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

Abstract

The negative electrode material for a lithium ion secondary battery of the present application has graphite particles and a low-crystallinity carbon layer that coats at least a portion of the surface of the graphite particles, and satisfies at least any one of the following A to C. A: a histogram of R values obtained by Raman spectroscopic imaging measurement has two or more peaks; B: the dispersion of the largest peak in the histogram of R values obtained by Raman spectroscopic imaging measurement is 2.0 or more; and C: the low-crystallinity carbon layer contains two or more carbon phases that differ in crystallinity.

Description

Technical Field

[0001] This invention relates to negative electrode materials for lithium-ion secondary batteries, negative electrodes for lithium-ion secondary batteries, and lithium-ion secondary batteries. Background Technology

[0002] Lithium-ion rechargeable batteries, with their so-called small size, lightweight, and high energy density, have long been widely used in electronic devices such as laptop PCs, mobile phones, smartphones, and tablet PCs. In recent years, against the backdrop of environmental problems such as global warming caused by CO2 emissions, clean electric vehicles (EVs) powered solely by batteries and hybrid electric vehicles (HEVs) combining gasoline engines and batteries have gradually become more common. Furthermore, the applications of lithium-ion rechargeable batteries have expanded to various fields, including energy storage.

[0003] Carbon materials are widely used as negative electrode materials in lithium-ion secondary batteries. The carbon materials used in negative electrode materials are broadly classified into graphite and low-crystallinity carbon (including amorphous carbon), which has lower crystallinity than graphite. Graphite, due to its regularly stacked hexagonal network structure of carbon atoms, allows lithium ions to intercalate and deintercalate from the ends of the hexagonal network, thus enabling charging and discharging.

[0004] Low-crystallinity carbon is either irregularly stacked with hexagonal meshes or lacks hexagonal meshes altogether. Therefore, lithium-ion insertion and extraction reactions occur across the entire surface of the negative electrode material. Consequently, it tends to have a lower energy density than graphite, but easily yields lithium-ion batteries with excellent input characteristics. Furthermore, its reactivity with the electrolyte is lower than that of graphite, resulting in superior battery lifespan.

[0005] In order to utilize the respective properties of graphite and low-crystallinity carbon, a negative electrode material obtained by coating the surface of graphite particles with low-crystallinity carbon has been proposed (for example, refer to International Publication No. 2012 / 015054). Summary of the Invention

[0006] The technical problem that the invention aims to solve

[0007] Anode materials using carbon generally exhibit a trade-off between input and storage characteristics. For example, reducing the particle size of the anode material increases the contact area with the electrolyte and improves input characteristics, but reduces storage characteristics because side reactions with the electrolyte are promoted.

[0008] The negative electrode material described in International Publication No. 2012 / 015054 suppresses the reduction of retention properties by coating the surface of graphite particles with low-crystallinity carbon, but there is a desire to develop negative electrode materials that can maintain retention properties more effectively.

[0009] One aspect of the present invention is made in view of the above facts, and its object is to provide a negative electrode material for lithium-ion secondary batteries that can maintain good storage characteristics, as well as a negative electrode for lithium-ion secondary batteries and a lithium-ion secondary battery using the material.

[0010] Means for solving technical problems

[0011] The specific means to solve the above-mentioned technical problems are as follows.

[0012] <1> A negative electrode material for lithium-ion secondary batteries has graphite particles and a low-crystallinity carbon layer covering at least a portion of the surface of the graphite particles, and the histogram of the R value obtained by Raman spectroscopy imaging has two or more peaks.

[0013] <2> A negative electrode material for lithium-ion secondary batteries has graphite particles and a low-crystallinity carbon layer covering at least a portion of the surface of the graphite particles, and the dispersion of the maximum peak in the histogram of the R value obtained by Raman spectroscopy imaging is 2.0 or higher.

[0014] <3> A negative electrode material for lithium-ion secondary batteries has graphite particles and a low-crystallinity carbon layer that coats at least a portion of the surface of the graphite particles, wherein the low-crystallinity carbon layer comprises two or more carbon phases with different crystallinities.

[0015] <4> The above <1> ~ <3> The negative electrode material for lithium-ion secondary batteries described in any one of the above examples has an average sphericity in the range of 0.8 to 1.0.

[0016] <5> A negative electrode for a lithium-ion secondary battery, comprising: a current collector; and a electrode disposed on the surface of the current collector and containing the aforementioned components. <1> ~ <4> The negative electrode layer of the negative electrode material for lithium-ion secondary batteries as described in any one of the following.

[0017] <6> A lithium-ion secondary battery having the above-mentioned features <5> The negative electrode used in the lithium-ion secondary battery.

[0018] Invention Effects

[0019] According to one aspect of the present invention, a negative electrode material for a lithium-ion secondary battery that can maintain its preservation characteristics well, as well as a negative electrode for a lithium-ion secondary battery and a lithium-ion secondary battery using the material, can be provided. Attached Figure Description

[0020] Figure 1 A cross-sectional view showing an example of the structure of a lithium-ion secondary battery. Detailed Implementation

[0021] The following describes in detail the methods for implementing the present invention. However, the present invention is not limited to the following embodiments. In the following embodiments, the constituent elements (including element steps, etc.) are not essential unless specifically stated otherwise. Similarly, numerical values ​​and their ranges do not limit the present invention. Furthermore, various changes and modifications can be made by those skilled in the art within the scope of the technical concept disclosed in this specification.

[0022] In this specification, the numerical range represented by “~” includes the minimum and maximum values ​​recorded before and after the “~”, respectively.

[0023] In this specification, the upper or lower limit of a numerical range described in stages can be replaced with the upper or lower limit of other numerical ranges described in stages. Furthermore, the upper or lower limit of a numerical range described in this specification can be replaced with the values ​​shown in the embodiments.

[0024] In this specification, the content and proportion of each component refer to the total content and proportion of the above-mentioned multiple substances unless otherwise stated.

[0025] In this specification, the particle size of each component refers to the value for a mixture of the aforementioned particles, unless otherwise stated. In the presence of multiple particles equivalent to each component, the particle size of each component refers to the value for a mixture of the aforementioned particles.

[0026] In this specification, the terms "layer" or "film" when referring to the area where the layer or film is present include not only the case where it is formed in the entire area, but also the case where it is formed only in a part of the area.

[0027] In this specification, the "solid components" of the positive electrode mixture or negative electrode mixture refer to the components remaining after removing volatile components such as organic solvents from the slurry of the positive electrode mixture or negative electrode mixture.

[0028] Negative electrode materials for lithium-ion secondary batteries

[0029] The negative electrode material for lithium-ion secondary batteries disclosed herein (hereinafter also referred to as negative electrode material) has graphite particles and a low-crystallinity 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 or C.

[0030] Condition A: The histogram of the R value obtained by Raman spectroscopy imaging has more than two peaks.

[0031] Condition B: The dispersion of the largest peak in the histogram of the R value obtained by Raman spectroscopy imaging is greater than 2.0.

[0032] Condition C: The low-crystallinity carbon layer contains two or more carbon phases with different crystallinities.

[0033] The inventors' research shows that a negative electrode material that satisfies at least one of conditions A, B, or C maintains its storage characteristics well compared to a negative electrode material that does not satisfy any of conditions A, B, or C. Furthermore, this negative electrode material maintains its good storage characteristics even when the particle size is reduced to improve input characteristics.

[0034] The reason is unclear, but it is believed to be due to the excellent coating properties of graphitic particles in low-crystallinity carbon layers that satisfy at least one of conditions A, B, or C.

[0035] (Graphite particles)

[0036] In this disclosure, graphitic particles refer to the interlayer (d) of carbon mesh in wide-angle X-ray diffraction. 002 Particles formed from carbon materials smaller than 0.340 nm.

[0037] carbon network interlayer (d) of carbon materials 002 The method can be to irradiate the sample with X-rays (CuKα rays), and then use the Bragg formula to calculate the diffraction pattern obtained by measuring the diffraction lines with a goniometer, based on the diffraction peaks of the carbon 002 plane that appear near the diffraction angle 2θ of 24° to 27°.

[0038] d 002 The determination can be carried out under the following conditions.

[0039] Source: CuKα rays (wavelength = 0.15418 nm)

[0040] Output power: 40kV, 20mA

[0041] Sampling amplitude: 0.010°

[0042] Scanning range: 10°~35°

[0043] Scanning speed: 0.5° / min

[0044] Bragg's formula: 2dsinθ=nλ

[0045] Here, d represents the length of one period, θ represents the diffraction angle, n represents the number of reflections, and λ represents the X-ray wavelength.

[0046] As graphitic particles, particles obtained by crushing lumpy natural graphite can be used. However, graphitic particles obtained by crushing lumpy natural graphite sometimes contain impurities, so it is preferable to use a refining process to increase the purity of natural graphite.

[0047] There are no particular limitations on the methods for refining natural graphite; appropriate methods can be selected from commonly used refining methods. Examples include flotation, electrochemical treatment, and chemical reagent treatment.

[0048] The purity of natural graphite, on a mass basis, is preferably 99.8% or higher (ash content 0.2% or less), more preferably 99.9% or higher (ash content 0.1% or less). A purity of 99.8% or higher tends to further improve battery safety and battery performance.

[0049] The purity of natural graphite can be determined, for example, by measuring the residual amount of ash after 100g of graphite has been left to stand in an ambient gas in an 800°C furnace for 48 hours.

[0050] As graphitic particles, particles obtained by pulverizing artificial graphite obtained by firing resin-based materials such as epoxy resin and phenolic resin, or asphalt-based materials obtained from petroleum, coal, etc. can also be used.

[0051] There are no particular limitations on the methods used to obtain artificial graphite. For example, a method can be used to calcine raw materials such as thermoplastic resins, naphthalene, anthracene, phenanthrene, coal tar, and tar pitch in an inert atmosphere at temperatures above 800°C to obtain artificial graphite as the calcined product. Next, the obtained calcined product is pulverized using known methods such as a jet mill, vibratory mill, pin mill, or hammer mill to adjust the average particle size to approximately 2 μm to 40 μm, thereby producing graphitic particles derived from artificial graphite. Alternatively, the raw materials can be pre-heat-treated before calcination. When heat-treating the raw materials, for example, pre-heat-treating can be performed using equipment such as an autoclave. After coarse pulverization using known methods, the raw materials, which have been heat-treated in an inert atmosphere at temperatures above 800°C as described above, are calcined. The resulting artificial graphite is then pulverized to adjust the average particle size to approximately 2 μm to 40 μm, thereby obtaining graphitic particles derived from artificial graphite.

[0052] (Low-crystallinity carbon layer)

[0053] In this disclosure, low-crystallinity carbon is a concept that includes amorphous carbon, and refers to the interlayer (d) of carbon mesh in wide-angle X-ray diffraction. 002 Carbon materials with a wavelength greater than 0.340 nm. In low-crystallinity carbon materials, sometimes the interplanar spacing (d) is... 002 Carbon materials with a wavelength greater than 0.340 nm and less than 0.350 nm are called soft carbon (easily graphitized carbon) and interfacial spacers (d). 002 Carbon materials with a wavelength of 0.350 nm or larger are called hard carbon (difficult-to-graphitize carbon).

[0054] The thickness of the low-crystallinity carbon layer (the maximum thickness when the thickness is not constant) is not particularly limited. For example, it can be selected from the range of 0.5 nm to 500 nm.

[0055] The thickness of a low-crystallinity carbon layer can be measured, for example, using a transmission electron microscope (TEM).

[0056] The ratio (mass ratio) of the low-crystallinity carbon layer to 1 part by mass of graphite particles is preferably 0.005 to 10, more preferably 0.005 to 5, and even more preferably 0.005 to 0.08. A ratio of 0.005 or higher tends to exhibit excellent initial charge / discharge efficiency and lifetime characteristics. Conversely, a ratio of 10 or lower tends to exhibit excellent output characteristics.

[0057] The aforementioned mass ratio can be calculated, for example, by measuring the weight change in the airflow using TG-DTA (Thermogravimetry-Differential Thermal Analysis), and determining the weight reduction rate from 500°C to 600°C. Furthermore, the weight change in the temperature range of 500°C to 600°C can be attributed to the weight change from materials other than graphite. On the other hand, the amount remaining after heat treatment can be attributed to graphite.

[0058] As a method of utilizing a low-crystallinity carbon layer to coat the surface of graphitic particles, one example is the method of heat-treating a mixture of a precursor containing graphitic particles and a low-crystallinity carbon layer.

[0059] Examples of precursors for low-crystallinity carbon layers include asphalt and organic polymers.

[0060] Examples of asphalt include ethylene tailings asphalt, crude oil asphalt, coal tar asphalt, pyrolysis asphalt, asphalt produced by thermally decomposing polyvinyl chloride, and asphalt produced by polymerizing naphthalene in the presence of superacids.

[0061] Examples of organic polymer compounds include thermoplastic resins such as polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate, and polyvinyl butyral, as well as natural substances such as starch and cellulose.

[0062] Anode materials that satisfy at least one of conditions A, B, or C can be obtained, for example, by using two or more types as precursors to a low-crystallinity carbon layer coated with graphitic particles.

[0063] The low-crystallinity carbon layer of the anode material that satisfies at least one of conditions A, B, or C is preferably formed using two or more precursors with different temperatures when they transform into the carbon phase. In this case, the precursor with the lower temperature when it transforms into the carbon phase first forms the carbon phase on the surface of the graphitic particles, followed by the precursor with the higher temperature when it transforms into the carbon phase forming the carbon phase, thereby forming a low-crystallinity carbon layer with better coating properties.

[0064] In one embodiment, the low-crystallinity carbon layer is preferably formed using precursors selected from bitumen and precursors selected from organic polymers, respectively.

[0065] When the low-crystallinity carbon layer contains two or more carbon phases with different crystallinities, the proportion of the carbon phase with the largest proportion in the low-crystallinity carbon layer is preferably 10% to 90% by mass of the entire low-crystallinity carbon layer, more preferably 20% to 80% by mass, and even more preferably 30% to 70% by mass.

[0066] Whether a low-crystallinity carbon layer contains two or more carbon phases with different crystallinities can be determined using a histogram of R values ​​obtained through Raman spectroscopy. Specifically, if the histogram of R values ​​obtained through Raman spectroscopy has two or more peaks, or if the dispersion of the largest peak in the histogram of R values ​​obtained through Raman spectroscopy is greater than 2.0, then the low-crystallinity carbon layer can be considered to meet condition A. The upper limit of the aforementioned dispersion can also be, for example, 5.0.

[0067] Raman spectroscopy imaging measurements were performed using a micro-Raman spectroscopy spectrometer (e.g., a DXR Raman spectroscopy imaging microscope, manufactured by Thermo Fisher Scientific). The conditions used included a 532 nm laser, a 100x lens, and a 25 μm aperture. An output power of 2.0 mW was sufficient to obtain adequate irradiation time and accumulation for the SN ratio. For example, an irradiation time of 2 seconds and 30 accumulations were performed. The R-value was determined by offsetting the measurement points on the particle by more than 1.5 μm or changing the particle by more than 100 points to obtain a histogram.

[0068] As a method for coating the surface of graphite particles with a low-crystallinity carbon layer obtained from two or more precursors, examples 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, then mixing it with other precursors and further heat-treating it. From the viewpoint of production efficiency, method (1) is preferred.

[0069] (Particle shape)

[0070] There are no particular limitations on the shape of the negative electrode material; it can be selected from spherical, flat, or amorphous shapes. From the viewpoint of balancing input and storage characteristics, a spherical or near-spherical shape is preferred. Furthermore, it is preferable that no secondary particles are formed.

[0071] In one embodiment, the average sphericity of the negative electrode material is preferably in the range of 0.8 to 1.0, more preferably in the range of 0.9 to 1.0, and even more preferably in the range of 0.95 to 1.0.

[0072] The average roundness of the negative electrode material can be determined using a wet-blowing particle size-shape analyzer (e.g., FPIA-3000, Malvern).

[0073] As a pretreatment for the assay, purified water containing 0.06 g of the negative electrode material and 0.2% by mass of a surfactant (trade name: Liponol T / 15, Lion Co., Ltd.) can be placed in a test tube (12mm × 120mm, Maruemu Co., Ltd.), stirred for 20 seconds in a test tube mixer (Pasolina NS-80, As One Co., Ltd.), and then ultrasonically stirred for 1 minute. As an ultrasonic cleaner, the US102 from SND Co., Ltd. (high-frequency output power 100W, oscillation frequency 38kHz) can be used.

[0074] The analysis of the standard deviation of roundness within a specific range based on the roundness measurement results can be carried out in accordance with the FPIA-3000 academic materials (2nd edition, published on August 31, 2006).

[0075] In addition, the measurement temperature was 25°C, the concentration of the sample was 10% by mass, and the number of particles counted was 10,000. Water was used as the dispersion solvent.

[0076] (Average particle size)

[0077] The average particle size of the negative electrode material is preferably 2 μm to 30 μm, more preferably 2.5 μm to 25 μm, further preferably 3 μm to 20 μm, and particularly preferably 5 μm to 20 μm. When the average particle size of the graphite particles is less than 30 μm, there is a tendency to improve the discharge capacity and discharge characteristics. When the average particle size of the graphite particles is greater than 2 μm, there is a tendency to improve the initial charge and discharge efficiency.

[0078] The average particle size of the negative electrode material is the volume average particle size obtained by measuring the particle size distribution based on the volume using a particle size distribution measuring device (manufactured by Shimadzu Corporation, SALD-3000) that utilizes laser diffraction / scattering method, and using the particle size distribution as d50 (median diameter).

[0079] In one embodiment, the average particle size of the negative electrode material can be 9 μm or less, or even 8 μm or less. When the average particle size of the negative electrode material is 9 μm or less, there is a tendency to obtain good input characteristics. Even when the average particle size of the negative electrode material disclosed herein is 9 μm or less, it can maintain good storage characteristics, thus improving both input characteristics and storage characteristics.

[0080] (R value)

[0081] 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.

[0082] The above R value is located at 1300 cm⁻¹ in the Raman spectra obtained by irradiating the negative electrode material with a laser of wavelength 532 nm. -1 ~1400cm -1 Peak intensity ID range at 1580 cm⁻¹ -1 ~1620cm -1 The ratio of peak intensity IG to peak intensity IG (ID / IG).

[0083] Raman spectrophotometry can be performed using Raman spectrometers (such as Thermo Fisher Scientific's DXR).

[0084] (BET specific surface area)

[0085] The preferred BET specific surface area of ​​the negative electrode material is 0.8 m². 2 / g~8m 2 / g, more preferably 1m 2 / g~7m 2 / g, further preferably 1.5m 2 / g~6m 2 / g.

[0086] The BET specific surface area of ​​the negative electrode material is 0.8 m². 2 When the surface area is above a certain value (e.g., g), there is a tendency to obtain excellent battery performance. Furthermore, the BET specific surface area of ​​the anode material is 8 m² / g. 2 When the density is below / g, the tap density tends to be easily increased, and the compatibility with other materials such as binders and conductive agents becomes better.

[0087] The BET specific surface area of ​​the negative electrode material can be determined from the nitrogen adsorption energy according to JIS Z 8830:2013. As an evaluation device, the AUTOSORB-1 (trade name) manufactured by QUANTACHROME can be used. When determining the BET specific surface area, it is considered that moisture adsorbed on the sample surface and in the structure will affect the gas adsorption energy; therefore, it is preferable to first perform a pretreatment to remove moisture by heating.

[0088] In the pretreatment, the pressure in the test cell containing 0.05g of the test sample was reduced to below 10Pa using a vacuum pump. The cell was then heated at 110°C and maintained at this temperature for at least 3 hours. Afterward, it was allowed to cool naturally to room temperature (25°C) while maintaining the reduced pressure. Following this pretreatment, the evaluation temperature was set to 77K, and the evaluation pressure range was set to less than 1 relative pressure (equilibrium pressure relative to saturated vapor pressure) for the measurement.

[0089] Negative electrode for lithium-ion secondary batteries

[0090] The negative electrode (negative electrode) for lithium-ion secondary batteries disclosed herein has a current collector and a negative electrode flux layer disposed on the surface of the current collector and comprising the negative electrode material for lithium-ion secondary batteries of this disclosure. Details of the current collector and the negative electrode flux layer will be described later.

[0091] Lithium-ion secondary batteries

[0092] The lithium-ion secondary battery disclosed herein is not particularly limited in its composition as long as it has a negative electrode comprising the negative electrode material of the lithium-ion secondary battery disclosed herein. The negative electrode material of the lithium-ion secondary battery disclosed herein may be contained in the negative electrode flux layer.

[0093] (Overview of lithium-ion secondary batteries)

[0094] 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 within its battery container. A separator is positioned between the positive and negative electrodes.

[0095] When charging a lithium-ion secondary battery, a charger is connected between the positive and negative electrodes. During charging, lithium ions embedded in the positive electrode active material are de-intercalated and released into the non-aqueous electrolyte. The released lithium ions move in the non-aqueous electrolyte, pass through the separator, and reach the negative electrode. These lithium ions that reach the negative electrode are then intercalated into the negative electrode active material that constitutes the negative electrode.

[0096] During discharge, an external load is connected between the positive and negative electrodes. During discharge, lithium ions embedded in the negative electrode active material are de-intercalated and 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 then move within the electrolyte, pass through the separator, and reach the positive electrode. These lithium ions that reach the positive electrode are intercalated into the positive electrode active material. Through the intercalation of lithium ions into the positive electrode active material, electrons flow into the positive electrode. Thus, discharge occurs through the movement of electrons from the negative electrode to the positive electrode.

[0097] Thus, lithium-ion secondary batteries can be charged and discharged by the insertion and extraction of lithium ions between the positive and negative electrode active materials. Furthermore, examples of the actual structure of lithium-ion secondary batteries will be described later (see, for example, [reference]). Figure 1 ).

[0098] Next, the positive electrode, negative electrode, non-aqueous electrolyte, separator, and other constituent components provided as needed will be described in turn as constituent elements of the lithium-ion secondary battery disclosed herein.

[0099] (positive electrode)

[0100] The lithium-ion secondary battery disclosed herein has a high-capacity and high-input-output positive electrode suitable for lithium-ion secondary batteries, as shown below. The positive electrode (positive plate) of this disclosure has a current collector (positive current collector) and a positive electrode flux layer disposed on its surface. The positive electrode flux layer is a layer disposed on the surface of the current collector and contains at least a positive electrode active material.

[0101] As a positive electrode active material, a layered lithium-nickel-manganese-cobalt composite oxide (also sometimes referred to as NMC) is preferred. NMC tends to have high capacity and excellent safety.

[0102] From the perspective of further improving safety, a mixture of NMC and spinel-type lithium manganese composite oxide (hereinafter sometimes referred to as sp-Mn) is preferred as the positive electrode active material.

[0103] From the viewpoint of increasing battery capacity, the NMC content is preferably 65% ​​by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more relative to the total amount of the positive electrode additive layer.

[0104] As an NMC, the substance shown in the following composition formula (chemical structural formula 1) is preferred.

[0105] Li (1+δ) Mn x Ni y Co (1-x-y-z) M z O2 (chemical structural formula 1)

[0106] In the composition formula (chemical structural 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.

[0107] Element M is selected from at least one element selected from Ti (titanium), Zr (zirconium), Nb (niobium), Mo (molybdenum), W (tungsten), Al (aluminum), Si (silicon), Ga (gallium), Ge (germanium), and Sn (tin).

[0108] Additionally, -0.15 < δ < 0.15, 0.1 <x≤0.5、0.6<x+y+z<1.0、0≤z≤0.1。

[0109] As sp-Mn, it is preferred to use a substance represented by the following composition formula (chemical structural formula 2).

[0110] Li (1+η) Mn (2-λ) M' λ O4 (chemical structural formula 2)

[0111] In the composition formula (chemical structural 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.

[0112] Element M' is preferably selected from at least one element chosen from Mg (magnesium), Ca (calcium), Sr (strontium), Al, Ga, Zn (zinc) and Cu (copper).

[0113] 0≤η≤0.2、0≤λ≤0.1.

[0114] As element M' in the composition formula (chemical structural formula 2), Mg or Al is preferred. Using Mg or Al tends to result in a longer battery life. Additionally, it tends to improve battery safety. Furthermore, by adding element M', the dissolution of Mn can be reduced, thus tending to improve storage characteristics and charge-discharge cycle characteristics.

[0115] In addition to NMC and sp-Mn, other substances can also be used as positive electrode active materials.

[0116] As positive electrode active materials other than NMC and sp-Mn, commonly used materials in this field can be used, such as lithium-containing complex metal oxides, olivine-type lithium salts, chalcogenides, and manganese dioxide.

[0117] Lithium-containing composite metal oxides are metal oxides containing lithium and transition metals, or metal oxides in which a portion of the transition metal is replaced by a different element. Examples of such different elements 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 different element may be used alone, or two or more may be used in combination.

[0118] Li can be cited as an example of a lithium-containing composite metal oxide other than NMC and sp-Mn. x CoO2, Lix 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 wherein M 1 represents at least one element selected from Na, Mg, Sc, Y, Mn, Fe, Ni, Cu, Zn, Al, Cr, Pb, Sb, V and B), Li x Ni 1-y M 2 y O z (Li x Ni 1-y M 2 y O z wherein M 2 is at least one element selected from Na, Mg, Sc, Y, Mn, Fe, Co, Cu, Zn, Al, Cr, Pb, Sb, V and B). 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. In addition, the value x, which represents the molar ratio of lithium, increases and decreases with charging and discharging.

[0119] In addition, examples of the olivine-type lithium salt include LiFePO₄ and the like. Examples of the chalcogenide include titanium disulfide, molybdenum disulfide and the like. The positive electrode active material may be used alone in one type, or may be used in combination of two or more types.

[0120] Next, the positive electrode mixture layer and the current collector will be described in detail. The positive electrode mixture layer contains the positive electrode active material, a binder and the like, and is disposed on the current collector. The formation method of the positive electrode mixture layer is not limited, and it is formed, for example, as follows. The positive electrode mixture layer can be formed by dry-mixing the positive electrode active material, the binder, and other materials such as a conductive agent and a thickener that are used as needed to obtain a sheet, and pressure-bonding the sheet to the current collector (dry method). Alternatively, the positive electrode mixture layer can be formed by dissolving or dispersing the positive electrode active material, the binder, and other materials such as a conductive agent and a thickener that are used as needed in a dispersion solvent to prepare a slurry of the positive electrode mixture, coating the slurry on the current collector and drying the slurry (wet method).

[0121] As described above, layered lithium-nickel-manganese-cobalt composite oxide (NMC) is preferably used as the positive electrode active material. The positive electrode active material is used or mixed in powder (granular) form.

[0122] As positive electrode active materials such as NMC and sp-Mn, particles with shapes such as block, polyhedral, spherical, ellipsoidal, plate, needle, and columnar can be used.

[0123] The average particle size (d50) of positive electrode active materials such as NMC and sp-Mn (when primary particles agglomerate to form secondary particles, it is the average particle size (d50) of the 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 viewpoint of tap density (filling properties) and the mixing with other materials during electrode formation. The average particle size (d50) of the positive electrode active material particles can be measured in the same way as that of graphite particles.

[0124] The preferred range for the BET specific surface area of ​​positive electrode active materials such as NMC and sp-Mn is 0.2 m². 2 / g~4.0m 2 / g, more preferably 0.3m 2 / g~2.5m 2 / g, further preferably 0.4m 2 / g~1.5m 2 / g.

[0125] If the BET specific surface area of ​​the positive electrode active material particles is 0.2 m², then... 2 A surface area of ​​4.0 m² / g or higher tends to result in superior battery performance. Additionally, when the BET specific surface area of ​​the positive electrode active material particles is 4.0 m² / g... 2 When the density is below a certain value (e.g.), the tap density tends to increase easily, and the mixability with other materials such as binders and conductive agents becomes better. The BET specific surface area can be measured in the same way as graphite particles.

[0126] Examples of conductive agents used in the positive electrode include metallic materials such as copper and nickel; graphite (natural graphite, artificial graphite, etc.); carbon black such as acetylene black; and carbonaceous materials such as needle coke and other amorphous carbon. Furthermore, a single conductive agent can be used for the positive electrode, or two or more can be used in combination.

[0127] The content of the conductive agent relative to the mass of the positive electrode binder layer is preferably 0.01% to 50% by mass, more preferably 0.1% to 30% by mass, and even more preferably 1% to 15% by mass. When the content of the conductive agent is 0.01% by mass or more, there is a tendency to easily obtain sufficient conductivity. When the content of the conductive agent is 50% by mass or less, there is a tendency to suppress the reduction of battery capacity.

[0128] There are no particular limitations on the binder used for the positive electrode. When forming the positive electrode binder layer using a wet method, materials with good solubility or dispersibility in the dispersion solvent should be selected. Specifically, 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); fluorinated polymers such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, polytetrafluoroethylene-vinylidene fluoride copolymer, and fluorinated polyvinylidene fluoride; and polymer compositions with ion conductivity of alkali metal ions (especially lithium ions). Furthermore, a single binder can be used for the positive electrode, or two or more can be used in combination.

[0129] From the perspective of the stability of the positive electrode, fluorine-based polymers such as polyvinylidene fluoride (PVdF) or polytetrafluoroethylene-vinylidene fluoride copolymer are preferred as binders.

[0130] The content of the binder relative to the mass of the positive electrode binder layer is preferably 0.1% to 60% by mass, more preferably 1% to 40% by mass, and even more preferably 3% to 10% by mass.

[0131] When the binder content is 0.1% by mass or higher, there is a tendency to improve battery performance, such as sufficient bonding of the positive electrode active material, adequate mechanical strength of the positive electrode binder layer, and improved cycle characteristics. When the binder content is 60% by mass or lower, there is a tendency to obtain sufficient battery capacity and conductivity.

[0132] Thickeners are effective in adjusting the viscosity of slurries. There are no particular limitations on thickeners; specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts. A single thickener can be used, or two or more can be used in combination.

[0133] From the viewpoint of input / output characteristics and battery capacity, the content of the thickener relative to the mass of the positive electrode compound layer when using a thickener is preferably 0.1% to 20% by mass, more preferably 0.5% to 15% by mass, and even more preferably 1% to 10% by mass.

[0134] As a dispersion solvent used to form the slurry, there are no restrictions on its type, as long as it can dissolve or disperse the positive electrode active material, binder, and conductive agent or thickener used as needed. Either aqueous or organic solvents can be used. Examples of aqueous solvents include water, alcohols, and mixtures of water and alcohols. 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. In particular, when using aqueous solvents, thickeners are preferred.

[0135] To increase the filling density of the positive electrode active material, the positive electrode compound layer formed on the current collector using wet or dry methods is preferably compacted by manual pressing or roll pressing.

[0136] From the perspective of further improving input-output characteristics and safety, the density of the compacted positive electrode mixture layer is preferably 2.5 g / cm³. 3 ~3.5g / cm 3 The range, more preferably 2.55 g / cm³, is [specific value missing]. 3 ~3.15g / cm 3 The range, more preferably 2.6 g / cm³, is [specific value missing]. 3 ~3.0g / cm 3 The range.

[0137] Furthermore, from the viewpoint of energy density and input / output characteristics, the single-sided coating amount of the positive electrode slurry on the current collector when forming the positive electrode mixture layer is preferably 30 g / m² based on the solid content of the positive electrode mixture. 2 ~170g / m 2 More preferably 40g / m 2 ~160g / m 2 A further preferred value is 40g / m 2 ~150g / m 2 .

[0138] Considering the amount of positive electrode slurry coated on one side of the current collector and the density of the positive electrode slurry layer, the average thickness of the positive electrode slurry 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 this disclosure, the average thickness of the slurry layer is the average of the thickness at any 10 locations.

[0139] There are no particular limitations on the material used as the current collector for the positive electrode, but metallic materials are preferred, and aluminum is more preferred. There are no particular limitations on the shape of the current collector, and materials processed into various shapes can be used. Regarding metallic materials, examples include metal foil, metal plate, metal film, and expanded metal, with metal film being preferred. Furthermore, the film can be appropriately formed into a mesh shape.

[0140] The average thickness of the current collector is not particularly limited, but from the viewpoint of obtaining the strength and good flexibility required for a 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.

[0141] (negative electrode)

[0142] The lithium-ion secondary battery disclosed herein has a high-capacity and high-input-output negative electrode suitable for lithium-ion secondary batteries, as shown below. The negative electrode (negative electrode plate) of this disclosure has a current collector (negative electrode current collector) and a negative electrode flux layer disposed on its surface. The negative electrode flux layer is a layer disposed on the surface of the current collector and contains at least a negative electrode active material. The negative electrode for lithium-ion secondary batteries of this disclosure can be used as the negative electrode.

[0143] The negative electrode active material contained in the negative electrode mixture layer of the lithium-ion secondary battery disclosed herein is used as the negative electrode material for lithium-ion secondary batteries disclosed herein.

[0144] From the viewpoint of increasing battery capacity, the content of the negative electrode material for lithium-ion secondary batteries disclosed herein is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, relative to the total amount of the negative electrode additive layer.

[0145] Next, the negative electrode mixture layer and the current collector will be described in detail. The negative electrode mixture layer contains negative electrode active material, binder, etc., and is disposed on the current collector. There are no restrictions on the method of forming the negative electrode mixture layer, for example, as follows: The negative electrode active material, binder, and other materials such as conductive agents and thickeners used as needed can be dissolved or dispersed in a dispersion solvent to prepare a negative electrode mixture slurry, which is then coated on the current collector and dried (wet method) to form the negative electrode mixture layer.

[0146] As a conductive agent for the negative electrode, natural graphite, artificial graphite, and other graphite (graphite particles) other than the graphitic particles of the lithium-ion secondary battery negative electrode material disclosed herein, as well as carbon black such as acetylene black, and amorphous carbon such as needle coke, can be used. One type of conductive agent can be used alone, or two or more can be used in combination. Thus, by adding a conductive agent, there is a tendency to achieve effects such as reducing electrode resistance.

[0147] 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 binder layer is preferably 1% to 45% by mass, more preferably 2% to 42% by mass, and even more preferably 3% to 40% by mass. When the content of the conductive agent is 1% by mass or more, there is a tendency to easily obtain sufficient conductivity. When the content of the conductive agent is 45% by mass or less, there is a tendency to suppress the reduction of battery capacity.

[0148] As a binder for the negative electrode, there are no particular limitations as long as it is a material stable to non-aqueous electrolytes or the dispersion solvent used during electrode formation. Specifically, 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 with ion conductivity of alkali metal ions (especially lithium ions). Furthermore, a single binder for the negative electrode can be used, or two or more can be used in combination. Fluorine-based polymers, such as SBR and polyvinylidene fluoride, are preferred.

[0149] The content of the binder relative to the mass of the negative electrode adhesive 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.

[0150] When the binder content is 0.1% by mass or more, there is a tendency to ensure sufficient bonding of the negative electrode active material and obtain sufficient mechanical strength of the negative electrode binder layer. When the binder content is 20% by mass or less, there is a tendency to obtain sufficient battery capacity and conductivity.

[0151] Furthermore, when using fluorinated polymers, such as polyvinylidene fluoride, as the main component, the content of the binder relative to the mass of the negative electrode binder layer is preferably 1% to 15% by mass, more preferably 2% to 10% by mass, and even more preferably 3% to 8% by mass.

[0152] Thickeners are used to adjust the viscosity of slurries. There are no particular limitations on thickeners; specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts. A single thickener can be used, or two or more can be used in combination.

[0153] From the viewpoint of input / output characteristics and battery capacity, the content of the thickener relative to the mass of the negative electrode binder layer when using a thickener is preferably 0.1% to 5% by mass, more preferably 0.5% to 3% by mass, and even more preferably 0.6% to 2% by mass.

[0154] As a dispersion solvent for forming the slurry, there are no restrictions on its type, as long as it can dissolve or disperse the negative electrode active material, binder, and conductive agent or thickener used as needed. Either aqueous or organic solvents can be used. Examples of aqueous solvents include water, alcohols, and mixtures of water and alcohols. 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. In particular, when using aqueous solvents, thickeners are preferred.

[0155] The density of the negative electrode mixture layer is preferably 0.7 g / cm³. 3 ~2g / cm 3 More preferably 0.8 g / cm 3 ~1.9g / cm 3 A further preferred value is 0.9 g / cm³. 3 ~1.8g / cm 3 .

[0156] The density of the negative electrode mixture layer is 0.7 g / cm³. 3 At the above levels, there is a tendency for increased conductivity between the negative electrode active materials, suppression of increased battery resistance, and increased capacity per unit volume. The density of the negative electrode binder layer is 2 g / cm³. 3 The following tends to reduce the risk of increased initial irreversible capacity and decreased discharge characteristics due to reduced permeability of non-aqueous electrolyte to the interface between the current collector and the negative electrode active material.

[0157] Furthermore, from the viewpoint of energy density and input / output characteristics, the amount of negative electrode slurry coated onto one side of the current collector when forming the negative electrode mixture layer is preferably 30 g / m³ based on the solid content of the negative electrode mixture. 2 ~150g / m 2 More preferably 40g / m 2 ~140g / m 2 A further preferred value is 45g / m 2 ~130g / m 2 .

[0158] Considering the amount of negative electrode slurry coated on one side of the current collector and the density of the negative electrode slurry layer, the average thickness of the negative electrode slurry layer is preferably 10 μm to 150 μm, more preferably 15 μm to 140 μm, and even more preferably 15 μm to 120 μm.

[0159] There are no particular limitations on the material used as the current collector for the negative electrode. Specific examples include copper, nickel, stainless steel, and nickel-plated copper. Among these, copper is preferred from the perspective of ease of processing and cost.

[0160] There are no particular limitations on the shape of the current collector, and materials processed into various shapes can be used. Specific examples include metal foil, metal plate, metal film, and expanded metal. Among these, metal film is preferred, and copper foil is more preferred. Copper foil can be rolled copper foil formed by rolling or electrolytic copper foil formed by electrolysis; either type is preferred as a current collector.

[0161] The average thickness of the current collector is not particularly limited. For example, it is preferably 5μm to 50μm, more preferably 8μm to 40μm, and even more preferably 9μm to 30μm.

[0162] Furthermore, when the average thickness of the current collector is less than 25 μm, its strength can be improved by using copper alloys that are stronger than pure copper (phosphor bronze, titanium copper, Cosun alloy, Cu-Cr-Zr alloy, etc.).

[0163] (Non-aqueous electrolyte)

[0164] Non-aqueous electrolytes generally contain non-aqueous solvents and lithium salts (electrolytes).

[0165] First, let's clarify the non-aqueous solvent.

[0166] Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, and cyclic sulfonates.

[0167] As a cyclic carbonate, it is preferable that the alkylene group constituting the cyclic carbonate has 2 to 6 carbon atoms, more preferably 2 to 4. Examples include ethylene carbonate, propylene carbonate, and butyl carbonate. Among these, ethylene carbonate and propylene carbonate are preferred.

[0168] As a chain carbonate, dialkyl carbonate is preferred, with the number of carbon atoms in the two alkyl groups preferably being 1 to 5, more preferably 1 to 4. Examples include symmetrical chain carbonates such as dimethyl carbonate, diethyl carbonate, and di-n-propyl carbonate; and asymmetrical chain carbonates such as methyl ethyl carbonate, methyl-n-propyl carbonate, and ethyl-n-propyl carbonate. Among these, dimethyl carbonate and methyl ethyl carbonate are preferred. Dimethyl carbonate has better oxidation and reduction resistance than diethyl carbonate, thus tending to improve cycle performance. Ethyl methyl carbonate has an asymmetrical molecular structure and a low melting point, thus tending to improve low-temperature performance. Mixed solvents containing ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate are particularly preferred as they ensure battery performance over a wide temperature range.

[0169] From the viewpoint of battery characteristics, the content of cyclic carbonates and chain carbonates, based on the total amount of non-aqueous solvents, is preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

[0170] Furthermore, from the viewpoint of battery characteristics, the mixing ratio of cyclic carbonate and chain carbonate when using cyclic carbonate and chain carbonate is preferably 1 / 9 to 6 / 4, more preferably 2 / 8 to 5 / 5.

[0171] Examples of cyclic sulfonates include 1,3-propanesulfonyl lactone, 1-methyl-1,3-propanesulfonyl lactone, 3-methyl-1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, 1,3-propenesulfonyl lactone, and 1,4-butenesulfonyl lactone. Among these, 1,3-propanesulfonyl lactone and 1,4-butanesulfonyl lactone are particularly preferred from the viewpoint of further reducing DC resistance.

[0172] Non-aqueous electrolytes may further contain chain esters, cyclic ethers, chain ethers, cyclic sulfones, etc.

[0173] Examples of chain esters include methyl acetate, ethyl acetate, propyl acetate, and methyl propionate. Among these, methyl acetate is preferred from the viewpoint of improving low-temperature properties.

[0174] Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, and tetrahydropyran.

[0175] Examples of chain ethers include dimethoxyethane and dimethoxymethane.

[0176] Examples of cyclic sulfones include sulfolane and 3-methylsulfolane.

[0177] Non-aqueous electrolytes may also contain silyl phosphate compounds.

[0178] 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, and phosphate. Octyl bis(trimethylsilyl) ester, diphenyltrimethylsilyl phosphate, phenyl bis(trimethylsilyl) phosphate, di(trifluoroethyl)(trimethylsilyl) phosphate, trifluoroethyl bis(trimethylsilyl) phosphate, compounds in which the trimethylsilyl group of the above-mentioned silyl phosphates is replaced by triethylsilyl, triphenylsilyl, tert-butyldimethylsilyl, etc., compounds with a so-called condensed phosphate ester structure formed by the condensation of phosphate esters with each other and phosphorus atoms bonded by oxygen, etc.

[0179] Tris(trimethylsilyl) phosphate (TMSP) is preferred. Compared with other silyl phosphate compounds, TMSP can suppress the increase in resistance with a smaller amount added.

[0180] These silyl phosphates can be used alone or in combination of two or more.

[0181] When the non-aqueous electrolyte contains silyl phosphate compound, the content of silyl phosphate compound relative to the total amount of the non-aqueous electrolyte is preferably 0.1% to 5% by mass, more preferably 0.3% to 3% by mass, and even more preferably 0.4% to 2% by mass.

[0182] In particular, when the non-aqueous electrolyte contains trimethylsilyl phosphate (TMSP), the content of trimethylsilyl phosphate (TMSP) relative to the total amount of the non-aqueous electrolyte is preferably 0.1% to 0.5% by mass, more preferably 0.1% to 0.4% by mass, and even more preferably 0.2% to 0.4% by mass. When the TMSP content is within the above range, it tends to improve lifetime characteristics through the effect of a thin SEI (Solid Electrolyte Interphase) and the like.

[0183] In addition, 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 the charging of the lithium-ion secondary battery. This coating has the effect of inhibiting the decomposition of the non-aqueous electrolyte on the surface of the negative electrode.

[0184] The content of vinylene carbonate relative to the total amount of non-aqueous electrolyte is preferably 0.3% to 1.6% by mass, more preferably 0.3% to 1.5% by mass, and even more preferably 0.3% to 1.3% by mass. When the content of vinylene carbonate is within the above range, it tends to improve lifespan characteristics and prevent the decomposition of excess VC during the charging and discharging of lithium-ion secondary batteries, which would reduce charging and discharging efficiency.

[0185] Next, lithium salts (electrolytes) will be explained.

[0186] As for lithium salts, there are no particular restrictions as long as they can be used as electrolytes in non-aqueous electrolytes for lithium-ion secondary batteries. Examples include inorganic lithium salts, fluorine-containing organic lithium salts, and oxalate borate.

[0187] Examples of inorganic lithium salts include inorganic fluoride salts such as LiPF6, LiBF4, LiAsF6, and LiSbF6; perhalate salts such as LiClO4, LiBrO4, and LiIO4; and inorganic chloride salts such as LiAlCl4.

[0188] Examples of fluorinated organic lithium salts include perfluoroalkane sulfonates such as LiCF3SO3; perfluoroalkane sulfonyl imide salts such as LiN(CF3SO2)2, LiN(CF3CF2SO2)2, and LiN(CF3SO2)(C4F9SO2); perfluoroalkane sulfonyl methyl salts such as LiC(CF3SO2)3; and fluoroalkyl fluorinated phosphates 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].

[0189] Examples of oxalate borates include lithium dioxalate borate and lithium difluorooxalate borate.

[0190] These lithium salts can be used alone or in combination of two or more. Among them, lithium hexafluorophosphate (LiPF6) is preferred when comprehensively considering factors such as solubility in solvents, charge-discharge characteristics, output characteristics, and cycle characteristics when used in lithium-ion secondary batteries.

[0191] There are no particular limitations on the electrolyte concentration in the non-aqueous electrolyte. The concentration range of the electrolyte 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. Furthermore, 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. When the electrolyte concentration is 0.5 mol / L or more, the conductivity of the non-aqueous electrolyte tends to become sufficient. When the electrolyte concentration is 2 mol / L or less, the increase in viscosity of the non-aqueous electrolyte tends to be suppressed, thus the conductivity tends to increase. The increase in the conductivity of the non-aqueous electrolyte tends to improve the performance of the lithium-ion secondary battery.

[0192] (Diaphragm)

[0193] There are no particular restrictions on the type of membrane, as long as it is electronically insulated between the positive and negative electrodes, has ion permeability, and is resistant to oxidation on the positive side and reduction on the negative side. Resins, inorganic materials, etc., are used as membrane materials that meet these characteristics.

[0194] As resins, olefin polymers, fluorinated polymers, cellulose polymers, polyimides, nylon, etc. are used. Materials that are stable to non-aqueous electrolytes and have excellent liquid retention properties are preferred, and porous sheets or nonwoven fabrics made from polyolefins such as polyethylene and polypropylene are particularly favored.

[0195] As inorganic materials, oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and glass are used. For example, the aforementioned inorganic materials in fibrous or particle shapes can be made into nonwoven fabrics, woven fabrics, or attached to a thin-film substrate such as a microporous membrane for use as a separator. As a thin-film substrate, a substrate with a pore size of 0.01 μm to 1 μm and an average thickness of 5 μm to 50 μm is preferred. In addition, the aforementioned inorganic materials in fibrous or particle shapes can be formed into a composite porous layer using a binder such as a resin for use as a separator. Furthermore, this composite porous layer can be formed on the surface of other separators to form a multilayer separator. Moreover, this composite porous layer can also be formed on the surface of the positive or negative electrode as a separator.

[0196] (Other constituent components)

[0197] As another component of a lithium-ion secondary battery, a split valve can also be installed. By opening the split valve, the pressure rise inside the battery can be suppressed, thus improving safety.

[0198] Additionally, a component can be included that releases an inert gas (such as carbon dioxide) as the temperature rises. By incorporating this component, the generation of the inert gas when the internal temperature of the battery rises can rapidly open the split valve, thus improving safety. Lithium carbonate, polyethylene carbonate, and polypropylene carbonate are preferred materials used in the aforementioned component.

[0199] 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 irreversible capacity of either the negative electrode or the positive electrode, whichever is greater]. Here, the [discharge capacity of the negative electrode] is defined as the value obtained using a charge-discharge apparatus when lithium ions embedded in the negative electrode active material are extracted. Similarly, the [initial charge capacity of the positive electrode] is defined as the value obtained using a charge-discharge apparatus when lithium ions are extracted from the positive electrode active material.

[0200] The capacity ratio of the negative electrode to the positive electrode can also be calculated, for example, by dividing the discharge capacity of the negative electrode by the discharge capacity of the lithium-ion secondary battery. The discharge capacity of the lithium-ion secondary battery can be measured, for example, under the following conditions: after charging at 4.2V, 0.1C–0.5C, and a termination time of 2–5 hours using constant current and constant voltage (CCCV), it is discharged to 2.7V using a constant current (CC) of 0.1C–0.5C. The discharge capacity of the negative electrode can be calculated as follows: the negative electrode from which the discharge capacity of the lithium-ion secondary battery has been measured is cut into a specified area. A single-electrode battery is fabricated using lithium metal as the counter electrode, separated by a separator impregnated with a non-aqueous electrolyte. After charging at 0V, 0.1C, and a termination current of 0.01C using constant current and constant voltage (CCCV), it is discharged to 1.5V using a constant current (CC) of 0.1C. The discharge capacity per unit area is then measured and converted to the total area used as the negative electrode of the lithium-ion secondary battery, thus yielding the capacity. In this unipolar battery, the direction in which lithium ions are inserted into the negative electrode active material is defined as charging, and the direction in which lithium ions already inserted into the negative electrode active material are extracted is defined as discharging. Furthermore, C refers to "current value (A) / battery discharge capacity (Ah)".

[0201] (Lithium-ion secondary battery)

[0202] Next, with reference to the accompanying drawings, an embodiment in which this disclosure is applied to an 18650 type cylindrical lithium-ion secondary battery will be described. Figure 1 A cross-sectional view of a lithium-ion secondary battery using the present disclosure.

[0203] like Figure 1As shown, the lithium-ion secondary battery 1 of this disclosure has a bottomed cylindrical battery container 6 made of nickel-plated steel. A strip-shaped positive electrode plate 2 and a negative electrode plate 3 are housed within the battery container 6, separated by a separator 4 made of porous polyethylene sheet wound into a spiral cross-section, forming an electrode winding assembly 5. The separator 4 is, for example, configured with a width of 58 mm and an average thickness of 30 μm. An aluminum strip-shaped positive electrode tab terminal, one end of which is fixed to the positive electrode plate 2, is extended from the upper end face of the electrode winding assembly 5. The other end of the positive electrode tab terminal is ultrasonically welded to the underside of a disc-shaped battery cover, which serves as the external positive electrode terminal, located on the upper side of the electrode winding assembly 5. On the other hand, a copper strip-shaped negative electrode tab terminal, one end of which is fixed to the negative electrode plate 3, is extended from the lower end face of the electrode winding assembly 5. The other end of the negative electrode tab terminal is resistance welded to the inner bottom of the battery container 6. Therefore, the positive and negative electrode tabs are led to opposite sides of the two end faces of the electrode winding assembly 5. Furthermore, an insulating coating (not shown) is applied to the entire circumference of the outer periphery of the electrode winding assembly 5. The battery cover is secured to the upper part of the battery container 6 by an insulating resin gasket. Therefore, the interior of the lithium-ion secondary battery 1 is sealed. Additionally, a non-aqueous electrolyte (not shown) is injected into the battery container 6.

[0204] Example

[0205] The present invention will now be described in more detail with reference to the embodiments. However, the present invention is not limited to the following embodiments.

[0206] [Fabrication of Anode Materials]

[0207] (Comparative Example 1)

[0208] 100 parts by mass of spherical natural graphite with a volume average particle size of 10.0 μm and 10 parts by mass of coal tar pitch (softening point of 90℃, carbon residue (carbonization rate) of 50%) were mixed to obtain a mixture. The mixture was then heat-treated to form a low-crystallinity carbon layer on the surface of the graphitic particles. The heat treatment was carried out as follows: under nitrogen flow, the temperature was increased from 25℃ to 1000℃ at a rate of 200℃ / hour, and held at 1000℃ for 1 hour. The resulting particles were pulverized using a shredder and sieved; the sieved portion was used as the negative electrode material. The physical properties of the obtained negative electrode material are shown in Table 1.

[0209] (Example 1)

[0210] A mixture was prepared by mixing 100 parts by mass of spherical natural graphite with a volume average particle size of 10.0 μm, 7 parts by mass of coal tar pitch (softening point of 90 °C, carbon residue (carbonization rate) of 50%), and 11 parts by mass of polyvinyl alcohol (carbon residue (carbonization rate) of 14%). A negative electrode material was obtained in the same manner as in Comparative Example 1, except that the negative electrode material was used. The physical properties of the obtained negative electrode material are shown in Table 1.

[0211] (Comparative Example 2)

[0212] 100 parts by mass of spherical natural graphite with a volume average particle size of 8.0 μm and 10 parts by mass of coal tar pitch (softening point of 90 °C and residual carbon content (carbonization rate) of 50%) were mixed to obtain a mixture. A negative electrode material was obtained in the same manner as in Comparative Example 1, except that the mixture was used. The physical properties of the obtained negative electrode material are shown in Table 1.

[0213] (Example 2)

[0214] A mixture was prepared by mixing 100 parts by mass of spherical natural graphite with a volume average particle size of 8.0 μm, 7 parts by mass of coal tar pitch (softening point of 90 °C, carbon residue (carbonization rate) of 50%), and 11 parts by mass of polyvinyl alcohol (carbon residue (carbonization rate) of 14%). A negative electrode material was obtained in the same manner as in Comparative Example 1, except that the negative electrode material was used. The physical properties of the obtained negative electrode material are shown in Table 1.

[0215] [Making the negative electrode plate]

[0216] Carboxymethyl cellulose (CMC) as a tackifier and styrene-butadiene rubber (SBR) as a binder were added to the negative electrode material. Their mass ratio was negative electrode material:CMC:SBR = 98:1:1. Purified water was added as a dispersing solvent and the mixture was kneaded to form the slurry of each embodiment and comparative example. A predetermined amount of this slurry was applied substantially equally and homogeneously to both sides of a rolled copper foil with an average thickness of 10 μm, serving as the current collector for the negative electrode. The density of the negative electrode mixture layer was 1.3 g / cm³. 3 .

[0217] [Making the positive electrode plate]

[0218] As the positive electrode active material, a layered lithium-nickel-manganese-cobalt composite oxide (NMC, BET specific surface area of ​​0.4 m²) is used. 2 / g, average particle size (d50) of 6.5μm). Acetylene black (trade name: HS-100, average particle size of 48nm (Denka Co., Ltd. catalog value), manufactured by Denka Co., Ltd.) as a conductive agent and polyvinylidene fluoride as a binder were added sequentially to this positive electrode active material, and mixed to obtain a mixture of positive electrode materials. The mass ratio of positive electrode active material: conductive agent: binder = 90:5:5. Then, N-methyl-2-pyrrolidone (NMP) as a dispersion solvent was added to the above mixture and kneaded to form a slurry. The slurry was substantially uniformly and homogeneously coated on both sides of an aluminum foil with an average thickness of 20μm as a current collector for the positive electrode. After that, a drying treatment was performed, and the mixture was pressed until the density reached 2.7g / cm³. 3 The mixture is then compacted. The single-sided coating weight of the positive electrode agent slurry, based on the solid content of the positive electrode agent, is 40 g / m². 2 .

[0219] [The fabrication of lithium-ion secondary batteries]

[0220] The positive and negative electrode plates are cut to the specified size. The cut positive and negative electrodes are then wound together with a single-layer polyethylene separator (trade name: Hipore, manufactured by Asahi Kasei Corporation, "Hipore" is a registered trademark) with an average thickness of 30 μm sandwiched between them, forming a rolled electrode body. The lengths of the positive and negative electrodes and the separator are then adjusted to achieve a diameter of 17.15 mm. A current-collecting lead is attached to this electrode body, which is then inserted into an 18650 battery case. A non-aqueous electrolyte is then injected into the battery case. The non-aqueous electrolyte is prepared by mixing ethylene carbonate (EC) (a cyclic carbonate), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (chain carbonates) in a 2:3:2 volume ratio, dissolving lithium hexafluorophosphate (LiPF6) (the lithium salt, the electrolyte) at a concentration of 1.2 mol / L, and adding 1.0% by mass of ethylene carbonate (VC). Finally, the battery case is sealed to complete the lithium-ion secondary battery.

[0221] The fabricated lithium-ion secondary battery was charged to 4.2V at a constant current of 0.5CA at 25°C. Once 4.2V was reached, it was charged at the same voltage until the current reached 0.01CA. Then, it was discharged to 2.7V at a constant current of 0.5CA. This constituted one cycle, and a total of three cycles were performed. A 30-minute rest period was included between each charge / discharge cycle. The lithium-ion secondary battery after three cycles was referred to as the initial state.

[0222] [Evaluation of Input Characteristics]

[0223] (1) Charge the initialized lithium-ion secondary battery to 4.2V with a constant current of 0.2CA, and then charge it with a constant voltage of 4.2V until the current reaches 0.02CA. The charging capacity at this time is taken as "charging capacity 1" (mAh).

[0224] (2) After a 30-minute rest period, discharge to 2.7V with a constant current of 0.2CA.

[0225] (3) After a 30-minute rest period, charge the battery to 4.2V with a constant current of 5CA. The charging capacity at this point is taken as “charging capacity 2” (mAh).

[0226] Input characteristic (%) = Charging capacity2 (mAh) / Charging capacity1 (mAh) × 100 (Equation 2)

[0227] [Evaluation of preservation characteristics]

[0228] (1) Charge the battery in its initial state to 4.2V with a constant current of 0.5CA, and then charge it with a constant voltage of 4.2V until the current value reaches 0.01CA.

[0229] (2) After a 30-minute rest period, discharge at a constant current of 0.5 CA to 2.7 V. Measure the discharge capacity at this point (mAh).

[0230] (3) After a 30-minute rest period, charge to 4.2V with a constant current of 0.5CA.

[0231] (4) Place (3) batteries at 60°C for 30 days.

[0232] (5) Discharge to 2.7V with a constant current of 0.5CA. Measure the discharge capacity at this point, 2 (mAh).

[0233] (6) The storage characteristics are obtained using Equation 2 below, based on the discharge capacity obtained in (2) and (5).

[0234] Storage characteristics (%) = Discharge capacity2 (mAh) / Discharge capacity1 (mAh) × 100 (Formula 2, Table 1)

[0235]

[0236] As shown in Table 1, the retention characteristics of Examples 1 and 2, which have negative electrode materials that meet conditions A to C, are better than those of Comparative Examples 1 and 2, which have negative electrode materials that do not meet conditions A to C.

[0237] Example 2, with an average particle size of 8.0 μm for the negative electrode material, showed superior input characteristics compared to Example 1, with an average particle size of 10.0 μm for the negative electrode material.

[0238] All documents, patent applications and technical standards described in this specification are incorporated herein by reference to the same extent as those specifically and separately described herein.

Claims

1. A negative electrode material for a lithium-ion secondary battery, comprising graphitic particles and a low-crystallinity carbon layer covering at least a portion of the surface of the graphitic particles, wherein the dispersion of the maximum peak in the histogram of the R value obtained by Raman spectroscopy is 2.0 or higher, wherein the low-crystallinity carbon layer is formed using two or more precursors that change to carbon phase at different temperatures.

2. The negative electrode material for lithium-ion secondary batteries according to claim 1, wherein, The histogram of the R value obtained by Raman spectroscopy imaging has more than two peaks.

3. The negative electrode material for lithium-ion secondary batteries according to claim 1 or 2, wherein, The low-crystallinity carbon layer contains two or more carbon phases with different crystallinities.

4. The negative electrode material for lithium-ion secondary batteries according to claim 1 or 2, wherein the average sphericity is in the range of 0.8 to 1.

0.

5. The negative electrode material for lithium-ion secondary batteries according to claim 3, wherein the average sphericity is in the range of 0.8 to 1.

0.

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

7. A lithium-ion secondary battery comprising the negative electrode for a lithium-ion secondary battery as described in claim 6.

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