Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

A negative electrode material for lithium ion secondary batteries, the negative electrode material comprising graphite particles and a low crystallinity carbon layer that covers at least a part of the surface of each one of the graphite particles, while satisfying at least one of the requirements A to C described below. A: The histogram of R values as obtained by Raman mapping measurement has two or more peaks. B: The maximum peak dispersion in the histogram of R values as obtained by Raman mapping measurement is 2.0 or more. C: The low crystallinity carbon layer comprises two or more carbon phases that have different crystallinities.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery. [Background technology]

[0002] Taking advantage of their small size, light weight, and high energy density, lithium-ion secondary batteries have been widely used in electronic devices such as notebook PCs, mobile phones, smartphones, and tablet PCs. In recent years, environmental issues such as global warming caused by CO2 emissions have led to the widespread use of clean electric vehicles (EVs) that run solely on batteries and hybrid electric vehicles (HEVs) that combine gasoline engines with batteries. Furthermore, the applications of lithium-ion secondary batteries are expanding to a wide range of fields, including power storage.

[0003] Carbon materials are widely used as the negative electrode material for lithium-ion secondary batteries. Carbon materials used for negative electrodes are broadly classified into graphite and low-crystalline carbon (including amorphous carbon), which has lower crystallinity than graphite. Graphite has a structure in which hexagonal mesh planes of carbon atoms are regularly stacked, allowing lithium ions to be inserted and extracted from the edges of the hexagonal mesh planes, resulting in charging and discharging.

[0004] Low-crystalline carbon has irregular hexagonal mesh layers or no hexagonal mesh layers. This allows the lithium ion insertion and desorption reactions to occur across the entire surface of the anode material. While this tends to result in lower energy density than graphite, it is easier to obtain lithium-ion batteries with excellent input characteristics. Furthermore, its reactivity with electrolytes is lower than that of graphite, resulting in superior battery storage characteristics (lifespan).

[0005] In order to utilize the respective properties of graphite and low-crystalline carbon, a negative electrode material in which the surface of graphite particles is coated with low-crystalline carbon has been proposed (see, for example, International Publication No. 2012 / 015054). Summary of the Invention [Problem to be solved by the invention]

[0006] Generally, anode materials using carbon materials have a trade-off between input characteristics and storage characteristics. For example, reducing the particle size of the anode material increases the contact area with the electrolyte, improving input characteristics, but accelerates side reactions with the electrolyte, degrading storage characteristics. The negative electrode material described in WO 2012 / 015054 prevents deterioration of storage characteristics by coating the surface of graphite particles with low-crystalline carbon, but there is a need for the development of a negative electrode material that can maintain storage characteristics more effectively.

[0007] One aspect of the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode material for a lithium ion secondary battery that maintains good storage characteristics, and a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery that use the same. [Means for solving the problem]

[0008] Specific means for solving the above problems are as follows. <1> A negative electrode material for a lithium-ion secondary battery, comprising graphite particles and a low-crystalline carbon layer covering at least a portion of the surface of the graphite particles, wherein a histogram of R values ​​obtained by mapping Raman measurement has two or more peaks. <2> A negative electrode material for a lithium ion secondary battery, comprising graphite particles and a low-crystalline carbon layer covering at least a portion of the surface of the graphite particles, wherein the variance of the maximum peak in an R value histogram obtained by mapping Raman measurement is 2.0 or more. <3> A negative electrode material for a lithium ion secondary battery, comprising graphite particles and a low-crystalline carbon layer covering at least a portion of the surface of the graphite particles, the low-crystalline carbon layer comprising two or more carbon phases with different crystallinity. <4> The average circularity is within the range of 0.8 to 1.0. <1> ~ <3> 10. The negative electrode material for a lithium ion secondary battery according to claim 9, wherein the negative electrode material is a lithium ion secondary battery material. <5> a current collector; and a metal electrode disposed on the surface of the current collector. <1> ~ <4> a negative electrode mixture layer containing the negative electrode material for a lithium ion secondary battery according to any one of the preceding claims; A negative electrode for a lithium ion secondary battery comprising: <6> <5> A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 1. [Effects of the Invention]

[0009] According to one aspect of the present invention, there are provided a negative electrode material for a lithium ion secondary battery that maintains good storage characteristics, and a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery that use the same. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view showing an example of the configuration of a lithium ion secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless specifically stated. The same applies to numerical values ​​and their ranges, and do not limit the present invention. Furthermore, various changes and modifications are possible by those skilled in the art within the scope of the technical ideas disclosed in this specification. In this specification, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the present specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the present specification, the upper or lower limit of a numerical range may be replaced with a value shown in the examples. In this specification, when there are multiple substances corresponding to each component, the content and ratio of each component means the total content and ratio of the multiple substances, unless otherwise specified. In this specification, when there are multiple types of particles corresponding to each component, the particle size of each component refers to the value for a mixture of the multiple types of particles, unless otherwise specified. In this specification, the terms "layer" and "film" include cases where the layer or film is formed over the entire area when the area in which the layer or film is present is observed, as well as cases where the layer or film is formed over only a part of the area. In this specification, the "solid content" of the positive electrode mixture or the negative electrode mixture means the remaining components after removing volatile components such as organic solvents from the positive electrode mixture slurry or the negative electrode mixture slurry.

[0012] <Anode material for lithium-ion secondary batteries> The negative electrode material for a lithium ion secondary battery (hereinafter also simply referred to as the negative electrode material) of the present disclosure has graphite particles and a low-crystalline carbon layer covering at least a portion of the surface of the graphite particles, and satisfies at least one of the following conditions A, B, and C:

[0013] Condition A: The histogram of R values ​​obtained by mapping Raman measurement has two or more peaks. Condition B: The variance of the maximum peak in the histogram of R values ​​obtained by mapping Raman measurement is 2.0 or more. Condition C: The low-crystalline carbon layer contains two or more carbon phases with different crystallinity.

[0014] As a result of investigations by the present inventors, it was found that a negative electrode material that satisfies at least one of condition A, condition B, and condition C maintains better storage characteristics than a negative electrode material that does not satisfy any of condition A, condition B, or condition C. Furthermore, it was found that this negative electrode material maintains good storage characteristics even when the particle size is reduced to improve input characteristics. The reason for this is not entirely clear, but it is thought that a low-crystalline carbon layer that satisfies at least one of condition A, condition B, and condition C has excellent coating properties for graphite particles.

[0015] (graphite particles) In the present disclosure, graphite particles are particles that are formed by carbon interlayer (d 002 ) is less than 0.340 nm. The carbon network interlayer (d 002 ) can be calculated using the Bragg equation from the diffraction peak corresponding to the carbon 002 plane, which appears at a diffraction angle 2θ of around 24° to 27°, from the diffraction profile obtained by irradiating a sample with X-rays (CuKα rays) and measuring the diffraction rays with a goniometer. d 002 Measurement can be performed under the following conditions: Radiation source: CuKα radiation (wavelength = 0.15418nm) Output: 40kV, 20mA Sampling width: 0.010° Scanning range: 10°~35° Scan speed: 0.5° / min

[0016] Bragg's equation: 2d sinθ=nλ Here, d is the length of one period, θ is the diffraction angle, n is the order of reflection, and λ is the X-ray wavelength.

[0017] The graphite particles may be obtained by pulverizing lump natural graphite. However, since the graphite particles obtained by pulverizing lump natural graphite may contain impurities, it is preferable to highly purify the natural graphite by a refining process. The method for refining natural graphite is not particularly limited and can be appropriately selected from commonly used refining methods, such as flotation, electrochemical treatment, and chemical treatment.

[0018] The purity of natural graphite is preferably 99.8% or more (ash content 0.2% or less) by mass, and more preferably 99.9% or more (ash content 0.1% or less). A purity of 99.8% or more tends to further improve the safety of the battery and further improve the battery performance. The purity of natural graphite can be calculated, for example, by leaving 100 g of graphite in an air atmosphere in a furnace at 800° C. for 48 hours or more, and then measuring the amount of residue derived from ash.

[0019] As the graphite particles, crushed artificial graphite obtained by burning resin materials such as epoxy resins and phenolic resins, or pitch materials obtained from petroleum, coal, etc. may be used.

[0020] The method for obtaining artificial graphite is not particularly limited, and examples include a method in which raw materials such as thermoplastic resin, naphthalene, anthracene, phenanthroline, coal tar, and tar pitch are calcined in an inert atmosphere at 800°C or higher to obtain a calcined product of artificial graphite. The resulting calcined product is then pulverized using a known method such as a jet mill, vibration mill, pin mill, or hammer mill, and the average particle size is adjusted to approximately 2 μm to 40 μm, thereby producing graphitic particles derived from artificial graphite. The raw materials may also be heat-treated before calcination. When heat-treating the raw materials, for example, the raw materials may be heat-treated in advance using an autoclave or other device, coarsely pulverized using a known method, and then calcined in an inert atmosphere at 800°C or higher in the same manner as above. The resulting calcined product, artificial graphite, is pulverized and the average particle size adjusted to approximately 2 μm to 40 μm, thereby obtaining graphitic particles derived from artificial graphite.

[0021] (Low crystalline carbon layer) In this disclosure, low-crystalline carbon is a concept that includes amorphous carbon, and is a carbon lattice interlayer (d 002 ) of 0.340 nm or more. 002 ) is 0.340 nm or more and less than 0.350 nm, it is soft carbon (easily graphitizable carbon), and the interplanar spacing (d 002 ) of 0.350 nm or more is sometimes called hard carbon (hard to graphitize carbon).

[0022] The thickness of the low-crystalline carbon layer (the maximum thickness when the thickness is not constant) is not particularly limited, and may be selected from the range of, for example, 0.5 nm to 500 nm. The thickness of the low-crystalline carbon layer can be measured, for example, using a transmission electron microscope (TEM).

[0023] The ratio (mass ratio) of the low-crystalline carbon layer to 1 part by mass of the graphite particles is preferably 0.005 to 10, more preferably 0.005 to 5, and even more preferably 0.005 to 0.08. If the ratio is 0.005 or more, the initial charge / discharge efficiency and life characteristics tend to be excellent. If the ratio is 10 or less, the output characteristics tend to be excellent. The mass ratio can be calculated from the weight loss ratio between 500°C and 600°C by measuring the weight change in an air stream using, for example, TG-DTA (Thermogravimetry-Differential Thermal Analysis, simultaneous differential thermal-thermogravimetric measurement). The weight change in the temperature range from 500°C to 600°C can be attributed to the weight change due to materials other than graphite. Meanwhile, the remainder after the end of the heat treatment can be attributed to the amount of graphite.

[0024] An example of a method for coating the surface of graphite particles with a low-crystalline carbon layer is a method in which a mixture containing graphite particles and a precursor of the low-crystalline carbon layer is heat-treated.

[0025] Examples of precursors of the low crystalline carbon layer include pitch and organic polymer compounds. Examples of pitch include ethylene heavy end pitch, crude oil pitch, coal tar pitch, asphalt cracking pitch, pitch produced by thermal decomposition of polyvinyl chloride or the like, and pitch produced by polymerizing naphthalene or the like in the presence of a super strong acid. Examples of organic polymeric compounds include thermoplastic resins such as polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate, and polyvinyl butyral, and natural substances such as starch and cellulose.

[0026] A negative electrode material that satisfies at least one of the conditions A, B, and C can be obtained, for example, by using two or more precursors of the low-crystalline carbon layer that coats the graphite particles.

[0027] The low-crystalline carbon layer of the negative electrode material that satisfies at least one of condition A, condition B, and condition C is preferably formed using two or more precursors that change into a carbon phase at different temperatures. In this case, it is believed that the precursor that changes into a carbon phase at a lower temperature first forms a carbon phase on the surface of the graphitic particles, and then the precursor that changes into a carbon phase at a higher temperature forms a carbon phase, thereby forming a low-crystalline carbon layer with better coverage. In one embodiment, the low-crystalline carbon layer is preferably formed using a precursor selected from pitch and a precursor selected from organic polymer compounds.

[0028] When the low-crystalline carbon layer contains two or more carbon phases with different crystallinity, the proportion of the carbon phase with the largest proportion in the low-crystalline carbon layer is preferably 10% by mass to 90% by mass, more preferably 20% by mass to 80% by mass, and even more preferably 30% by mass to 70% by mass of the entire low-crystalline carbon layer.

[0029] Whether or not the low-crystalline carbon layer contains two or more carbon phases with different crystallinity can be determined using a histogram of R values ​​obtained by mapping Raman measurement. Specifically, if the histogram of R values ​​obtained by mapping Raman measurement has two or more peaks, or if the variance of the largest peak in the histogram of R values ​​obtained by mapping Raman measurement is 2.0 or more, it can be determined that the low-crystalline carbon layer satisfies condition A. The upper limit of the variance may be, for example, 5.0.

[0030] For mapping Raman measurements, a micro-Raman spectrometer (e.g., DXR Imaging Raman Microscope, manufactured by Thermo Fisher Scientific) is used. The conditions are a 532 nm laser, a 100x lens, and a 25 μm diameter aperture. The output is 2.0 mW, and the irradiation time and integration are set to a value sufficient to obtain a sufficient signal-to-noise ratio. For example, the irradiation time is set to 2 seconds and the integration time is set to 30. The R value is measured at 100 or more points by shifting the measurement point on the particle by 1.5 μm or more or by changing the particle, and a histogram is obtained.

[0031] Methods for coating the surfaces of graphite particles with a low-crystalline carbon layer obtained from two or more precursors include (1) a method of heat-treating a mixture containing graphitic particles and two or more precursors, and (2) a method of heat-treating a mixture containing graphitic particles and one of the precursors, followed by mixing with another precursor and further heat-treating. From the viewpoint of production efficiency, method (1) is preferred.

[0032] (particle shape) The shape of the negative electrode material is not particularly limited and can be selected from spherical, flat, amorphous, etc. From the viewpoint of achieving both input characteristics and storage characteristics, a spherical shape or a shape close to spherical is preferred. It is also preferred that the negative electrode material does not form secondary particles. In one embodiment, the average circularity of the negative electrode material is preferably within the range of 0.8 to 1.0, more preferably within the range of 0.9 to 1.0, and even more preferably within the range of 0.95 to 1.0.

[0033] The average circularity of the negative electrode material can be measured using a wet flow particle size and shape analyzer (for example, FPIA-3000, Malvern Instruments). As a pretreatment for the measurement, 0.06 g of anode material and purified water containing 0.2% by weight of a surfactant (trade name: Liponol T / 15, Lion Corporation) can be placed in a test tube (12 mm x 120 mm, Maruem Corporation) and stirred for 20 seconds using a test tube mixer (Pasolina NS-80, AS ONE Corporation), followed by ultrasonic stirring for 1 minute. An ultrasonic cleaner such as SND Corporation's US102 (high-frequency output 100 W, oscillation frequency 38 kHz) can be used. Analysis of the standard deviation of circularity in a specific range based on the measurement results of circularity can be performed based on the FPIA-3000 academic materials (2nd edition published on August 31, 2006). The measurement temperature was 25°C, the concentration of the measurement sample was 10 mass%, and the number of particles to be counted was The number is set to 10,000. Water is used as the dispersion solvent.

[0034] (Average particle size) The average particle size of the negative electrode material is preferably 2 μm to 30 μm, more preferably 2.5 μm to 25 μm, even more preferably 3 μm to 20 μm, and particularly preferably 5 μm to 20 μm. When the average particle size of the graphite particles is 30 μm or less, the discharge capacity and discharge characteristics tend to be improved. When the average particle size of the graphite particles is 2 μm or more, the initial charge / discharge efficiency tends to be improved. The average particle size of the negative electrode material is the volume-average particle size determined as d50 (median diameter) by measuring the volume-based particle size distribution using a particle size distribution analyzer (SALD-3000, manufactured by Shimadzu Corporation) that utilizes the laser diffraction / scattering method.

[0035] In one embodiment, the average particle size of the negative electrode material may be 9 μm or less, or may be 8 μm or less. When the average particle size of the negative electrode material is 9 μm or less, good input characteristics tend to be obtained. The negative electrode material of the present disclosure maintains good storage characteristics even when the average particle size is 9 μm or less, so both the input characteristics and storage characteristics can be improved.

[0036] (R value) The R value of the negative electrode material is preferably 0.10 to 0.60, more preferably 0.15 to 0.55, and even more preferably 0.20 to 0.50. The above R value is the value at 1580 cm in the Raman spectrum obtained by irradiating the negative electrode material with laser light of 532 nm wavelength. -1 ~1620cm -1 1300 cm for the peak intensity IG in the range -1 ~1400cm -1 is the ratio of peak intensities ID in the range of The Raman spectroscopy spectrum can be measured using a Raman spectroscopy device (for example, DXR manufactured by Thermo Fisher Scientific).

[0037] (BET specific surface area) The BET specific surface area of ​​the negative electrode material is 0.8m 2 / g~8m 2 / g, and 1m 2 / g~7m 2 / g, more preferably 1.5m 2 / g~6m 2 / g is more preferred. The BET specific surface area of ​​the negative electrode material is 0.8m 2 / g or more, excellent battery performance tends to be obtained. 2 When the porosity is 1 / g or less, the tap density is likely to increase, and the mixability with other materials such as binders and conductive agents tends to improve. The BET specific surface area of ​​the negative electrode material can be measured from the nitrogen adsorption capacity in accordance with JIS Z 8830:2013. The evaluation device used can be an AUTOSORB-1 (product name) manufactured by QUANTACHROME. When measuring the BET specific surface area, it is preferable to first perform a pretreatment to remove moisture by heating, since moisture adsorbed on the sample surface and in the structure is thought to affect the gas adsorption capacity. In pretreatment, a measurement cell containing 0.05 g of sample is depressurized to 10 Pa or less using a vacuum pump, heated to 110°C, and held for at least 3 hours, after which it is naturally cooled to room temperature (25°C) while maintaining the reduced pressure. After this pretreatment, measurements are performed with an evaluation temperature of 77 K and an evaluation pressure range of less than 1 in relative pressure (equilibrium pressure relative to saturated vapor pressure).

[0038] <Anode for lithium-ion secondary batteries> The negative electrode for a lithium ion secondary battery (negative electrode) of the present disclosure includes a current collector and a negative electrode mixture layer disposed on the surface of the current collector and containing the negative electrode material for a lithium ion secondary battery of the present disclosure. Details of the current collector and the negative electrode mixture layer will be described later.

[0039] <Lithium-ion secondary battery> The lithium ion secondary battery of the present disclosure is not particularly limited in its configuration as long as it has a negative electrode containing the negative electrode material for lithium ion secondary batteries of the present disclosure. The negative electrode material for lithium ion secondary batteries of the present disclosure may be contained in a negative electrode mixture layer.

[0040] (Outline of lithium-ion secondary batteries) First, a brief overview of lithium-ion secondary batteries will be provided. A lithium-ion secondary battery contains a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution within a battery container. A separator is disposed between the positive electrode and the negative electrode. When charging a lithium-ion secondary battery, a charger is connected between the positive electrode and the negative electrode. During charging, lithium ions inserted into the positive electrode active material are released into the non-aqueous electrolyte. The lithium ions released into the non-aqueous electrolyte move through the non-aqueous electrolyte, pass through the separator, and reach the negative electrode. The lithium ions that reach the negative electrode are inserted into the negative electrode active material that constitutes the negative electrode.

[0041] When discharging, an external load is connected between the positive electrode and the negative electrode. During discharging, lithium ions inserted into the negative electrode active material are released into the non-aqueous electrolyte. At this time, electrons are released from the negative electrode. The lithium ions released into the non-aqueous electrolyte move through the non-aqueous electrolyte, pass through the separator, and reach the positive electrode. The lithium ions that reach the positive electrode are inserted into the positive electrode active material that constitutes the positive electrode. As lithium ions are inserted into the positive electrode active material, electrons flow into the positive electrode. In this way, electrons move from the negative electrode to the positive electrode, thereby causing discharging.

[0042] In this way, lithium ion secondary batteries can be charged and discharged by inserting and extracting lithium ions between the positive electrode active material and the negative electrode active material. An example of the configuration of an actual lithium ion secondary battery will be described later (see, for example, FIG. 1). Next, the components of the lithium ion secondary battery of the present disclosure, namely, the positive electrode, the negative electrode, the non-aqueous electrolyte, the separator, and other components that may be provided as needed, will be described in order.

[0043] (positive electrode) The lithium-ion secondary battery of the present disclosure has the following positive electrode applicable to high-capacity, high-input / output lithium-ion secondary batteries. The positive electrode (positive electrode plate) of the present disclosure has a current collector (positive electrode current collector) and a positive electrode mixture layer disposed on the current collector. The positive electrode mixture layer is a layer containing at least a positive electrode active material disposed on the surface of the current collector.

[0044] The positive electrode active material preferably contains layered lithium-nickel-manganese-cobalt composite oxide (hereinafter sometimes referred to as NMC), which tends to have high capacity and excellent safety. From the viewpoint of further improving safety, it is preferable to use a mixture of NMC and spinel-type lithium manganese composite oxide (hereinafter sometimes referred to as sp-Mn) as the positive electrode active material. From the viewpoint of increasing the capacity of the battery, the content of NMC is preferably 65 mass % or more, more preferably 70 mass % or more, and even more preferably 80 mass % or more, based on the total amount of the positive electrode mixture layer.

[0045] It is preferable to use NMC represented by the following composition formula (Chemical Formula 1). Li (1+δ) Mn x Ni y Co (1-x-y-z) M z O2…(chemical 1) In the composition formula (Chemical Formula 1), (1+δ) represents the composition ratio of Li (lithium), x represents the composition ratio of Mn (manganese), y represents the composition ratio of Ni (nickel), and (1-xyz) represents the composition ratio of Co (cobalt). z represents the composition ratio of element M. The composition ratio of O (oxygen) is 2. The element M is at least one element selected from the group consisting of Ti (titanium), Zr (zirconium), Nb (niobium), Mo (molybdenum), W (tungsten), Al (aluminum), Si (silicon), Ga (gallium), Ge (germanium), and Sn (tin). Also, -0.15<δ<0.15, 0.1 <x≦0.5、0.6<x+y+z<1.0、0≦z≦0.1である。

[0046] As sp-Mn, it is preferable to use one represented by the following composition formula (Chemical Formula 2). Li (1+η) Mn (2-λ) M' λ O4…(chemical 2) In the composition formula (Chemical Formula 2), (1+η) represents the composition ratio of Li, (2-λ) represents the composition ratio of Mn, and λ represents the composition ratio of element M'. The composition ratio of O (oxygen) is 4. The element M' is preferably at least one element selected from the group consisting of Mg (magnesium), Ca (calcium), Sr (strontium), Al, Ga, Zn (zinc), and Cu (copper). 0≦η≦0.2, 0≦λ≦0.1. As the element M' in the composition formula (Chemical Formula 2), it is preferable to use Mg or Al. By using Mg or Al, the life of the battery tends to be extended. In addition, the safety of the battery tends to be improved. Furthermore, by adding the element M', the elution of Mn can be reduced, which tends to improve the storage characteristics and charge / discharge cycle characteristics.

[0047] Furthermore, the positive electrode active material may be other than NMC and sp-Mn. Positive electrode active materials other than NMC and sp-Mn can be materials commonly used in this field, such as lithium-containing composite metal oxides other than NMC and sp-Mn, olivine-type lithium salts, chalcogen compounds, and manganese dioxide. The lithium-containing composite metal oxide is a metal oxide containing lithium and a transition metal, or a metal oxide in which a part of the transition metal in the metal oxide is substituted with a different element. Here, examples of the different element include Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B, with Mn, Al, Co, Ni, and Mg being preferred. One type of different element may be used alone, or two or more types may be used in combination. Lithium-containing composite metal oxides other than NMC and sp-Mn include Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1 1-y O z (Li x Co y M 1 1-y O z Medium, M 1 represents at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B), Li x Ni 1-y M 2y O z (Li x Ni 1-y M 2 y O z Among them, M 2 represents at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Cu, Zn, Al, Cr, Pb, Sb, V, and B. ) and the like can be mentioned. Here, x is in the range of 0 < x ≦ 1.2, y is in the range of 0 to 0.9, and z is in the range of 2.0 to 2.3. Also, the x value indicating the molar ratio of lithium increases or decreases by charge and discharge. Further, examples of the olivine-type lithium salt include LiFePO4 and the like. Examples of the chalcogen compound include titanium disulfide, molybdenum disulfide and the like. The positive electrode active material may be used alone or in combination of two or more.

[0048] Next, the positive electrode mixture layer and the current collector will be described in detail. The positive electrode mixture layer contains a positive electrode active material, a binder, etc. and is disposed on the current collector. There is no limitation on the method of forming the positive electrode mixture layer. For example, it can be formed as follows. The positive electrode active material, the binder, and other materials such as a conductive agent and a thickener used as needed are dry-mixed into a sheet shape and pressed onto the current collector (dry method) to form the positive electrode mixture layer. Also, the positive electrode active material, the binder, and other materials such as a conductive agent and a thickener used as needed are dissolved or dispersed in a dispersion solvent to form a slurry of the positive electrode mixture, which is applied to the current collector and dried (wet method) to form the positive electrode mixture layer. As the positive electrode active material, as described above, it is preferable to use a layered lithium-nickel-manganese-cobalt composite oxide (NMC). The positive electrode active material is used and mixed in a powder (granular) form. As particles of the positive electrode active material such as NMC and sp-Mn, those having shapes such as块状, polyhedral, spherical, ellipsoidal, plate-like, needle-like, and columnar can be used. The average particle size (d50) of particles of the positive electrode active material such as NMC or sp-Mn (or the average particle size (d50) of the secondary particles when primary particles aggregate to form secondary particles) is preferably 1 μm to 30 μm, more preferably 3 μm to 25 μm, and even more preferably 5 μm to 15 μm, from the viewpoints of tap density (packing ability) and mixability with other materials when forming an electrode. The average particle size (d50) of the particles of the positive electrode active material can be measured in the same manner as for graphite particles.

[0049] The BET specific surface area of ​​particles of positive electrode active materials such as NMC and sp-Mn is in the range of 0.2 m 2 / g~4.0m 2 / g, and 0.3m 2 / g~2.5m 2 / g, more preferably 0.4m 2 / g~1.5m 2 / g is more preferred. The BET specific surface area of ​​the positive electrode active material particles is 0.2 m 2 / g or more, excellent battery performance tends to be obtained. 2 When the BET specific surface area is 0.05 / g or less, the tap density is likely to increase, and the mixability with other materials such as binders, conductive agents, etc. tends to be good. The BET specific surface area can be measured in the same manner as in the case of graphite particles.

[0050] Examples of conductive agents for the positive electrode include metal materials such as copper and nickel, graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, and carbonaceous materials such as amorphous carbon such as needle coke. The conductive agents for the positive electrode may be used alone or in combination of two or more. The content of the conductive agent relative to the mass of the positive electrode mixture layer is preferably 0.01% by mass to 50% by mass, more preferably 0.1% by mass to 30% by mass, and even more preferably 1% by mass to 15% by mass. If the content of the conductive agent is 0.01% by mass or more, sufficient conductivity tends to be easily obtained. If the content of the conductive agent is 50% by mass or less, a decrease in battery capacity tends to be suppressed.

[0051] The binder for the positive electrode is not particularly limited, and when the positive electrode mixture layer is formed by a wet method, a material with good solubility or dispersibility in the dispersion solvent is selected. Specific examples include resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polyimide, and cellulose; rubber-like polymers such as SBR (styrene-butadiene rubber) and NBR (acrylonitrile-butadiene rubber); fluorine-based polymers such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, polytetrafluoroethylene-vinylidene fluoride copolymer, and fluorinated polyvinylidene fluoride; and polymer compositions having ionic conductivity for alkali metal ions (especially lithium ions). The binder for the positive electrode may be used alone or in combination of two or more. From the viewpoint of the stability of the positive electrode, it is preferable to use a fluorine-based polymer such as polyvinylidene fluoride (PVdF) or a polytetrafluoroethylene-vinylidene fluoride copolymer as the binder. The content of the binder relative to the mass of the positive electrode mixture layer is preferably 0.1% by mass to 60% by mass, more preferably 1% by mass to 40% by mass, and even more preferably 3% by mass to 10% by mass. When the binder content is 0.1% by mass or more, the positive electrode active material can be sufficiently bound, sufficient mechanical strength of the positive electrode mixture layer can be obtained, and battery performance such as cycle characteristics tends to be improved.When the binder content is 60% by mass or less, sufficient battery capacity and conductivity tend to be obtained.

[0052] The thickener is effective for adjusting the viscosity of the slurry. There are no particular limitations on the thickener, and specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphated starch, casein, and salts thereof. One type of thickener may be used alone, or two or more types may be used in combination. When a thickener is used, the content of the thickener relative to the mass of the positive electrode mixture layer is preferably 0.1% by mass to 20% by mass, more preferably 0.5% by mass to 15% by mass, and even more preferably 1% by mass to 10% by mass, from the viewpoints of input / output characteristics and battery capacity.

[0053] The dispersion solvent for forming the slurry is not limited to any particular type, as long as it can dissolve or disperse the positive electrode active material, binder, and optionally used conductive agent or thickener. Either an aqueous or organic solvent may be used. Examples of aqueous solvents include water, alcohol, and a mixture of water and alcohol. Examples of organic solvents include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethyl sulfoxide, benzene, xylene, and hexane. When using an aqueous solvent, it is preferable to use a thickener.

[0054] The positive electrode mixture layer formed on the current collector by a wet method or a dry method is preferably compacted by a hand press, a roller press, or the like in order to improve the packing density of the positive electrode active material. The density of the compacted positive electrode mixture layer is set to 2.5 g / cm from the viewpoint of further improving input / output characteristics and safety. 3 ~3.5g / cm 3 and preferably in the range of 2.55 g / cm 3 ~3.15g / cm 3 More preferably, it is in the range of 2.6 g / cm 3 ~3.0g / cm3 It is more preferable that the range is: In addition, the amount of the positive electrode mixture slurry applied to one side of the current collector when forming the positive electrode mixture layer is set to 30 g / m as the solid content of the positive electrode mixture from the viewpoint of energy density and input / output characteristics. 2 ~170g / m 2 It is preferable that the thickness is 40 g / m 2 ~160g / m 2 More preferably, it is 40 g / m 2 ~150g / m 2 It is more preferable that: Considering the amount of positive electrode mixture slurry applied to one side of the current collector and the density of the positive electrode mixture layer, the average thickness of the positive electrode mixture layer is preferably 19 μm to 68 μm, more preferably 23 μm to 64 μm, and even more preferably 36 μm to 60 μm. In the present disclosure, the average thickness of the mixture layer is the average value of thicknesses at any 10 points.

[0055] The material of the current collector for the positive electrode is not particularly limited, and among them, metal materials are preferred, and aluminum is more preferred. The shape of the current collector is not particularly limited, and materials processed into various shapes can be used. Examples of metal materials include metal foil, metal plate, metal thin film, and expanded metal, and among them, it is preferable to use a metal thin film. The thin film may be formed into a mesh shape as appropriate. The average thickness of the current collector is not particularly limited, but from the viewpoint of obtaining the strength and good flexibility required for the current collector, it is preferably 1 μm to 1 mm, more preferably 3 μm to 100 μm, and even more preferably 5 μm to 100 μm.

[0056] (Negative electrode) The lithium ion secondary battery of the present disclosure has the following negative electrode applicable to high-capacity, high-input / output lithium ion secondary batteries. The negative electrode (negative electrode plate) of the present disclosure has a current collector (negative electrode current collector) and a negative electrode mixture layer disposed on the surface thereof. The negative electrode mixture layer is a layer containing at least a negative electrode active material disposed on the surface of the current collector. The negative electrode for a lithium ion secondary battery of the present disclosure can be used as the negative electrode. The negative electrode material for lithium ion secondary batteries of the present disclosure is used as the negative electrode active material contained in the negative electrode mixture layer of the lithium ion secondary battery of the present disclosure. From the viewpoint of increasing the capacity of the battery, the content of the negative electrode material for a lithium ion secondary battery according to the present disclosure is preferably 80 mass % or more, more preferably 85 mass % or more, and even more preferably 90 mass % or more, relative to the total amount of the negative electrode mixture layer.

[0057] Next, the negative electrode mixture layer and the current collector will be described in detail. The negative electrode mixture layer contains a negative electrode active material, a binder, etc., and is disposed on the current collector. There are no limitations on the method for forming the negative electrode mixture layer, and it can be formed, for example, as follows. The negative electrode active material, the binder, and other materials used as needed, such as a conductive agent and a thickener, are dissolved or dispersed in a dispersion solvent to form a negative electrode mixture slurry, which is then applied to the current collector and dried (wet method), thereby forming the negative electrode mixture layer.

[0058] As the conductive agent for the negative electrode, graphite (graphite) such as natural graphite or artificial graphite other than the graphite particles according to the negative electrode material for lithium ion secondary batteries of the present disclosure, carbon black such as acetylene black, amorphous carbon such as needle coke, etc., can be used. The conductive agent for the negative electrode may be used alone or in combination of two or more. In this way, the addition of the conductive agent tends to have the effect of reducing the resistance of the electrode, etc.

[0059] From the viewpoint of improving conductivity and reducing initial irreversible capacity, the content of the conductive agent relative to the mass of the negative electrode mixture layer is preferably 1% by mass to 45% by mass, more preferably 2% by mass to 42% by mass, and even more preferably 3% by mass to 40% by mass. When the content of the conductive agent is 1% by mass or more, sufficient conductivity tends to be easily obtained. When the content of the conductive agent is 45% by mass or less, a decrease in battery capacity tends to be suppressed.

[0060] The binder for the negative electrode is not particularly limited as long as it is a material that is stable against the nonaqueous electrolyte or the dispersion solvent used in forming the electrode. Specific examples include resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, cellulose, and nitrocellulose; rubber-like polymers such as SBR (styrene-butadiene rubber) and NBR (acrylonitrile-butadiene rubber); fluorine-based polymers such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, and fluorinated polyvinylidene fluoride; and polymer compositions having ionic conductivity for alkali metal ions (especially lithium ions). The binder for the negative electrode may be used alone or in combination of two or more. Among these, fluorine-based polymers such as SBR and polyvinylidene fluoride are preferred.

[0061] The content of the binder relative to the mass of the negative electrode mixture layer is preferably 0.1 to 20% by mass, more preferably 0.5 to 15% by mass, and even more preferably 0.6 to 10% by mass. When the binder content is 0.1% by mass or more, the negative electrode active material can be sufficiently bound, and sufficient mechanical strength of the negative electrode mixture layer tends to be obtained.When the binder content is 20% by mass or less, sufficient battery capacity and conductivity tend to be obtained.

[0062] When a fluorine-based polymer such as polyvinylidene fluoride is used as the main component of the binder, the content of the binder relative to the mass of the negative electrode mixture layer is preferably 1 mass % to 15 mass %, more preferably 2 mass % to 10 mass %, and even more preferably 3 mass % to 8 mass %.

[0063] The thickener is used to adjust the viscosity of the slurry. There are no particular limitations on the thickener, and specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphated starch, casein, and salts thereof. One type of thickener may be used alone, or two or more types may be used in combination.

[0064] When a thickener is used, the content of the thickener relative to the mass of the negative electrode mixture layer is preferably 0.1 mass % to 5 mass %, more preferably 0.5 mass % to 3 mass %, and even more preferably 0.6 mass % to 2 mass %, from the viewpoints of input / output characteristics and battery capacity.

[0065] The dispersion solvent for forming the slurry is not limited to any particular type, and may be either an aqueous or organic solvent, as long as it can dissolve or disperse the negative electrode active material, binder, and optional conductive agent or thickener. Examples of aqueous solvents include water, alcohol, and a mixture of water and alcohol. Examples of organic solvents include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethyl sulfoxide, benzene, xylene, and hexane. When using an aqueous solvent, it is preferable to use a thickener.

[0066] The density of the negative electrode mixture layer is 0.7 g / cm 3 ~2g / cm 3 and preferably 0.8 g / cm 3 ~1.9g / cm 3 More preferably, it is 0.9 g / cm 3 ~1.8g / cm 3 It is more preferable that: The density of the negative electrode mixture layer is 0.7 g / cm 3 When the density of the negative electrode mixture layer is 2 g / cm or more, the conductivity between the negative electrode active materials is improved, the increase in battery resistance can be suppressed, and the capacity per unit volume tends to be improved. 3 If the content is not more than this, there is a tendency to reduce the risk of deterioration of discharge characteristics due to an increase in the initial irreversible capacity and a decrease in the permeability of the non-aqueous electrolyte solution to the vicinity of the interface between the current collector and the negative electrode active material. In addition, the amount of the negative electrode mixture slurry applied to one side of the current collector when forming the negative electrode mixture layer is set to 30 g / m as the solid content of the negative electrode mixture from the viewpoint of energy density and input / output characteristics. 2 ~150g / m 2 It is preferable that the thickness is 40 g / m 2 ~140g / m 2 More preferably, it is 45 g / m 2 ~130g / m 2 It is more preferable that: Considering the amount of negative electrode mixture slurry applied to one side of the current collector and the density of the negative electrode mixture layer, the average thickness of the negative electrode mixture layer is preferably 10 μm to 150 μm, more preferably 15 μm to 140 μm, and even more preferably 15 μm to 120 μm.

[0067] The material of the negative electrode current collector is not particularly limited, and specific examples include metal materials such as copper, nickel, stainless steel, nickel-plated steel, etc. Among these, copper is preferred from the viewpoints of ease of processing and cost.

[0068] The shape of the current collector is not particularly limited, and materials processed into various shapes can be used. Specific examples include metal foil, metal plate, metal thin film, and expanded metal. Among these, metal thin film is preferred, and copper foil is more preferred. Copper foil includes rolled copper foil formed by a rolling method and electrolytic copper foil formed by an electrolytic method, and both are suitable as current collectors. The average thickness of the current collector is not particularly limited, but is preferably 5 μm to 50 μm, more preferably 8 μm to 40 μm, and even more preferably 9 μm to 30 μm, for example. If the average thickness of the current collector is less than 25 μm, its strength can be improved by using a stronger copper alloy (phosphor bronze, titanium copper, Corson alloy, Cu-Cr-Zr alloy, etc.) rather than pure copper.

[0069] (Non-aqueous electrolyte) The non-aqueous electrolyte generally contains a non-aqueous solvent and a lithium salt (electrolyte). First, the non-aqueous solvent will be described. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, and cyclic sulfonates. The cyclic carbonate is preferably one in which the number of carbon atoms in the alkylene group constituting the cyclic carbonate is 2 to 6, more preferably 2 to 4. Examples include ethylene carbonate, propylene carbonate, butylene carbonate, etc. Among these, ethylene carbonate and propylene carbonate are preferred. The chain carbonate is preferably a dialkyl carbonate, preferably one in which the carbon number of each of the two alkyl groups is 1 to 5, more preferably 1 to 4. Examples include symmetric chain carbonates such as dimethyl carbonate, diethyl carbonate, and di-n-propyl carbonate; and asymmetric chain carbonates such as ethyl methyl carbonate, methyl-n-propyl carbonate, and ethyl-n-propyl carbonate. Among these, dimethyl carbonate and ethyl methyl carbonate are preferred. Dimethyl carbonate has better oxidation resistance and reduction resistance than diethyl carbonate, and therefore tends to improve cycle characteristics. Ethyl methyl carbonate has an asymmetric molecular structure and a low melting point, and therefore tends to improve low-temperature characteristics. A mixed solvent combining ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is particularly preferred, as it can ensure battery characteristics over a wide temperature range. From the viewpoint of battery characteristics, the content of the cyclic carbonate and chain carbonate is preferably 85 mass % or more, more preferably 90 mass % or more, and even more preferably 95 mass % or more, based on the total amount of the non-aqueous solvent. Furthermore, when a cyclic carbonate and a chain carbonate are used in combination, the mixing ratio of the cyclic carbonate and the chain carbonate is preferably cyclic carbonate / chain carbonate (volume ratio) of 1 / 9 to 6 / 4, more preferably 2 / 8 to 5 / 5, from the viewpoint of battery characteristics. Examples of cyclic sulfonic acid esters include 1,3-propane sultone, 1-methyl-1,3-propane sultone, 3-methyl-1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, 1,4-butene sultone, etc. Among these, 1,3-propane sultone and 1,4-butane sultone are particularly preferred from the viewpoint of being able to further reduce DC resistance. The non-aqueous electrolyte may further contain a chain ester, a cyclic ether, a chain ether, a cyclic sulfone, or the like. Examples of the chain ester include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, etc. Among these, it is preferable to use methyl acetate from the viewpoint of improving low-temperature properties. Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, and tetrahydropyran. Examples of the chain ether include dimethoxyethane and dimethoxymethane. Examples of cyclic sulfones include sulfolane and 3-methylsulfolane.

[0070] The non-aqueous electrolyte may contain a silyl phosphate compound. Specific examples of silyl phosphate compounds include tris(trimethylsilyl) phosphate, dimethyltrimethylsilyl phosphate, methyl bis(trimethylsilyl) phosphate, diethyltrimethylsilyl phosphate, ethyl bis(trimethylsilyl) phosphate, dipropyltrimethylsilyl phosphate, propyl bis(trimethylsilyl) phosphate, dibutyltrimethylsilyl phosphate, butyl bis(trimethylsilyl) phosphate, dioctyltrimethylsilyl phosphate, octylbis(trimethylsilyl) phosphate, diphenyltrimethylsilyl phosphate, phenyl bis(trimethylsilyl) phosphate, di(trifluoroethyl)(trimethylsilyl) phosphate, trifluoroethyl bis(trimethylsilyl) phosphate, compounds in which the trimethylsilyl group of the aforementioned silyl phosphate esters is substituted with a triethylsilyl group, a triphenylsilyl group, a t-butyldimethylsilyl group, or the like, and compounds having a so-called condensed phosphate ester structure in which phosphate esters are condensed and the phosphorus atom is bonded via oxygen. Among these, tris(trimethylsilyl) phosphate (TMSP) is preferably used. Tris(trimethylsilyl) phosphate can suppress the increase in resistance with a smaller amount added than other silyl phosphate ester compounds. These silyl phosphates may be used alone or in combination of two or more. When the non-aqueous electrolyte solution contains a phosphate silyl ester compound, the content of the phosphate silyl ester compound is preferably 0.1 mass % to 5 mass %, more preferably 0.3 mass % to 3 mass %, and even more preferably 0.4 mass % to 2 mass %, relative to the total amount of the non-aqueous electrolyte solution. In particular, when the non-aqueous electrolyte contains tris(trimethylsilyl) phosphate (TMSP), the content of tris(trimethylsilyl) phosphate (TMSP) is preferably 0.1 to 0.5 mass%, more preferably 0.1 to 0.4 mass%, and even more preferably 0.2 to 0.4 mass%, relative to the total amount of the non-aqueous electrolyte. When the TMSP content is within the above range, the effect of a thin SEI (Solid Electrolyte Interphase) and other factors tends to improve the life characteristics.

[0071] The non-aqueous electrolyte may also contain vinylene carbonate (VC). By using VC, a stable coating is formed on the surface of the negative electrode during charging of the lithium-ion secondary battery. This coating has the effect of suppressing decomposition of the non-aqueous electrolyte on the surface of the negative electrode. The content of vinylene carbonate is preferably 0.3 mass % to 1.6 mass %, more preferably 0.3 mass % to 1.5 mass %, and even more preferably 0.3 mass % to 1.3 mass %, relative to the total amount of the non-aqueous electrolyte. When the content of vinylene carbonate is within the above range, the life characteristics can be improved and the action of decomposing excess VC during charging and discharging of the lithium ion secondary battery, which reduces the charge and discharge efficiency, tends to be prevented.

[0072] Next, the lithium salt (electrolyte) will be described. The lithium salt is not particularly limited as long as it is usable as an electrolyte for a non-aqueous electrolyte solution for a lithium ion secondary battery, and examples thereof include inorganic lithium salts, fluorine-containing organic lithium salts, and oxalatoborate salts shown below. Examples of inorganic lithium salts include inorganic fluoride salts such as LiPF6, LiBF4, LiAsF6, and LiSbF6, perhalogen salts such as LiClO4, LiBrO4, and LiIO4, and inorganic chloride salts such as LiAlCl4. Examples of fluorine-containing organic lithium salts include perfluoroalkanesulfonates such as LiCF3SO3; perfluoroalkanesulfonylimide salts such as LiN(CF3SO2)2, LiN(CF3CF2SO2)2, and LiN(CF3SO2)(C4F9SO2); perfluoroalkanesulfonylmethide salts such as LiC(CF3SO2)3; and fluoroalkyl fluorophosphates such as Li[PF5(CF2CF2CF3)], Li[PF4(CF2CF2CF3)2], Li[PF3(CF2CF2CF3)3], Li[PF5(CF2CF2CF2CF3)], Li[PF4(CF2CF2CF2CF3)2], and Li[PF3(CF2CF2CF2CF3)3]. Examples of oxalatoborate salts include lithium bis(oxalato)borate and lithium difluorooxalatoborate. These lithium salts may be used alone or in combination of two or more. Among them, lithium hexafluorophosphate (LiPF6) is preferred when judging comprehensively the solubility in a solvent, and the charge / discharge characteristics, output characteristics, and cycle characteristics when used in a lithium ion secondary battery.

[0073] There are no particular limitations on the concentration of the electrolyte in the non-aqueous electrolyte. The electrolyte concentration range is as follows: The lower limit of the concentration is 0.5 mol / L or more, preferably 0.6 mol / L or more, and more preferably 0.7 mol / L or more. The upper limit of the concentration is 2 mol / L or less, preferably 1.8 mol / L or less, and more preferably 1.7 mol / L or less. If the electrolyte concentration is 0.5 mol / L or more, the electrical conductivity of the non-aqueous electrolyte tends to be sufficient. If the electrolyte concentration is 2 mol / L or less, an increase in the viscosity of the non-aqueous electrolyte is suppressed, so the electrical conductivity tends to increase. Increasing the electrical conductivity of the non-aqueous electrolyte tends to improve the performance of the lithium ion secondary battery.

[0074] (separator) The separator is not particularly limited as long as it is ion-permeable while providing electronic insulation between the positive electrode and the negative electrode, and is resistant to oxidation on the positive electrode side and reduction on the negative electrode side. Materials (components) that satisfy these properties include resins, inorganic substances, and the like. Examples of resins that can be used include olefin polymers, fluorine polymers, cellulose polymers, polyimides, nylon, etc. It is preferable to select a material that is stable to the non-aqueous electrolyte and has excellent liquid retention, and it is preferable to use a porous sheet or nonwoven fabric made from a polyolefin such as polyethylene or polypropylene. Examples of inorganic materials include oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and glass. For example, the above-mentioned inorganic materials in fibrous or particulate form can be attached to a thin-film substrate such as a nonwoven fabric, woven fabric, or microporous film, and the resulting material can be used as a separator. Thin-film substrates preferably have a pore size of 0.01 μm to 1 μm and an average thickness of 5 μm to 50 μm. The above-mentioned inorganic materials in fibrous or particulate form can also be used as a separator by forming a composite porous layer using a binder such as a resin. This composite porous layer can also be formed on the surface of another separator to form a multilayer separator. Furthermore, this composite porous layer can also be formed on the surface of a positive electrode or a negative electrode to form a separator.

[0075] (Other components) A cleavage valve may be provided as another component of the lithium ion secondary battery. When the cleavage valve opens, it is possible to suppress a pressure increase inside the battery, thereby improving safety. A component that releases an inert gas (e.g., carbon dioxide) as the temperature rises may also be provided. By providing such a component, when the temperature inside the battery rises, the inert gas is generated, allowing the cleavage valve to open quickly, improving safety. Materials used for the above component are preferably lithium carbonate, polyethylene carbonate, polypropylene carbonate, etc.

[0076] In this disclosure, the negative electrode capacity of a lithium ion secondary battery refers to the discharge capacity of the negative electrode. Furthermore, in this disclosure, the positive electrode capacity of a lithium ion secondary battery refers to the initial charge capacity of the positive electrode minus the greater irreversible capacity of either the negative electrode or the positive electrode. Here, the discharge capacity of the negative electrode is defined as the capacity calculated by a charge / discharge device when lithium ions inserted into the negative electrode active material are deintercalated. Furthermore, the initial charge capacity of the positive electrode is defined as the capacity calculated by a charge / discharge device when lithium ions are deintercalated from the positive electrode active material. The capacity ratio of the negative electrode to the positive electrode can also be calculated, for example, from "negative electrode discharge capacity / lithium ion secondary battery discharge capacity." The discharge capacity of a lithium ion secondary battery can be measured under conditions such as constant current / constant voltage (CCCV) charging at 4.2 V, 0.1 C to 0.5 C, and a cut-off time of 2 to 5 hours, followed by constant current (CC) discharging at 0.1 C to 0.5 C to 2.7 V. The discharge capacity of the negative electrode can be calculated by cutting the negative electrode, for which the discharge capacity of the lithium ion secondary battery has been measured, into a predetermined area, and fabricating a single-electrode cell using lithium metal as the counter electrode and a separator impregnated with a nonaqueous electrolyte, and then performing constant current / constant voltage (CCCV) charging at 0 V, 0.1 C, and a cut-off current of 0.01 C, followed by constant current (CC) discharging at 0.1 C to 1.5 V, measuring the discharge capacity per predetermined area, and converting this to the total area used as the negative electrode of the lithium ion secondary battery. In this single-electrode cell, the direction in which lithium ions are inserted into the negative electrode active material is defined as charging, and the direction in which the lithium ions inserted into the negative electrode active material are removed is defined as discharging. Note that C means "current value (A) / battery discharge capacity (Ah)".

[0077] (lithium-ion secondary battery) Next, an embodiment in which the present disclosure is applied to a cylindrical lithium ion secondary battery of 18650 type will be described with reference to the drawings. Fig. 1 is a cross-sectional view of a lithium ion secondary battery to which the present disclosure is applied. As shown in FIG. 1 , the lithium-ion secondary battery 1 of the present disclosure has a cylindrical battery container 6 made of nickel-plated steel and with a bottom. The battery container 6 contains an electrode winding group 5, in which a strip-shaped positive electrode plate 2 and a negative electrode plate 3 are wound in a spiral cross section with a polyethylene porous sheet separator 4 sandwiched between them. The separator 4 has a width of 58 mm and an average thickness of 30 μm, for example. A ribbon-shaped aluminum positive electrode tab terminal extends from the upper end surface of the electrode winding group 5, one end of which is fixed to the positive electrode plate 2. The other end of the positive electrode tab terminal is joined by ultrasonic welding to the underside of a disk-shaped battery lid disposed above the electrode winding group 5 and serving as a positive electrode external terminal. Meanwhile, a ribbon-shaped copper negative electrode tab terminal extends from the lower end surface of the electrode winding group 5, one end of which is fixed to the negative electrode plate 3. The other end of the negative electrode tab terminal is joined by resistance welding to the inner bottom of the battery container 6. Therefore, the positive electrode tab terminal and the negative electrode tab terminal are respectively led out to opposite end faces of the electrode wound group 5. The entire outer periphery of the electrode wound group 5 is covered with an insulating coating (not shown). The battery lid is fixed to the top of the battery container 6 by crimping via an insulating resin gasket. Therefore, the inside of the lithium ion secondary battery 1 is sealed. A nonaqueous electrolyte (not shown) is poured into the battery container 6. [Example]

[0078] The present embodiment will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0079] [Preparation of negative electrode material] (Comparative Example 1) A mixture was obtained by mixing 100 parts by mass of spherical natural graphite having a volume average particle diameter of 10.0 μm with 10 parts by mass of coal tar pitch (softening point 90°C, residual carbon rate (carbonization rate) 50%). The mixture was then heat-treated to form a low-crystalline carbon layer on the surface of the graphite particles. The heat treatment was carried out by increasing the temperature from 25°C to 1000°C at a rate of 200°C / hour under a nitrogen flow and holding at 1000°C for 1 hour. The obtained particles were crushed using a cutter mill and sieved, and the under-sieved portion was used as the negative electrode material. The physical properties of the obtained negative electrode material are shown in Table 1.

[0080] Example 1 A mixture was obtained by mixing 100 parts by mass of spherical natural graphite having a volume average particle size of 10.0 μm, 7 parts by mass of coal tar pitch (softening point 90° C., residual carbon rate (carbonization rate) 50%), and 11 parts by mass of polyvinyl alcohol (residual carbon rate (carbonization rate) 14%). A negative electrode material was obtained in the same manner as in Comparative Example 1, except that this mixture was used. The physical properties of the obtained negative electrode material are shown in Table 1.

[0081] (Comparative Example 2) A mixture was obtained by mixing 100 parts by mass of spherical natural graphite having a volume average particle size of 8.0 μm and 10 parts by mass of coal tar pitch (softening point 90° C., residual carbon rate (carbonization rate) 50%). A negative electrode material was obtained in the same manner as in Comparative Example 1, except for using this mixture. The physical properties of the obtained negative electrode material are shown in Table 1.

[0082] Example 2 A mixture was obtained by mixing 100 parts by mass of spherical natural graphite having a volume average particle size of 8.0 μm, 7 parts by mass of coal tar pitch (softening point 90° C., residual carbon rate (carbonization rate) 50%), and 11 parts by mass of polyvinyl alcohol (residual carbon rate (carbonization rate) 14%). A negative electrode material was obtained in the same manner as in Comparative Example 1, except that this mixture was used. The physical properties of the obtained negative electrode material are shown in Table 1.

[0083] [Preparation of negative electrode plate] Carboxymethyl cellulose (CMC) as a thickener and styrene butadiene rubber (SBR) as a binder were added to the negative electrode material. The mass ratio of these was negative electrode material:CMC:SBR=98:1:1. Purified water as a dispersion solvent was added to this and kneaded to form a slurry for each example and comparative example. A predetermined amount of this slurry was applied substantially evenly and homogeneously to both sides of a rolled copper foil with an average thickness of 10 μm, which served as a current collector for the negative electrode. The density of the negative electrode mixture layer was 1.3 g / cm 3 It was decided.

[0084] [Preparation of positive electrode plate] The positive electrode active material was a layered lithium-nickel-manganese-cobalt composite oxide (NMC, with a BET specific surface area of ​​0.4 m2 / g, average particle diameter (d50) 6.5 μm). To this positive electrode active material, acetylene black (product name: HS-100, average particle diameter 48 nm (Denka Co., Ltd. catalog value), manufactured by Denka Co., Ltd.) as a conductive agent and polyvinylidene fluoride as a binder were sequentially added and mixed to obtain a mixture of positive electrode materials. The mass ratio was positive electrode active material: conductive agent: binder = 90:5:5. Further, N-methyl-2-pyrrolidone (NMP) as a dispersion solvent was added to the above mixture, and the mixture was kneaded to form a slurry. This slurry was applied substantially evenly and homogeneously to both sides of aluminum foil with an average thickness of 20 μm, which served as a current collector for the positive electrode. Thereafter, a drying process was performed to obtain a density of 2.7 g / cm. 3 The amount of the positive electrode mixture slurry applied to one side was 40 g / m2 in terms of the solid content of the positive electrode mixture. 2 It was decided.

[0085] [Fabrication of lithium-ion secondary batteries] The positive and negative electrode plates were each cut to a predetermined size, and the cut positive and negative electrodes were wound with a polyethylene single-layer separator (product name: Hipore, manufactured by Asahi Kasei Corporation, "Hipore" is a registered trademark) having an average thickness of 30 μm sandwiched between them to form a rolled electrode assembly. The lengths of the positive and negative electrodes and separator were adjusted so that the diameter of the electrode assembly was 17.15 mm. A current collecting lead was attached to this electrode assembly, which was then inserted into an 18650-type battery case, and a nonaqueous electrolyte was then poured into the battery case. The non-aqueous electrolyte was a mixture of cyclic carbonate ethylene carbonate (EC) and chain carbonates dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 2:3:2, to which lithium hexafluorophosphate (LiPF6) was dissolved at a concentration of 1.2 mol / L as the lithium salt (electrolyte), and 1.0 mass% vinylene carbonate (VC) was added. Finally, the battery case was sealed to complete the lithium-ion secondary battery.

[0086] The fabricated lithium-ion secondary battery was charged at a constant current of 0.5 CA to 4.2 V in a 25°C environment, and then charged at a constant voltage of 0.01 CA from the time the voltage reached 4.2 V. The battery was then discharged at a constant current of 0.5 CA to 2.7 V. This constitutes one cycle, and a total of three cycles were performed. A 30-minute break was taken between each charge and discharge. The lithium-ion secondary battery after three cycles is referred to as its initial state.

[0087] [Input characteristic evaluation] (1) The initialized lithium-ion secondary battery was charged to 4.2 V at a constant current of 0.2 CA, and then constant voltage charged at 4.2 V until the current reached 0.02 CA. The charge capacity at this time was designated as "charge capacity 1" (mAh). (2) After a 30-minute rest period, the battery was discharged to 2.7 V at a constant current of 0.2 CA. (3) After a 30-minute rest period, the battery was charged at a constant current of 5 CA up to 4.2 V. The charge capacity at this time was designated as "charge capacity 2" (mAh). Input characteristics (%) = Charge capacity 2 (mAh) / Charge capacity 1 (mAh) x 100 ... (Formula 2)

[0088] [Evaluation of storage characteristics] (1) The battery in the initial state was charged to 4.2 V at a constant current of 0.5 CA, and then constant voltage charged at 4.2 V until the current reached 0.01 CA. (2) After a 30-minute rest period, the battery was discharged at a constant current of 0.5 CA to 2.7 V. The discharge capacity (mAh) at this time was measured. (3) After a 30-minute rest period, the battery was charged to 4.2 V at a constant current of 0.5 CA. (4) The battery (3) was left at 60°C for 30 days. (5) The battery was discharged at a constant current of 0.5 CA down to 2.7 V. The discharge capacity (mAh) was measured. (6) The storage characteristics were calculated from the discharge capacity obtained in (2) and the discharge capacity obtained in (5) using the following formula 2. Storage characteristics (%) = Discharge capacity 2 (mAh) / Discharge capacity 1 (mAh) × 100 ... Formula 2

[0089] [Table 1]

[0090] As shown in Table 1, Examples 1 and 2, in which the negative electrode materials satisfied conditions A to C, had better evaluation results for storage characteristics than Comparative Examples 1 and 2, in which the negative electrode materials did not satisfy conditions A to C. Example 2, in which the average particle size of the negative electrode material was 8.0 μm, exhibited better input characteristics than Example 1, in which the average particle size of the negative electrode material was 10.0 μm.

[0091] All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. 1. A negative electrode material for a lithium ion secondary battery, comprising: graphite particles; and a low-crystalline carbon layer covering at least a portion of the surface of the graphite particles, wherein the variance of the largest peak in an R-value histogram obtained by mapping Raman measurement is 2.0 or more; and wherein the negative electrode material satisfies at least one of the following conditions 1 and 2: Condition 1: The low-crystalline carbon layer contains a carbon material having a carbon lattice plane interlayer distance (d 002 ) of 0.340 nm or more as determined by wide-angle X-ray diffraction. Condition 2: Average circularity is within the range of 0.8 to 1.0

2. A negative electrode material for a lithium ion secondary battery, comprising: graphite particles; and a low-crystalline carbon layer covering at least a portion of the surface of the graphite particles, wherein the low-crystalline carbon layer contains two or more carbon phases with different crystallinity; and wherein the negative electrode material satisfies at least one of the following conditions 1 and 2: Condition 1: The low-crystalline carbon layer contains a carbon material having a carbon lattice plane interlayer distance (d 002 ) of 0.340 nm or more as determined by wide-angle X-ray diffraction. Condition 2: Average circularity is within the range of 0.8 to 1.0

3. The low-crystalline carbon layer is a carbon lattice interlayer (d ００２ 3. The negative electrode material for a lithium ion secondary battery according to claim 1, comprising a carbon material having a particle size of 0.340 nm or more.

4. The negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 3, wherein the average circularity is within a range of 0.8 to 1.

0.

5. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the thickness of the low-crystalline carbon layer is selected from the range of 0.5 nm to 500 nm.

6. A current collector; a negative electrode mixture layer disposed on a surface of the current collector and including the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5; A negative electrode for a lithium ion secondary battery comprising:

7. A lithium ion secondary battery comprising the negative electrode for lithium ion secondary batteries according to claim 6.

8. 6. A method for producing the negative electrode material for a lithium ion secondary battery according to claim 1, comprising heat-treating a mixture containing graphite particles and two or more types of precursors of a low-crystalline carbon layer, thereby coating surfaces of the graphite particles with the low-crystalline carbon layer.

9. 6. A method for producing the negative electrode material for a lithium ion secondary battery according to claim 1, comprising: heat-treating a mixture containing graphite particles and one of two or more precursors of a low-crystalline carbon layer; and then mixing another precursor into the mixture and further heat-treating the mixture, thereby coating at least a portion of the surface of the graphite particles with the low-crystalline carbon layer.

10. 10. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 8 or claim 9, wherein the precursor of the low crystalline carbon layer includes a precursor selected from pitch and a precursor selected from organic polymer compounds.

11. A method for producing a negative electrode material for a lithium ion secondary battery, which comprises graphite particles and a low-crystalline carbon layer covering at least a portion of the surface of the graphite particles, and wherein the dispersion of the largest peak in a histogram of R values ​​obtained by mapping Raman measurement is 2.0 or more, or the low-crystalline carbon layer contains two or more carbon phases with different crystallinity, A method for producing a negative electrode material for a lithium ion secondary battery, comprising: heat-treating a mixture containing graphite particles and one of two or more precursors of a low-crystalline carbon layer; then mixing another precursor into the mixture and further heat-treating the mixture to coat at least a portion of the surface of the graphite particles with the low-crystalline carbon layer.

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