metal composite oxides or the like, specifically, those having the formula: Liｘ Ni １－ｙ－ｚ Co ｙ M ｚ O ２ (wherein 0.97≦x≦1.2, 0≦y≦0.2, 0<z, and M contains at least one element selected from the group consisting of Mn, W, Mg, Mo, Nb, Ti, Si, and Al) Examples of lithium-containing metal composite oxides include those represented by the formula: 0.97≦x≦1.2, 0≦y≦0.2, 0<z. The positive electrode active material B preferably satisfies 0.85≦1-y-z in the above formula, for example, in order to increase the capacity of the battery. Satisfying 0.85≦1-y-z generally facilitates Ni elution during high-temperature charge-discharge cycles, but the use of the nonaqueous electrolyte of this embodiment can suppress Ni elution. The positive electrode active material A preferably contains Mn and satisfies 0.045≦z in the above formula, for example, in order to facilitate production. However, containing Mn and satisfying 0.045≦z generally facilitates Mn elution during high-temperature charge-discharge cycles, but the use of the nonaqueous electrolyte of this embodiment can suppress Mn elution.

[0034] For example, in order to enhance the packing property of the positive electrode active material or to improve the rapid charging characteristics, the average particle size (D Ａ ) is the positive electrode active material B (D Ｂ ) and D Ｂ / D Ａ is more preferably 2 or more and 6 or less, more preferably 3 or more and 5 or less, and even more preferably 3 or more and 4 or less. The average particle size of the positive electrode active material A made of single-crystal particles may be, for example, in the range of 2 μm to 20 μm, and the average particle size of the positive electrode active material B made of polycrystalline particles may be, for example, in the range of 5 μm to 25 μm. The average particle size is the volume-average particle size measured by laser diffraction, and is the median diameter at which the volume integrated value in the particle size distribution is 50%. The average particle size can be measured by laser diffraction using, for example, a Microtrac Bell MT3000II.

[0035] The total mass of the positive electrode active material A and the positive electrode active material B (M Ａ +M Ｂ ) the mass of the positive electrode active material A (M Ａ ) ratio ((MＡ / (M Ａ +M Ｂ )) × 100) is preferably 10 mass % or more and 40 mass % or less, more preferably 20 mass % or more and 40 mass % or less, and even more preferably 20 mass % or more and 30 mass % or less, for example, in terms of being able to improve rapid charging characteristics, etc. The total mass of the positive electrode active material A and the positive electrode active material B is preferably 90 mass % or more and more preferably 95 mass % or more with respect to the total mass of the positive electrode mixture layer.

[0036] The method for producing single-crystal particles and polycrystalline particles is not particularly limited, but for example, they can be obtained by firing a mixture of a transition metal-containing compound and a Li compound and pulverizing the resulting fired product. Depending on the firing temperature of the mixture, single-crystal particles or polycrystalline particles can be obtained. To obtain polycrystalline particles, firing at a temperature below 750°C is preferred, although this depends on the composition of the raw materials. On the other hand, to obtain single-crystal particles, firing at a temperature above 750°C is preferred, although this depends on the composition of the raw materials.

[0037] Examples of conductive materials contained in the positive electrode composite layer include carbon materials such as carbon black, acetylene black, ketjen black, graphite, and carbon nanotubes. Among these, carbon nanotubes are preferred in terms of electronic conductivity, etc. Examples of carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes. Single-walled carbon nanotubes (SWCNTs) are carbon nanostructures in which a single layer of graphene sheets forms a cylindrical shape, while multi-walled carbon nanotubes (MWCNTs) are carbon nanostructures in which two or more layers of graphene sheets are concentrically stacked to form a cylindrical shape. Note that a graphene sheet refers to a layer in which carbon atoms of sp2 hybrid orbitals that form a graphite crystal are located at the vertices of a regular hexagon.

[0038] The metal catalyst used in the production of the carbon nanotubes may remain inside the carbon nanotubes. The elution of the metal catalyst may lead to an increase in battery resistance during high-temperature charge-discharge cycles. However, the use of the nonaqueous electrolyte solution of this embodiment suppresses the elution of the metal catalyst.

[0039] As the binder contained in the positive electrode mixture layer, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, styrene butadiene rubber, carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like may be used in combination.

[0040] [Negative Electrode] The negative electrode 12 includes a negative electrode current collector and a negative electrode composite layer formed on the negative electrode current collector. The negative electrode current collector can be, for example, a foil of a metal stable in the potential range of the negative electrode, such as copper or a copper alloy, or a film with such a metal disposed on the surface. The negative electrode composite layer contains, for example, a negative electrode active material, a binder, etc. The negative electrode composite layer is formed on one or both sides of the negative electrode current collector. The negative electrode 12 can be manufactured, for example, by applying a negative electrode composite slurry containing a negative electrode active material, a binder, etc., onto the negative electrode current collector, drying and rolling the coating, and forming a negative electrode composite layer on the negative electrode current collector. Examples of the binder include the same materials as those used for the positive electrode.

[0041] The negative electrode active material preferably contains a Si-containing material in order to increase the capacity of the battery. Since Si-containing materials exhibit large volume expansion and contraction during charge and discharge, their use as a negative electrode active material generally tends to increase battery resistance during high-temperature charge and discharge cycles. However, by using the nonaqueous electrolyte solution of the present embodiment described above, it is possible to suppress the increase in battery resistance during high-temperature charge and discharge cycles, even when a Si-containing material is used as a negative electrode active material. The content of the Si-containing material is preferably 1% by mass or more, more preferably 5% by mass or more, relative to the total mass of the negative electrode active material in terms of increasing the capacity of the secondary battery. The upper limit of the content of the Si-containing material may be, for example, 30% by mass or less.

[0042] Examples of the Si-containing material include Si, Si alloys, and Si compounds. The Si-containing material may also be a composite particle containing an ion-conducting phase and a silicon phase (silicon particles in one respect) dispersed within the ion-conducting phase. Examples of the ion-conducting phase include a silicate phase, a carbon phase, and a silicon oxide phase.

[0043] The carbon phase may be composed of, for example, amorphous carbon. Examples of amorphous carbon constituting the carbon layer include hard carbon, soft carbon, and other amorphous carbon. Amorphous carbon has an average interplanar spacing d of (002) planes measured by, for example, X-ray diffraction. ００２ The carbon material has a particle size of more than 0.34 nm.

[0044] The main component of the silicon oxide phase (for example, 95% by mass or more and 100% by mass or less) may be silicon dioxide. The composition of the composite particles containing the silicon oxide phase and the silicon phase dispersed therein is generally SiO ｘ It can be expressed as: SiO ｘ is a material in which silicon particles are amorphous SiO ２ The oxygen content ratio x to silicon is preferably, for example, 0.5≦x<2.0, and more preferably 0.8≦x≦1.5.

[0045] The silicate phase is preferably a silicate phase containing lithium (hereinafter, sometimes referred to as a "lithium silicate phase"), for example, in terms of a small irreversible capacity and a high initial charge / discharge efficiency. The lithium silicate phase may be an oxide phase containing Li, Si, and O, and may also contain other elements. The lithium silicate phase has the formula: Li ２ｚ SiO （２＋ｚ） The lithium silicate phase may contain or be composed of a lithium silicate phase represented by (0<z<2). Preferably, z satisfies the relationship 0<z<1, and z=1 / 2 (i.e., Li ２ Si ２ O ５ ) is more preferred.

[0046] The Si-containing material may also include composite particles containing an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase, and a coating layer covering at least a portion of the surface of the composite particles.

[0047] The coating layer present on the surface of the composite particle may include, for example, a conductive layer. Forming a conductive layer on the surface of the composite particle may increase the conductivity of the Si-containing material. A conductive material containing carbon is preferred as the conductive material constituting the conductive layer. Examples of conductive materials containing carbon include conductive carbon materials. Examples of conductive carbon materials include carbon black, graphite, and amorphous carbon with low crystallinity (amorphous carbon). Amorphous carbon may be either easily graphitized carbon (soft carbon) or difficultly graphitized carbon (hard carbon). Examples of carbon black include acetylene black and ketjen black. The thickness of the conductive layer may be, for example, in the range of 1 to 200 nm. The thickness of the conductive layer can be measured by observing the cross section of the Si-containing material using a SEM or TEM (transmission electron microscope).

[0048] The negative electrode active material may contain an active material other than the Si-containing material, for example, a carbon material, etc. Examples of the carbon material include graphite such as natural graphite and artificial graphite.

[0049] The weight of the negative electrode mixture layer on one side of the negative electrode current collector is, for example, 140 g / m from the viewpoint of being able to further suppress an increase in battery resistance accompanying high-temperature charge / discharge cycles. ２ It is preferable that:

[0050] Charge state of the negative electrode 12 (5 mV vs. Li / Li ＋ ) The thickness of the negative electrode at T1, the discharge state of the negative electrode 12 (1 V vs. Li / Li ＋ When the negative electrode thickness at the time of charging / discharging is T2, it is preferable that T1 / T2 is 0.15 or more and 0.30 or less. When the negative electrode thickness satisfies the above range, it is possible to further suppress the increase in battery resistance accompanying high-temperature charge / discharge cycles. The negative electrode thickness of the negative electrode 12 in the charged state can be adjusted by the type and amount of negative electrode active material. The negative electrode thickness is measured as follows.

[0051] The nonaqueous electrolyte secondary battery 10 is subjected to constant current discharge at a current of 0.5 C until the state of charge (SOC) reaches 0%, and then subjected to constant voltage discharge at a voltage of 0% SOC until the current reaches 0.02 C, thereby entering a discharged state. The nonaqueous electrolyte secondary battery 10 in this discharged state is disassembled, and the negative electrode 12 is removed. A three-electrode test battery is assembled using the removed negative electrode 12 as the working electrode and metallic lithium as the counter electrode and reference electrode. The assembled three-electrode test battery is then subjected to a current of 0.5 C until the voltage reaches 5 mV (vs. Li / Li ＋ The test battery in this charged state is disassembled, and the removed negative electrode 12 is used as the negative electrode in the charged state. The thickness of the negative electrode in the charged state is measured at one point near the center, and this is used as the negative electrode in the charged state (5 mV vs. Li / Li ＋ The negative electrode thickness when the voltage is 5 mV (vs. Li / Li) is T1. ＋ ), then at a current of 0.5 C, the voltage was 1 V (vs. Li / Li ＋ ) and constant current discharge is performed until the negative electrode 12 is removed from the test battery in this discharged state. The thickness of the negative electrode in the discharged state is measured at one point near the center, and this is the negative electrode in the discharged state (1 V vs. Li / Li ＋ ) is defined as the negative electrode thickness T2.

[0052] [Separator] Fig. 2 is a schematic cross-sectional view showing an example of a separator according to this embodiment. As shown in Fig. 2, the separator 13 preferably includes a substrate 30 having a first surface 30a and a second surface 30b, and a functional layer 32 disposed on the first surface 30a of the substrate 30. The functional layer 32 may be disposed on both the first surface 30a and the second surface 30b.

[0053] The substrate 30 is, for example, a porous sheet having ion permeability and insulating properties, and specific examples thereof include a microporous thin film, a woven fabric, a nonwoven fabric, etc. The material of the substrate 30 is not particularly limited, and examples thereof include polyolefins such as polyethylene, polypropylene, and copolymers of polyethylene and α-olefin, acrylic resins, polystyrene, polyesters, cellulose, polyimides, polyphenylene sulfide, polyether ether ketones, and fluororesins.

[0054] The functional layer 32 includes, for example, a heat-resistant layer 34 containing inorganic particles and resin particles 36 dispersed in the heat-resistant layer 34. Some of the resin particles 36 form protrusions 36a protruding from the surface of the heat-resistant layer 34. The outer surface of the functional layer 32, i.e., the surface opposite to the surface facing the substrate 30, is formed by the surface of the heat-resistant layer 34 and the protrusions 36a protruding from the surface of the heat-resistant layer 34.

[0055] The outer surface of the functional layer 32 is the surface of the first separator facing the positive electrode 11 or the surface of the second separator facing the negative electrode 12, preferably the surface of the first separator facing the positive electrode 11, and preferably has a ten-point mean roughness (Rz) of 2.7 μm or more. For example, the outer surface of the functional layer 32 can be roughened and the ten-point mean roughness (Rz) can be increased by increasing the average particle diameter (D50) of the resin particles 36 or the amount of resin particles 36 dispersed in the heat-resistant layer 34. Roughening the outer surface of the functional layer 32 to a ten-point mean roughness (Rz) of 2.7 μm or more facilitates penetration of the nonaqueous electrolyte between the electrode and the separator. As a result, when high-temperature charge-discharge cycles are performed, the decrease in the amount of nonaqueous electrolyte in the electrode assembly 14 is suppressed, and an increase in battery resistance may be suppressed.

[0056] The ten-point mean roughness (Rz) of the outer surface of the functional layer 32 is preferably 2.7 μm or more, more preferably 3 to 10, and even more preferably 3.5 to 8, in order to further suppress the increase in battery resistance associated with high-temperature charge-discharge cycles. The ten-point mean roughness (Rz) is calculated by extracting a reference length from the roughness curve in the direction of the mean line, measuring the average absolute value of the elevations (Yp) of the five highest peaks and the average absolute value of the elevations (Yv) of the five lowest valleys in the direction of the longitudinal magnification from the mean line of the extracted portion, and expressing this value in micrometers (μm). The larger the ten-point mean roughness (Rz), the rougher the entire surface, and the smaller the ten-point mean roughness (Rz), the smoother the entire surface. For example, the surface of the functional layer 32 can be observed with a laser microscope (OLYMPUS OLS4100) and the ten-point average roughness (Rz) can be measured by a method in accordance with JIS B0601:2001.

[0057] The average particle diameter (D50) of the resin particles 36 is preferably, for example, in the range of 1.0 μm or more and 8.0 μm or less. D50 refers to the particle size at which the cumulative frequency of the smallest particle size in a volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution of the resin particles 36 can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrac Bell Co., Ltd.) using water as a dispersion medium.

[0058] The content of the resin particles 36 is preferably, for example, in a range of 4:96 to 20:80 in terms of the mass ratio of the resin particles 36 to the heat-resistant layer 34 (resin particles:heat-resistant layer).

[0059] The resin particles 36 may be, for example, a known polymer that can be used as a binder when forming the functional layer 32. Examples of monomer units constituting the resin particles 36 (polymer) include aromatic vinyl monomer units, (meth)acrylic acid ester monomer units, and fluorine-containing monomer units. In this disclosure, "(meth)acrylic" refers to acrylic and / or methacrylic. The statement that the resin particles 36 (polymer) "contain monomer units" means that the polymer obtained using the monomer contains repeating units derived from the monomer.

[0060] Examples of aromatic vinyl monomers capable of forming aromatic vinyl monomer units include, but are not limited to, styrene, α-methylstyrene, styrenesulfonic acid, butoxystyrene, vinylnaphthalene, etc. Examples of (meth)acrylic acid ester monomers capable of forming (meth)acrylic acid ester monomer units include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, butyl acrylates such as n-butyl acrylate and t-butyl acrylate, octyl acrylates such as pentyl acrylate, hexyl acrylate, heptyl acrylate and 2-ethylhexyl acrylate, acrylic acrylates such as nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate and stearyl acrylate. acid alkyl esters; and methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, butyl methacrylates such as n-butyl methacrylate and t-butyl methacrylate, octyl methacrylates such as pentyl methacrylate, hexyl methacrylate, heptyl methacrylate and 2-ethylhexyl methacrylate, and methacrylic acid alkyl esters such as nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate and stearyl methacrylate.

[0061] Furthermore, examples of fluorine-containing monomers capable of forming fluorine-containing monomer units are not particularly limited, and include, for example, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, vinyl trifluoride chloride, vinyl fluoride, and perfluoroalkyl vinyl ether.

[0062] In addition to the above-described monomer units, the resin particles 36 may also contain crosslinkable monomer units. Here, the crosslinkable monomer units are monomers that can form crosslinked structures during or after polymerization by heating or irradiation with energy rays. Examples of monomers that can form crosslinkable monomer units include polyfunctional monomers having two or more polymerization reactive groups. Examples of such polyfunctional monomers include divinyl compounds such as allyl methacrylate and divinylbenzene; di(meth)acrylic acid ester compounds such as diethylene glycol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, and 1,3-butylene glycol diacrylate; tri(meth)acrylic acid ester compounds such as trimethylolpropane trimethacrylate and trimethylolpropane triacrylate; and ethylenically unsaturated monomers containing epoxy groups such as allyl glycidyl ether and glycidyl methacrylate.

[0063] The resin particles 36 can be prepared by polymerizing a monomer composition containing the above-mentioned monomers in an aqueous solvent such as water. The polymerization method is not particularly limited, and may be, for example, a suspension polymerization method, an emulsion polymerization aggregation method, or a pulverization method. The polymerization reaction may be any reaction such as radical polymerization or living radical polymerization.

[0064] The monomer composition used in preparing the resin particles 36 may contain other additives such as chain transfer agents, polymerization regulators, polymerization reaction retarders, reactive fluidizing agents, fillers, flame retardants, antioxidants, and colorants in any desired amounts.

[0065] Examples of inorganic particles contained in the heat-resistant layer 34 include metal oxide particles, metal nitride particles, metal fluoride particles, and metal carbide particles.

[0066] Examples of metal oxide particles include aluminum oxide, titanium oxide, magnesium oxide, zirconium oxide, nickel oxide, silicon oxide, and manganese oxide. Examples of metal nitride particles include titanium nitride, boron nitride, aluminum nitride, magnesium nitride, and silicon nitride. Examples of metal fluoride particles include aluminum fluoride, lithium fluoride, sodium fluoride, magnesium fluoride, calcium fluoride, and barium fluoride. Examples of metal carbide particles include silicon carbide, boron carbide, titanium carbide, and tungsten carbide.

[0067] The inorganic particles are zeolite (M ２／ｎ O.Al ２ O ３ xSiO ２ ・yH ２ O, M is a metal element, n is the valence of M, x≧2, y≧0), porous aluminosilicates such as talc (Mg ３ Si ４ O １０ (OH) ２ ), layered silicates such as barium titanate (BaTiO ３ ), strontium titanate (SrTiO ３ ), cerium oxide (Ce ｘ O ｙ These may be used alone or in combination of two or more.

[0068] The heat-resistant layer 34 preferably further contains a binder. The binder functions to bond, for example, individual inorganic particles to each other and to the inorganic particles and the substrate 30. Examples of binders include fluorine-based resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, and polyvinyl alcohol (PVA). These may be used alone or in combination of two or more. The content of the inorganic particles contained in the heat-resistant layer 34 is preferably, for example, 400% by mass or more and 9900% by mass or less, relative to the mass of the binder. Furthermore, the content of the binder contained in the heat-resistant layer 34 is preferably, for example, 3% by mass or more and 30% by mass or less, relative to the total mass of the heat-resistant layer 34.

[0069] An example of a method for producing the separator 13 of this embodiment will be described. For example, inorganic particles, resin particles 36, water as a dispersion medium, and other components (e.g., a binder) that are used as needed are mixed to prepare a functional layer slurry composition. The functional layer slurry is then applied to a substrate and dried, thereby producing the separator 13 of this embodiment.

[0070] The heat-resistant layer 34 containing inorganic particles is not an essential component of the functional layer 32. The functional layer 32 may have, for example, a layer containing a known additive, and examples of the functional layer 32 other than the heat-resistant layer include an antistatic layer, an adhesive layer, a sliding layer, a leveling layer, a flame-retardant layer, a layer for improving compatibility with an electrolyte, an antioxidant layer, and a lubricating / softening layer.

[0071] As a method for adjusting the ten-point average roughness (Rz) of the separator surface to 2.7 μm or more, for example, rolling with an uneven pressure roller can be used in addition to using the above-mentioned resin particles 36. For example, after forming a functional layer such as the heat-resistant layer 34 on the substrate 30, the surface of the functional layer is rolled with an uneven pressure roller to adjust the ten-point average roughness (Rz) of the separator surface to 2.7 μm or more.

[0072] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0073] Example 1 [Positive electrode] [Ni ０．８８ Co ０．０９ Mn ０．０３ ](OH) ２ and LiOH were mixed in an Ishikawa-type mortar so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.1:1.0. The mixture was then fired at 730°C for 50 hours in an oxygen atmosphere, and the fired product was pulverized to obtain LiNi. ０．８８ Co ０．０９ Mn ０．０３ O ２ The composition of the positive electrode active material A was analyzed by ICP emission spectroscopy.

[0074] The obtained positive electrode active material A was embedded in a resin, and a cross section of the particle was prepared by cross-section polishing (CP), and the cross section was observed by SEM. As a result, the positive electrode active material A was polycrystalline particles in a state where several hundred or more primary particles were aggregated.

[0075] In addition, [Ni ０．８０ Co ０．１５ Mn ０．０５ ](OH) ２ and LiOH were mixed in an Ishikawa-type mortar so that the molar ratio of Li to the total amount of Ni, Co, and Al was 1.1:1.0. Then, this mixture was fired at 800°C for 50 hours in an oxygen atmosphere, and the fired product was pulverized to obtain LiNi. ０．８０ Co ０．１５ Mn ０．０５ O ２ The composition of the positive electrode active material B was analyzed by ICP emission spectroscopy.

[0076] The obtained positive electrode active material B was embedded in a resin, and a cross section of the particle was prepared by cross-section polishing (CP), and the cross section was observed by SEM. As a result, the positive electrode active material B was a single crystal particle that existed in a state where it was completely separated into individual primary particles or in a state where 2 to 10 primary particles existed together.

[0077] 98 parts by mass of a mixture of single-crystal particle positive electrode active material B and polycrystalline particle positive electrode active material A in a mass ratio of 20:80 was mixed with 1 part by mass of acetylene black and 1 part by mass of PVDF, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and the coating was dried. The coating was rolled using a roller and then cut to a predetermined electrode size, and a positive electrode having a positive electrode composite layer formed on both sides of the positive electrode current collector was produced.

[0078] [Negative electrode] A negative electrode active material was prepared by mixing graphite powder and a Si-containing material in a mass ratio of 94:6, a dispersion of styrene butadiene rubber, and sodium carboxymethyl cellulose in a solids mass ratio of 98:1:1, and water was used as a dispersion medium to prepare a negative electrode composite slurry. Next, the negative electrode composite slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried. The coating was rolled using a roller and then cut to a predetermined electrode size, producing a negative electrode in which a negative electrode composite layer was formed on both sides of the negative electrode current collector.

[0079] [Non-aqueous electrolyte] A mixed solvent of cyclic carbonate having ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in a volume ratio of 36.4:63.6, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 10:70:20 was used. ６ A lithium salt (LiFSI: 35.5 mol %) consisting of the above was dissolved in the nonaqueous electrolyte at a ratio of 1.55 mol / L to prepare a nonaqueous electrolyte.

[0080] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] (1) A positive electrode lead was attached to the positive electrode, and a negative electrode lead was attached to the negative electrode. A separator made of a polyethylene microporous film was then interposed between the positive and negative electrodes, and the resulting structure was wound to produce a wound electrode assembly. (2) Insulating plates were placed above and below the electrode assembly, and the negative electrode lead was welded to the case body and the positive electrode lead was welded to the sealing member, and the electrode assembly was housed in the case body. (3) The non-aqueous electrolyte solution was injected into the case body under reduced pressure, and the opening of the case body was then sealed with the sealing member via a gasket. This resulted in a non-aqueous electrolyte secondary battery.

[0081] Example 2: LiFSI and LiPF were added to a mixed solvent of fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 7:73:20. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 50.0 mol %) consisting of the compound (I) at a ratio of 1.80 mol / L was used.

[0082] Example 3 A mixed solvent of cyclic carbonate (ethylene carbonate) and fluoroethylene carbonate (FEC) in a volume ratio of 58.8:41.2, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 10:70:20 was added to the mixed solvent. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 35.5 mol %) consisting of the compound (I) at a ratio of 1.55 mol / L was used.

[0083] Example 4: LiFSI and LiPF were added to a mixed solvent of fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 7:73:20. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 75.6 mol %) consisting of the compound (I) at a ratio of 2.00 mol / L was used.

[0084] Example 5 A mixed solvent of cyclic carbonate (ethylene carbonate) and fluoroethylene carbonate (FEC) in a volume ratio of 58.8:41.2, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 10:70:20 was added to the mixed solvent. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 35.5 mol %) consisting of the compound (I) at a ratio of 2.50 mol / L was used.

[0085] Example 6 A mixed solvent of cyclic carbonate (ethylene carbonate) and fluoroethylene carbonate (FEC) in a volume ratio of 36.4:63.6, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 10:70:20 was added to the mixed solvent, and LiFSI and LiPF ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 20.0 mol %) consisting of the compound (I) at a ratio of 1.65 mol / L was used.

[0086] Comparative Example 1: A mixed solvent of cyclic carbonate having ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in a volume ratio of 66.7:33.3, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 20:60:20 was added to the mixed solvent. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 1.5 mol %) consisting of the compound (I) at a ratio of 1.30 mol / L was used.

[0087] Comparative Example 2: A mixed solvent of cyclic carbonate having ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in a volume ratio of 66.7:33.3, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 21:60:19 was added to the mixed solvent. ６A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 25.9 mol %) consisting of the compound (I) at a ratio of 1.30 mol / L was used.

[0088] Comparative Example 3: A mixed solvent of cyclic carbonate having ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in a volume ratio of 36.4:63.6, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 10:70:20 was added to the mixed solvent. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 25.9 mol %) consisting of the compound (I) at a ratio of 1.30 mol / L was used.

[0089] Comparative Example 4: A mixed solvent of cyclic carbonate having ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in a volume ratio of 58.8:41.2, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 20:60:20 was added to the mixed solvent. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 1.5 mol %) consisting of the compound (I) at a ratio of 1.30 mol / L was used.

[0090] Comparative Example 5: A mixed solvent of cyclic carbonate having ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in a volume ratio of 66.7:33.3, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 20:60:20 was added to the mixed solvent. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 50.0 mol %) consisting of the compound (I) at a ratio of 1.30 mol / L was used.

[0091] Comparative Example 6: A mixed solvent of cyclic carbonate having ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in a volume ratio of 66.7:33.3, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 20:60:20 was added to the mixed solvent. ６ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a nonaqueous electrolyte solution prepared by dissolving a lithium salt (LiFSI: 75.6 mol %) consisting of the compound (I) at a ratio of 1.30 mol / L was used.

[0092] [Measurement of Initial Resistance] In an environment of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.5 C to an SOC of 50%. The voltage at this time was designated as V0. Next, the batteries were discharged at a constant current of 0.5 C for 10 seconds. The voltage at this time was designated as V1. The direct current resistance (DCR) was then calculated using the following formula. This was designated as the initial resistance: DCR = (V0 - V1) / 0.5 C

[0093] [Measurement of Resistance Increase Rate in High-Temperature Charge-Discharge Cycle Test] In a 45°C environment, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.5 C until the voltage reached 4.2 V, and then charged at a constant voltage of 0.05 C. Thereafter, the batteries were discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This charge-discharge cycle constitutes one cycle, and 100 cycles were performed. Then, the DC resistance of the nonaqueous electrolyte secondary batteries of each Example and Comparative Example after 100 cycles was measured using the same method as above. This was defined as the resistance after the high-temperature charge-discharge cycle. The initial resistance and the resistance after the high-temperature charge-discharge cycle were then applied to the following formula to calculate the resistance increase rate: Resistance increase rate = (resistance after charge-discharge cycles / initial resistance) × 100

[0094] The results of the initial resistance and the resistance increase rate in the high-temperature charge-discharge cycle test for each example and each comparative example are summarized in Table 1. However, the initial resistance and resistance increase rate shown in Table 1 are shown relative to the initial resistance and resistance increase rate of Comparative Example 1, which is set as the reference (100). When the resistance increase rate is less than 100, it can be evaluated that the increase in battery resistance due to the high-temperature charge-discharge cycle was suppressed.

[0095]

[0096] As can be seen from Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 to 6, which used nonaqueous electrolyte solutions that satisfied all four requirements: (1) the total content of cyclic carbonates was 10% by volume or less, based on the total volume of the organic solvent; (2) the content of fluorine-free cyclic carbonates was 60% by volume or less, based on the total volume of the cyclic carbonates; (3) the concentration of the electrolyte salt relative to the organic solvent was 1.5 mol / L or more and 2.5 mol / L or less; and (4) the content of LiFSI was 10 mol% or more of the total electrolyte salt, showed a suppressed increase in battery resistance during high-temperature charge-discharge cycling compared to the nonaqueous electrolyte secondary batteries of Comparative Examples 1 to 6, which did not satisfy at least one of the above requirements (1) to (4).

[0097] Furthermore, among Examples 1 to 6, the nonaqueous electrolyte secondary batteries of Examples 1 to 4 and 6, in which the concentration of the electrolyte salt relative to the organic solvent was 1.5 mol / L or more and 2.0 mol / L or less, had lower initial resistance values ​​than the nonaqueous electrolyte secondary battery of Example 5.

[0098] The present disclosure is further described by the following embodiments. Aspect 1: A non-aqueous electrolyte solution comprising an electrolyte salt and an organic solvent, wherein the total content of cyclic carbonates in the organic solvent is 10 vol% or less relative to the total volume of the organic solvent, and the content of fluorine-free cyclic carbonates belonging to the cyclic carbonates is 60 vol% or less relative to the total volume of the cyclic carbonates, and the concentration of the electrolyte salt relative to the organic solvent is 1.5 mol / L or more and 2.5 mol / L or less, and the electrolyte salt contains lithium bis(fluorosulfonyl)imide, and the content of the lithium bis(fluorosulfonyl)imide is 10 mol% or more of the total electrolyte salt. Aspect 2: The non-aqueous electrolyte solution according to Aspect 1, wherein the content of the lithium bis(fluorosulfonyl)imide is 80 mol% or less of the total electrolyte salt. Aspect 3: The non-aqueous electrolyte solution according to Aspect 1 or 2, wherein the concentration of the electrolyte salt relative to the organic solvent is 1.5 mol / L or more and 2.0 mol / L or less. Configuration 4: The nonaqueous electrolyte according to any one of Configurations 1 to 3, wherein the content of ethylene carbonate, which belongs to the cyclic carbonates, is 5% by volume or less relative to the total volume of the organic solvent. Configuration 5: The nonaqueous electrolyte according to Configuration 4, wherein the content of ethylene carbonate, which belongs to the cyclic carbonates, is 0% by volume relative to the total volume of the organic solvent. Configuration 6: The nonaqueous electrolyte according to any one of Configurations 1 to 5, wherein the content of fluoroethylene carbonate, which belongs to the cyclic carbonates, is 2% by volume or more and 10% by volume or less relative to the total volume of the organic solvent. Configuration 7: The nonaqueous electrolyte according to any one of Configurations 1 to 6, wherein the organic solvent contains a chain carbonate, and the total content of the chain carbonates in the organic solvent is 50% by volume or more relative to the total volume of the organic solvent. Configuration 8: The nonaqueous electrolyte according to Configuration 7, wherein the content of ethyl methyl carbonate, which belongs to the chain carbonates, is 15% by volume or more relative to the total volume of the organic solvent. Configuration 9: The nonaqueous electrolyte solution of any one of Configurations 1 to 8, wherein the electrolyte salt includes lithium hexafluorophosphate.Configuration 10: A non-aqueous electrolyte secondary battery comprising: a positive electrode having a positive electrode composite layer, a negative electrode having a negative electrode composite layer, a separator disposed between the positive electrode and the negative electrode, and the non-aqueous electrolyte solution according to any one of Configurations 1 to 9. Configuration 11: The non-aqueous electrolyte secondary battery according to Configuration 10, wherein the negative electrode composite layer contains a negative electrode active material, and the negative electrode active material contains a Si-containing material. Configuration 12: The non-aqueous electrolyte secondary battery according to Configuration 10 or 11, wherein the separator comprises a substrate having a first surface and a second surface opposite to the first surface, and a functional layer disposed on at least the first surface of the first surface and the second surface of the substrate, the outer surface of the functional layer being a first separator surface facing the positive electrode or a second separator surface facing the negative electrode, and having a ten-point average roughness (Rz) of 2.7 μm or more. Configuration 13: The positive electrode mixture layer includes a positive electrode active material, the positive electrode active material including a positive electrode active material A of single crystal particles and a positive electrode active material B of polycrystalline particles, both of which have the formula: Li ｘ Ni １－ｙ－ｚ Co ｙ M ｚ O ２ (wherein 0.97≦x≦1.2, 0≦y≦0.2, 0<z, and M contains at least one element selected from the group consisting of Mn, W, Mg, Mo, Nb, Ti, Si, and Al). Configuration 14: The nonaqueous electrolyte secondary battery of Configuration 13, wherein the positive electrode active material B satisfies 0.85≦1-y-z in the formula. Configuration 15: The nonaqueous electrolyte secondary battery of Configuration 13 or 14, wherein the positive electrode active material A contains Mn and satisfies 0.045≦z in the formula. Configuration 16: The nonaqueous electrolyte secondary battery of any one of Configurations 10 to 15, wherein the positive electrode mixture layer contains a conductive material, and the conductive material contains carbon nanotubes. Aspect 17: The nonaqueous electrolyte secondary battery according to Aspect 11, wherein the Si-containing material comprises composite particles including an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase. Aspect 18: The nonaqueous electrolyte secondary battery according to Aspect 11, wherein the negative electrode has a state of charge (5 mV vs. Li / Li ＋ ) The thickness of the negative electrode at T1, the discharge state of the negative electrode (1 V vs. Li / Li＋ 19. The nonaqueous electrolyte secondary battery according to any one of claims 10 to 17, wherein T1 / T2 is 0.15 or more and 0.30 or less, where T2 is the thickness of the negative electrode when the negative electrode mixture layer is formed on one or both sides of the negative electrode current collector, and the weight of the negative electrode mixture layer on one side of the negative electrode current collector is 140 g / m or less. ２ The nonaqueous electrolyte secondary battery according to any one of claims 10 to 18, wherein:

[0099] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding portion, 23 Filter, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Substrate, 30a First surface, 30b Second surface, 32 Functional layer, 34 Heat-resistant layer, 36 Resin particles, 36a Convex portion.

Claims

1. A non-aqueous electrolyte solution comprising an electrolyte salt and an organic solvent, wherein the total content of cyclic carbonates in the organic solvent is 10% by volume or less relative to the total volume of the organic solvent, and the content of fluorine-free cyclic carbonates belonging to the cyclic carbonates is 60% by volume or less relative to the total volume of the cyclic carbonates, the concentration of the electrolyte salt in the organic solvent is 1.5 mol / L or more and 2.5 mol / L or less, and the electrolyte salt contains lithium bis(fluorosulfonyl)imide, and the content of the lithium bis(fluorosulfonyl)imide is 10 mol% or more of the total electrolyte salt.

2. The nonaqueous electrolyte solution according to claim 1, wherein the content of the lithium bis(fluorosulfonyl)imide is 80 mol % or less of the total amount of the electrolyte salt.

3. The nonaqueous electrolyte solution according to claim 1 or 2, wherein the concentration of the electrolyte salt relative to the organic solvent is 1.5 mol / L or more and 2.0 mol / L or less.

4. The nonaqueous electrolyte according to claim 1 or 2, wherein the content of ethylene carbonate, which belongs to the cyclic carbonates, is 5% by volume or less based on the total volume of the organic solvent.

5. The nonaqueous electrolyte solution according to claim 4, wherein the content of ethylene carbonate, which belongs to the cyclic carbonates, is 0% by volume relative to the total volume of the organic solvent.

6. The nonaqueous electrolyte solution according to claim 1 or 2, wherein the content of fluoroethylene carbonate, which belongs to the cyclic carbonates, is 2% by volume or more and 10% by volume or less relative to the total volume of the organic solvent.

7. The nonaqueous electrolyte solution according to claim 1 or 2, wherein the organic solvent contains a chain carbonate, and the total content of the chain carbonate in the organic solvent is 50% by volume or more relative to the total volume of the organic solvent.

8. The nonaqueous electrolyte solution according to claim 7, wherein the content of ethyl methyl carbonate, which belongs to the chain carbonate, is 15% by volume or more relative to the total volume of the organic solvent.

9. The nonaqueous electrolyte solution according to claim 1 or 2, wherein the electrolyte salt includes lithium hexafluorophosphate.

10. A non-aqueous electrolyte secondary battery comprising: a positive electrode having a positive electrode mixture layer; a negative electrode having a negative electrode mixture layer; a separator disposed between the positive electrode and the negative electrode; and the non-aqueous electrolyte solution according to claim 1 or 2.

11. The nonaqueous electrolyte secondary battery according to claim 10, wherein the negative electrode mixture layer contains a negative electrode active material, and the negative electrode active material contains a Si-containing material.

12. The nonaqueous electrolyte secondary battery according to claim 10, wherein the separator comprises a substrate having a first surface and a second surface opposite to the first surface, and a functional layer disposed on at least the first surface of the first surface and the second surface of the substrate, the outer surface of the functional layer being a first separator surface facing the positive electrode or a second separator surface facing the negative electrode, and having a ten-point average roughness (Rz) of 2.7 μm or more.

13. The positive electrode mixture layer includes a positive electrode active material, and the positive electrode active material includes a positive electrode active material A of single crystal particles and a positive electrode active material B of polycrystalline particles, and both the positive electrode active material A and the positive electrode active material B have a formula: Li ｘ Ni １－ｙ－ｚ Co ｙ M ｚ O ２ 11. The nonaqueous electrolyte secondary battery according to claim 10, wherein: (wherein 0.97≦x≦1.2, 0≦y≦0.2, 0<z, and M contains at least one element selected from the group consisting of Mn, W, Mg, Mo, Nb, Ti, Si, and Al).

14. The nonaqueous electrolyte secondary battery according to claim 13, wherein the positive electrode active material B satisfies the relationship 0.85≦1−y−z in the formula.

15. The nonaqueous electrolyte secondary battery according to claim 13, wherein in said positive electrode active material A, M in said formula contains Mn and satisfies 0.045≦z.

16. The nonaqueous electrolyte secondary battery according to claim 10, wherein the positive electrode mixture layer contains a conductive material, and the conductive material contains carbon nanotubes.

17. The nonaqueous electrolyte secondary battery according to claim 11, wherein the Si-containing material comprises composite particles including an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase.

18. State of charge of the negative electrode (5 mV vs. Li / Li ＋ ) The thickness of the negative electrode at T1, the discharge state of the negative electrode (1 V vs. Li / Li ＋ 11. The nonaqueous electrolyte secondary battery according to claim 10, wherein T1 / T2 is 0.15 or more and 0.30 or less, where T2 is the thickness of the negative electrode when 19. The negative electrode mixture layer is disposed on one or both sides of the negative electrode current collector, and the weight of the negative electrode mixture layer on one side of the negative electrode current collector is 140 g / m ２ The nonaqueous electrolyte secondary battery according to claim 10 , wherein:

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