Non-aqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode has a negative electrode core and a negative electrode composite material layer formed on the surface of the negative electrode core. The negative electrode composite material layer contains a negative electrode active material and a third amorphous carbon as a conductive material, the negative electrode active material forms a coating layer containing a first amorphous carbon and a second amorphous carbon on the surface of a graphite particle, and the pore volume is 0.5 ml / g or less. The nonaqueous electrolyte contains a difluorophosphate and a lithium salt having an oxalate complex as an anion.

Description

Non-aqueous electrolyte secondary battery Technical Field

[0001] This application relates to non-aqueous electrolyte secondary batteries. Background Technology

[0002] Previously, non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, were widely used as power sources for mobile information terminals like phones and laptops. In addition, non-aqueous electrolyte secondary batteries are also used as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs). The negative electrode active material of non-aqueous electrolyte secondary batteries typically uses highly crystalline carbon materials such as natural graphite or artificial graphite; or amorphous carbon materials.

[0003] In non-aqueous electrolyte secondary batteries, the negative electrode active material and the non-aqueous electrolyte significantly affect battery performance such as low-temperature characteristics and durability. For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery that improves battery durability (storage characteristics, cycle characteristics) by using lithium dioxaborate and lithium difluorophosphate as additives in the electrolyte. Patent Document 2 discloses 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 composite material layer containing a negative electrode active material. The negative electrode active material comprises graphite particles coated with a coating layer containing a first amorphous carbon and a second amorphous carbon. The negative electrode composite material layer comprises the graphite particles and a third amorphous carbon as a conductive material. The non-aqueous electrolyte comprises difluorophosphate and a lithium salt with an oxalate complex as an anion.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2007-180015

[0007] Patent Document 2: Japanese Patent Application Publication No. 2018-163833 Summary of the Invention

[0008] The problem the invention aims to solve

[0009] In previous non-aqueous electrolyte secondary batteries, including Patent Document 1 and Patent Document 2, there is still room for improvement in terms of low-temperature characteristics and durability. In addition, lithium deposition sometimes occurs in the negative electrode, and there is still room for improvement in suppressing lithium deposition.

[0010] The non-aqueous electrolyte secondary battery described in this application is a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte. The aforementioned negative electrode has a negative electrode core and a negative electrode composite material layer formed on the surface of the aforementioned negative electrode core. The aforementioned negative electrode composite material layer contains a negative electrode active material and a third amorphous carbon as a conductive material. The negative electrode active material forms a covering layer containing a first amorphous carbon and a second amorphous carbon on the surface of graphite particles, and the pore capacity is less than 0.5 ml / g. The non-aqueous electrolyte contains difluorophosphate and lithium salt with oxalate complex as an anion.

[0011] The non-aqueous electrolyte secondary battery described in this application is less prone to lithium deposition and exhibits excellent low-temperature characteristics and durability. Attached Figure Description

[0012] Figure 1 is a perspective view showing the appearance of a non-aqueous electrolyte secondary battery as an example of an embodiment.

[0013] Figure 2 is a perspective view of an electrode body as an example of an implementation method.

[0014] Figure 3 is a cross-sectional view of an electrode body as an example of an embodiment. Detailed Implementation

[0015] As described above, it can be assumed that by adding lithium oxalate borate and lithium difluorophosphate to the non-aqueous electrolyte, a coating derived from them can be formed on the surface of the negative electrode active material, thereby improving battery durability. However, according to the research results of the inventors, this coating increases the resistance of the negative electrode, hindering the smooth absorption of lithium ions into the negative electrode active material, and lithium is easily deposited on the negative electrode surface.

[0016] In order to solve the above-mentioned problems, the inventors conducted in-depth research and found that in a non-aqueous electrolyte secondary battery containing difluorophosphate and lithium salt with oxalate complex as anion, by using first to third amorphous carbon in the negative electrode and controlling the pore capacity of the negative electrode active material to below 0.5 ml / g, lithium deposition is highly suppressed, and low-temperature characteristics and durability are greatly improved.

[0017] The three types of amorphous carbon enhance the electronic conductivity of the negative electrode and suppress the increase in plate resistance caused by coating formation, playing a crucial role in suppressing lithium deposition and improving low-temperature characteristics and durability. Furthermore, setting the pore size of the negative electrode active material to below 0.5 ml / g specifically improves these properties. This can be attributed to the fact that reducing the pore size increases the electronic conductivity within the particles of the negative electrode active material and reduces the amount of electrolyte penetrating into the particles, thus suppressing side reactions.

[0018] Hereinafter, with reference to the accompanying drawings, a detailed description will be provided for one embodiment of the non-aqueous electrolyte secondary battery described in this application. It should be noted that from the outset, it is envisioned that the various embodiments and modifications illustrated below be selectively combined. Furthermore, in this specification, unless otherwise specified, the designation "numerical value A to numerical value B" means "numerical value A or higher and numerical value B or lower".

[0019] Figure 1 is a perspective view showing the appearance of a non-aqueous electrolyte secondary battery 10 as an example of an embodiment, and Figure 2 is a perspective view of the electrode body 11 constituting the non-aqueous electrolyte secondary battery 10. In the non-aqueous electrolyte secondary battery 10 shown in Figure 1, the outer casing includes a bottom-shaped cylindrical outer can 14, but the outer casing is not limited to this. The non-aqueous electrolyte secondary battery described in this application can be, for example, a cylindrical battery with a bottom-shaped cylindrical outer can, a coin-shaped battery with a coin-shaped outer can, or a laminated battery with an outer casing composed of a laminate containing a metal layer and a resin layer.

[0020] As shown in Figures 1 and 2, the non-aqueous electrolyte secondary battery 10 includes an electrode body 11, a non-aqueous electrolyte, a bottomed rectangular outer container 14 for housing the electrode body 11 and the non-aqueous electrolyte, and a sealing plate 15 for sealing the opening of the outer container 14. The non-aqueous electrolyte secondary battery 10 is a so-called square battery. The electrode body 11 has a wound structure formed by winding a positive electrode 20 and a negative electrode 30 with a separator 40 between them. The positive electrode 20, the negative electrode 30, and the separator 40 are all strip-shaped bodies, and the positive electrode 20 and the negative electrode 30 are stacked with the separator 40 between them and wound around a winding shaft. It should be noted that the electrode body can be a stacked type in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators between them.

[0021] The non-aqueous electrolyte secondary battery 10 includes a positive terminal 12 electrically connected to a positive electrode 20 via a positive current collector 25, and a negative terminal 13 electrically connected to a negative electrode 30 via a negative current collector 35. In this embodiment, the sealing plate 15 has an elongated rectangular shape, with the positive terminal 12 disposed at one end along the length of the sealing plate 15 and the negative terminal 13 disposed at the other end along the length of the sealing plate 15. The positive terminal 12 and the negative terminal 13 are external connection terminals for electrical connection with other non-aqueous electrolyte secondary batteries 10, various electronic devices, etc., and are mounted on the sealing plate 15 by means of insulating members.

[0022] For ease of explanation, the height direction of the outer can 14 is defined as the "vertical direction" of the non-aqueous electrolyte secondary battery 10, the side of the sealing plate 15 is defined as "up", and the bottom side of the outer can 14 is defined as "down". In addition, the direction along the length of the sealing plate 15 is defined as the "lateral direction" of the non-aqueous electrolyte secondary battery 10.

[0023] The outer can 14 is a bottomed, rectangular metal container. An opening formed at the top of the outer can 14 is sealed by, for example, welding a sealing plate 15 to the edge of the opening. The sealing plate 15 typically includes an injection port 16 for injecting non-aqueous electrolyte, an exhaust valve 17 for opening a valve to release gas in case of battery malfunction, and a current-blocking mechanism. The outer can 14 and the sealing plate 15 are made of, for example, a metallic material with aluminum as the main component.

[0024] The electrode body 11 is a flat, wound-type electrode body including a flat portion and a pair of curved portions. The electrode body 11 is housed in the outer can 14 with its width direction along the battery height direction and the pair of curved portions arranged side-by-side along the winding axis direction. In this embodiment, a current collector is formed at one axial end of the electrode body 11 to form a positive electrode side where the core exposed portion 23 of the positive electrode 20 is stacked, and at the other axial end to form a negative electrode side where the core exposed portion 33 of the negative electrode 30 is stacked. Each current collector is electrically connected to a terminal via a current collector. It should be noted that an insulating electrode body frame (insulating sheet) can be disposed between the electrode body 11 and the inner surface of the outer can 14.

[0025] Hereinafter, with reference to FIG3, the positive electrode 20, the negative electrode 30 and the separator 40 constituting the electrode body 11 will be described in detail, especially the negative electrode 30. In addition, the non-aqueous electrolyte will be described in detail.

[0026] [positive electrode]

[0027] As shown in Figure 3, the positive electrode 20 has a positive electrode core 21 and a positive electrode composite material layer 22 formed on the surface of the positive electrode core 21. The positive electrode core 21 can be a foil of a metal that is stable within the potential range of the positive electrode 20, such as aluminum or aluminum alloy, or a thin film of the metal disposed on the surface. The positive electrode composite material layer 22 preferably contains a positive electrode active material, a conductive material, and a binder material, and is formed on both sides of the positive electrode core 21. In this embodiment, a core exposed portion 23 is formed at one end of the positive electrode 20 in the width direction, which protrudes from the core surface along the length direction. The positive electrode 20 can be manufactured, for example, by coating a positive electrode composite material slurry containing a positive electrode active material, a conductive material, and a binder material onto the positive electrode core 21, drying the coating, and then compressing it to form the positive electrode composite material layer 22 on both sides of the positive electrode core 21.

[0028] Lithium transition metal composite oxides can be used as positive electrode active materials. Examples of metal elements contained in lithium transition metal composite oxides include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Preferably, at least one of Ni, Co, and Mn is included. 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.

[0029] Examples of conductive materials included in the positive electrode composite layer 22 include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. Examples of binders included in the positive electrode composite layer 22 include fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF); polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. Alternatively, these resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, or polyethylene oxide (PEO).

[0030] [negative electrode]

[0031] The negative electrode 30 has a negative electrode core 31 and a negative electrode composite material layer 32 formed on the surface of the negative electrode core 31. The negative electrode core 31 can be a foil of a metal that is stable within the potential range of the negative electrode 30, such as copper, or a thin film of the metal disposed on the surface. In this embodiment, a core exposed portion 33 is formed at one end of the negative electrode 30 in the width direction, which protrudes from the core surface along the length direction. Furthermore, the positive electrode 20 and the negative electrode 30 are stacked with the core exposed portions 23 and 33 located on opposite sides of the axial direction of the electrode body 11, separated by a separator 40. The negative electrode 30 can be manufactured, for example, by coating a negative electrode composite material slurry containing a negative electrode active material onto the negative electrode core 31, drying the coating, and then compressing it to form the negative electrode composite material layer 32 on both sides of the negative electrode core 31.

[0032] The negative electrode composite material layer 32 comprises a negative electrode active material and a third amorphous carbon as a conductive material. The negative electrode active material forms a capping layer containing a first and a second amorphous carbon on the surface of the graphite particles, and the pore size is less than 0.5 ml / g. Furthermore, the negative electrode composite material layer 32 preferably includes a binder material and is formed on both sides of the negative electrode core 31. The graphite constituting the negative electrode active material is natural graphite such as flake graphite, block graphite, or amorphous graphite; or artificial graphite such as blocky graphite (MAG) or graphitized mesophase carbon microspheres (MCMB). It should be noted that, as the negative electrode active material, metals such as Si and Sn alloyed with lithium, or their compounds, can also be used in combination.

[0033] As described above, the negative electrode active material is a core-shell particle with graphite particles as the core and a capping layer containing first amorphous carbon and second amorphous carbon as the shell. The capping layer may contain other materials without impairing the purpose of this application, or it may be substantially composed only of first amorphous carbon and second amorphous carbon. Furthermore, the capping layer has a structure in which second amorphous carbon particles are dispersed within the layered first amorphous carbon. For example, the first amorphous carbon is formed over a wide area on the surface of the graphite particles, and the second amorphous carbon is dispersed on the surface of the graphite particles.

[0034] The first amorphous carbon is preferably present in an amount of 0.5 to 8% by mass relative to the negative electrode active material, more preferably 1 to 5% by mass. Furthermore, the second amorphous carbon is preferably present in an amount of 1 to 15% by mass relative to the negative electrode active material, more preferably 2 to 10% by mass. The content of the second amorphous carbon may be equal to or less than the content of the first amorphous carbon, but is preferably more than the content of the first amorphous carbon.

[0035] The first amorphous carbon can be, for example, a calcined product of asphalt (petroleum asphalt, coal tar pitch), a calcined product of resins that undergo carbonization such as phenolic resin, or a calcined product of heavy oil. Among these, a calcined product of asphalt is preferred. It should be noted that the first amorphous carbon can be formed on the surface of graphite particles using a CVD method employing acetylene, methane, etc. The first amorphous carbon also functions as a binder material for fixing the second amorphous carbon to the surface of the graphite particles.

[0036] The second amorphous carbon preferably has higher conductivity than the first amorphous carbon. The second amorphous carbon has particulate shapes such as granular (spherical), blocky, needle-like, or fibrous. The second amorphous carbon can be, for example, acetylene black, Ketjen black, or carbon black. Carbon black is preferred. The higher conductivity of the second amorphous carbon compared to the first amorphous carbon more effectively improves the electronic conductivity of the negative electrode 30.

[0037] The median particle size (hereinafter referred to as "D50") of the negative electrode active material in a volume-based manner is, for example, 3 μm to 30 μm, preferably 5 μm to 15 μm. The negative electrode composite material layer 32 may contain two or more active materials with different D50s. D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution reaches 50% from the smaller particle size side, and is also known as the median particle size. The particle size distribution of the negative electrode active material can be measured using a laser diffraction particle size distribution measuring device (e.g., MICROTRAC-BELL MT3000II) with water as the dispersion medium.

[0038] The negative electrode active material has pores within the graphite particles. By using first to third amorphous carbon and controlling the pore capacity of the negative electrode active material to below 0.5 ml / g, lithium deposition is highly suppressed, and low-temperature characteristics and durability are specifically improved. The pore capacity of the negative electrode active material can be measured using a mercury porosimeter (MICROMERITICS, AUTOPORE IV9510 model).

[0039] The lower limit of the pore capacity of the negative electrode active material is not particularly limited, but is preferably 0.01 ml / g, more preferably 0.05 ml / g. Suitable pore capacity ranges are, for example, 0.01–0.5 ml / g or 0.05–0.5 ml / g. The pore capacity of the negative electrode active material can be adjusted to below 0.5 ml / g by, for example, compressing the graphite particles with a stronger force than that used in the compression process of the negative electrode composite layer 32 to flatten the pores. The compression of the graphite particles is preferably performed before the formation of the capping layer.

[0040] The negative electrode active material can be manufactured, for example, by attaching a first and amorphous carbon to the surface of graphite particles whose porosity has been reduced due to compression, followed by calcination of the mixture. The graphite particles and amorphous carbon can be mixed using existing known mixers, such as rotary mixers like planetary mills, air-jet mixers, screw mixers, and kneaders. Calcination is carried out for several hours, for example, in an inactive atmosphere at a temperature of 700°C to 900°C. It should be noted that through this calcination, the asphalt carbonizes, resulting in a mass reduction of approximately 30%.

[0041] As described above, the negative electrode composite material layer 32 comprises a binder and a third amorphous carbon as a conductive material. The conductive material may contain other materials without prejudice to the purpose of this application, or may consist substantially only of the third amorphous carbon. The third amorphous carbon is used in the same manner as, for example, the second amorphous carbon, such as acetylene black, Ketjen black, or carbon black. The second and third amorphous carbons may use the same materials. The content of the third amorphous carbon relative to the mass of the negative electrode composite material layer 32 is preferably 1 to 10% by mass, more preferably 2 to 5% by mass.

[0042] The binder material included in the negative electrode composite layer 32 is similar to that in the positive electrode 20; it can also be fluoropolymer, PAN, polyimide, acrylic resin, polyolefin, etc., with styrene-butadiene rubber (SBR) being preferred. Furthermore, the negative electrode composite layer 32 preferably also includes CMC or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc. It is suitable to use SBR in combination with CMC or its salts, or PAA or its salts.

[0043] [Separator]

[0044] The separator 40 can be a porous sheet with ion permeability and insulation. Specific examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 40 include polyethylene, polypropylene, copolymers of ethylene and α-olefins, cellulose, etc. The separator 40 can be either a single-layer structure or a multilayer structure. A heat-resistant layer containing inorganic particles or a heat-resistant layer composed of resins with high heat resistance, such as aromatic polyamide resins, polyimides, and polyamide-imides, can be formed on the surface of the separator 40.

[0045] [Non-aqueous electrolytes]

[0046] Non-aqueous electrolytes comprise non-aqueous solvents and electrolyte salts. Non-aqueous solvents can include, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these solvents. Non-aqueous solvents may contain halogen-substituted derivatives, where at least a portion of the hydrogen atoms in these solvents are replaced by halogen atoms such as fluorine. Examples of halogen-substituted derivatives include fluorocyclic carbonates such as fluoroethylene carbonate (FEC), fluorochain carbonates, and fluorochain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0047] Examples of the aforementioned esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butyl 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. Preferably, at least one solvent selected from EC, EMC, and DMC is used, and a mixture of EC, EMC, and DMC is particularly preferred.

[0048] Examples of the aforementioned ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-eucalyptol, and crown ethers; and cyclic ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, and dihexyl ether. Ethers, including ethyl vinyl ether, butyl vinyl ether, methyl phenyl 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, tetraethylene glycol dimethyl ether, and other chain ethers.

[0049] The non-aqueous electrolyte further contains difluorophosphate and a lithium salt having an oxalate complex as an anion. By adding these to the non-aqueous electrolyte, a protective film is formed on the surface of the negative electrode active material, and the durability of the battery is improved. When the protective film is formed, problems such as an increase in the resistance of the electrode plate, a decrease in the low-temperature characteristics, and easy precipitation of lithium may be considered. However, by improving the negative electrode 30 as described above, these problems can be addressed, and good battery performance can be obtained. It should be noted that the difluorophosphate and the lithium salt having an oxalate complex as an anion are dissolved in the non-aqueous solvent.

[0050] The difluorophosphate contains a counter cation selected from, for example, lithium, sodium, potassium, magnesium, and calcium. Among them, lithium difluorophosphate (LiPF2O2) having lithium as a counter cation is preferred. It should be noted that lithium difluorophosphate may be coordinated with other compounds, or other difluorophosphates may be used in combination.

[0051] 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. If 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, and the durability of the battery is improved. The concentration of the difluorophosphate is preferably lower than the concentration of the lithium salt having an oxalate complex as an anion.

[0052] Examples of the lithium salt having an oxalate complex as an anion include lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium tris(oxalato)phosphate, lithium difluoro-bis(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, etc. Among them, lithium bis(oxalato)borate (LiBOB) is preferred.

[0053] The concentration of the lithium salt having an oxalate complex as an 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, and the durability of the battery is improved. The concentration of the lithium salt having an oxalate complex as an anion is preferably higher than the concentration of the difluorophosphate, for example, 1.5 to 3 times the concentration of the difluorophosphate.

[0054] The non-aqueous electrolyte preferably further contains other lithium salts as electrolyte salts on the basis of containing the above-mentioned lithium salts such as LiPF2O2 and LiBOB. Specific examples of the other lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB10 Cl 10 Borates such as LiCl, LiBr, LiI, lithium chloroborane, lower aliphatic carboxylic acids, and Li₂B₄O₇ are preferred. LiPF₆ is the most preferred. The concentration of LiPF₆ is preferably higher than that of LiPF₂O₂ and LiBOB, for example, 0.5M to 1.5M.

[0055] <Example>

[0056] The present application is further illustrated below by way of examples, but the present application is not limited to these examples.

[0057] <Example 1>

[0058] [The production of the positive electrode]

[0059] As the positive electrode active material, LiNi is used 0.35 Co 0.35 Mn 0.30 The lithium nickel cobalt manganese composite oxide shown in O2. Positive electrode active material, polyvinylidene fluoride, and carbon black were mixed in a solid component mass ratio of 91:3:6, using N-methyl-2-pyrrolidone (NMP) as the dispersion medium to prepare a positive electrode composite slurry. Next, portions for connecting positive electrode leads were left on both sides of the positive electrode core formed from aluminum foil, and the positive electrode composite slurry was coated on these portions. After drying and calendering, the coating was cut to the specified electrode size, resulting in a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core. It should be noted that the filling density of the positive electrode composite layer was set to 2.65 g / cm³. 3 .

[0060] [Preparation of negative electrode active material]

[0061] Graphite particles, formed by modifying natural graphite into spherical shapes, are compressed with a stronger force than the calendering process described later for the negative electrode, thereby flattening the pores within the particles. Then, carbon black (a second conductive material) is mixed and mechanically fused, causing the carbon black to adhere to the surface of the graphite particles. Next, the graphite particles with carbon black adhering to their surface are mixed with pitch, causing the pitch to adhere to the particle surface. At this point, the mass ratio of graphite particles, pitch, and carbon black is set to 90:3:7. Next, this mixture is calcined at 1250°C in an inactive gas atmosphere for 24 hours, and then the calcined material is crushed to obtain a negative electrode active material with a coating layer containing carbon black and pitch formed on the surface of the graphite particles.

[0062] The negative electrode active material has a D50 of 9 μm and a pore capacity of 0.4 ml / g. As mentioned above, the pore capacity of the negative electrode active material was calculated using a mercury pressure porosimeter (MICROMERITICS, AUTOPORE IV9510 model) based on the amount of mercury injected when the pressure was increased from 4 kPa to 400 MPa.

[0063] During the calcination process of the mixture, the pitch carbonizes, reducing its mass by approximately 30%, but the mass of the graphite particles and carbon black remains essentially unchanged. A coating layer is formed on the surface of the graphite particles by binding the carbon black particles with the calcined pitch (carbide). That is, the surface of the graphite particles is covered by a coating layer formed from the calcined pitch, in which carbon black is dispersed.

[0064] [Making the negative electrode]

[0065] The obtained negative electrode active material, carbon black (third conductive material), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a solid component mass ratio of 94.45:4.45:0.7:0.4, using water as the dispersion medium to prepare a negative electrode composite slurry. Next, portions for connecting negative electrode leads were left on both sides of the negative electrode core formed from copper foil, and the negative electrode composite slurry was coated on these portions. After drying and calendering the coating, it was cut into specified electrode sizes, resulting in a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core. It should be noted that the filling density of the negative electrode composite layer was set to 1.10 g / cm³. 3 .

[0066] The filler density of the negative electrode composite layer is calculated as follows: a 10cm section is cut from the negative electrode. 2 After preparing the sample sheet, the mass A and thickness C of the sample sheet were measured, and the core was measured at 10 cm. 2 The mass B and thickness D are calculated using the following formula.

[0067] Filling density (g / ml) = (AB) / [(CD) × 10cm 2 ]

[0068] [Preparation of non-aqueous electrolytes]

[0069] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4 (25°C, 1 atm). LiPF6, LiPF2O2, and LiBOB were added to the mixed solvent at concentrations of 1.2M, 0.05M, and 0.10M, respectively. Then, ethylene carbonate was added at a concentration of 0.3% by mass relative to the total mass of the non-aqueous electrolyte to prepare the non-aqueous electrolyte.

[0070] [Construction of a non-aqueous electrolyte secondary battery]

[0071] By winding the fabricated positive and negative electrodes together with a polyolefin separator and pressing them into a flat shape, a flat, wound electrode body is obtained. The positive and negative electrodes are wound such that the core of the positive electrode is exposed at one end along the winding axis of the electrode body, and the core of the negative electrode is exposed at the other end.

[0072] An external insulating member is disposed on the outer surface of the battery around the positive terminal mounting hole in the sealing plate, and an internal insulating member and the base of the positive current collector are disposed on the inner surface of the battery around the positive terminal mounting hole. The positive terminal is inserted from the outside of the battery into the through hole in the external insulating member, the positive terminal mounting hole, the through hole in the internal insulating member, and the through hole in the base of the positive current collector, and the front end of the positive terminal is riveted to the base of the positive current collector. Thus, the positive terminal and the positive current collector are fixed to the sealing plate. The riveted portion of the positive terminal is then welded to the base.

[0073] An external insulating member is disposed on the outer surface of the battery around the negative terminal mounting hole in the sealing plate, and an internal insulating member and the base of the negative current collector are disposed on the inner surface of the battery around the negative terminal mounting hole. The negative terminal is inserted from the outside of the battery into the through hole in the external insulating member, the negative terminal mounting hole, the through hole in the internal insulating member, and the through hole in the base of the negative current collector, and the front end of the negative terminal is riveted to the base of the negative current collector. Thus, the negative terminal and the negative current collector are fixed to the sealing plate. Furthermore, the riveted portion of the negative terminal is welded to the base.

[0074] Next, the positive current collector is welded to the exposed part of the positive electrode core, and the negative current collector is welded to the exposed part of the negative electrode core. The electrode body with the current collector is then covered with a resin sheet and inserted into a square outer can. A sealing plate is welded to the edge of the outer can's opening to seal it. Non-aqueous electrolyte is then injected through the injection hole in the sealing plate, and the injection hole is sealed with a sealing plug. This yields a non-aqueous electrolyte secondary battery with a capacity of 5.5 Ah.

[0075] The performance of the fabricated non-aqueous electrolyte secondary battery was evaluated using 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 values ​​of Comparative Example 1 battery are set to 100.

[0076] [Evaluation of Low-Temperature Properties]

[0077] The non-aqueous electrolyte secondary battery was charged to 50% state of charge (SOC) at 25°C. Then, at -30°C, it was charged for 10 seconds at constant currents of 1.6 It, 3.2 It, 4.8 It, 6.4 It, 8.0 It, and 9.6 It, respectively, and the battery voltage was measured. The battery voltage was plotted relative to each current value, and the low-temperature regeneration characteristics (electrical power (W) at 4.3V charging) were calculated by multiplying the current value by the battery voltage value (4.3V).

[0078] [Evaluation of Lithium Deposition]

[0079] The non-aqueous electrolyte secondary battery was charged to 60% SOC at 25°C. Then, at 25°C, it was charged at a constant current of 38 ItO for 10 seconds, discharged at a constant current of 6.8 ItO for 55.9 seconds, followed by a 300-second pause. This was considered one cycle, and 1000 charge-discharge cycles were performed. Afterward, the battery was disassembled, and the presence of lithium deposition on the negative electrode surface was visually confirmed.

[0080] Evaluation of Cyclic Performance (Capacity Retention)

[0081] At 25°C, the battery was charged with a constant current of 1 It until the battery voltage reached 4.1V. Then, it was charged with a constant voltage of 4.1V for 1.5 hours. After a 10-second pause, it was discharged with a constant current of 1 It until the battery voltage reached 2.5V. The discharge capacity at this point is set as the battery capacity before high-temperature cycling.

[0082] Next, the battery was charged at a constant current of 2It at 60°C until the battery voltage reached 4.1V. After a 10-second pause, it was discharged at a constant current of 2It until the battery voltage reached 3.0V. This was considered one cycle, and 400 charge-discharge cycles were performed. After 400 cycles, the battery was charged at a constant current of 1It at 25°C until the battery voltage reached 4.1V. It was then charged at a constant voltage of 4.1V for 1.5 hours. After a 10-second pause, it was discharged at a constant current of 1It until the battery voltage reached 2.5V. The discharge capacity at this point was taken as the battery capacity after high-temperature cycling, and the capacity retention rate was calculated using the following formula.

[0083] Capacity retention = Battery capacity after high-temperature cycling / Battery capacity before high-temperature cycling

[0084] [Evaluation of preservation characteristics (capacity retention after preservation test)]

[0085] At 25°C, charge the battery with a constant current of 1 It until the battery voltage reaches 4.1V. Then, charge it with a constant voltage of 4.1V for 1.5 hours. After a 10-second pause, discharge the battery with a constant current of 1 It until the battery voltage reaches 2.5V. Set the discharge capacity at this point as the battery capacity before storage.

[0086] Next, the battery was charged to 80% SOC at 25°C and stored at 70°C for 56 days. Afterward, the battery was discharged to 2.5V. Then, it was charged with a constant current of 1It until the battery voltage reached 4.1V, and then charged at a constant voltage of 4.1V for 1.5 hours. Afterward, it was discharged with a constant current of 1It until the battery voltage reached 2.5V. The discharge capacity at this point is taken as the battery capacity after storage, and the capacity retention rate after the storage test is calculated using the following formula.

[0087] Capacity retention rate = Battery capacity after storage / Battery capacity before storage

[0088] <Example 2>

[0089] 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. In the preparation of the negative electrode composite slurry, the negative electrode active material, carbon black, CMC and SBR were mixed in a solid component mass ratio of 93.46:5.44:0.7:0.4. Otherwise, the same procedure as in Example 1 was followed to prepare a negative electrode and a non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0090] <Example 3>

[0091] 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. In the preparation of the negative electrode composite slurry, the negative electrode active material, carbon black, CMC and SBR were mixed in a solid component mass ratio of 95.44:3.46:0.7:0.4. Otherwise, the same procedure as in Example 1 was followed to prepare the negative electrode and non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0092] <Example 4>

[0093] In the preparation of the negative electrode active material, graphite particles were compressed to achieve a pore capacity of 0.5 ml / g. Otherwise, the same procedure as in Example 1 was followed to prepare the negative electrode and non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0094] <Example 5>

[0095] In the preparation of the negative electrode active material, graphite particles were compressed to achieve a pore capacity of 0.1 ml / g. Otherwise, the same procedure as in Example 1 was followed to prepare the negative electrode and non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0096] <Comparative Example 1>

[0097] In the preparation of the negative electrode active material, the graphite particles were not compressed (pore capacity was 0.8 ml / g), and the graphite particles were mixed with pitch at 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 at a solid component mass ratio of 98.9:0.7:0.4. Furthermore, in the preparation of the non-aqueous electrolyte, LiPF2O2 and LiBOB were not added. Otherwise, the same procedures as in Example 1 were followed to prepare the negative electrode and non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0098] <Comparative Example 2>

[0099] In the preparation of the negative electrode active material, graphite particles were compressed to achieve a pore capacity of 0.4 ml / g. Otherwise, the same procedure as in Comparative Example 1 was followed to prepare a negative electrode and a non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0100] <Comparative Example 3>

[0101] In the preparation of the non-aqueous electrolyte, LiPF2O2 and LiBOB were added at concentrations of 0.05M and 0.10M, respectively. Otherwise, the same procedure as in Comparative Example 1 was followed to prepare a negative electrode and a non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0102] <Comparative Example 4>

[0103] In the preparation of the negative electrode active material, graphite particles, pitch and carbon black were mixed in a mass ratio of 90:3:7. Otherwise, the same procedure as in Comparative Example 2 was followed to prepare a negative electrode and a non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0104] <Comparative Example 5>

[0105] In the preparation of the negative electrode composite slurry, the negative electrode active material, carbon black, CMC and SBR were mixed in a solid component mass ratio of 94.45:4.45:0.7:0.4. Otherwise, the same procedure as in Comparative Example 2 was followed to prepare a negative electrode and a non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0106] <Comparative Example 6>

[0107] In the preparation of the non-aqueous electrolyte, LiPF2O2 and LiBOB were added at concentrations of 0.05M and 0.10M, respectively. Otherwise, the same procedure as in Comparative Example 2 was followed to prepare the negative electrode and the non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0108] <Comparative Example 7>

[0109] In the preparation of the non-aqueous electrolyte, LiPF2O2 and LiBOB were not added. Otherwise, the same procedure as in Example 1 was followed to fabricate the negative electrode and non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0110] <Comparative Example 8>

[0111] In the preparation of the negative electrode composite slurry, no carbon black was added. The negative electrode active material, CMC and SBR were mixed in a solid component mass ratio of 98.9:0.7:0.4. Otherwise, the same procedure as in Example 1 was followed to prepare a negative electrode and a non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0112] <Comparative Example 9>

[0113] In the preparation of the negative electrode active material, no carbon black was added. Graphite particles and pitch were mixed at a mass ratio of 98:2. Otherwise, the same procedure as in Example 1 was followed to prepare the negative electrode and non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0114] <Comparative Example 10>

[0115] In the preparation of the negative electrode active material, the graphite particles were not compressed (pore capacity was 0.8 ml / g). Otherwise, the same procedure as in Example 1 was followed to prepare the negative electrode and non-aqueous electrolyte secondary battery, and the performance was evaluated.

[0116] [Table 1]

[0117]

[0118] As shown in Table 1, compared with the comparative example batteries, the example batteries exhibit superior low-temperature characteristics and durability (cycle characteristics, storage characteristics). Furthermore, lithium deposition was observed on the negative electrode surface in the comparative example batteries, but not in the example batteries. In other words, by using a non-aqueous electrolyte containing LiPF2O2 and LiBOB, a coating layer containing calcined pitch and carbon black formed on the surface of graphite particles, a negative electrode active material with a pore capacity of 0.5 ml / g or less, and the addition of carbon black to the negative electrode composite layer, a non-aqueous electrolyte secondary battery that specifically improves storage and low-temperature characteristics and highly suppresses lithium deposition can be obtained.

[0119] The comparison examples were examined as follows.

[0120] Comparative Example 2: Compared to the battery in Comparative Example 1, the reduced pore capacity of the negative electrode active material leads to improved electron conductivity and low-temperature performance. Furthermore, reduced side reactions with the electrolyte result in improved cycle and storage characteristics. However, its characteristics differ significantly from those of the battery in the examples.

[0121] Comparative Example 3: Compared to the battery in Comparative Example 1, the cycle characteristics and storage characteristics were improved by adding LiPF2O2 and LiBOB to the non-aqueous electrolyte. However, the coating became a resistive component, resulting in decreased low-temperature performance.

[0122] Comparative Example 4: Compared with the battery in Comparative Example 2, the addition of carbon black to the coating layer improves the electronic conductivity of the negative electrode plate and the low-temperature characteristics, but increases the side reactions with the electrolyte and reduces the storage characteristics.

[0123] Comparative Example 5: Compared with the battery in Comparative Example 2, the addition of carbon black to the negative electrode composite material layer improved the electronic conductivity and low-temperature characteristics of the negative electrode, but increased the side reactions with the electrolyte and reduced the storage characteristics.

[0124] Comparative Example 6: Compared to the battery in Comparative Example 2, the cycle characteristics and storage characteristics were improved by adding LiPF2O2 and LiBOB to the non-aqueous electrolyte. However, the coating became a resistive component, resulting in decreased low-temperature performance.

[0125] Comparative Example 7: Compared with the battery of Comparative Example 2, by adding carbon black to the capping layer and the negative electrode composite material layer, the electronic conductivity of the negative electrode is improved and the low-temperature characteristics are improved, but the side reactions with the electrolyte increase and the storage characteristics decrease.

[0126] Comparative Example 8: Compared to the battery of Comparative Example 6, the addition of carbon black to the coating layer improves the electronic conductivity of the negative electrode plate, as well as its low-temperature characteristics, cycle characteristics, and storage characteristics. However, its characteristics differ significantly from those of the battery in the Examples.

[0127] Comparative Example 9: Compared to the battery in Comparative Example 6, the addition of carbon black to the negative electrode composite material layer improved the electronic conductivity of the negative electrode, as well as its low-temperature characteristics, cycle characteristics, and storage characteristics. However, its characteristics differed significantly from those of the battery in the Examples.

[0128] Comparative Example 10: This battery has the same structure as the battery of Example 1, except that the pore volume of the negative electrode active material exceeds 0.5 ml / g. However, compared to the battery of Example 1, its low-temperature characteristics, cycle characteristics, and storage characteristics are poor. Furthermore, lithium deposition was observed on the surface of the negative electrode.

[0129] Explanation of reference numerals in the attached figures

[0130] 10 Non-aqueous electrolyte secondary batteries

[0131] 11 Electrode Body

[0132] 12 Positive Extremes

[0133] 13 Negative extremes

[0134] 14 Outer packaging tanks

[0135] 15 Sealing board

[0136] 16 Liquid injection part

[0137] 17. Exhaust valve

[0138] 20 Positive Electrode

[0139] 21 Positive electrode core

[0140] 22 Positive electrode composite material layer

[0141] 23, 33 Core Exposed Parts

[0142] 25 Positive current collector

[0143] 30 Negative electrode

[0144] 31 Negative electrode core

[0145] 32 Negative electrode composite material layer

[0146] 35 Negative current collector

[0147] 40 Separators

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode core and a negative electrode composite material layer formed on the surface of the negative electrode core, the negative electrode composite material layer comprising a negative electrode active material and a third amorphous carbon as a conductive material, wherein the negative electrode active material has a capping layer containing a first amorphous carbon and a second amorphous carbon formed on the surface of graphite particles, and the pore capacity is 0.01 ml / g or more and 0.5 ml / g or less, and the graphite constituting the negative electrode active material is compressed graphite. The first amorphous carbon is a calcined asphalt product, and the first amorphous carbon exists in an amount of 0.5 to 8% by mass relative to the negative electrode active material. The second amorphous carbon and the third amorphous carbon are carbon black, the second amorphous carbon exists in an amount of 1 to 15% by mass relative to the negative electrode active material, and the third amorphous carbon exists in an amount of 1 to 10% by mass relative to the negative electrode composite material layer. The non-aqueous electrolyte contains difluorophosphate and lithium salt with oxalate complex as an anion.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein, The covering layer has a structure in which particles of the second amorphous carbon are dispersed in the first amorphous carbon formed in a layered manner.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The conductivity of the second amorphous carbon is higher than that of the first amorphous carbon.

4. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The difluorophosphate is lithium difluorophosphate.

5. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The lithium salt with oxalate complex as an anion is lithium bis(oxalateborate).

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