Non-aqueous electrolyte secondary battery

Abstract

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 current collector and a negative electrode mixed layer formed on a surface of the negative electrode current collector, the negative electrode mixed layer contains artificial graphite particles and natural graphite particles, the negative electrode mixed layer has a first layer disposed on the negative electrode current collector side and a second layer disposed on the first layer, the second layer contains artificial graphite particles and natural graphite particles, a particle shape index X of the artificial graphite particles satisfies 0.3 ≤ X < 0.9 on the second layer, a particle shape index Y of the natural graphite particles satisfies 0.3 ≤ Y < 0.9 in the second layer, the particle shape index X is an average value of a ratio (ax / bx) of a minor axis ax to a major axis bx of the artificial graphite particles, and the particle shape index Y is an average value of a ratio (ay / by) of a minor axis ay to a major axis by of the natural graphite particles.

Description

Nonaqueous electrolyte secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-153950, filed on September 6, 2024, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0003] Patent Document 1 proposes "a negative electrode for a non-aqueous electrolyte secondary battery, comprising a negative electrode core and a negative electrode mixture layer formed on the negative electrode core, wherein the negative electrode mixture layer contains graphite particles as a negative electrode active material and has a lower layer formed on the negative electrode core side and an upper layer formed on the surface side of the negative electrode mixture layer, the graphite particles include first graphite particles having a circularity of less than 0.92 and second graphite particles having a circularity higher than that of the first graphite particles, the ratio of the fracture strength of the first graphite particles to the fracture strength of the second graphite particles is 2 to 5, and the content of the first graphite particles relative to the graphite particles in the upper layer is 30 mass % or more and is higher than the content of the first graphite particles relative to the graphite particles in the lower layer."

[0004] International Publication No. 2022 / 176650

[0005] In the case of Patent Document 1, the porosity in the upper layer of the negative electrode mixture layer decreases, which may result in a decrease in the rapid charge performance of the non-aqueous electrolyte secondary battery. One of the objects of the present disclosure is to improve the rapid charge performance of non-aqueous electrolyte secondary batteries.

[0006] One aspect of the present disclosure provides a battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector, the negative electrode mixture layer including artificial graphite particles and natural graphite particles, the negative electrode mixture layer having a first layer disposed on the negative electrode current collector side and a second layer disposed on the first layer, the second layer including the artificial graphite particles and the natural graphite particles, and in the second layer, the particle shape index X of the artificial graphite particles satisfies 0.3≦X<0.9, and in the second layer, the particle shape index Y of the natural graphite particles satisfies 0.3≦Y<0.9, the particle shape index X being the average value of the ratios (ax / bx) of the minor axis ax to the major axis bx of the artificial graphite particles, and the particle shape index Y being the average value of the ratios (ay / by) of the minor axis ay to the major axis by of the natural graphite particles.

[0007] According to the present disclosure, it is possible to provide a nonaqueous electrolyte secondary battery with excellent rapid charging performance.

[0008] 1 is a longitudinal sectional view schematically illustrating the structure of an example of a nonaqueous electrolyte secondary battery.

[0009] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit.

[0011] The non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. A separator may be disposed between the positive electrode and the negative electrode. The non-aqueous electrolyte secondary battery includes at least a lithium ion secondary battery. The non-aqueous electrolyte of the lithium ion secondary battery has lithium ion conductivity.

[0012] The negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer is usually formed in the form of a film. The negative electrode mixture layer is supported on one or both surfaces of the negative electrode current collector. The negative electrode mixture layer may also be referred to as a "negative electrode active material layer."

[0013] The negative electrode mixture layer is composed of a negative electrode mixture. The negative electrode mixture contains a negative electrode active material as an essential component and may contain optional components such as a binder, a conductive additive, and a thickener. The negative electrode active material reversibly absorbs and releases lithium ions.

[0014] The negative electrode mixture layer is formed, for example, by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied film. The dried coating may be rolled.

[0015] The negative electrode active material according to the present disclosure includes artificial graphite particles and natural graphite particles. The artificial graphite particles are synthesized by heat-treating a carbonaceous raw material in a high-temperature electric furnace or the like. The artificial graphite particles are characterized by easy quality control, ease of reducing impurities, and ease of designing the shape and characteristics. The natural graphite particles are obtained by refining mined natural graphite ore. The carbon mass content of natural graphite particles is generally refined to 99% or more, but the impurity concentration tends to be higher than that of artificial graphite particles. The artificial graphite particles are harder and have higher particle fracture strength than natural graphite particles.

[0016] Generally, graphite particles refer to carbon materials having a (002) plane spacing d002 of, for example, 0.340 nm or less as measured by X-ray diffraction. The crystallite size Lc(002) of the graphite particles as measured by X-ray diffraction is, for example, 5 nm or more, or may be 5 nm or more and 300 nm or less, or 10 nm or more and 200 nm or less.

[0017] Artificial graphite particles and natural graphite particles can be distinguished from each other based on differences in micrographs (appearance), Vickers hardness, impurity type, impurity concentration, etc. For example, artificial graphite particles can be distinguished from natural graphite particles by analyzing the impurity elements in SEM-EDX (scanning electron microscope-energy dispersive X-ray analysis) analysis of a cross section of the negative electrode mixture layer. Natural graphite particles have, for example, a scaly (flake-like) shape and are silver-black in color. On the other hand, artificial graphite particles are manufactured in a controlled manner so as to have a uniform particle shape. Artificial graphite particles often have a uniform particle shape and size.

[0018] In the negative electrode according to the present disclosure (hereinafter also referred to as "negative electrode (N)"), the negative electrode mixture layer is composed of two or more layers. Specifically, the negative electrode mixture layer of the negative electrode (N) has a first layer disposed on the negative electrode current collector side and a second layer disposed on the first layer. Note that at least one other layer may be interposed between the first layer and the second layer.

[0019] The first layer may be the layer closest to the surface of the negative electrode current collector. The configuration of the first layer significantly affects the adhesion between the negative electrode current collector and the negative electrode mixture layer, the capacity, and the like.

[0020] The second layer may be the layer farthest from the surface of the negative electrode current collector. Because the second layer is closest to the separator holding the positive electrode and the non-aqueous electrolyte, the configuration of the second layer significantly affects the performance of the non-aqueous electrolyte secondary battery. In particular, the liquid circulation (permeability) of the non-aqueous electrolyte in the second layer determines the rate of lithium ion migration (diffusion), and therefore significantly affects rapid charging performance.

[0021] To significantly improve the liquid circulation of the non-aqueous electrolyte in the second layer, it is effective to include artificial graphite particles and natural graphite particles in the second layer. This is because artificial graphite particles have high hardness and high particle fracture strength, making them difficult to pack densely and making it easier to ensure gaps between the particles. The gaps between the particles serve as paths for the non-aqueous electrolyte to move.

[0022] On the other hand, if only artificial graphite particles were used in the second layer, the adhesion between the hard artificial graphite particles would be insufficient, making it difficult to ensure the strength of the second layer. In contrast, if the second layer contains both artificial graphite particles and natural graphite particles, the natural graphite particles improve the adhesion between the particles, making it possible to increase the strength of the second layer while ensuring gaps between the particles.

[0023] If only natural graphite particles are used, the second layer will be dense and the gaps between the particles will be very small, making it difficult for the non-aqueous electrolyte to move through the second layer, which will tend to cause a shortage of non-aqueous electrolyte in the first layer and result in a decrease in battery performance.

[0024] As described above, in order to improve the rapid charging performance of a nonaqueous electrolyte secondary battery while maintaining the strength of the second layer, it is necessary for the second layer to contain both artificial graphite particles and natural graphite particles. However, even if the second layer contains both artificial graphite particles and natural graphite particles, it is not necessarily possible to ensure sufficient gaps between the particles. To form sufficient gaps between the particles, it is necessary to control the particle shape index of both the artificial graphite particles and the natural graphite particles.

[0025] In the present disclosure, the particle shape index X of the artificial graphite particles is controlled to satisfy the following condition (A1), and the particle shape index Y of the natural graphite particles is controlled to satisfy the following condition (B1).

[0026] (A1) In the second layer, the particle shape index X of the artificial graphite particles satisfies 0.3≦X<0.9, where X is the average value of the ratio (ax / bx) of the minor axis ax to the major axis bx of the artificial graphite particles.

[0027] (B1) In the second layer, the particle shape index Y of the natural graphite particles satisfies 0.3≦Y<0.9, where Y is the average value of the ratio (ay / by) of the minor axis ay to the major axis by of the natural graphite particles.

[0028] The average value of the ax / bx ratio in the second layer may be determined as the arithmetic mean value of the ax / bx ratios of any 100 or more artificial graphite particles constituting the second layer. The major axis bx of the artificial graphite particle is the maximum diameter of the artificial graphite particle. The minor axis ax of the artificial graphite particle is the maximum diameter of the artificial graphite particle in the direction perpendicular to the major axis bx.

[0029] The average ay / by ratio in the second layer may be determined as the arithmetic mean value of the ay / by ratios of any 100 or more natural graphite particles constituting the second layer. The major axis by of the natural graphite particle is the maximum diameter of the natural graphite particle. The minor axis ay of the natural graphite particle is the maximum diameter of the natural graphite particle in the direction perpendicular to the major axis by.

[0030] According to the above configuration, sufficient voids are formed within the second layer, which has sufficient strength, improving the circulation of the nonaqueous electrolyte in the second layer, thereby improving the rapid charge performance of the nonaqueous electrolyte secondary battery. On the other hand, when at least one of the particle shape indices X and Y is 0.9 or greater, the filling rate of the artificial graphite particles and natural graphite particles in the second layer increases, reducing the number of voids formed within the second layer. As a result, rapid charge performance is not improved. Furthermore, when at least one of the particle shape indices X and Y is less than 0.3, the voids formed within the second layer become too large, making it difficult to design the density of the second layer and resulting in reduced battery performance.

[0031] From the viewpoint of forming more voids in the second layer and further improving the rapid charging performance of the nonaqueous electrolyte secondary battery, it is preferable that the particle shape indices of both the artificial graphite particles and the natural graphite particles be controlled so as to satisfy the following conditions (A2) and (B2):

[0032] (A2) In the second layer, the particle shape index X of the artificial graphite particles satisfies 0.3≦X≦0.5.

[0033] (B2) In the second layer, the particle shape index Y of the natural graphite particles satisfies 0.3≦Y≦0.5.

[0034] The larger the particle shape index, the closer the particle shape becomes to a shape suitable for close packing, thereby reducing the number of gaps formed within the second layer. On the other hand, if the particle shape index is too small, the particle shape becomes closer to a shape that is more likely to be oriented, thereby also reducing the number of gaps formed within the second layer. Therefore, in order to significantly improve fast charging performance, it is necessary for both the artificial graphite particles and the natural graphite particles to have appropriately controlled particle shape indices.

[0035] Next, the particle sizes of the artificial graphite particles and the natural graphite particles are not particularly limited, and may be any common particle size. Specifically, the average particle size Dx of the artificial graphite particles is, for example, 10 μm to 25 μm, and may be 15 μm to 20 μm. The average particle size Dy of the natural graphite particles is, for example, 10 μm to 25 μm, and may be 15 μm to 20 μm.

[0036] The ratio of the average particle size Dx of the artificial graphite particles to the average particle size Dy of the natural graphite particles is, for example, Dx / Dy=2.0 to 0.5, and may be 1.5 to 0.75.

[0037] The particle shape indices X and Y and the average particle sizes Dx and Dy of the artificial graphite particles and the natural graphite particles can be determined, for example, by the following method.

[0038] (1) A battery in a discharged state (DOD = 0 to 10%) (e.g., a battery that has been charged and discharged several times to less than 10 times) is disassembled, and a portion of the negative electrode is cut away to expose the cross section of the negative electrode mixture layer. For example, a method for exposing the cross section includes cutting away a portion of the negative electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode mixture layer.

[0039] (2) A backscattered electron image of the cross section of the exposed negative electrode mixture layer is taken using a scanning electron microscope (SEM). The magnification when taking the backscattered electron image may be, for example, 3,000 to 5,000 times.

[0040] (3) The obtained backscattered electron image is input into a computer and binarized using image analysis software (e.g., "ImageJ" manufactured by the National Institutes of Health, USA) to obtain a binarized image in which the cross sections of the particles in the electron image are colored black and voids present in the cross sections of the particles are colored white. The binarized image is sized so that 10 or more (preferably 100 or more) artificial graphite particles or natural graphite particles contained in the second layer can be observed.

[0041] (4) All artificial graphite particles or natural graphite particles are selected from the binarized image, and the area enclosed by the outline of the cross-section of the particle (particle cross-sectional area) is calculated. The diameter of an equivalent circle having the same area as the particle cross-sectional area is then regarded as the particle size of the particle, and a volume-based particle size distribution is calculated, and the median diameter (D50) at which the cumulative volume reaches 50% is calculated. The median diameter (D50) of the artificial graphite particles is the average particle size Dx of the artificial graphite particles, and the median diameter (D50) of the natural graphite particles is the average particle size Dy of the natural graphite particles.

[0042] (5) Ten artificial graphite particles or natural graphite particles having a particle diameter as close as possible to the median diameter (D50) are selected, and the minor axis ax or ay and major axis bx or by are determined for all selected particles, the ax / bx ratio or ay / by ratio is determined, and the average value is calculated.

[0043] (6) Using 10 or more different binarized images, the average value of the ax / bx ratio or ay / by ratio obtained for all the binarized images is calculated. The finally calculated ax / bx ratio or ay / by ratio is the particle shape index X, Y of the artificial graphite particles or natural graphite particles.

[0044] When artificial graphite particles and natural graphite particles are available in their raw state, the average particle sizes Dx and Dy may be determined using a laser diffraction / scattering particle size distribution analyzer. In this case, the volumetric particle size distribution is calculated, and the median diameter (D50) at which the cumulative volume reaches 50% is taken as the average particle size Dx or Dy. To determine the particle shape indices X and Y, 10 artificial graphite particles or natural graphite particles having a particle size as close as possible to the median diameter (D50) are selected, all selected particles are observed under a microscope, the minor diameter ax or ay and the major diameter bx or by are determined, the ax / bx ratio or ay / by ratio is determined, and the average values ​​are calculated.

[0045] The Vickers hardness of the artificial graphite particles may be, for example, in the range of 20 Hv to 50 Hv. Meanwhile, the Vickers hardness of the natural graphite particles may be, for example, in the range of 10 Hv to 30 Hv. However, the artificial graphite particles and natural graphite particles that coexist in the second layer are selected so that the Vickers hardness of the artificial graphite particles is sufficiently greater than that of the natural graphite particles. The Vickers hardness of the artificial graphite particles is preferably 10 Hv or more (e.g., 10 Hv to 20 Hv) greater than that of the natural graphite particles. It is desirable that the samples of each graphite particle used to measure the Vickers hardness of the artificial graphite particles and the natural graphite particles have a maximum diameter that is ±5% of the average particle diameter D50 of each graphite particle. Vickers hardness may be measured using a measuring device conforming to JIS Z 2244-1 (2020).

[0046] The graphite particles to be measured can be prepared by disassembling a secondary battery, removing the negative electrode, peeling off the negative electrode mixture from the negative electrode, washing the negative electrode mixture with anhydrous ethyl methyl carbonate or dimethyl carbonate, pulverizing the negative electrode mixture in a mortar, and separating the components by a method such as centrifugation. The pulverized negative electrode mixture sample is dried in a dry atmosphere for 1 hour, immersed in gently boiling 6 M hydrochloric acid for 10 minutes, and then washed with ion-exchanged water, filtered, and dried at 200°C for 1 hour. Centrifugation or other methods can then be performed.

[0047] From the viewpoint of forming a negative electrode mixture layer with sufficient strength and forming more voids in the second layer to further improve the rapid charge performance of the nonaqueous electrolyte secondary battery, it is preferable to satisfy the following condition (C1), and it is more preferable to satisfy the following condition (C2):

[0048] (C1) In the second layer, the ratio of the mass content Cx2 of the artificial graphite particles to the mass content Cy2 of the natural graphite particles is Cx2 / Cy2 = 10 / 90 or more and 90 / 10 or less.

[0049] (C2) In the second layer, the ratio of the mass content Cx2 of the artificial graphite particles to the mass content Cy2 of the natural graphite particles is Cx2 / Cy2 = 50 / 50 or more and 90 / 10 or less.

[0050] From the viewpoint of improving capacity, strength, and high-speed charging performance in a balanced manner, it is preferable to further satisfy the following condition (D1), and it is even more preferable to satisfy the following condition (D2).

[0051] (D1) The ratio of the thickness T2 of the second layer to the thickness T1 of the first layer is T2 / T1 = 3 / 7 or more and 6 / 4 or less.

[0052] (D2) The ratio of the thickness T2 of the second layer to the thickness T1 of the first layer is T2 / T1 = 3 / 7 or more and 5 / 5 or less.

[0053] The negative electrode mixture layer typically has a two-layer structure. When the negative electrode mixture layer has a three-layer structure or more, the ratio of the total of T1 and T2 to the thickness T of the negative electrode mixture layer is preferably 60% or more, and more preferably 80% or more.

[0054] The configuration of the negative electrode (N) is particularly effective when the negative electrode mixture layer is formed thick. When the negative electrode mixture layer is thin, the influence of the liquid permeability of the second layer on battery performance such as rapid charging performance is small. On the other hand, the thicker the negative electrode mixture layer, the greater the influence of the liquid permeability of the second layer on battery performance such as rapid charging performance. This is because the thicker the negative electrode mixture layer, the more likely it is that the nonaqueous electrolyte supplied to the first layer will be insufficient. The configuration of the negative electrode (N) can significantly improve battery performance such as rapid charging performance when the thickness of the negative electrode mixture layer is, for example, 10 μm or more, or even 100 μm or more. Note that if the negative electrode mixture layer is too thick, the effect of improving battery performance will be reduced, so the thickness of the negative electrode mixture layer is preferably, for example, 150 μm or less.

[0055] The configuration of the first layer is not particularly limited, but from the viewpoint of ensuring a high capacity and high adhesion between the negative electrode mixture layer and the negative electrode current collector, it is preferable that the first layer satisfy the following condition (E):

[0056] (E) In the first layer, the mass content Cy1 of the natural graphite particles is greater than the mass content Cx1 of the artificial graphite particles and greater than the mass content Cy2 of the natural graphite particles in the second layer described above.

[0057] In other words, the first layer preferably contains more natural graphite particles than artificial graphite particles on a mass basis. Even when the first layer contains artificial graphite particles, the mass content Cx1 of the artificial graphite particles in the first layer may be small, and the mass content Cx1 may be 10 mass% or less, or even 0 mass%. The Cy1 / Cy2 ratio is preferably 0.3 or more, and more preferably in the range of 0.5 to 2.0.

[0058] The particle shape index X of the artificial graphite particles and the particle shape index Y of the natural graphite particles in the first layer may be selected from the same ranges as the particle shape index X and particle shape index Y of the second layer. The particle shape index X of the artificial graphite particles in the first layer and the second layer may be the same or different, for example, the particle shape index X of the artificial graphite particles in the first layer may be in the range of 0.8 to 1.2 times the particle shape index X of the artificial graphite particles in the second layer. The particle shape index Y of the natural graphite particles in the first layer may be the same or different, for example, the particle shape index Y of the natural graphite particles in the first layer may be in the range of 0.8 to 1.2 times the particle shape index Y of the natural graphite particles in the second layer.

[0059] At least a portion of the artificial graphite particles may be coated with amorphous carbon. Also, at least a portion of the natural graphite particles may be coated with amorphous carbon. More specifically, at least a portion of the surface of the artificial graphite particles or natural graphite particles may be covered with amorphous carbon. The method for covering the artificial or natural graphite particles with amorphous carbon is not particularly limited, but may involve mixing a petroleum-derived material such as pitch or tar with the graphite particles and then carbonizing the petroleum-derived material.

[0060] The negative electrode active material may contain other active material particles in addition to the artificial graphite particles and natural graphite particles. The other active material particles are preferably Si-containing materials. The proportion of the Si-containing material in the entire negative electrode active material is, for example, 1% by mass to 30% by mass, or may be 3% by mass to 30% by mass, or may be 3% by mass to 15% by mass. In this case, a good balance between improved cycle characteristics and increased capacity can be achieved.

[0061] Examples of the Si-containing material include simple Si, silicon alloys, and silicon compounds. As the Si-containing material, particles containing a lithium ion conductive phase and a silicon phase dispersed in the lithium ion conductive phase may be used. The lithium ion conductive phase may be SiO ２ phases, silicate phases (eg, lithium silicate phases), carbon phases, and the like may be used.

[0062] SiO ２ phase and SiO ２The particles containing the silicon phase dispersed in the SiO ｘ (0.5≦x<2 (e.g., 0.5≦x<1.6)). ２ In the particles containing the lithium silicate phase and the silicon phase dispersed in the lithium silicate phase, the lithium silicate phase is ２ｚ SiO ２＋ｚ (0<z<1 (e.g., z=1 / 2)).

[0063] Examples of binders include resin materials, for example, fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0064] Examples of the conductive additive include carbons such as acetylene black, carbon fibers (carbon nanotubes (CNT), carbon fibers other than CNT), metal fibers, metal powders such as aluminum, etc. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0065] Examples of thickeners include carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salts), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), and saponified polymers having vinyl acetate units such as polyvinyl alcohol. One type of thickener may be used alone, or two or more types may be used in combination.

[0066] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0067] Examples of thickeners include carboxymethyl cellulose (CMC) and modified products thereof (including salts such as the Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.); saponified polymers having vinyl acetate units such as polyvinyl alcohol; and polyethers (polyalkylene oxides such as polyethylene oxide, etc.).

[0068] An example of a method for producing a negative electrode will be described below, taking as an example a case where a negative electrode mixture layer having a two-layer structure is formed.

[0069] A first slurry is prepared by dispersing a first negative electrode mixture in a dispersion medium, and a second slurry is prepared by dispersing a second negative electrode mixture in a dispersion medium. Each of the first negative electrode mixture and the second negative electrode mixture contains a predetermined amount of graphite and an optional component (for example, at least one of a binder, a conductive additive, and a thickener).

[0070] The dispersion medium is not particularly limited, but examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0071] A first slurry is applied to a negative electrode current collector to form a first coating film and dry it, and a second slurry is applied to the first coating film to form a second coating film and dry it. The dried laminate of the first and second coating films is rolled to obtain a two-layer negative electrode mixture layer composed of a first layer and a second layer on the negative electrode current collector. Three or more slurries with different contents may be prepared and coated in layers.

[0072] 1 is a longitudinal cross-sectional view of a cylindrical nonaqueous electrolyte secondary battery (hereinafter also referred to as a "secondary battery 10") that is an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0073] The secondary battery 10 includes an electrode group 18, a nonaqueous electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode 15 and a negative electrode 16 with a separator 17 interposed therebetween.

[0074] [Positive Electrode] The positive electrode comprises, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying it. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of a sheet-shaped positive electrode current collector. The positive electrode mixture contains a positive electrode active material as an essential component and may contain a binder, a conductive agent, etc. as optional components. NMP, etc., is used as the dispersion medium for the positive electrode slurry.

[0075] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal such as Ni, Co, or Mn can be used. ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Liａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４、 LiMPO ４、 Li ２ MPO ４ F (M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B). Here, 0<a≦1.2, 0<b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0076] Among them, Li ａ Ni ｂ Me １－ｂ O ２ (M is at least one selected from the group consisting of Mn, Co, and Al, and 0<a≦1.2, and 0.3≦b<1) is preferred. From the viewpoint of increasing capacity, it is more preferred that 0.85≦b<1 is satisfied. From the viewpoint of stability of the crystal structure, Li-nickel composite oxides containing Co and Al as M are preferred. ａ Ni ｂ Co ｃ Al ｄ O ２ (0<a≦1.2, 0.85≦b<1, 0<c<0.15, 0<d≦0.1, b+c+d=1) is more preferable.

[0077] The binder and conductive agent may be the same as those exemplified for the negative electrode. As the conductive agent, graphite such as natural graphite or artificial graphite may be used.

[0078] The positive electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the positive electrode current collector include stainless steel, aluminum, an aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0079] [Non-aqueous electrolyte] The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a gel electrolyte. The liquid electrolyte is, for example, an electrolytic solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolytic solution may contain known additives.

[0080] The gel electrolyte contains a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0081] For example, a liquid non-aqueous electrolyte is prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that ionizes in the electrolyte, and may include, for example, a lithium salt. The electrolyte may contain various additives. The electrolyte is usually used in its liquid state, but its fluidity may be limited by a gelling agent or the like.

[0082] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonates having an unsaturated bond, such as vinylene carbonate (VC), may also be used. Cyclic carbonates having a fluorine atom, such as fluoroethylene carbonate (FEC), may also be used. Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0083] Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ The lithium salt may be used alone or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0084] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0085] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a nonaqueous electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0086] (Additional Notes) The above description discloses the following technology: (Technology 1) A battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, wherein the negative electrode mixture layer contains artificial graphite particles and natural graphite particles, wherein the negative electrode mixture layer has a first layer disposed on the negative electrode current collector side and a second layer disposed on the first layer, wherein the second layer contains the artificial graphite particles and the natural graphite particles, wherein the particle shape index X of the artificial graphite particles in the second layer satisfies 0.3≦X<0.9, and wherein the particle shape index Y of the natural graphite particles in the second layer satisfies 0.3≦Y<0.9, wherein the particle shape index X is an average value of the ratio (ax / bx) of the minor axis ax to the major axis bx of the artificial graphite particles, a nonaqueous electrolyte secondary battery according to Technology 1, wherein the particle shape index X of the artificial graphite particles in the second layer satisfies 0.3≦X≦0.5, and the particle shape index Y of the natural graphite particles in the second layer satisfies 0.3≦Y≦0.5. (Technology 3) The nonaqueous electrolyte secondary battery according to Technology 1 or 2, wherein the ratio Cx2 / Cy2 of the mass content Cx2 of the artificial graphite particles to the mass content Cy2 of the natural graphite particles, relative to the total of the artificial graphite particles and the natural graphite particles, is 10 / 90 or more and 90 / 10 or less. (Technology 4) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 3, wherein the ratio T2 / T1 of the thickness T2 of the second layer to the thickness T1 of the first layer is 3 / 7 or more and 6 / 4 or less. (Technology 5) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 4, wherein in the first layer, the mass content Cy1 of the natural graphite particles, relative to the total of the artificial graphite particles and the natural graphite particles, is greater than the mass content Cx1 of the artificial graphite particles and is also greater than the mass content Cy2 of the natural graphite particles in the second layer. (Technology 6) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 5, wherein the mass content Cx1 of the artificial graphite particles in the first layer is 10 mass% or less.(Technology 7) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 6, wherein at least a portion of the artificial graphite particles is coated with amorphous carbon, and / or at least a portion of the natural graphite particles is coated with amorphous carbon.

[0087] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0088] Examples 1 to 12, Comparative Examples 1 to 10 [Preparation of Negative Electrode] Second Slurry Artificial graphite particles (Vickers hardness 40 Hv) and natural graphite particles (Vickers hardness 20 Hv) having particle shape indices X and Y shown in Table 1 were mixed in the mass ratio shown in Table 1 and used as the negative electrode active material. A first negative electrode mixture was prepared by mixing 1 part by mass of styrene-butadiene copolymer rubber (SBR) as a binder and 1 part by mass of carboxymethyl cellulose (CMC) as a thickener with 100 parts by mass of the negative electrode active material. An appropriate amount of water was added to the first negative electrode mixture and mixed to prepare a second slurry.

[0089] The average particle size Dx of the artificial graphite particles is 15 μm, and the average particle size Dy of the natural graphite particles is 15 μm. It is estimated that the particle shape index and the average particle size obtained from the binarized image using the method described above will be approximately the same value.

[0090] <First Slurry> A first slurry was prepared in the same manner as the second slurry, except that only the same natural graphite particles as those used in the second slurry were used as the negative electrode active material.

[0091] <Formation of Negative Electrode Mixture Layer> The first slurry was applied to both sides of a copper foil (thickness 10 μm) serving as a negative electrode current collector, forming a first coating film, which was then dried. Furthermore, the second slurry was applied to the first coating film formed on both sides of the copper foil, forming a second coating film, which was then dried. The coating thicknesses of the first and second slurries were controlled so that the ratio (T2 / T1) of the thickness T2 of the second layer to the thickness T1 of the first layer was the value shown in Table 1. The thickness of the negative electrode mixture layer (total thickness of the first and second layers) was 130 to 180 μm.

[0092] The dried laminate of the first coating film and the second coating film was rolled to obtain a negative electrode having a negative electrode mixture layer consisting of the first layer and the second layer on both sides of the negative electrode current collector.

[0093] [Preparation of Positive Electrode] An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture to obtain a positive electrode slurry. The positive electrode mixture was a mixture of a lithium-containing composite oxide as a positive electrode active material, graphite as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder. The lithium-containing composite oxide was LiNi ０．８０ Co ０．１０ Mn ０．１０ O ２ In the positive electrode mixture, the mass ratio of the lithium-containing composite oxide, graphite, and PVDF was 100:1:0.9.

[0094] The positive electrode slurry was applied to both sides of an aluminum foil (thickness 15 μm) serving as a positive electrode current collector, and the coating was dried and rolled to form a positive electrode active material layer (density 3.6 g / cm ３ ) was formed to obtain a positive electrode.

[0095] [Preparation of non-aqueous electrolyte] Vinylene carbonate (VC) was added to a non-aqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:3, and LiPF ６ The content of VC in the entire nonaqueous electrolyte was 5 mass %. ６ The concentration was set to 1.5 mol / L.

[0096] [Fabrication of Secondary Battery] An electrode group was fabricated by spirally winding a positive electrode and a negative electrode with a separator (a polyethylene microporous film) interposed therebetween in an inert gas atmosphere. The electrode group was housed in a bag-shaped exterior body formed of a laminate sheet with an Al layer, and the nonaqueous electrolyte was injected. The exterior body was then sealed to complete batteries A1 to A12 and B1 to B10. Batteries A1 to A12 are examples, and batteries B1 to B10 are comparative examples.

[0097] [Evaluation: Permeability of non-aqueous electrolyte in negative electrode mixture layer] 3 μL of propylene carbonate (PC) was dropped onto one surface of the negative electrode (negative electrode mixture layer), and the time from the dropping of PC until all of the PC penetrated into the negative electrode mixture layer was measured. The shorter this time, the better the permeability of the non-aqueous electrolyte in the negative electrode mixture layer and the better the rapid charging performance. The values ​​in Table 1 are indexes with the time (seconds) measured in Comparative Example 1 set to 100, and the smaller the index, the better the rapid charging characteristics.

[0098]

[0099] From Table 1, it can be seen that the rapid charging performance is significantly affected by the particle shape indices X and Y. It can also be seen that when 0.3≦X≦0.5 and 0.3≦Y≦0.5 are satisfied, the rapid charging performance is particularly significantly improved.

[0100] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0101] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode, 15a: Positive electrode lead, 16: Negative electrode, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove portion, 23: First insulating plate, 24: Second insulating plate

[0102] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

Claims

1. A battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, wherein the negative electrode mixture layer contains artificial graphite particles and natural graphite particles, and the negative electrode mixture layer has a first layer disposed on the negative electrode current collector side and a second layer disposed on the first layer, wherein the second layer contains the artificial graphite particles and the natural graphite particles, wherein the artificial graphite particles in the second layer have a particle shape index X that satisfies 0.3≦X<0.9, and the natural graphite particles in the second layer have a particle shape index Y that satisfies 0.3≦Y<0.9, and the particle shape index X is an average value of the ratio (ax / bx) of the minor axis ax to the major axis bx of the artificial graphite particles, the particle shape index Y is an average value of the ratio (ay / by) of the minor axis ay to the major axis by of the natural graphite particles.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the particle shape index X of the artificial graphite particles in the second layer satisfies 0.3≦X≦0.5, and the particle shape index Y of the natural graphite particles in the second layer satisfies 0.3≦Y≦0.

5.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the second layer, the ratio of the mass content Cx2 of the artificial graphite particles to the mass content Cy2 of the natural graphite particles: Cx2 / Cy2 is 10 / 90 or more and 90 / 10 or less.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the thickness T2 of the second layer to the thickness T1 of the first layer: T2 / T1 is 3 / 7 or more and 6 / 4 or less.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the first layer, the mass content Cy1 of the natural graphite particles is greater than the mass content Cx1 of the artificial graphite particles and greater than the mass content Cy2 of the natural graphite particles in the second layer.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the mass content Cx1 of the artificial graphite particles in the first layer is 10 mass % or less.

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein at least a portion of the artificial graphite particles is coated with amorphous carbon, and / or at least a portion of the natural graphite particles is coated with amorphous carbon.

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