Non-aqueous electrolyte secondary batteries

Abstract

This nonaqueous electrolyte secondary battery is provided with a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode has a negative electrode core body and a negative electrode mixture layer formed on the surface of the negative electrode core body. The negative electrode mixture layer contains: a negative electrode active material which has a pore volume of 0.5 ml / g or less and in which a coating layer containing a first amorphous carbon and a second amorphous carbon is formed on the surfaces of graphite particles; and a third amorphous carbon serving as a conductive material. The nonaqueous electrolyte contains difluorophosphate and a lithium salt including an oxalate complex as an anion.

Description

[Technical Field]

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

[0002] Conventionally, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been widely used as driving power sources for mobile information terminals such as mobile phones and laptops. Non-aqueous electrolyte secondary batteries are also used as driving power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs), etc. Generally, highly crystalline carbon materials such as natural graphite and artificial graphite, or amorphous carbon materials, are used as the negative electrode active material for non-aqueous electrolyte secondary batteries.

[0003] In non-aqueous electrolyte secondary batteries, the negative electrode active material and non-aqueous electrolyte have a significant effect on battery performance, such as low-temperature characteristics and durability. For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery in which the durability (storage characteristics and cycle characteristics) of the battery is improved by using lithium bis(oxalato)borate and lithium difluorophosphate as additives to the electrolyte. Furthermore, Patent Document 2 discloses a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode having a negative electrode composite layer containing a negative electrode active material, and a non-aqueous electrolyte, in which the negative electrode active material includes coated graphite particles in which the surfaces of graphite particles are coated with a coating layer containing first and second amorphous carbons, the negative electrode composite layer includes the coated graphite particles and a third amorphous carbon as a conductive material, and the non-aqueous electrolyte includes a difluorophosphate and a lithium salt having an oxalate complex as an anion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-180015 [Patent Document 2] Japanese Patent Application Publication No. 2018-163833 Summary of the Invention

[0005] Conventional nonaqueous electrolyte secondary batteries, including those described in Patent Documents 1 and 2, still have room for improvement in terms of low-temperature characteristics and durability. In addition, lithium deposition may occur in the negative electrode, and there is also room for improvement in suppressing lithium deposition.

[0006] 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, wherein the negative electrode has a negative electrode core and a negative electrode composite layer formed on the surface of the negative electrode core, the negative electrode composite layer including a negative electrode active material having a pore volume of 0.5 ml / g or less and a coating layer containing first amorphous carbon and second amorphous carbon formed on the surface of graphite particles, and third amorphous carbon as a conductive material, and the nonaqueous electrolyte includes a lithium salt having a difluorophosphate and an oxalate complex as an anion.

[0007] The nonaqueous electrolyte secondary battery according to the present disclosure is less susceptible to lithium deposition and has excellent low-temperature characteristics and durability. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view showing the appearance of a nonaqueous electrolyte secondary battery as an example of the embodiment. [Figure 2] FIG. 2 is a perspective view of an electrode assembly according to an embodiment. [Figure 3] FIG. 3 is a cross-sectional view of an electrode assembly according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] As described above, it is believed that adding lithium bis(oxalato)borate and lithium difluorophosphate to a non-aqueous electrolyte forms a coating on the surface of the negative electrode active material, improving the durability of the battery. However, as a result of investigations by the present inventors, it was found that such a coating increases the resistance of the negative electrode, hinders smooth absorption of lithium ions into the negative electrode active material, and makes it easier for lithium to deposit on the negative electrode surface.

[0010] As a result of intensive research aimed at solving the above problems, the present inventors have found that in a nonaqueous electrolyte secondary battery containing a lithium salt having difluorophosphate and oxalate complexes as anions, by using the first to third amorphous carbons in the negative electrode and controlling the pore volume of the negative electrode active material to 0.5 ml / g or less, lithium deposition is highly suppressed and low-temperature characteristics and durability are greatly improved.

[0011] The three types of amorphous carbon improve the electron conductivity of the anode, suppressing the increase in electrode resistance due to the formation of a coating, and play an important role in suppressing lithium deposition and improving low-temperature characteristics and durability. Furthermore, these properties are significantly improved when the pore volume of the anode active material is reduced to 0.5 ml / g or less. This is thought to be because reducing the pore volume increases the electron conductivity within the particles of the anode active material and reduces the amount of electrolyte that penetrates into the particles, thereby suppressing side reactions.

[0012] 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. It is anticipated from the beginning that multiple embodiments and variations exemplified below may be selectively combined. Furthermore, in this specification, the expression "numerical value A to numerical value B" means "numerical value A or more and numerical value B or less" unless otherwise specified.

[0013] Fig. 1 is a perspective view showing the appearance of a nonaqueous electrolyte secondary battery 10 according to an embodiment, and Fig. 2 is a perspective view of an electrode assembly 11 constituting the nonaqueous electrolyte secondary battery 10. The nonaqueous electrolyte secondary battery 10 shown in Fig. 1 includes a bottomed, rectangular cylindrical outer can 14 as an outer casing, but the outer casing is not limited to this. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a cylindrical battery including a bottomed, cylindrical outer can, a coin-shaped battery including a coin-shaped outer can, or a laminate battery including an outer casing made of a laminate sheet including a metal layer and a resin layer.

[0014] As shown in FIGS. 1 and 2 , a nonaqueous electrolyte secondary battery 10 includes an electrode assembly 11, a nonaqueous electrolyte, a bottomed rectangular cylindrical outer can 14 that houses the electrode assembly 11 and the nonaqueous electrolyte solution, and a sealing plate 15 that closes the opening of the outer can 14. The nonaqueous electrolyte secondary battery 10 is a so-called prismatic battery. The electrode assembly 11 has a wound structure in which a positive electrode 20 and a negative electrode 30 are wound with a separator 40 interposed therebetween. The positive electrode 20, the negative electrode 30, and the separator 40 are all strip-shaped, long, strip-like bodies, and the positive electrode 20 and the negative electrode 30 are stacked with the separator 40 interposed therebetween and wound around a winding axis. The electrode assembly may also be a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with separators interposed therebetween.

[0015] The nonaqueous electrolyte secondary battery 10 includes a positive electrode terminal 12 electrically connected to the positive electrode 20 via a positive electrode current collector 25, and a negative electrode terminal 13 electrically connected to the negative electrode 30 via a negative electrode current collector 35. In this embodiment, the sealing plate 15 has an elongated rectangular shape, with the positive electrode terminal 12 disposed at one longitudinal end of the sealing plate 15 and the negative electrode terminal 13 disposed at the other longitudinal end of the sealing plate 15. The positive electrode terminal 12 and the negative electrode terminal 13 are external connection terminals electrically connected to other nonaqueous electrolyte secondary batteries 10, various electronic devices, etc., and are attached to the sealing plate 15 via insulating members.

[0016] For ease of explanation, the height direction of the outer can 14 will be referred to as the "vertical direction" of the nonaqueous electrolyte secondary battery 10, the sealing plate 15 side will be referred to as the "top," and the bottom side of the outer can 14 will be referred to as the "bottom." Additionally, the direction along the longitudinal direction of the sealing plate 15 will be referred to as the "lateral direction" of the nonaqueous electrolyte secondary battery 10.

[0017] The outer can 14 is a metal container in the shape of a rectangular cylinder with a bottom. An opening formed at the top end of the outer can 14 is closed, for example, by welding a sealing plate 15 to the edge of the opening. The sealing plate 15 is generally provided with a liquid injection portion 16 for injecting a non-aqueous electrolyte, a gas exhaust valve 17 that opens to exhaust gas in the event of a battery abnormality, and a current interruption mechanism. The outer can 14 and the sealing plate 15 are made of a metal material containing, for example, aluminum as a main component.

[0018] The electrode body 11 is a flat, wound electrode body including a flat portion and a pair of curved portions. The electrode body 11 is housed in the outer can 14 with the winding axis direction aligned with the lateral direction of the outer can 14 and the width direction of the electrode body 11, along which the pair of curved portions are aligned, aligned with the height direction of the battery. In this embodiment, a positive electrode side current collector formed by laminating the substrate exposed portion 23 of the positive electrode 20 at one axial end of the electrode body 11 and a negative electrode side current collector formed by laminating the substrate exposed portion 33 of the negative electrode 30 at the other axial end are formed, and each current collector is electrically connected to a terminal via a current collector. Note that an insulating electrode body holder (insulating sheet) may be disposed between the electrode body 11 and the inner surface of the outer can 14.

[0019] 3, the positive electrode 20, negative electrode 30, and separator 40 that constitute the electrode assembly 11, particularly the negative electrode 30, will be described in detail below. The non-aqueous electrolyte will also be described in detail.

[0020] [Positive electrode] As shown in FIG. 3 , the positive electrode 20 includes a positive electrode core 21 and a positive electrode composite layer 22 formed on the surface of the positive electrode core 21. The positive electrode core 21 can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 20, or a film with such a metal disposed on its surface. The positive electrode composite layer 22 contains a positive electrode active material, a conductive material, and a binder, and is preferably formed on both sides of the positive electrode core 21. In this embodiment, a core exposed portion 23, in which the core surface is exposed along the longitudinal direction, is formed at one end in the width direction of the positive electrode 20. The positive electrode 20 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a conductive material, a binder, and the like onto the positive electrode core 21, drying the coating, and then compressing it to form the positive electrode composite layer 22 on both sides of the positive electrode core 21.

[0021] A lithium transition metal composite oxide is used as the positive electrode active material. Metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al.

[0022] Examples of the conductive material contained in the positive electrode mixture layer 22 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 22 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resin, acrylic resin, and polyolefin resin. These resins may also be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO).

[0023] [Negative electrode] The negative electrode 30 has a negative electrode core 31 and a negative electrode composite layer 32 formed on the surface of the negative electrode core 31. The negative electrode core 31 can be a foil of a metal such as copper that is stable within the potential range of the negative electrode 30, or a film with such a metal disposed on its surface. In this embodiment, a core exposed portion 33, in which the core surface is exposed along the longitudinal direction, is formed at one end in the width direction of the negative electrode 30. The positive electrode 20 and the negative electrode 30 are stacked via a separator 40 so that the core exposed portions 23, 33 are located on opposite sides of the electrode body 11 in the axial direction. The negative electrode 30 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material and the like onto the negative electrode core 31, drying the coating, and then compressing it to form negative electrode composite layers 32 on both sides of the negative electrode core 31.

[0024] The negative electrode mixture layer 32 includes a negative electrode active material having a pore volume of 0.5 ml / g or less, in which a coating layer containing first and second amorphous carbons is formed on the surface of graphite particles, and a third amorphous carbon as a conductive material. The negative electrode mixture layer 32 preferably includes a binder and is formed on both sides of the negative electrode substrate 31. The graphite constituting the negative electrode active material is natural graphite such as flake graphite, lump graphite, or amorphous graphite, or artificial graphite such as lump artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB). Metals that alloy with lithium, such as Si and Sn, or compounds thereof, may also be used as the negative electrode active material.

[0025] As described above, the negative electrode active material is a core-shell particle having a graphite particle as the core and a coating layer containing first and second amorphous carbon as the shell. The coating layer may contain other materials as long as the object of the present disclosure is not impaired, or may be composed essentially of the first and second amorphous carbons. The coating layer has a structure in which second amorphous carbon particles are dispersed in the first amorphous carbon formed in a layer. For example, the first amorphous carbon is formed over a wide area on the surface of the graphite particle, and the second amorphous carbon is scattered on the surface of the graphite particle.

[0026] The first amorphous carbon is present in an amount of preferably 0.5 to 8 mass% and more preferably 1 to 5 mass% relative to the mass of the negative electrode active material. The second amorphous carbon is present in an amount of preferably 1 to 15 mass% and more preferably 2 to 10 mass% relative to the mass of the negative electrode active material. The content of the second amorphous carbon may be equal to or less than the content of the first amorphous carbon, but is preferably greater than the content of the first amorphous carbon.

[0027] The first amorphous carbon may be, for example, a burned product of pitch (petroleum pitch, coal pitch), a burned product of a carbonizable resin such as a phenolic resin, or a burned product of heavy oil. Among these, a burned product of pitch is preferred. The first amorphous carbon may be formed on the surface of graphite particles by a CVD method using acetylene, methane, or the like. The first amorphous carbon also functions as a binder that fixes the second amorphous carbon to the surface of the graphite particles.

[0028] The second amorphous carbon preferably has a higher electrical conductivity than the first amorphous carbon. The second amorphous carbon has a particle shape such as granular (spherical), blocky, needle-like, or fibrous. Examples of the second amorphous carbon include acetylene black, ketjen black, and carbon black. Of these, carbon black is preferred. The second amorphous carbon has a higher electrical conductivity than the first amorphous carbon, and more effectively improves the electronic conductivity of the negative electrode 30.

[0029] The volume-based median diameter (hereinafter referred to as "D50") of the negative electrode active material is, for example, 3 μm to 30 μm, and preferably 5 μm to 15 μm. The negative electrode composite layer 32 may contain two or more active materials with different D50s. D50 refers to the particle size at which the cumulative frequency of particles in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the negative electrode active material can be measured using a laser diffraction particle size distribution analyzer (for example, MT3000II manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0030] The negative electrode active material has voids within the graphite particles, but by using the first to third amorphous carbons and controlling the pore volume of the negative electrode active material to 0.5 ml / g or less, lithium precipitation is highly suppressed, resulting in significant improvements in low-temperature characteristics and durability. The pore volume of the negative electrode active material can be measured using a mercury porosimeter (Autopore IV9510, manufactured by MicroMetitex).

[0031] The lower limit of the pore volume of the negative electrode active material is not particularly limited, but is preferably 0.01 ml / g, and more preferably 0.05 ml / g. A suitable range of the pore volume is, for example, 0.01 to 0.5 ml / g, or 0.05 to 0.5 ml / g. The pore volume of the negative electrode active material can be adjusted to 0.5 ml / g or less, for example, by compressing the graphite particles with a force stronger than that used in the compression step of the negative electrode mixture layer 32 to eliminate voids. The compression of the graphite particles is preferably performed before forming the coating layer.

[0032] The negative electrode active material can be produced, for example, by adhering first and second amorphous carbons to the surfaces of graphite particles whose void volume has been reduced by compression, and then calcining the mixture. A conventional mixer can be used to mix the graphite particles and amorphous carbon, and examples include a rotating container mixer such as a planetary ball mill, an airflow mixer, a screw blender, and a kneader. The calcination is carried out, for example, in an inert atmosphere at a temperature of 700°C to 900°C for several hours. The calcination carbonizes the pitch, reducing its mass by approximately 30%.

[0033] As described above, the negative electrode mixture layer 32 contains a third amorphous carbon as a conductive material and a binder. The conductive material may contain other materials as long as the purpose of the present disclosure is not impaired, or may be composed essentially of the third amorphous carbon alone. As with the second amorphous carbon, the third amorphous carbon may be, for example, acetylene black, ketjen black, carbon black, or the like. The second and third amorphous carbons may be made of the same material. The content of the third amorphous carbon is preferably 1 to 10 mass % and more preferably 2 to 5 mass % relative to the mass of the negative electrode mixture layer 32.

[0034] As in the case of the positive electrode 20, the binder contained in the negative electrode mixture layer 32 can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene-butadiene rubber (SBR). Furthermore, it is preferable that the negative electrode mixture layer 32 further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use a combination of SBR with CMC or a salt thereof, or PAA or a salt thereof.

[0035] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 40. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 40 include polyethylene, polypropylene, polyolefins such as copolymers of ethylene and α-olefins, and cellulose. The separator 40 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 40.

[0036] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0037] 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. Among these, it is preferable to use at least one selected from EC, EMC, and DMC, and it is particularly preferable to use a mixed solvent of EC, EMC, and DMC.

[0038] 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, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, and the like. Examples of suitable ethers include chain ethers such as ethyl ether, 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.

[0039] The non-aqueous electrolyte further contains a difluorophosphate salt and a lithium salt having an oxalate complex as the anion. Adding these to the non-aqueous electrolyte forms a protective coating on the surface of the negative electrode active material, improving the durability of the battery. The formation of a protective coating can lead to problems such as increased plate resistance, reduced low-temperature characteristics, and increased lithium deposition. However, by improving the negative electrode 30 as described above, these problems can be addressed, resulting in excellent battery performance. The difluorophosphate salt and the lithium salt having an oxalate complex as the anion are dissolved in the non-aqueous solvent.

[0040] The difluorophosphate salt contains a counter cation selected from, for example, lithium, sodium, potassium, magnesium, and calcium. Among them, lithium difluorophosphate (LiPF2O2) having lithium as the counter cation is preferred. Note that other compounds may be coordinated to the lithium difluorophosphate salt, and other difluorophosphate salts may be used in combination.

[0041] The concentration of the difluorophosphate is preferably 0.01 M to 0.20 M, more preferably 0.02 M to 0.15 M, and particularly preferably 0.03 M to 0.10 M. When the concentration of the difluorophosphate is within this range, a high-quality protective film is formed on the surface of the negative electrode active material, improving the durability of the battery. The concentration of the difluorophosphate is preferably lower than the concentration of the lithium salt having an oxalate complex as the anion.

[0042] Examples of lithium salts having an oxalate complex as the anion include lithium bis(oxalate)borate, lithium difluoro(oxalate)borate, lithium tris(oxalate)phosphate, lithium difluoro(bis(oxalate)phosphate, lithium tetrafluoro(oxalate)phosphate, etc. Among these, lithium bis(oxalate)borate (LiBOB) is preferred.

[0043] The concentration of the lithium salt having an oxalate complex as the anion is preferably 0.01 M to 0.50 M, more preferably 0.02 M to 0.30 M, and particularly preferably 0.05 M to 0.20 M. In this case, a high-quality protective film is formed on the surface of the negative electrode active material, improving the durability of the battery. The concentration of the lithium salt having an oxalate complex as the anion is preferably higher than the concentration of the difluorophosphate, for example, 1.5 to 3 times the concentration of the difluorophosphate.

[0044] The non-aqueous electrolyte preferably contains other lithium salts as electrolyte salts in addition to the above lithium salts such as LiPF2O2 and LiBOB. Specific examples of other lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, and LiPF 6-x (C n F 2n+1 ) x (1 <x<6,nは1または2)、LiB 10 Cl 10 Examples of suitable lithium salts include LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylates, and borates such as Li2B4O7. Of these, LiPF6 is preferred. The concentration of LiPF6 is preferably higher than the concentrations of LiPF2O2 and LiBOB, and is, for example, 0.5M to 1.5M.

[0045] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0046] Example 1 [Preparation of positive electrode] The positive electrode active material is LiNi 0.35 Co 0.35 Mn 0.30 A lithium nickel cobalt manganese composite oxide represented by O2 was used. The positive electrode active material, polyvinylidene fluoride, and carbon black were mixed in a solids mass ratio of 91:3:6, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was applied to both sides of a positive electrode core made of aluminum foil, leaving only the area where the positive electrode lead was connected. The coating was then dried and rolled, and cut to the specified electrode size to obtain a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core. The packing density of the positive electrode composite layer was 2.65 g / cm. 3 It was decided.

[0047] [Preparation of negative electrode active material] Graphite particles, which were made by modifying natural graphite into spherical shapes, were compressed with a force stronger than that used in the rolling process for the negative electrode described below to eliminate the voids within the particles. Carbon black (second conductive material) was then mixed and mechanically fused to adhere the carbon black to the surfaces of the graphite particles. Next, the graphite particles with carbon black adhered to their surfaces were mixed with pitch, and the pitch was adhered to the particle surfaces. The mass ratio of the graphite particles, pitch, and carbon black was 90:3:7. This mixture was then fired at 1250°C in an inert gas atmosphere for 24 hours, after which the fired material was crushed to obtain a negative electrode active material in which a coating layer of carbon black and pitch was formed on the surfaces of the graphite particles.

[0048] The negative electrode active material had a D50 of 9 μm and a pore volume of 0.4 ml / g. The pore volume of the negative electrode active material was calculated from the amount of mercury intrusion measured when the pressure was increased from 4 kPa to 400 MPa using a mercury porosimeter (Autopore IV9510, manufactured by Micrometitex).

[0049] During the firing process of the mixture, the pitch is carbonized and its mass is reduced by approximately 30%, but the mass of the graphite particles and carbon black is not reduced at all. The coating layer is formed on the surface of the graphite particles by binding the carbon black particles with the fired pitch (carbonized product). In other words, the surface of the graphite particles is covered with a coating layer made of the fired pitch, and the carbon black is dispersed in the coating layer.

[0050] [Preparation of negative electrode] The obtained negative electrode active material, carbon black (third conductive material), carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed in a solid mass ratio of 94.45:4.45:0.7:0.4, 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 core made of copper foil, leaving only the area where the negative electrode lead was connected. The coating was then dried and rolled, and cut to a predetermined electrode size to obtain a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core. The packing density of the negative electrode composite layer was 1.10 g / cm. 3 It was decided.

[0051] The packing density of the negative electrode mixture layer is 2 After cutting out the sample piece, the mass A and thickness C of the sample piece are measured, and the core is 10 cm 2 The mass B and thickness D of the sample were measured and calculated using the following formula.

[0052] Packing density (g / ml)=(AB) / [(CD)×10cm 2 ] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4 (25°C, 1 atmosphere). LiPF, LiPFO, and LiBOB were added to this mixed solvent to concentrations of 1.2 M, 0.05 M, and 0.10 M, respectively. Vinylene carbonate was then added to a concentration of 0.3 mass% relative to the total mass of the nonaqueous electrolyte to form a nonaqueous electrolyte.

[0053] [Fabrication of non-aqueous electrolyte secondary battery] The positive and negative electrodes were wound with a polyolefin separator between them, and the wound electrode was pressed into a flat shape, so that the exposed portion of the positive electrode core was located at one end of the electrode assembly in the direction of the winding axis, and the exposed portion of the negative electrode core was located at the other end.

[0054] An external insulating member is placed on the outer surface of the battery around the positive terminal mounting hole provided in the sealing plate, and an internal insulating member and the base portion of the positive electrode current collector are placed on the inner surface of the battery around the positive terminal mounting hole. Then, from the outside of the battery, a positive electrode terminal is inserted through the through hole in the external insulating member, the positive electrode terminal mounting hole, the through hole in the internal insulating member, and the through hole in the base portion of the positive electrode current collector, and the tip of the positive electrode terminal is crimped to the base portion of the positive electrode current collector. This secures the positive electrode terminal and positive electrode current collector to the sealing plate. The crimped portion of the positive electrode terminal is then welded to the base portion.

[0055] An external insulating member is placed on the outer surface of the battery around the negative terminal mounting hole provided in the sealing plate, and an internal insulating member and the base portion of the negative electrode current collector are placed on the inner surface of the battery around the negative terminal mounting hole. Then, from the outside of the battery, a negative electrode terminal is inserted through the through hole in the external insulating member, the negative electrode terminal mounting hole, the through hole in the internal insulating member, and the through hole in the base portion of the negative electrode current collector, and the tip of the negative electrode terminal is crimped to the base portion of the negative electrode current collector. This fixes the negative electrode terminal and the negative electrode current collector to the sealing plate. The crimped portion of the negative electrode terminal is then welded to the base portion.

[0056] Next, a positive electrode current collector was welded to the exposed portion of the positive electrode core, and a negative electrode current collector was welded to the exposed portion of the negative electrode core. The electrode assembly with the current collectors attached was then covered with a resin sheet and inserted into a rectangular outer can. A sealing plate was then welded to the periphery of the opening of the outer can to close the opening, and a nonaqueous electrolyte solution was poured through the filling hole in the sealing plate, which was then sealed with a sealing plug. This resulted in a nonaqueous electrolyte secondary battery with a battery capacity of 5.5 Ah.

[0057] The fabricated nonaqueous electrolyte secondary batteries were subjected to performance evaluation by the following method, and the evaluation results are shown in Table 1. The values ​​of low-temperature characteristics, cycle characteristics, and storage characteristics shown in Table 1 are relative values ​​when the value of the battery of Comparative Example 1 is set to 100.

[0058] [Evaluation of low-temperature characteristics] The nonaqueous electrolyte secondary batteries were charged to a state of charge (SOC) of 50% at 25°C. Next, they were charged at constant currents of 1.6 It, 3.2 It, 4.8 It, 6.4 It, 8.0 It, and 9.6 It for 10 seconds each at -30°C, and the battery voltage was measured and plotted against each current value. The low-temperature regeneration characteristics (power (W) when charging to 4.3 V) were calculated as the product of the current value and the battery voltage value (4.3 V).

[0059] [Evaluation of lithium deposition] The nonaqueous electrolyte secondary battery was charged at 25°C until the SOC reached 60%. It was then charged at a constant current of 38 It for 10 seconds at 25°C, discharged at a constant current of 6.8 It for 55.9 seconds, and then rested for 300 seconds. This constitutes one cycle, and 1000 charge / discharge cycles were performed. The battery was then disassembled, and the presence or absence of lithium deposition on the negative electrode surface was visually confirmed.

[0060] [Evaluation of cycle characteristics (capacity retention rate)] At 25°C, the battery was charged at a constant current of 1 It until the battery voltage reached 4.1 V. Then, constant voltage charging was performed at a constant voltage of 4.1 V for 1.5 hours. After a 10-second pause, the battery was discharged at a constant current of 1 It until the battery voltage reached 2.5 V. The discharge capacity at this time was recorded as the battery capacity before the high-temperature cycle.

[0061] Next, the battery was charged at a constant current of 2 It at 60°C until the battery voltage reached 4.1 V. After a 10-second pause, the battery was discharged at a constant current of 2 It until the battery voltage reached 3.0 V. This constitutes one cycle, and 400 charge / discharge cycles were performed. After 400 cycles, the battery was charged at a constant current of 1 It at 25°C until the battery voltage reached 4.1 V. Then, the battery was charged at a constant voltage of 4.1 V for 1.5 hours. After a 10-second pause, the battery was discharged at a constant current of 1 It until the battery voltage reached 2.5 V. The discharge capacity at this time was taken as the battery capacity after the high-temperature cycle, and the capacity retention rate was calculated using the following formula.

[0062] Capacity retention rate = Battery capacity after high-temperature cycling / Battery capacity before high-temperature cycling [Evaluation of storage characteristics (capacity retention rate after storage test)] At 25°C, the battery was charged at a constant current of 1 It until the battery voltage reached 4.1 V. Then, constant voltage charging was performed at a constant voltage of 4.1 V for 1.5 hours. After a 10-second break, the battery was discharged at a constant current of 1 It until the battery voltage reached 2.5 V. The discharge capacity at this time was recorded as the battery capacity before storage.

[0063] Next, the battery was charged at 25°C until the SOC reached 80% and then stored at 70°C for 56 days. The battery was then discharged to 2.5V. Subsequently, the battery was charged at a constant current of 1 It until the battery voltage reached 4.1V, and then charged at a constant voltage of 4.1V for 1.5 hours. The battery was then discharged at a constant current of 1 It until the battery voltage reached 2.5V. The discharge capacity at this time was taken as the battery capacity after storage, and the capacity retention rate after the storage test was calculated using the following formula:

[0064] Capacity retention rate = Battery capacity after storage / Battery capacity before storage <Example 2> A negative electrode and a nonaqueous electrolyte secondary battery were fabricated and their performance was evaluated in the same manner as in Example 1, except that in the preparation of the negative electrode active material, graphite particles, pitch, and carbon black were mixed in a mass ratio of 90:1:9, and in the preparation of the negative electrode composite slurry, the negative electrode active material, carbon black, CMC, and SBR were mixed in a solid content mass ratio of 93.46:5.44:0.7:0.4.

[0065] Example 3 A negative electrode and a nonaqueous electrolyte secondary battery were fabricated and their performance was evaluated in the same manner as in Example 1, except that in the preparation of the negative electrode active material, graphite particles, pitch, and carbon black were mixed in a mass ratio of 90:5:5, and in the preparation of the negative electrode composite slurry, the negative electrode active material, carbon black, CMC, and SBR were mixed in a solid content mass ratio of 95.44:3.46:0.7:0.4.

[0066] Example 4 A negative electrode and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 1, except that in producing the negative electrode active material, the graphite particles were compressed so that the pore volume became 0.5 ml / g, and performance evaluation was carried out.

[0067] <Example 5> A negative electrode and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 1, except that in producing the negative electrode active material, the graphite particles were compressed so that the pore volume became 0.1 ml / g, and performance evaluation was carried out.

[0068] <Comparative Example 1> A negative electrode and a non-aqueous electrolyte secondary battery were fabricated and their performance was evaluated in the same manner as in Example 1, except that in the preparation of the negative electrode active material, the graphite particles were not compressed (pore volume 0.8 ml / g), and the graphite particles and pitch were mixed in a mass ratio of 98:2 (no carbon black was added), in the preparation of the negative electrode composite slurry, no carbon black was added, and the negative electrode active material, CMC, and SBR were mixed in a solid content mass ratio of 98.9:0.7:0.4, and further in the preparation of the non-aqueous electrolyte, LiPF2O2 and LiBOB were not added.

[0069] <Comparative Example 2> A negative electrode and a non-aqueous electrolyte secondary battery were produced in the same manner as in Comparative Example 1, except that in producing the negative electrode active material, the graphite particles were compressed so that the pore volume became 0.4 ml / g, and performance evaluation was carried out.

[0070] <Comparative Example 3> A negative electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Comparative Example 1, except that in preparing the nonaqueous electrolyte, LiPF2O2 and LiBOB were added to give concentrations of 0.05 M and 0.10 M, respectively, and performance evaluations were carried out.

[0071] <Comparative Example 4> A negative electrode and a nonaqueous electrolyte secondary battery were produced in the same manner as in Comparative Example 2, except that in producing the negative electrode active material, graphite particles, pitch, and carbon black were mixed in a mass ratio of 90:3:7, and performance evaluation was carried out.

[0072] <Comparative Example 5> A negative electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Comparative Example 2, except that in preparing the negative electrode mixture slurry, the negative electrode active material, carbon black, CMC, and SBR were mixed in a solid content mass ratio of 94.45:4.45:0.7:0.4, and performance evaluation was performed.

[0073] <Comparative Example 6> A negative electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Comparative Example 2, except that in preparing the nonaqueous electrolyte, LiPF2O2 and LiBOB were added to give concentrations of 0.05 M and 0.10 M, respectively, and performance evaluations were carried out.

[0074] <Comparative Example 7> A negative electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that LiPF2O2 and LiBOB were not added in the preparation of the nonaqueous electrolyte solution, and performance evaluations were carried out.

[0075] <Comparative Example 8> A negative electrode and a nonaqueous electrolyte secondary battery were fabricated and their performance was evaluated in the same manner as in Example 1, except that in preparing the negative electrode composite slurry, carbon black was not added and the negative electrode active material, CMC, and SBR were mixed in a solid content mass ratio of 98.9:0.7:0.4.

[0076] <Comparative Example 9> A negative electrode and a nonaqueous electrolyte secondary battery were produced in the same manner as in Example 1, except that in producing the negative electrode active material, carbon black was not added and graphite particles and pitch were mixed in a mass ratio of 98:2, and performance evaluation was carried out.

[0077] <Comparative Example 10> A negative electrode and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 1, except that the graphite particles were not compressed (pore volume 0.8 ml / g) in producing the negative electrode active material, and their performance was evaluated.

[0078] [Table 1]

[0079] As shown in Table 1, all of the batteries of the examples have superior low-temperature characteristics and durability (cycle characteristics and storage characteristics) compared to the batteries of the comparative examples. Furthermore, lithium deposition was observed on the negative electrode surface of the batteries of the comparative examples, but lithium deposition was not observed in the batteries of the examples. In other words, when a negative electrode active material having a pore volume of 0.5 ml / g or less, in which LiPF2O2 and LiBOB are contained in the nonaqueous electrolyte and a coating layer made of burned pitch and carbon black is formed on the surface of graphite particles, and carbon black is added to the negative electrode composite layer, a nonaqueous electrolyte secondary battery can be obtained that exhibits significantly improved storage characteristics and low-temperature characteristics and highly suppresses lithium deposition.

[0080] The batteries of the comparative examples are considered as follows.

[0081] Comparative Example 2: Compared to the battery of Comparative Example 1, the pore volume of the negative electrode active material was reduced, improving the electronic conductivity within the active material and the low-temperature characteristics. Furthermore, the side reaction with the electrolyte was reduced, improving the cycle characteristics and storage characteristics. However, there was a significant difference in the characteristics compared to the battery of the Example.

[0082] Comparative Example 3: Addition of LiPF2O2 and LiBOB to the non-aqueous electrolyte improved the cycle characteristics and storage characteristics compared to the battery of Comparative Example 1. However, the coating became a resistance component, and the low-temperature characteristics deteriorated.

[0083] Comparative Example 4: Compared to the battery of Comparative Example 2, the addition of carbon black to the coating layer improved the electronic conductivity of the negative electrode plate and the low-temperature characteristics, but side reactions with the electrolyte increased, resulting in a decrease in storage characteristics.

[0084] Comparative Example 5: Compared to the battery of Comparative Example 2, the addition of carbon black to the negative electrode composite layer improved the electronic conductivity of the negative electrode and the low-temperature characteristics, but side reactions with the electrolyte increased, resulting in a decrease in storage characteristics.

[0085] Comparative Example 6: Addition of LiPF2O2 and LiBOB to the non-aqueous electrolyte improved the cycle characteristics and storage characteristics compared to the battery of Comparative Example 2. However, the coating became a resistance component, and the low-temperature characteristics deteriorated.

[0086] Comparative Example 7: Compared to the battery of Comparative Example 2, the addition of carbon black to the coating layer and the negative electrode composite layer improved the electronic conductivity of the negative electrode and the low-temperature characteristics, but side reactions with the electrolyte increased, resulting in a decrease in storage characteristics.

[0087] Comparative Example 8: Compared to the battery of Comparative Example 6, the addition of carbon black to the coating layer improved the electronic conductivity of the negative electrode plate, and improved low-temperature characteristics, cycle characteristics, and storage characteristics. However, there was a significant difference in the characteristics compared to the batteries of the Examples.

[0088] Comparative Example 9: Compared to the battery of Comparative Example 6, the addition of carbon black to the negative electrode composite layer improved the electronic conductivity of the negative electrode, and improved the low-temperature characteristics, cycle characteristics, and storage characteristics. However, there was a significant difference in the characteristics compared to the batteries of the Examples.

[0089] Comparative Example 10: This battery had the same structure as that of Example 1, except that the pore volume of the negative electrode active material exceeded 0.5 ml / g. However, the low-temperature characteristics, cycle characteristics, and storage characteristics were inferior to those of the battery of Example 1. In addition, lithium deposition was observed on the surface of the negative electrode. [Explanation of symbols]

[0090] 10 Nonaqueous electrolyte secondary battery 11 Electrode body 12 Positive terminal 13 Negative terminal 14 Outer can 15 Sealing plate 16 Injection section 17 Gas exhaust valve 20 positive electrode 21 Positive electrode core 22 Positive electrode mixture layer 23,33 Exposed core part 25 Positive electrode current collector 30 negative electrode 31 Negative electrode core 32 Negative electrode composite layer 35 Negative electrode current collector 40 Separator

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, the negative electrode has a negative electrode core and a negative electrode mixture layer formed on a surface of the negative electrode core, the negative electrode mixture layer comprises a negative electrode active material in which a coating layer containing first amorphous carbon and second amorphous carbon is formed on the surface of a graphite particle, and the negative electrode mixture layer has a pore volume of 0.5 ml / g or less as calculated from the amount of mercury intrusion when the pressure is increased from 4 kPa to 400 MPa using a mercury porosimeter, and a third amorphous carbon as a conductive material; the non-aqueous electrolyte contains a lithium salt having a difluorophosphate and an oxalate complex as an anion; the first amorphous carbon is a burned product of pitch, the second amorphous carbon and the third amorphous carbon are carbon black.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the coating layer has a structure in which particles of the second amorphous carbon are dispersed in the first amorphous carbon formed in a layer shape.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the second amorphous carbon has a higher conductivity than the first amorphous carbon.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the difluorophosphate is lithium difluorophosphate.

5. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium salt having an oxalate complex as an anion is lithium bis(oxalate)borate.

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