Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery (10) includes a positive electrode (11), a negative electrode (12), and a nonaqueous electrolyte solution containing a lithium salt and a nonaqueous solvent. The nonaqueous solvent contains a cyclic carbonate and a chain carbonate. The chain carbonate contains a first chain carbonate having a melting point of at most 0°C in an amount of 10-24 mass% with respect to the mass of the nonaqueous solvent.

Description

Nonaqueous electrolyte secondary battery

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

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion batteries have been widely used in applications requiring high capacity, such as in-vehicle applications and power storage applications. Since non-aqueous electrolyte secondary batteries are expected to be used not only in room temperature environments but also in low-temperature environments such as −20° C., they are required to have good discharge performance even in low-temperature environments. For example, Patent Document 1 proposes a non-aqueous electrolyte solution containing a fluorinated carboxylic acid ester having hydrogen at the α-position and lithium fluorosulfate, with the aim of improving low-temperature discharge performance and high-temperature storage characteristics.

[0003] JP 2017-69184 A

[0004] However, attempts to improve the discharge performance of non-aqueous electrolyte secondary batteries in low-temperature environments generally result in an increase in DC resistance in room-temperature environments, thereby degrading the discharge performance in room-temperature environments. That is, there is a trade-off between the discharge performance in room-temperature environments and the discharge performance in low-temperature environments. An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that exhibits excellent discharge performance in low-temperature environments while suppressing an increase in DC resistance in room-temperature environments.

[0005] The nonaqueous electrolyte secondary battery according to the present disclosure is a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte solution containing a lithium salt and a nonaqueous solvent, wherein the nonaqueous solvent contains a cyclic carbonate and a chain carbonate, and the chain carbonate contains a first chain carbonate having a melting point of 0°C or lower in an amount of 10% by mass or more and 24% by mass or less relative to the mass of the nonaqueous solvent.

[0006] The nonaqueous electrolyte secondary battery according to the present disclosure can improve discharge performance in a low-temperature environment while suppressing an increase in DC resistance in a normal-temperature environment.

[0007] 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention;

[0008] As described above, nonaqueous electrolyte secondary batteries are expected to be used not only in room temperature environments but also in low temperature environments such as −20° C. Therefore, it is important to improve discharge performance in low temperature environments while suppressing an increase in DC resistance in room temperature environments. In addition, other battery performances such as charge / discharge cycle performance must also be considered.

[0009] As a result of extensive research into the above-mentioned problems, the present inventors have succeeded in improving discharge performance in low-temperature environments while suppressing an increase in DC resistance in room-temperature environments by using a nonaqueous electrolyte solution containing a cyclic carbonate and a chain carbonate, with a predetermined amount of the chain carbonate having a melting point of 0°C or lower. The nonaqueous electrolyte secondary battery according to the present disclosure can effectively achieve discharge performance in both room-temperature and low-temperature environments. Furthermore, by improving the negative electrode, DC resistance in room-temperature environments can be reduced, making the above-mentioned effects more pronounced.

[0010] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

[0011] In the embodiment described below, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified as a nonaqueous electrolyte secondary battery, but the battery outer can is not limited to a cylindrical outer can. Other embodiments of the nonaqueous electrolyte secondary battery according to the present disclosure include a prismatic battery equipped with a prismatic outer can, a pouch-type battery equipped with an outer can made of a laminate sheet including a metal layer and a resin layer, and the like. Furthermore, the electrode assembly is not limited to a wound type, and may be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0012] FIG. 1 is a schematic diagram illustrating an axial and radial cross section of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte solution, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte solution. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 has a cylindrical shape with a bottom and an open end in the axial direction, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0013] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal and width directions. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0014] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in FIG. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal. The outer peripheral surface of the electrode body 14 may be provided with an exposed portion in which the surface of the negative electrode core constituting the negative electrode 12 is exposed. In this case, the exposed portion may be in contact with the inner peripheral surface of the outer can 16, electrically connecting the negative electrode 12 and the outer can 16.

[0015] The outer can 16 is a cylindrical metal container with a bottom. A gasket 28 is provided between the outer can 16 and the sealing body 17, ensuring the sealing of the interior of the battery and preventing electrical contact between the outer can 16 and the sealing body 17. The outer can 16 has a groove 22 formed on its side surface that protrudes inward and supports the sealing body 17. The groove 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the groove 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0016] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, 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 breaks, and gas is discharged from the opening of the cap 27.

[0017] 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, particularly the negative electrode 12 and the nonaqueous electrolyte solution.

[0018] [Positive Electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core. The positive electrode core can be a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface. The thickness of the positive electrode core is, for example, 10 μm to 30 μm. The positive electrode mixture layer preferably contains a positive electrode active material, a conductive agent, and a binder, and is provided on both sides of the positive electrode core. The thickness of the positive electrode mixture layer is, for example, 60 μm to 120 μm on one side of the positive electrode core. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0019] The positive electrode active material is a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, Mn, and Al. Examples of metal elements contained in the composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. Among these, it is preferable to contain at least one of Ni, Co, Mn, and Al. The lithium transition metal composite oxide has, for example, a layered rock salt structure. One type of lithium transition metal composite oxide may be used alone, or multiple types may be used in combination. The content of the positive electrode active material is, for example, 90% by mass or more and 99.8% by mass or less relative to the mass of the positive electrode mixture layer.

[0020] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. One type of conductive agent may be used alone, or multiple types may be used in combination. The content of the conductive agent is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0021] Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer; and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The content of the binder is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less, relative to the mass of the positive electrode mixture layer.

[0022] [Negative Electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer provided on the negative electrode core. The negative electrode core can be a foil of a metal stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with such a metal disposed on the surface layer. The thickness of the negative electrode core is, for example, 5 μm to 20 μm. The negative electrode mixture layer contains a negative electrode active material and a binder and is preferably provided on both sides of the negative electrode core. The thickness of the negative electrode mixture layer is, for example, 50 μm to 100 μm on one side of the negative electrode core. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0023] As with the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, an olefin resin, PAN, a polyimide, a polyamide, an acrylic resin, or the like. However, polyvinyl acetate, styrene-butadiene rubber (SBR), or the like may also be used. Among these, SBR is preferably used. One type of binder may be used alone, or multiple types may be used in combination. The negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. The content of the binder is not particularly limited, but is, for example, 0.05% by mass or more and 3% by mass or less, or 0.1% by mass or more and 1% by mass or less, relative to the mass of the negative electrode mixture layer 31.

[0024] The negative electrode mixture layer may contain a conductive agent such as CNT. CNT effectively suppresses the occurrence of the negative electrode active material being isolated from the conductive path of the negative electrode mixture layer, contributing to lowering the resistance of the battery and improving the charge / discharge cycle performance. The CNT content is preferably 0.01% by mass or more and 0.1% by mass or less relative to the mass of the negative electrode active material. The CNT may be either a single-walled CNT (SWCNT) or a multi-walled CNT (MWCNT).

[0025] The negative electrode mixture layer contains a carbon material that reversibly absorbs and releases lithium ions as a negative electrode active material. The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon. Among these, graphite is preferably used. The graphite may be artificial graphite such as massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, or amorphous graphite, or a mixture thereof.

[0026] The volume-based D50 of graphite is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less. The D50 of the negative electrode active material particles means the particle size at which the cumulative frequency of the particles in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the negative electrode active material 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.

[0027] The graphite may be artificial graphite or natural graphite whose particle surfaces are coated with amorphous carbon. Hereinafter, such graphite may be referred to as "amorphous carbon-coated graphite." Amorphous carbon-coated graphite has a core-shell structure including graphite particles (parent particles) and an amorphous carbon coating formed on the surface of the parent particles. The amorphous carbon coating is a carbon coating in an amorphous or microcrystalline turbostratic state in which the graphite crystal structure is not developed, and is composed of, for example, carbon whose d(002) interplanar spacing determined by X-ray diffraction is greater than 0.340 nm. The amorphous carbon coating is preferably formed on the entire particle surface.

[0028] Amorphous carbon-coated graphite is harder than graphite without an amorphous carbon coating and is less likely to be crushed during the compression process of the negative electrode mixture layer. The amorphous carbon coating, for example, increases the hardness of the graphite particles and inhibits decomposition of the nonaqueous electrolyte. Furthermore, amorphous carbon is characterized by its isotropic structure and large intercalation openings for lithium ions. Therefore, by covering the graphite particle surfaces with amorphous carbon, lithium ion diffusibility is improved and DC resistance is reduced.

[0029] Specific examples of amorphous carbon coatings include hard carbon, soft carbon, carbon black such as acetylene black, ketjen black, thermal black, and furnace black, carbon fiber, and activated carbon. An example of a suitable range for the thickness of the amorphous carbon coating is 10 nm to 200 nm. The amorphous carbon coating can be formed by mixing coal tar, tar pitch, naphthalene, anthracene, phenanthrolene, or the like with graphite and heat treating the mixture at a temperature of 800°C to 1200°C, or by a chemical vapor deposition (CVD) method using a hydrocarbon gas or the like. The amorphous carbon coating is formed, for example, in an amount of 0.5% by mass to 15% by mass relative to the mass of the base particles.

[0030] From the viewpoint of increasing the capacity of the battery, it is preferable to use a Si-containing material as the negative electrode active material. Furthermore, from the viewpoint of achieving both high capacity and good charge / discharge cycle performance, it is preferable to use graphite and a Si-containing material in combination. Examples of Si-containing materials that function as negative electrode active materials include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred. The D50 of the Si-containing material is, for example, smaller than the D50 of graphite, and is 1 μm or more and 20 μm or less.

[0031] When graphite and a Si-containing material are used in combination as the negative electrode active material, the graphite content is, for example, 80% by mass to 99% by mass, 85% by mass to 97% by mass, or 90% by mass to 95% by mass of the negative electrode active material. The Si-containing material content is, for example, 1% by mass to 20% by mass, 3% by mass to 15% by mass, or 5% by mass to 10% by mass of the negative electrode active material. When the contents of graphite and Si-containing material are within these ranges, it becomes easier to achieve both high capacity and high durability (good cycle performance) of the battery.

[0032] The Si-containing material is preferably a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicide phase is a phase of a compound consisting of Si and an element more electropositive than Si, such as NiSi, Mg ２ Si, TiSi ２ The Si phase is formed by dispersing Si in the form of fine particles. The ion-conducting phase is a continuous phase formed by an aggregation of particles finer than the Si phase. The Si content in the Si-containing material is preferably 40% by mass or more and 70% by mass or less.

[0033] The Si-containing material may have a conductive layer covering the surface of the ion-conducting phase. The conductive layer is made of a material with higher conductivity than the ion-conducting layer and forms a good conductive path in the negative electrode mixture layer. The conductive layer is, for example, a carbon coating made of a conductive carbon material. Examples of the conductive carbon material include carbon black such as acetylene black and ketjen black, graphite, and amorphous carbon with low crystallinity.

[0034] An example of a suitable Si-containing material includes an amorphous silicon oxide phase, a Si phase dispersed in the silicon oxide phase, and has the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The silicon oxide phase may be doped with Li. The oxygen to Si content (x) is, for example, 0.5≦x<2.0, and preferably 0.8≦x≦1.5.

[0035] Another example of a suitable Si-containing material is a composite particle containing an amorphous silicate phase and a Si phase dispersed in the silicate phase. A suitable silicate phase is a lithium silicate phase containing Li. The lithium silicate phase can be, for example, represented by the general formula Li ２ｚ SiO （２＋ｚ） (0<z<2). The lithium silicate phase is a composite oxide phase represented by the formula: Li ２ SiO ３ (Z=1) or Li ２ Si ２ O ５ It is preferable that (Z=1 / 2) is used as the main component.

[0036] Another example of a suitable Si-containing material is a composite particle containing an amorphous carbon phase and a Si phase dispersed in the amorphous carbon phase. The composite particle has a sea-island structure in which fine Si phases are dispersed in a continuous phase composed of amorphous carbon. The Si phase changes volume with charge and discharge, but the stress caused by the volume change of the Si phase is alleviated by the amorphous carbon phase. Therefore, by using the composite particle, the volume change of the negative electrode 12 with charge and discharge can be minimized. The amorphous carbon phase is, for example, composed of carbon having a d(002) interplanar spacing of greater than 0.340 nm by X-ray diffraction, and is obtained by heat treating a pitch having a softening point of 200°C or higher at a temperature of 700°C or higher and 900°C or lower.

[0037] The negative electrode mixture layer may have a single layer structure, but preferably has a multilayer structure. When the negative electrode mixture layer has a predetermined multilayer structure, for example, the permeability of the non-aqueous electrolyte solution in the negative electrode mixture layer is improved, and the DC resistance in normal temperature and low temperature environments can be more effectively reduced by a synergistic effect with the non-aqueous electrolyte solution described below.

[0038] 2 is a cross-sectional view of the negative electrode 12 cut in the width direction. As shown in FIG. 2, the negative electrode 12 preferably has a negative electrode core 30, a first negative electrode mixture layer 31 provided on the negative electrode core 30, and a second negative electrode mixture layer 32 provided on the negative electrode core 30 with the first negative electrode mixture layer 31 interposed therebetween. The negative electrode 12 has a two-layer negative electrode mixture layer 33 including the first negative electrode mixture layer 31 as a lower layer and the second negative electrode mixture layer 32 as an upper layer. The negative electrode mixture layer 33 may include a third negative electrode mixture layer as long as the object of the present disclosure is not impaired. However, a two-layer structure is preferred from the viewpoint of improving productivity, etc.

[0039] Both the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 contain the negative electrode active material and the binder. However, the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 preferably contain different types of negative electrode active materials. In this case, the physical properties of each mixture layer, such as tortuosity and porosity, are different from each other. The tortuosity is an index indicating the degree of curvature of the voids (pores) formed in the negative electrode mixture layer 33 through which the nonaqueous electrolyte passes. A smaller tortuosity indicates a less tortuous path of the voids. As will be described in detail later, the second negative electrode mixture layer 32 preferably contains amorphous carbon-coated graphite in an amount of 50% by mass or more relative to the mass of the negative electrode active material. The first negative electrode mixture layer 31 may or may not contain amorphous carbon-coated graphite.

[0040] The tortuosity τ of the negative electrode mixture layer 33 is calculated by the following formula: τ = f / s, where f is the path length along the central axis (medial axis) of the void (hereinafter abbreviated as "path length"), and s is the length of the straight line connecting the start point and end point of each path (hereinafter abbreviated as "straight line distance between the start and end points"). f and s are the average values ​​of the path length and straight line distance obtained for multiple paths. The tortuosity is evaluated using a negative electrode 12 in a fully discharged state. The path length and straight line distance are determined by cross-sectional observation and image analysis of the negative electrode mixture layer 33 using a 3D scanning electron microscope (3D SEM, for example, Ethos NX-5000 manufactured by Hitachi High-Technologies Corporation).

[0041] A specific method for calculating the tortuosity is as follows. (1) Construction of the three-dimensional structure of the negative electrode mixture layer 33: The negative electrode mixture layer 33 is placed on the sample stage of a 3D SEM, and continuous cross-sectional slicing and cross-sectional observation are alternately performed. Observation is performed at an acceleration voltage of 5 kV. The obtained two-dimensional continuous images are binarized using three-dimensional image analysis software (e.g., EXFACT VR manufactured by Nippon Visual Science Co., Ltd.), and the images are then joined together to construct a three-dimensional structure. The size of the three-dimensional structure is preferably 100 μm × 100 μm × 100 μm or more. (2) Determining the boundary between the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32: From the three-dimensional structure image obtained in (1) above, the boundary between the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 is determined as the boundary line between the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32, based on the boundary between the regions where the voids differ in the thickness direction. (3) Determination of Path Length (f) The image of the three-dimensional structure acquired in (1) above was divided into the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 along the boundary determined in (2) above. The extracted voids were then thinned by binarization to determine the axis (medial axis) passing through the center of the void. The medial axis present within the cube, which penetrates in a direction perpendicular to the surface of the negative electrode core 30, was extracted, and for paths with branches, the shortest path was determined as the path length. (4) Calculation of Tortuosity (τ) The tortuosity of the negative electrode mixture layer 33 was calculated using the average path length determined in (3) above and the average linear distance connecting the end and start points of each path using the above formula. The tortuosity can be calculated not only for the negative electrode mixture layer 33, but also for each of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32.

[0042] The negative electrode mixture layer 33 has interparticle voids through which the nonaqueous electrolyte solution permeates between particles of the negative electrode active material. When the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 have different interparticle void ratios, the interparticle void ratio of the second negative electrode mixture layer 32 is preferably higher than the interparticle void ratio of the first negative electrode mixture layer 31. The interparticle void ratio of the mixture layer refers to the ratio of the area of ​​the interparticle voids of the negative electrode active material to the cross-sectional area of ​​the mixture layer, and voids within the active material particles are not counted in the interparticle void ratio. Increasing the interparticle void ratio of the second negative electrode mixture layer 32 improves the permeability of the electrolyte solution throughout the negative electrode mixture layer 33.

[0043] The interparticle porosity of the negative electrode mixture layer 33 is determined by the following method. (1) A cross section of the negative electrode mixture layer 33 is exposed using an ion milling device (IM4000PLUS, manufactured by Hitachi High-Technologies Corporation), and a backscattered electron image of the exposed cross section of the negative electrode mixture layer 33 is taken at a magnification of 800x. (2) The obtained cross-sectional image is imported into a computer, and the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 are cut out. (3) The cut-out cross-sectional image is binarized using image analysis software to obtain a binarized image in which the particle cross sections of the negative electrode active material in the cross-sectional image are converted to white and the interparticle voids are converted to black. The image analysis software used is ImageJ manufactured by the National Institutes of Health. Note that the binarization is performed so that voids inside the particles are white. (4) The interparticle void ratio (area of ​​voids between black particles × 100 / (area of ​​voids between black particles + area of ​​white negative electrode active material particles)) is calculated from the ratio of the white and black areas.

[0044] As described above, the second negative electrode mixture layer 32 preferably contains amorphous carbon-coated graphite in an amount of 50 mass% or more relative to the mass of the negative electrode active material. The nonaqueous electrolyte secondary battery 10 aims to achieve both good discharge performance at room temperature and low temperature. To achieve this goal, it is preferable to use a nonaqueous electrolyte solution containing a large amount of ethyl methyl carbonate, as described below. However, in this case, the viscosity of the nonaqueous electrolyte solution tends to increase. When amorphous carbon-coated graphite is used for the second negative electrode mixture layer 32, the synergistic effect of the nonaqueous electrolyte solution and the amorphous carbon-coated graphite can more effectively achieve good discharge performance at room temperature and low temperature.

[0045] Adding amorphous carbon-coated graphite to the second negative electrode mixture layer 32 can reduce DC resistance, particularly in room temperature environments. As described above, amorphous carbon-coated graphite has good lithium ion diffusibility into the particles. Furthermore, since amorphous carbon-coated graphite is harder than graphite without an amorphous carbon coating and less likely to be crushed during the compression process of the negative electrode mixture layer, interparticle voids in the second negative electrode mixture layer 32 can be secured, and the interparticle void ratio can be increased. As a result, the permeability of the nonaqueous electrolyte throughout the negative electrode mixture layer 33 is improved, effectively reducing DC resistance.

[0046] Substantially all of the graphite contained in the second negative electrode mixture layer 32 may be amorphous carbon-coated graphite. The base particles of the amorphous carbon-coated graphite may be either artificial graphite or natural graphite, and the second negative electrode mixture layer 32 may contain amorphous carbon-coated artificial graphite and amorphous carbon-coated natural graphite. The amorphous carbon-coated graphite may be contained throughout the entire negative electrode mixture layer 33, but the content of amorphous carbon-coated graphite in the first negative electrode mixture layer 31 is preferably lower than the content of amorphous carbon-coated graphite in the second negative electrode mixture layer 32.

[0047] The amorphous carbon-coated graphite may be contained only in the second negative electrode mixture layer 32, and not in the first negative electrode mixture layer 31. In this case, the interparticle porosity of the second negative electrode mixture layer 32 is increased to improve the permeability of the nonaqueous electrolyte, while the tortuosity of the first negative electrode mixture layer 31 can be reduced by appropriately crushing the graphite particles in the compression process of the mixture layer. As a result, rapid charging performance is improved, and discharge performance in both room temperature and low temperature environments can be more effectively achieved.

[0048] A suitable example of the negative electrode mixture layer 33 is a two-layer structure having a first negative electrode mixture layer 31 containing substantially only natural graphite as graphite, and a second negative electrode mixture layer 32 containing substantially only amorphous carbon-coated graphite. Alternatively, the negative electrode mixture layer 33 may have a two-layer structure having a first negative electrode mixture layer 31 containing amorphous carbon-coated natural graphite, and a second negative electrode mixture layer 32 containing amorphous carbon-coated artificial graphite.

[0049] The thickness ratio between the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 is not particularly limited, but for example, the thickness of the second negative electrode mixture layer 32 is preferably 10% or more, and more preferably 30% or more, of the total thickness of the negative electrode mixture layer 33. A suitable thickness ratio between the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 is 10:90 to 90:10, or 30:70 to 70:30, or 40:60 to 60:40, or may be 50:50.

[0050] The tortuosity (τ) of the negative electrode mixture layer 33 is, for example, 1 or more and 10 or less, or 2 or more and 8 or less. The tortuosity (τ1) of the first negative electrode mixture layer 31 is preferably smaller than the tortuosity (τ2) of the second negative electrode mixture layer 32, for example, 0.2≦(τ1 / τ2)≦0.5. The interparticle porosity (S) of the negative electrode mixture layer 33 is, for example, 28% or more and 32% or less. The interparticle porosity (S2) of the second negative electrode mixture layer 32 is preferably larger than the interparticle porosity (S1) of the first negative electrode mixture layer 31, for example, 1.05≦(S2 / S1)≦1.25. In this case, discharge performance in both room temperature and low temperature environments can be more effectively achieved.

[0051] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. The separator 13 may have, for example, a multi-layer structure including a thermoplastic resin layer such as polyolefin and a cellulose fiber layer, a two-layer structure of polyethylene (PE) / polypropylene (PP), or a three-layer structure of PE / PP / PE.

[0052] A filler layer containing an inorganic filler may be disposed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13. In addition, a highly heat-resistant resin layer (heat-resistant layer) such as an aramid resin may be disposed on the surface of the separator 13. The separator 13 may have, for example, a substrate made of a porous sheet and a filler layer or heat-resistant layer disposed on the substrate.

[0053] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a lithium salt and a non-aqueous solvent. The non-aqueous solvent contains at least a cyclic carbonate and a chain carbonate. As will be described in detail later, the chain carbonate contains a first chain carbonate having a melting point of 0°C or less in an amount of 10% by mass or more and 24% by mass or less relative to the mass of the non-aqueous solvent. By using the non-aqueous electrolyte of this embodiment, it is possible to improve discharge performance in low-temperature environments while suppressing an increase in DC resistance in room-temperature environments. In particular, by using the non-aqueous electrolyte together with the above-mentioned multi-layered negative electrode 12, it is possible to more effectively achieve good discharge performance at both room temperature and low-temperature environments.

[0054] An example of a lithium salt is 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), LiB １０ 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 As the lithium salt, one of these may be used alone or a plurality of types may be used in combination. Among these, LiPF ６ The concentration of the lithium salt is, for example, 0.8 mol or more and 2.0 mol or less, or 1.0 mol or more and 1.5 mol or less per liter of the non-aqueous solvent.

[0055] The nonaqueous electrolyte may contain, as a lithium salt, imide salts such as lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonyl nonafluorobutanesulfonyl imide, lithium bispentafluoroethanesulfonyl imide, and lithium bisfluorosulfonylimide (LiFSI). Among these, it is preferable to use LiFSI. By adding LiFSI, a protective film with high ion conductivity is formed on the surface of the negative electrode 12, enhancing the resistance reduction effect. LiFSI is LiPF ６ It is preferable to use LiFSI in combination with other lithium salts such as the above. The concentration of LiFSI (imide salts) is, for example, 0.005 mol or more and 0.5 mol or less, or 0.01 mol or more and 0.1 mol or less per 1 L of the non-aqueous solvent.

[0056] Examples of non-aqueous solvents that can be used include ethers, esters, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The cyclic and chain carbonates are classified as esters. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of halogen-substituted products include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0057] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0058] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0059] As described above, the nonaqueous electrolyte of this embodiment contains at least a cyclic carbonate and a chain carbonate as a nonaqueous solvent, and further contains a chain carbonate having a melting point of 0°C or lower. The carbonate, which is an essential component of the nonaqueous electrolyte, is a saturated carbonate. Cyclic carbonates, for example, generally have a high relative dielectric constant and form a high-quality protective coating on the surface of the negative electrode 12, thereby contributing to improved charge-discharge cycle performance. The cyclic carbonate includes a cyclic carbonate having a melting point exceeding 0°C. The nonaqueous electrolyte contains, for example, only a cyclic carbonate having a melting point exceeding 0°C as the cyclic carbonate.

[0060] The chain carbonate includes a first chain carbonate having a melting point of 0°C or less and a second chain carbonate having a melting point of above 0°C. Use of the second chain carbonate can reduce the viscosity of the non-aqueous electrolyte, particularly in a room temperature environment, and achieve good discharge performance. The first chain carbonate is preferably present in a smaller amount than the second chain carbonate, preferably 30% by mass or less relative to the total mass of the chain carbonates. On the other hand, the first chain carbonate is preferably present in an amount of 15% by mass or more, more preferably 20% by mass or more, relative to the total mass of the chain carbonates. In this case, good discharge performance can be more effectively achieved in both room temperature and low temperature environments. A preferred mass ratio of the first chain carbonate to the second chain carbonate is 15:85 to 30:70, or 20:80 to 25:75.

[0061] Although even a small amount of cyclic carbonate contributes to improving charge-discharge cycle performance, its content is preferably 10% by mass or more, more preferably 15% by mass or more, relative to the mass of the nonaqueous solvent. On the other hand, if the amount of cyclic carbonate is too large, the viscosity of the nonaqueous electrolyte increases, so the content of cyclic carbonate is preferably 30% by mass or less. The preferred mass ratio of cyclic carbonate to chain carbonate is 10:90 to 30:70, 15:85 to 28:72, or 18:82 to 27:73. The nonaqueous electrolyte of this embodiment substantially contains only cyclic carbonate and chain carbonate as the nonaqueous solvent.

[0062] The nonaqueous electrolyte preferably contains at least ethylene carbonate (EC) as a cyclic carbonate, and may further contain fluoroethylene carbonate (FEC). The combined use of EC and FEC can more effectively improve the charge-discharge cycle performance of the battery. The nonaqueous electrolyte of this embodiment contains substantially only EC, or only EC and FEC, as the cyclic carbonate.

[0063] When EC and FEC are used in combination as the cyclic carbonate, the preferred mass ratio of EC to FEC is 63:37 to 87:13, or 65:35 to 85:15. If the FEC content is too high, gas is likely to be generated due to decomposition of the non-aqueous electrolyte. If the mass ratio of EC to FEC is within this range, good charge-discharge cycle performance can be achieved while suppressing gas generation. Note that the addition of FEC is thought to have no substantial effect on discharge performance at room temperature or in low-temperature environments.

[0064] The melting point of the first chain carbonate is preferably -20°C or lower, and more preferably -40°C or lower. Specifically, it is preferably at least one selected from ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). In particular, it is preferable to use EMC alone as the first chain carbonate. By using a predetermined amount of EMC, the above effect becomes more pronounced. Furthermore, the second chain carbonate is preferably dimethyl carbonate (DMC). As described above, the preferred mass ratio of EMC to DMC is 10:90 to 30:70, or 15:85 to 27:73. In this case, good discharge performance in both room temperature and low temperature environments can be more effectively achieved.

[0065] The non-aqueous electrolyte may contain additives such as unsaturated carbonates such as vinylene carbonate, acid anhydrides such as succinic anhydride, isocyanate compounds such as toluene diisocyanate, phenol compounds, benzene compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0066] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0067] <Experimental Example 1> [Fabrication of Positive Electrode] As a positive electrode active material, LiNi ９０ Co ５ Mn ５ O ２ A lithium transition metal composite oxide represented by the formula (I) was used. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 98:1:1, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode mixture slurry was applied to both sides of a positive electrode core made of aluminum foil by a doctor blade method, and the coating was dried and compressed using a rolling roller. Thereafter, the positive electrode core was cut to a predetermined electrode size, and a positive electrode having a positive electrode mixture layer formed on both sides of the positive electrode core was obtained.

[0068] [Fabrication of Negative Electrode] As the negative electrode active material, natural graphite C having a D50 of 18.3 μm and SiO Ｘ A mixture of the negative electrode active material and a Si-containing material represented by (X = 1) in a mass ratio of 93:7 was used. The negative electrode active material, a dispersion of SBR, and CMC-Na were mixed in a solid content mass ratio of 98:1:1, and water was used as the dispersion medium to prepare a first negative electrode mixture slurry. Furthermore, as the negative electrode active material, amorphous carbon-coated artificial graphite B having a D50 of 17 μm, natural graphite C, and SiO ｘ A second negative electrode mixture slurry was prepared in the same manner as the first negative electrode mixture slurry, except that a mixture of the first negative electrode mixture slurry and a Si-containing material represented by (X=1) was used in a mass ratio of 46.5:46.5:7.

[0069] The first negative electrode mixture slurry was applied to both sides of a copper foil negative electrode core by a doctor blade method to form a first coating film, and after drying the first coating film, the second negative electrode mixture slurry was applied to the first coating film to form a second coating film, and the second coating film was dried. The coating film was then compressed using a rolling roller, and the negative electrode core was cut to a predetermined electrode size, resulting in a negative electrode in which a two-layer structure negative electrode mixture layer including a first layer (lower layer) and a second layer (upper layer) was formed on both sides of the negative electrode core. The thickness ratio of the upper layer to the lower layer was 50:50.

[0070] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a mass ratio of 17.7:6.7:22.8:52.7 to prepare a mixed solvent in which the ratio of EC to the mass of cyclic carbonate (EC + FEC) (EC / (EC + FEC)) was 0.73, the ratio of cyclic carbonate to the mass of cyclic and chain carbonate (cyclic carbonate / (cyclic carbonate + chain carbonate)) was 0.24, and the ratio of EMC to the total mass of EMC and DMC (EMC / (EMC + DMC)) was 0.30. LiPF ６ A non-aqueous electrolyte solution was obtained by adding 1.35 mol / L of ethylenediaminetetraacetic acid (EC) and 0.02 mol / L of LiFSI, and vinylene carbonate (VC) to a concentration of 1.5% by mass. Table 1 shows the composition of the non-aqueous electrolyte solution. The mass ratio of EC to FEC was 73:23, the mass ratio of cyclic carbonate to chain carbonate was 24:76, and the mass ratio of EMC to DMC was 30:70.

[0071] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] The positive electrode, in which the positive electrode lead was welded to the positive electrode core, and the negative electrode, in which the negative electrode lead was welded to the negative electrode core, were spirally wound with a separator interposed therebetween to prepare a wound electrode assembly. Insulating plates were placed on the top and bottom of the electrode assembly, and the negative electrode lead was welded to the inner bottom of a cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member, and the electrode assembly was then housed in the outer can. The non-aqueous electrolyte was injected into the outer can under reduced pressure, and the opening of the outer can was sealed with a sealing member via a gasket to obtain test cell X1.

[0072] Examples 2 and 3 Test cells X2 and X3 were prepared in the same manner as in Example 1, except that VC was not used in preparing the nonaqueous electrolyte solution and the mixing ratio of each component constituting the mixed solvent was changed to the values ​​shown in Table 1.

[0073] Example 4 A test cell X4 was produced in the same manner as in Example 1, except that in preparing the nonaqueous electrolyte solution, propene sultone (PRS) was used at a concentration of 1.0 mass % instead of VC, and the mixing ratios of the components constituting the mixed solvent were changed to the values ​​shown in Table 1.

[0074] Examples 5 and 6 Test cells X5 and X6 were produced in the same manner as in Example 1, except that VC and FEC were not used in preparing the nonaqueous electrolyte solution, and the mixing ratios of the components constituting the mixed solvent were changed to the values ​​shown in Table 1.

[0075] Example 7 A test cell X7 was produced in the same manner as in Example 1, except that in preparing the nonaqueous electrolyte solution, VC was not used, diethyl carbonate (DEC) was used instead of EMC, and the mixing ratios of the components constituting the mixed solvent were changed to the values ​​shown in Table 1.

[0076] Example 8 A test cell X8 was produced in the same manner as in Example 1, except that VC and LiFSI were not used in preparing the nonaqueous electrolyte solution, and the mixing ratios of the components constituting the mixed solvent were changed to the values ​​shown in Table 1.

[0077] Comparative Examples 1 and 2 Test cells Y1 and Y2 were prepared in the same manner as in Example 1, except that VC was not used in preparing the nonaqueous electrolyte solution and the mixing ratio of each component constituting the mixed solvent was changed to the values ​​shown in Table 1.

[0078] The performance of each test cell of the examples and comparative examples was evaluated by the following method, and the evaluation results are shown in Table 1 together with the composition of the non-aqueous electrolyte.

[0079] [Evaluation of low-temperature discharge characteristics] In a temperature environment of 25°C, the test cell to be evaluated was subjected to low-voltage charging until the state of charge (SOC) reached 100%, and then constant-current discharge was performed until the SOC reached 20%. Subsequently, in a temperature environment of -20°C, constant-power discharge was performed for 10 seconds at a constant power of 15 W. Whether or not the cell could be discharged for 10 seconds was used as an evaluation index.

[0080] [Evaluation of Direct Current Resistance (DCIR)] In a temperature environment of 25°C, the test cell to be evaluated was charged at a constant current until the state of charge (SOC) reached 50%. Subsequently, a constant voltage discharge was performed for 10 seconds at a constant current of 0.5 C. The difference between the open circuit voltage (OCV) and the closed circuit voltage (CCV) 10 seconds after discharge was divided by the discharge current 10 seconds after discharge to calculate the DCIR (mΩ) at 25°C. Similar charge and discharge were also performed in a temperature environment of -20°C, and the DCIR (mΩ) at -20°C was calculated.

[0081]

[0082] As shown in Table 1, the test cells of the examples have a DCIR at 25 ° C. equivalent to that of test cell Y2 of Comparative Example 2, and a DCIR at -20 ° C. significantly lower than that of test cell Y2. Furthermore, the test cells of the examples have a DCIR at 25 ° C. significantly lower than that of test cell Y1 of Comparative Example 1. Test cell Y2 of Comparative Example 2, in which the EMC content in the non-aqueous electrolyte is less than 10% by mass, had a high DCIR at -20 ° C., making discharge difficult in the evaluation of low-temperature discharge characteristics. Furthermore, test cell Y1 of Comparative Example 1, in which the EMC content in the non-aqueous electrolyte is greater than 24% by mass, had a low DCIR at -20 ° C., but a high DCIR at 25 ° C., resulting in poor discharge performance in a room-temperature environment. From the evaluation results shown in Table 1, it can be seen that the test cells of the examples can effectively achieve both discharge performance at room temperature and low-temperature environments.

[0083] Experimental Example 9 [Fabrication of Negative Electrode] Amorphous carbon-coated natural graphite A having a D50 of 18.2 μm and SiO Ｘ A mixture of the negative electrode active material, a dispersion of SBR, and CMC-Na in a solid content mass ratio of 98:1:1 was used, and a first negative electrode mixture slurry was prepared using water as a dispersion medium. Furthermore, amorphous carbon-coated natural graphite A, amorphous carbon-coated artificial graphite B, and SiO were used as negative electrode active materials. ｘA second negative electrode mixture slurry was prepared in the same manner as the first negative electrode mixture slurry, except that a mixture of the first negative electrode mixture slurry and a Si-containing material represented by (X=1) was used in a mass ratio of 46.5:46.5:7.

[0084] The first negative electrode mixture slurry was applied to both sides of a copper foil negative electrode core by a doctor blade method to form a first coating film, and after drying the first coating film, the second negative electrode mixture slurry was applied to the first coating film to form a second coating film, and the second coating film was dried. The coating film was then compressed using a rolling roller, and the negative electrode core was cut to a predetermined electrode size, resulting in a negative electrode in which a two-layer structure negative electrode mixture layer including a first layer (lower layer) and a second layer (upper layer) was formed on both sides of the negative electrode core. The thickness ratio of the upper layer to the lower layer was 40:60.

[0085] Test cell X9 was produced in the same manner as in Example 2, except that the electrode body was produced using the above negative electrode.

[0086] Experimental Example 10 A test cell X10 was produced in the same manner as in Example 9, except that in preparing the first negative electrode mixture slurry, natural graphite C was used instead of amorphous carbon-coated natural graphite A. The thickness ratio of the upper layer to the lower layer was 45:55.

[0087] Experimental Example 11 A test cell X11 was produced in the same manner as in Example 10, except that no Si-containing material was used in preparing the first and second negative electrode mixture slurries. The thickness ratio of the upper layer to the lower layer was 45:55.

[0088] Reference Example 1 A test cell R1 was produced in the same manner as in Example 2, except that in preparing the second negative electrode mixture slurry, artificial graphite D having a D50 of 17.5 μm was used instead of natural graphite A and amorphous carbon-coated artificial graphite B. The thickness ratio of the upper layer to the lower layer was 50:50.

[0089] The performance of each test cell of the Examples and Reference Examples was evaluated by the following method, and the evaluation results, along with the structure of the negative electrode, are shown in Table 2. The tortuosity of the negative electrode mixture layer was calculated by the method described above.

[0090]

[0091] As shown in Table 2, the test cells of the examples using amorphous carbon-coated graphite in the upper layer of the negative electrode mixture layer had lower DCIR at 25°C and superior discharge performance in a room temperature environment compared to test cell R1 of Reference Example 1. In particular, when the content of amorphous carbon-coated graphite in the upper layer was increased (test cells X9 to X11 of Examples 9 to 11), DCIR at 25°C could be more effectively reduced. Furthermore, when amorphous carbon-coated graphite was used only in the upper layer and natural graphite without an amorphous carbon coating was used in the lower layer (test cells X10 and X11), the tortuosity of the negative electrode mixture layer was smaller, and the DCIR reduction effect was more significant.

[0092] The present disclosure is further described by the following embodiments. Configuration 1: A nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and an electrolyte solution containing a lithium salt and a nonaqueous solvent, wherein the nonaqueous solvent contains a cyclic carbonate and a chain carbonate, and the chain carbonate contains a first chain carbonate having a melting point of 0°C or lower in an amount of 10% by mass or more and 24% by mass or less, relative to the mass of the nonaqueous solvent. Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein the mass ratio of the cyclic carbonate to the chain carbonate is 10:90 to 30:70. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the chain carbonate includes a second chain carbonate having a melting point above 0°C, and the mass ratio of the first chain carbonate to the second chain carbonate is 15:85 to 30:70. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the first chain carbonate is ethyl methyl carbonate and the second chain carbonate is dimethyl carbonate.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the cyclic carbonate includes ethylene carbonate and fluoroethylene carbonate, and the mass ratio of the ethylene carbonate to the fluoroethylene carbonate is 63:37 to 87:13.Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the negative electrode has a negative electrode core, a first negative electrode mixture layer provided on the negative electrode core, and a second negative electrode mixture layer provided on the negative electrode core via the first negative electrode mixture layer, and the second negative electrode mixture layer contains graphite whose particle surfaces are coated with amorphous carbon in an amount of 50 mass% or more relative to the mass of the negative electrode active material. Configuration 7: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the first and second negative electrode mixture layers contain a Si-containing material as the negative electrode active material.

[0093] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Negative electrode core, 31 First negative electrode mixture layer, 32 Second negative electrode mixture layer, 33 Negative electrode mixture layer

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte solution containing a lithium salt and a non-aqueous solvent, wherein the non-aqueous solvent contains a cyclic carbonate and a chain carbonate, and the chain carbonate contains a first chain carbonate having a melting point of 0°C or lower in an amount of 10% by mass or more and 24% by mass or less relative to the mass of the non-aqueous solvent.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the mass ratio of said cyclic carbonate to said chain carbonate is 10:90 to 30:

70.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the chain carbonate includes a second chain carbonate having a melting point exceeding 0°C, and the mass ratio of the first chain carbonate to the second chain carbonate is 15:85 to 30:

70.

4. The nonaqueous electrolyte secondary battery according to claim 3, wherein the first chain carbonate is ethyl methyl carbonate, and the second chain carbonate is dimethyl carbonate.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the cyclic carbonate includes ethylene carbonate and fluoroethylene carbonate, and the mass ratio of the ethylene carbonate to the fluoroethylene carbonate is 63:37 to 87:

13.

6. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the negative electrode comprises a negative electrode core, a first negative electrode mixture layer provided on the negative electrode core, and a second negative electrode mixture layer provided on the negative electrode core via the first negative electrode mixture layer, and the second negative electrode mixture layer contains graphite whose particle surfaces are coated with amorphous carbon in an amount of 50 mass% or more relative to the mass of the negative electrode active material.

7. The nonaqueous electrolyte secondary battery according to claim 6, wherein the first and second negative electrode mixture layers contain a Si-containing material as the negative electrode active material.

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