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

Abstract

To provide a negative electrode material for a lithium ion secondary battery that enables the production of the lithium ion secondary battery that has excellent input / output characteristics and cycle characteristics.SOLUTION: A negative electrode material for a lithium-ion secondary battery, containing a carbon material that satisfies the following (1) and (2). (1) The volume-based particle diameter D90 / D10 is greater than 2.0 and less than 4.3. (2) N / S, which is the number of particles N with a number-based equivalent circle diameter of 5 μm or less out of a total measured particle count of 10,000 particles, divided by the specific surface area S determined by nitrogen adsorption measurement at 77 K, is 750 (particles / g / cm2) or more.SELECTED DRAWING: None

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 personal computers (PCs), mobile phones, smartphones, tablet PCs, etc. In recent years, against the backdrop of environmental issues such as global warming caused by CO2 emissions, clean electric vehicles (EVs) that run solely on batteries, hybrid electric vehicles (HEVs) that combine gasoline engines with batteries, and plug-in hybrid electric vehicles (PHEVs) have become popular, and the development of lithium-ion secondary batteries (automotive lithium-ion secondary batteries) to be installed in these vehicles is underway.

[0003] The performance of the negative electrode material in lithium-ion secondary batteries significantly affects the characteristics of the battery. Carbon materials are widely used as the material for the negative electrode of lithium-ion secondary batteries. Carbon materials used in negative electrode materials are broadly classified into graphite and carbon materials with lower crystallinity than graphite (e.g., amorphous carbon). Graphite has a structure in which hexagonal mesh planes of carbon atoms are regularly stacked. When graphite is used as the negative electrode material for lithium-ion secondary batteries, lithium ion insertion and desorption reactions proceed from the edges of the hexagonal mesh planes, allowing charging and discharging to occur.

[0004] Amorphous carbon has irregular stacking of hexagonal mesh planes or no hexagonal mesh planes. Therefore, in anode materials using amorphous carbon, the intercalation and deintercalation reactions of lithium ions proceed across the entire surface of the anode material. As a result, lithium ion batteries with superior input / output characteristics are more likely to be obtained than those using graphite as the anode material (see, for example, Patent Documents 1 and 2). On the other hand, amorphous carbon has lower crystallinity than graphite, and therefore has a lower energy density than graphite. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 4-370662 [Patent Document 2] Japanese Patent Application Publication No. 5-307956 [Patent Document 3] International Publication No. 2012 / 015054 Summary of the Invention [Problem to be solved by the invention]

[0006] Taking into consideration the characteristics of carbon materials as described above, a negative electrode material has been proposed that combines amorphous carbon and graphite to maintain high energy density while improving input / output characteristics, and that coats graphite with amorphous carbon to reduce surface reactivity, thereby improving input / output characteristics while maintaining good initial charge / discharge efficiency (see, for example, Patent Document 3).

[0007] Lithium-ion secondary batteries used in EVs, HEVs, and other vehicles are required to have a high energy density in order to improve driving comfort and extend continuous driving distances. High-energy-density lithium-ion secondary batteries are also required to have a shorter charging time. To shorten the charging time of lithium-ion secondary batteries, it is necessary to increase the current density. However, increasing the current density to quickly charge a lithium-ion secondary battery tends to cause lithium deposition at the negative electrode, which can deteriorate input characteristics and cycle characteristics.

[0008] Increasing the amount of lithium ions that can be charged and discharged can increase the energy density of lithium-ion secondary batteries, so thickening the anode film has been investigated. However, when attempting to increase the energy density of lithium-ion secondary batteries by thickening the anode film, although the battery has a high capacity at the beginning of the charge-discharge cycle, a solid electrolyte interphase (SEI) film gradually grows with repeated charge-discharge. This clogs the voids in the anode, impairing the mobility of lithium ions and resulting in a decrease in cycle performance.

[0009] An object of one aspect of the present invention is to provide a negative electrode material for a lithium ion secondary battery and a negative electrode for a lithium ion secondary battery that can be used to manufacture a lithium ion secondary battery that has excellent input / output characteristics and cycle characteristics. Furthermore, an object of one embodiment of the present invention is to provide a lithium ion secondary battery that is excellent in input / output characteristics and cycle characteristics. [Means for solving the problem]

[0010] Specific means for solving the above problems include the following aspects. <1> A negative electrode material for lithium-ion secondary batteries, comprising a carbon material that satisfies the following (1) and (2): (1) The volume-based particle size D90 / D10 is greater than 2.0 and less than 4.3. (2) The number of particles with a circular equivalent diameter of 5 μm or less based on the number of particles (N) out of a total of 10,000 particles measured is divided by the specific surface area (S) determined by nitrogen adsorption measurement at 77 K. N / S is 750 (particles·g / m 2 )That's all. <2> The carbon material satisfies the following (3): <1> The negative electrode material for a lithium ion secondary battery according to claim 1. (3) The percentage of particles with a circular equivalent diameter of 5 μm or less based on the number of particles measured is 45% or more out of a total of 10,000 particles. <3> The carbon material satisfies the following (4): <1> or <2> The negative electrode material for a lithium ion secondary battery according to claim 1. (4) When X is the number-based equivalent circle diameter at 99% of the cumulative total of a particle size distribution limited to particle sizes of 5 μm or more, and Y is the circularity at 1% of the cumulative total of a circularity distribution limited to particle sizes of 5 μm or more, the following formula (a) is satisfied: Y ≧ 0.3×log 10 (X)+(0.5-0.3×log 10 6) (a) <4> The carbon material satisfies the following (5): <1> ~ <3> 10. The negative electrode material for a lithium ion secondary battery according to claim 9, wherein the negative electrode material is a material for a lithium ion secondary battery. (5) Tap density is 0.80 g / cm 3 ~0.95g / cm 3 is. <5> The carbon material satisfies the following (6): <1> ~ <4> 10. The negative electrode material for a lithium ion secondary battery according to claim 9, wherein the negative electrode material is a material for a lithium ion secondary battery. (6) The average circularity is 0.90 to 0.93. <6> The average interplanar spacing d obtained by X-ray diffraction for the carbon material 002 is 3.34Å to 3.38Å, <1> ~ <5> 10. The negative electrode material for a lithium ion secondary battery according to claim 9, wherein the negative electrode material is a material for a lithium ion secondary battery. <7> The carbon material has an R value of 0.1 to 0.4 in Raman spectroscopy. <1> ~ <6> 10. The negative electrode material for a lithium ion secondary battery according to claim 9, wherein the negative electrode material is a material for a lithium ion secondary battery. <8> The specific surface area of ​​the carbon material determined by nitrogen adsorption measurement at 77 K was 14 m 2 / g or less, <1> ~ <7> 10. The negative electrode material for a lithium ion secondary battery according to claim 9, wherein the negative electrode material is a material for a lithium ion secondary battery. <9> the carbon material does not have two or more exothermic peaks in a temperature range of 300°C to 1000°C in a differential thermal analysis in an air stream; <1> ~ <8> 10. The negative electrode material for a lithium ion secondary battery according to claim 9, wherein the negative electrode material is a material for a lithium ion secondary battery. <10> <1> ~ <9> 10. A negative electrode for a lithium ion secondary battery, comprising: a negative electrode material layer containing the negative electrode material for a lithium ion secondary battery according to any one of 1 to 8; and a current collector. <11> <10> A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 1, a positive electrode, and an electrolyte solution. [Effects of the Invention]

[0011] According to one aspect of the present invention, it is possible to provide a negative electrode material for a lithium ion secondary battery and a negative electrode for a lithium ion secondary battery, which can be used to manufacture a lithium ion secondary battery having excellent input / output characteristics and cycle characteristics. Furthermore, according to one embodiment of the present invention, a lithium-ion secondary battery with excellent input / output characteristics and cycle characteristics can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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 otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present invention. In the present disclosure, the term "process" includes not only a process that is independent of other processes, but also a process that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved. In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced by the upper or lower limit value of another numerical range described in stages. Also, in the numerical ranges described in this disclosure, the upper or lower limit value of the numerical range may be replaced by the value shown in each test. In the present disclosure, each component may contain multiple substances corresponding to the component. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, the composition may contain multiple types of particles corresponding to each component. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component means the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified. In the present disclosure, the term "layer" includes cases where the layer is formed over the entire area when the area in which the layer exists is observed, as well as cases where the layer is formed over only a portion of the area. In this disclosure, the term "lamination" refers to stacking layers, and two or more layers may be bonded together, or two or more layers may be detachable.

[0013] <Anode material for lithium-ion secondary batteries> A negative electrode material for lithium-ion secondary batteries, comprising a carbon material that satisfies the following (1) and (2): (1) The volume-based particle size D90 / D10 is greater than 2.0 and less than 4.3. (2) The number of particles with a circular equivalent diameter of 5 μm or less based on the number of particles (N) out of a total of 10,000 particles measured is divided by the specific surface area (S) determined by nitrogen adsorption measurement at 77 K. N / S is 750 (particles·g / m 2 )That's all.

[0014] When the negative electrode material for a lithium ion secondary battery satisfies the above (1) and (2), it becomes possible to manufacture a lithium ion secondary battery with excellent input / output characteristics and cycle characteristics. The reason for this is not clear, but is presumed to be as follows.

[0015] In the above (1), when the D90 / D10 ratio is greater than 2.0 and less than 4.3, the packing property of the carbon material tends to be high. This increases the electrode density when the negative electrode material for lithium ion secondary batteries is applied to a current collector, and the pressing pressure required to achieve the desired electrode density in the negative electrode for lithium ion secondary batteries tends to be reduced. Lowering the pressing pressure reduces the lateral orientation of the carbon material, facilitating the insertion and removal of lithium ions during charging and discharging. This tends to result in the production of lithium ion secondary batteries with superior input / output characteristics. In particular, spherical graphite obtained by spheroidizing natural graphite is prone to generating hollow particles during the spheroidization process, and these hollow particles are prone to deformation and lateral orientation due to the pressing pressure. On the other hand, the negative electrode material for lithium ion secondary batteries of the present disclosure allows the pressing pressure required to be reduced to achieve the desired electrode density, thereby suppressing an increase in the lateral orientation of the carbon material, even when the carbon material contained in the negative electrode material is spherical graphite.

[0016] In the above (1), when D90 / D10 is less than 4.3, the impediment to the circulation of the liquid in the voids between the carbon materials is suppressed, which tends to enable the production of a lithium-ion secondary battery with excellent input / output characteristics and cycle characteristics.

[0017] In (2) above, a high number of particles N with a circular equivalent diameter of 5 μm or less based on the number of particles measured (10,000 particles) indicates that more carbon materials have small particle sizes. Smaller particle sizes tend to shorten the diffusion distance of lithium ions within the solid, resulting in better output characteristics. Furthermore, the specific surface area S increases in inverse proportion to the particle size of the carbon material, as shown in (2) above, which tends to further improve the input / output characteristics of lithium-ion secondary batteries. However, electrolysis reactions are more likely to occur at the interface between carbon materials with large specific surface areas and the electrolyte, which can easily induce current concentration due to factors such as electrolyte depletion. As a result, lithium deposition is more likely to occur, leading to a deterioration in cycle characteristics.

[0018] In the negative electrode material for lithium ion secondary batteries of the present disclosure, the ratio N / S of the number of particles N to the specific surface area S is set to 750 (particles·g / m 2 ) or more, the number of particles N and the specific surface area S are well balanced. This makes it possible to improve the input / output characteristics of the lithium ion secondary battery while suppressing a decrease in the cycle characteristics of the lithium ion secondary battery.

[0019] In addition, excellent input / output characteristics are synonymous with low resistance, which has the effect of suppressing heat generation due to resistance. Therefore, by improving the input / output characteristics of lithium-ion secondary batteries, it is expected that high-temperature storage characteristics will be improved by suppressing heat generation.

[0020] Generally, in lithium-ion secondary batteries, carbon materials repeatedly expand and contract during charge and discharge, which can easily cause disconnection of the conduction path due to interfacial peeling between the carbon material and the current collector, peeling between carbon materials, etc. Furthermore, the cycle characteristics of lithium-ion secondary batteries may be degraded due to a decrease in charge and discharge capacity caused by deactivated negative electrode active material and an increase in current density in the active negative electrode active material.

[0021] On the other hand, the negative electrode material for lithium-ion secondary batteries of the present disclosure satisfies the above (1) and (2), and therefore tends to increase the number of contact points between the carbon material, which is the negative electrode active material, and the current collector, and between the carbon materials themselves. By using the negative electrode material for lithium-ion secondary batteries of the present disclosure, it is possible to manufacture lithium-ion secondary batteries in which the conductive paths between the carbon material and the current collector, and the conductive paths between the carbon materials, are suitably maintained, even when the carbon material repeatedly expands and contracts due to charging and discharging. As a result, it tends to be possible to manufacture lithium-ion secondary batteries that are excellent in life characteristics such as cycle characteristics, input / output characteristics, and the like.

[0022] [Carbon materials] The negative electrode material for lithium-ion secondary batteries (hereinafter also referred to simply as "negative electrode material") of the present disclosure contains a carbon material (hereinafter also referred to as "specific carbon material") that satisfies the above (1) and (2). The content of the specific carbon material in the negative electrode material is not particularly limited, and is, for example, preferably 50 mass% or more, more preferably 80 mass% or more, even more preferably 90 mass% or more, and particularly preferably 100 mass%.

[0023] The negative electrode material may contain other carbon materials in addition to the specific carbon material. Examples of other carbon materials include, but are not limited to, natural graphite in the form of scales, clay, or spheres; graphite such as artificial graphite; amorphous carbon; carbon black; fibrous carbon; and nanocarbon. One or more of the other carbon materials may be used alone or in combination. The negative electrode material may also contain particles containing an element capable of absorbing and desorbing lithium ions. Examples of elements capable of absorbing and desorbing lithium ions include, but are not limited to, Si, Sn, Ge, and In.

[0024] The specific carbon material has a volume-based particle size D90 / D10 ratio of greater than 2.0 and smaller than 4.3. From the viewpoint of making it easier to ensure conductive paths between the carbon material and the current collector, conductive paths between carbon materials, etc., thereby obtaining a lithium ion secondary battery with a high capacity retention rate and excellent cycle characteristics, and from the viewpoint of making it easier to improve the tap density, D90 / D10 is preferably greater than 2.2, more preferably greater than 2.5, and even more preferably greater than 3.0. From the viewpoint of suitably suppressing the obstruction of liquid circulation in the gaps between the carbon materials and obtaining a lithium ion secondary battery with superior input / output characteristics and cycle characteristics, it is preferably smaller than 4.1, and more preferably smaller than 3.6. The value of D90 / D10 may be changed within the above range, taking into consideration the amount of the negative electrode material composition applied to the current collector, the thickness of the negative electrode, and the like.

[0025] The particle size (D10) of a carbon material is the particle size at which the cumulative volume distribution curve of the carbon material reaches 10% of its peak when plotted from the smallest diameter side in the particle size distribution of the carbon material. The particle size (D90) of a carbon material is the particle size at which the cumulative volume distribution curve of the carbon material reaches 90% of its peak when plotted from the smallest diameter side in the particle size distribution of the carbon material. The particle size (D10) and particle size (D90) can be measured by dispersing the carbon material in purified water containing a surfactant and using a laser diffraction particle size analyzer (e.g., SALD-3100, manufactured by Shimadzu Corporation).

[0026] The specific carbon material preferably has an average particle size (D50) of 22 μm or less. The average particle size (D50) of the specific carbon material is more preferably 21 μm or less, and even more preferably 20 μm or less, from the viewpoint of suppressing an increase in the diffusion distance of lithium from the surface to the interior of the negative electrode material and further improving the input / output characteristics of the lithium ion secondary battery. The average particle diameter (D50) of the specific carbon material is preferably 10 μm or more, more preferably 12 μm or more, even more preferably 15 μm or more, and particularly preferably 17 μm or more, from the viewpoint of suppressing a deterioration in the high-temperature storage characteristics of the lithium-ion secondary battery due to an increase in the specific surface area.

[0027] The average particle size (D50) of a carbon material is the particle size at which the cumulative volume distribution curve of the carbon material reaches 50% of its peak when plotted from the smallest diameter side in the particle size distribution of the carbon material. The average particle size (D50) can be measured by dispersing the carbon material in purified water containing a surfactant and using a laser diffraction particle size analyzer (e.g., SALD-3100, manufactured by Shimadzu Corporation).

[0028] For specific carbon materials, the N / S ratio, calculated by dividing the number of particles N with a circular equivalent diameter of 5 μm or less out of a total of 10,000 particles measured by nitrogen adsorption at 77 K, is 750 (particles·g / m 2 )That's all. From the viewpoint of the input / output characteristics of the lithium ion secondary battery, N / S is preferably 900 or more, and more preferably 1200 or more. N / S may be 2000 or less, or may be 1800 or less.

[0029] In the present disclosure, the number and proportion of particles of a carbon material having a circle-equivalent diameter of 5 μm or less based on the number of particles can be measured using a wet flow particle size and shape analyzer (FPIA-3000 manufactured by Malvern Instruments) under the same conditions as those for measuring the average circularity described below.

[0030] In the present disclosure, the specific surface area determined by nitrogen adsorption measurement of a carbon material at 77K can be determined from the adsorption isotherm obtained by nitrogen adsorption measurement at 77K using the BET method.

[0031] The specific carbon material preferably satisfies the following condition (3). (3) The percentage of particles with a circular equivalent diameter of 5 μm or less based on the number of particles measured is 45% or more out of a total of 10,000 particles.

[0032] From the viewpoint of achieving superior input / output characteristics of a lithium-ion secondary battery, the specific carbon material preferably has the above-mentioned particle number ratio of 45% or more, more preferably 53% or more, and even more preferably 60% or more. By increasing the amount of carbon material with an equivalent circle diameter of 5 μm or less, the diffusion within the solid through which lithium ions move is shortened, and the number of oxidation-reduction reaction interfaces of lithium ions at the interface between the carbon material and the electrolyte increases, which tends to improve the current density.

[0033] The upper limit of the particle number ratio is not particularly limited, but from the viewpoint of making it easier for the carbon material to satisfy the conditions (1) and (2) above and the condition (5) described below, it is preferably 95% or less, and more preferably 90% or less.

[0034] The specific surface area of ​​a specific carbon material determined by nitrogen adsorption measurement at 77 K is 2 m 2 / g~14m 2 / g, and 3m2 / g~8m 2 / g, more preferably 4m 2 / g~6m 2 When the specific surface area is within the above range, a good balance between the input / output characteristics and the initial charge / discharge efficiency of the lithium ion secondary battery tends to be obtained.

[0035] From the viewpoint of achieving better input / output characteristics and cycle characteristics of the lithium ion secondary battery, the specific carbon material preferably satisfies the following (4): (4) When X is the number-based equivalent circle diameter at 99% of the cumulative total of a particle size distribution limited to particle sizes of 5 μm or more, and Y is the circularity at 1% of the cumulative total of a circularity distribution limited to particle sizes of 5 μm or more, the following formula (a) is satisfied: Y ≧ 0.3×log 10 (X)+(0.5-0.3×log 10 6) (a)

[0036] The right side of the above formula (a) represents an equation of a line segment (note that the particle size is on a logarithmic scale) passing through two points where (particle size (μm), circularity) are (6 μm, 0.5) and (50, 0.8). Satisfying the above formula (4) means that the specific carbon material does not contain particles with a small circularity (e.g., flat shape) and a large particle size, or contains only a small amount of such particles.

[0037] For a particular carbon material, Y-[0.3×log 10 (X)+(0.5-0.3×log 10 6)] is preferably 0 or more, and may be 0 or more and 0.15 or less, or 0 or more and 0.10 or less, from the viewpoint of achieving better input / output characteristics and cycle characteristics of the lithium ion secondary battery.

[0038] The planes of particles with low circularity (e.g., flat) and large particle diameters tend to be aligned parallel to the plane of the current collector, and because these parallel planes are perpendicular to the axis of lithium ion migration, the path lengths of the lithium ions, electrolyte, etc. that move during charge and discharge are extended, degrading the overall charge and discharge characteristics. On the other hand, when the above condition (4) is satisfied, the movement of the lithium ions and electrolyte in the interparticle voids becomes smooth, and the input / output characteristics and cycle characteristics of the lithium ion secondary battery tend to be improved.

[0039] Since the average circularity is the average value of the circularities of individual particles and can represent the bulk powder shape, it is effective to consider it from the perspective of improving the input / output characteristics and cycle characteristics of lithium-ion secondary batteries. Therefore, by using a specific carbon material that satisfies the above (4) and the following (6), it becomes easier to obtain a lithium-ion secondary battery that is particularly excellent in input / output characteristics and cycle characteristics.

[0040] In the present disclosure, for carbon materials, the number-based circle-equivalent diameter at 99% of the cumulative total of a distribution limited to particle diameters of 5 μm or more and the circularity at 1% of the cumulative total of a circularity distribution limited to particle diameters of 5 μm or more can be measured using a wet flow particle size and shape analyzer.

[0041] The specific carbon material preferably satisfies the following condition (5). (5) Tap density is 0.80 g / cm 3 ~0.95g / cm 3 is.

[0042] The tap density of the specific carbon material is set to 0.80 g / cm from the viewpoint of achieving superior input / output characteristics and energy density in a lithium ion secondary battery. 3 It is preferable that the value is 0.85 / cm or more. 3 More preferably, it is 0.90 / cm or more. 3 The tap density of the specific carbon material is more preferably 0.95 / cm or more from the viewpoint of improving the yield of the negative electrode material and the cycle characteristics of the lithium ion secondary battery. 3 It is preferable that:

[0043] The tap density of carbon materials tends to increase when the average particle size (D50) of the carbon material, the average circularity of the carbon material, etc., are increased, or when carbon materials with large particle sizes and flat shapes are removed.

[0044] In this disclosure, the tap density of a carbon material is defined as a volume of 150 cm 3 100 cm of sample powder was placed in a graduated flat-bottom test tube (Kuramochi Scientific Instruments Manufacturing Co., Ltd., KRS-406). 3 is placed in the graduated flat-bottomed test tube, the graduated flat-bottomed test tube is then stopped up, and the graduated flat-bottomed test tube is then dropped from a height of 5 cm 250 times, after which the value is determined from the mass and volume of the sample powder.

[0045] From the viewpoint of achieving better input / output characteristics and cycle characteristics in a lithium ion secondary battery, the specific carbon material preferably satisfies the following (6). (6) The average circularity is 0.90 to 0.93.

[0046] By ensuring that the average circularity is 0.90 or higher, the alignment of specific carbon materials parallel to the current collector plane is suppressed, which shortens the path length of lithium ions, electrolyte, etc. that move during charge and discharge, and smooths the movement of lithium ions and electrolyte through interparticle voids, tending to improve the input / output characteristics and cycle characteristics of lithium-ion secondary batteries.

[0047] By having an average circularity of 0.93 or less, point contact between particles is suppressed, thereby suppressing grain boundary delamination due to expansion and contraction during charge and discharge. As a result, the cycle characteristics of lithium-ion secondary batteries tend to be improved. Furthermore, when the first carbon material, which is a raw material for a specific carbon material, is obtained by a spheroidization process, processing roughness of the first carbon material can be reduced, which tends to improve the high-temperature storage characteristics of lithium-ion secondary batteries. In addition, since excessive spheroidization is not required, the yield during the spheroidization process also tends to be improved.

[0048] The average circularity of the specific carbon material is preferably 0.90 to 0.93, and more preferably 0.905 to 0.925.

[0049] In the present disclosure, the circularity of the carbon material can be measured using a wet flow particle size and shape analyzer.

[0050] (average spacing d 002 ) The average interplanar spacing d obtained by X-ray diffraction for a specific carbon material 002 The average interplanar spacing d is preferably 3.34 Å to 3.38 Å. 002 When the thickness is 3.38 Å or less, the initial charge / discharge efficiency and energy density of the lithium ion secondary battery tend to be excellent. Average spacing d 002 The theoretical value of 3.354 Å for graphite crystals is 3.354 Å, and the closer this value is to the theoretical value, the greater the energy density tends to be.

[0051] Average interplanar spacing d of carbon material 002 The diffraction peak corresponding to the carbon 002 plane, which appears at a diffraction angle 2θ of approximately 24° to 27°, can be calculated using the Bragg equation by irradiating a sample with X-rays (CuKα rays) and measuring the diffraction rays with a goniometer to obtain a diffraction profile.

[0052] Average spacing d 002 The measurement can be carried out by X-ray diffraction. Specifically, the negative electrode material for lithium ion secondary batteries is filled into the recess of a quartz sample holder, which is then set on the measurement stage, and a wide-angle X-ray diffractometer (for example, manufactured by Rigaku Corporation) is used to measure 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

[0053] Average interplanar spacing d of carbon material 002The value of d tends to decrease, for example, by increasing the temperature of the heat treatment when preparing the negative electrode material. Therefore, by adjusting the temperature of the heat treatment when preparing the negative electrode material, the average interplanar spacing d of the carbon material can be reduced. 002 tend to be able to control

[0054] (R value of Raman spectroscopy) The R value of the specific carbon material measured by Raman spectroscopy is preferably 0.1 to 0.4, and from the viewpoint of achieving excellent high-temperature storage characteristics of the lithium-ion secondary battery, is more preferably 0.1 to 0.33, even more preferably 0.1 to 0.3, and particularly preferably 0.1 to 0.25. When the R value is 0.1 or more, there are sufficient graphite lattice defects used for the insertion and removal of lithium ions, and a decrease in the input / output characteristics of the lithium-ion secondary battery tends to be suppressed. When the R value is 0.4 or less, the decomposition reaction of the electrolyte is sufficiently suppressed, and a decrease in the initial efficiency of the lithium-ion secondary battery tends to be suppressed.

[0055] The R value is determined by Raman spectroscopy at 1580 cm -1 The intensity Ig of the maximum peak near 1360 cm -1 The intensity ratio (Id / Ig) is defined as the intensity ratio of the maximum peak Id near 1580 cm -1 The peak appearing around 1530 cm is usually identified as corresponding to the graphite crystal structure, for example, -1 ~1630cm -1 This also refers to the peak observed at 1360 cm -1 The peak appearing around 1300 cm is usually identified as corresponding to the amorphous structure of carbon, for example, -1 ~1400cm -1 This refers to the peak observed at

[0056] In the present disclosure, the R value is measured using a Raman spectrum measuring device (for example, XploRA PLUS by Horiba, Ltd.), and the spectrum obtained is measured under the following conditions with the following range as the baseline. Laser wavelength: 532nm Laser power: 100mW or more Neutral density filter: 1% ·Irradiation intensity: 1mW Measurement range: 1000cm -1 ~1800cm -1 Irradiation time: 30 seconds ·Irradiation area: 1μm 2 Baseline (D band): 1100cm -1 ~1470cm -1 Baseline (G band): 1450cm -1 ~1710cm -1 Number of times per particle is accumulated: 2 times Number of particles measured: 30 particles

[0057] It is preferable that the specific carbon material does not have two or more exothermic peaks in a temperature range of 300° C. to 1000° C. in differential thermal analysis (DTA) in an air stream, which tends to further improve the input / output characteristics and high-temperature storage characteristics of the lithium ion secondary battery.

[0058] Here, "the carbon material does not have two or more exothermic peaks" means that the carbon material does not have multiple distinguishable exothermic peaks in the temperature range of 300° C. to 1000° C., that is, the carbon material has no distinguishable exothermic peaks or has one distinguishable exothermic peak. Here, "having multiple distinguishable exothermic peaks" means that the carbon material has multiple exothermic peaks whose peak values ​​are separated by at least 5° C.

[0059] In the present disclosure, differential thermal analysis (DTA analysis) can be performed using a thermogravimetric and differential thermal analyzer (for example, EXSTAR TG / DTA6200 manufactured by Seiko Instruments Inc.) Specifically, using α-alumina as a reference, measurement is performed in a dry air flow of 300 mL / min at a temperature increase rate of 2.5°C / min, and the presence or absence of an exothermic DTA peak between 300°C and 1000°C is confirmed.

[0060] The specific carbon material and other carbon materials (hereinafter also collectively referred to as "carbon materials") are not particularly limited, and examples thereof include graphite, low-crystalline carbon, amorphous carbon, mesophase carbon, etc. Examples of graphite include artificial graphite, natural graphite, graphitized mesophase carbon, graphitized carbon fiber, etc. As the carbon material, spherical graphite particles are preferred, and spherical artificial graphite, spherical natural graphite, etc. are more preferred, in view of excellent charge / discharge capacity and excellent tap density in lithium ion secondary batteries. Furthermore, by using spherical graphite particles, aggregation of the graphite particles can be suppressed, and when the graphite particles are coated with a carbon material with lower crystallinity (e.g., amorphous carbon), the graphite particles can be coated favorably. Furthermore, when a carbon material that has aggregated during coating is used to prepare a negative electrode material composition, exposure of the regions not coated with the carbon material is suppressed when the carbon material aggregates and is broken down by stirring. As a result, when a lithium-ion secondary battery is prepared, the decomposition reaction of the electrolyte on the surface of the carbon material tends to be suppressed, and a decrease in initial efficiency tends to be suppressed. The carbon material contained in the negative electrode material may be one type alone or two or more types.

[0061] The carbon material may include a first carbon material as a core and a second carbon material present on at least a portion of the surface of the first carbon material and having lower crystallinity than the first carbon material. The first carbon material and the second carbon material are not particularly limited as long as they satisfy the condition that the second carbon material has lower crystallinity than the first carbon material, and may be appropriately selected from the examples of carbon materials mentioned above, for example. The first carbon material and the second carbon material may each be one type alone or two or more types. The presence of the second carbon material on the surface of the first carbon material can be confirmed by observation with a transmission electron microscope.

[0062] In order to improve the input / output characteristics of the lithium ion secondary battery, the second carbon material preferably contains at least one of crystalline carbon and amorphous carbon, specifically at least one selected from the group consisting of carbonaceous substances and carbonaceous particles obtained from organic compounds that can be converted into carbonaceous materials by heat treatment (hereinafter also referred to as precursors of the second carbon material).

[0063] The precursor of the second carbon material is not particularly limited, and examples thereof include pitch, organic polymer compounds, etc. 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, etc., and pitch produced by polymerizing naphthalene, etc. in the presence of a super strong acid. Examples of organic polymer compounds include thermoplastic resins such as polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate, and polyvinyl butyral, and natural substances such as starch and cellulose.

[0064] The carbonaceous particles used as the second carbon material are not particularly limited, and examples thereof include particles of acetylene black, oil furnace black, ketjen black, channel black, thermal black, soil graphite, and the like.

[0065] When the carbon material contains a first carbon material and a second carbon material, the ratio of the amount of the first carbon material to the amount of the second carbon material in the carbon material is not particularly limited. From the viewpoint of improving the input / output characteristics of the lithium ion secondary battery, the ratio of the amount of the second carbon material to the total mass of the carbon material is preferably 0.1% by mass to 15% by mass, more preferably 1% by mass to 10% by mass, and even more preferably 1% by mass to 5% by mass.

[0066] When calculating the amount of the second carbon material in the carbon material from the amount of the precursor of the second carbon material, it can be calculated by multiplying the amount of the precursor of the second carbon material by its residual carbon percentage (mass %). The residual carbon percentage of the precursor of the second carbon material can be calculated by heat-treating the precursor of the second carbon material alone (or in the form of a mixture of the precursor of the second carbon material and the first carbon material in a predetermined ratio) at a temperature at which the precursor of the second carbon material can be converted to carbonaceous matter, and then calculating the residual carbon percentage by thermogravimetric analysis or the like from the mass of the precursor of the second carbon material before the heat treatment and the mass of the carbonaceous substance derived from the precursor of the second carbon material after the heat treatment.

[0067] The method for producing the negative electrode material of the present disclosure is not particularly limited. In order to efficiently produce a negative electrode material that satisfies the above-mentioned conditions, when a carbon material is produced using precursors of the first carbon material and the second carbon material, it is preferable to produce the carbon material by the following method for producing a negative electrode material.

[0068] <Method of manufacturing negative electrode material for lithium-ion secondary batteries> In one embodiment of the present invention, a method for producing a negative electrode material for a lithium ion secondary battery may include a step of producing a specific carbon material by heat-treating a mixture containing a first carbon material serving as a core and a precursor of a second carbon material having lower crystallinity than the first carbon material.

[0069] According to the above method, the above-mentioned negative electrode material can be produced efficiently. In the above method, the details and preferred embodiments of the first carbon material, the precursor of the second carbon material, and the specific carbon material are the same as those explained above in the section on the negative electrode material for lithium ion secondary batteries.

[0070] The first carbon material is preferably spherical graphite particles, and more preferably spherical artificial graphite, spherical natural graphite, etc. Spherical graphite obtained by subjecting non-spherical graphite particles such as flake graphite particles (for example, flake natural graphite particles) to a spheronization treatment may also be used as the first carbon material.

[0071] Spheroidized graphite can be obtained by subjecting non-spherical graphite particles such as flake graphite particles to spheroidization treatment under specific treatment conditions using a commercially available spheroidization treatment device. If necessary, non-spherical graphite particles such as flake graphite particles may be subjected to a pulverization treatment and then subjected to a spheroidization treatment, or the spheroidization treatment may also serve as a pulverization treatment.

[0072] The lower limit of the rotor peripheral speed during spheronization is preferably 65 m / min or more, more preferably 70 m / min or more, from the viewpoint of making it easier for the D90 / D10 of the specific carbon material to satisfy the above-mentioned (1).The upper limit of the rotor peripheral speed during spheronization is preferably 100 m / min or less, more preferably 90 m / min or less, from the viewpoint of making it easier for the specific surface area determined by nitrogen adsorption measurement at 77 K of the specific carbon material to satisfy the above-mentioned (2).

[0073] The processing time for the sphering treatment is preferably 2.0 to 7.0 minutes, more preferably 2.5 to 6.0 minutes, from the viewpoint of making it easier for the D90 / D10 of the specific carbon material to satisfy the above (1) and (2).

[0074] During the spheronization treatment, the product of the rotor peripheral speed and the treatment time is preferably 130 to 500, more preferably 150 to 450, from the viewpoint of making it easier for the D90 / D10 of the specific carbon material to satisfy the above (1) and (2).

[0075] From the viewpoint of suppressing electrolysis of the electrolyte in a lithium ion secondary battery and maintaining high-temperature storage characteristics, it is preferable to prevent the specific surface area of ​​the specific carbon material from becoming too high. From the viewpoint of suppressing an increase in the specific surface area that is independent of the particle size of the specific carbon material, it is preferable to suppress the occurrence of cracks and surface irregularities that occur due to grinding, spheroidization, etc. of the graphite particles. Specifically, it is preferable to adjust the particle size, thickness, etc. of the graphite particles, or to subject the graphite particles to spheroidization treatment under the conditions of the rotor peripheral speed and treatment time as described above.

[0076] The spherical graphite obtained by the spheroidizing treatment may be classified to remove fine particles, and the resulting spherical graphite may be used as the first carbon material. The classification method is not particularly limited, and examples thereof include classification using a cyclone and classification by sieving.

[0077] The classification point is not particularly limited, and may be 1 μm to 10 μm, 1 μm to 5 μm, or 1 μm to 3 μm.

[0078] From the viewpoint of increasing the proportion of particles having a circular equivalent diameter of 5 μm or less based on the number of particles per 10,000 total measured particles to more easily satisfy the conditions (2) and (3) above, the classification point may be made smaller (for example, 1 μm to 3 μm), or the fine powder removed by classification (for example, classification using a cyclone) may be further sieved to remove finer fine powders, such as 1 μm or less, and the by-product particles obtained by adding these to the spheroidized graphite from which the fine powders have been removed may be used as the first carbon material.

[0079] The graphite particles to be spheronized are preferably natural graphite, which does not require graphitization power and is a cheaper material. However, conventionally, the material yield in the spheronization process of natural graphite has been around 30%, and approximately 70% of the material has been removed, resulting in an extremely low yield. The removed by-product graphite is used, for example, by mixing it with resin as pencil lead material. However, with the recent spread of electric vehicles, by-product graphite has rapidly increased, resulting in an oversupply of by-product graphite relative to demand. Therefore, it is desirable to improve the material yield in the spheronization process of natural graphite. Note that natural graphite is contained in ores at approximately 10% by mass in the Heilongjiang Province region of the People's Republic of China, for example, and since there is a significant amount of ore by-products, it is desirable to improve the yield in the spheronization process of natural graphite.

[0080] In the manufacturing method of the negative electrode material for lithium ion secondary batteries of the present disclosure, the classification point may be reduced (for example, to 1 μm to 3 μm), or by-product particles from which fine particles smaller than 1 μm, for example, have been removed may be used as the first carbon material. This makes it possible to improve the yield during the spheroidizing treatment. Furthermore, certain carbon materials are more likely to satisfy the above conditions (2) and (3), which makes it easier to manufacture lithium ion secondary batteries with excellent input / output characteristics and high-temperature storage characteristics.

[0081] From the viewpoint of improving the input / output characteristics of the lithium ion secondary battery, the temperature at which the mixture is heat-treated is preferably 950° C. to 1500° C., more preferably 1000° C. to 1300° C., and even more preferably 1050° C. to 1250° C. The temperature at which the mixture is heat-treated may be constant from the start to the end of the heat treatment, or may vary.

[0082] In the above method, the contents of the first carbon material and the precursor of the second carbon material in the mixture before heat treatment are not particularly limited. From the viewpoint of improving the input / output characteristics of the lithium ion secondary battery, the content of the first carbon material is preferably 85% by mass to 99.9% by mass, more preferably 90% by mass to 99% by mass, and even more preferably 95% by mass to 99% by mass, relative to the total mass of the mixture. On the other hand, from the viewpoint of improving the input / output characteristics of the lithium ion secondary battery, the content of the precursor of the second carbon material is preferably 0.1% by mass to 15% by mass, more preferably 1% by mass to 10% by mass, and even more preferably 1% by mass to 5% by mass, relative to the total mass of the mixture.

[0083] <Anode for lithium-ion secondary batteries> The negative electrode for a lithium ion secondary battery of the present disclosure includes a negative electrode layer containing the above-described negative electrode material for a lithium ion secondary battery of the present disclosure, and a current collector. The negative electrode for a lithium ion secondary battery may include other components as necessary in addition to the negative electrode layer containing the above-described negative electrode material and the current collector.

[0084] The negative electrode for a lithium ion secondary battery can be produced, for example, by kneading the negative electrode material and binder with a solvent to prepare a slurry-like negative electrode material composition, which is then applied to a current collector to form a negative electrode material layer, or by forming the negative electrode material composition into a shape such as a sheet or pellet and integrating it with a current collector. Kneading can be performed using a dispersing device such as a stirrer, ball mill, super sand mill, or pressure kneader.

[0085] The binder used in preparing the negative electrode material composition is not particularly limited. Examples of binders include styrene-butadiene copolymers, polymers of ethylenically unsaturated carboxylic acid esters such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, hydroxyethyl acrylate, and hydroxyethyl methacrylate, polymers of ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid, and polymeric compounds with high ionic conductivity such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, and polyacrylonitrile. When the negative electrode material composition contains a binder, the amount thereof is not particularly limited. The content of the binder may be, for example, 0.5 to 20 parts by mass per 100 parts by mass of the total of the negative electrode material and binder.

[0086] The solvent is not particularly limited as long as it can dissolve or disperse the binder. Specific examples include organic solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, and γ-butyrolactone. The amount of solvent used is not particularly limited as long as it can turn the negative electrode material composition into a desired state such as a paste. The amount of solvent used is preferably, for example, 60 parts by mass or more but less than 150 parts by mass per 100 parts by mass of the negative electrode material.

[0087] The negative electrode material composition may contain a thickener. Examples of thickeners include carboxymethyl cellulose or a salt thereof, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid or a salt thereof, alginic acid or a salt thereof, oxidized starch, phosphated starch, and casein. When the negative electrode material composition contains a thickener, the amount thereof is not particularly limited. The content of the thickener may be, for example, 0.1 to 5 parts by mass per 100 parts by mass of the negative electrode material.

[0088] The negative electrode material composition may contain a conductive auxiliary material. Examples of the conductive auxiliary material include carbon materials such as natural graphite, artificial graphite, and carbon black (acetylene black, thermal black, furnace black, etc.), conductive oxides, and conductive nitrides. When the negative electrode material composition contains a conductive auxiliary material, the amount thereof is not particularly limited. The content of the conductive auxiliary material may be, for example, 0.5 to 15 parts by mass per 100 parts by mass of the negative electrode material.

[0089] The material of the current collector is not particularly limited and can be selected from aluminum, copper, nickel, titanium, stainless steel, etc. The state of the current collector is not particularly limited and can be selected from foil, perforated foil, mesh, etc. Porous materials such as porous metal (foamed metal) and carbon paper can also be used as the current collector.

[0090] When the negative electrode material composition is applied to a current collector to form a negative electrode material layer, the method is not particularly limited, and known methods such as metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade coating, comma coating, gravure coating, and screen printing can be used. After the negative electrode material composition is applied to the current collector, the solvent contained in the negative electrode material composition is removed by drying. Drying can be performed using, for example, a hot air dryer, an infrared dryer, or a combination of these devices. A rolling treatment may be performed as necessary. The rolling treatment can be performed using a plate press, a calendar roll, or the like.

[0091] When the negative electrode material composition formed into a shape such as a sheet or pellet is integrated with a current collector to form a negative electrode material layer, the integration method is not particularly limited. For example, it can be performed using a roll, a flat press, or a combination of these means. The pressure during integration is preferably, for example, 1 MPa to 200 MPa.

[0092] <Lithium-ion secondary battery> The lithium ion secondary battery of the present disclosure includes the above-described negative electrode for lithium ion secondary batteries of the present disclosure (hereinafter also simply referred to as "negative electrode"), a positive electrode, and an electrolyte solution.

[0093] The positive electrode can be obtained by forming a positive electrode material layer on a current collector in the same manner as the negative electrode described above. The current collector can be made of a metal or alloy such as aluminum, titanium, or stainless steel, and can be in the form of a foil, perforated foil, mesh, or the like.

[0094] The positive electrode material used to form the positive electrode layer is not particularly limited. For example, metal compounds (metal oxides, metal sulfides, etc.) that can dope or intercalate lithium ions and conductive polymer materials can be mentioned. More specifically, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), and their double oxides (LiCo x Ni y Mn z O2, x+y+z=1), and a double oxide containing an additional element M' (LiCo a Ni b Mn c M' d O2, a+b+c+d=1, M': Al, Mg, Ti, Zr or Ge), spinel-type lithium manganese oxide (LiMn2O4), lithium vanadium compounds, V2O5, V6O 13Examples of the positive electrode material include lithium-containing compounds such as VO2, MnO2, TiO2, MoV2O8, TiS2, V2S5, VS2, MoS2, MoS3, Cr3O8, Cr2O5, and olivine-type LiMPO4 (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene, and porous carbon. The positive electrode material may be one type alone or two or more types.

[0095] The electrolytic solution is not particularly limited, and for example, a solution in which a lithium salt as an electrolyte is dissolved in a non-aqueous solvent (a so-called organic electrolytic solution) can be used. Examples of lithium salts include LiClO4, LiPF6, LiAsF6, LiBF4, LiSO3CF3, etc. The lithium salt may be one type alone or two or more types. Examples of non-aqueous solvents include ethylene carbonate, fluoroethylene carbonate, chloroethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, cyclohexylbenzene, sulfolane, propane sultone, 3-methyl sulfolane, 2,4-dimethyl sulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, trimethyl phosphate, triethyl phosphate, etc. The non-aqueous solvents may be used alone or in combination of two or more.

[0096] The state of the positive electrode and the negative electrode in the lithium ion secondary battery is not particularly limited. For example, the positive electrode, the negative electrode, and a separator disposed between the positive electrode and the negative electrode as needed may be spirally wound or may be stacked in the form of flat plates.

[0097] The separator is not particularly limited, and may be, for example, a resin nonwoven fabric, cloth, microporous film, or a combination thereof. Resins include those primarily composed of polyolefins such as polyethylene and polypropylene. If the structure of the lithium-ion secondary battery does not allow the positive electrode and negative electrode to come into direct contact with each other, a separator may not be used.

[0098] The shape of the lithium ion secondary battery is not particularly limited, and examples thereof include laminated type batteries, paper type batteries, button type batteries, coin type batteries, stacked type batteries, cylindrical type batteries, and prismatic type batteries.

[0099] The lithium ion secondary battery of the present disclosure is suitable as a large-capacity lithium ion secondary battery for use in electric vehicles, power tools, power storage devices, etc. In particular, it is suitable as a lithium ion secondary battery for use in electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc., which require large current charging and discharging to improve acceleration performance and brake regeneration performance. [Example]

[0100] The present invention will be specifically explained below based on the following test results, but the present invention is not limited to these test results.

[0101] Example 1 (Preparation of spherical natural graphite) Spherical natural graphite was produced by spheronizing flake natural graphite (produced in Heilongjiang Province, China) with an average particle size of 50 μm using a spheronizing treatment device (Nara Machinery Manufacturing Co., Ltd., Hybridization NHS-0) at a rotor peripheral speed of 75.0 m / min for 2.5 minutes. The produced spherical natural graphite was then subjected to cyclone classification using a cyclone classifier with a classification point set to 1 μm.

[0102] Example 2 Spherical natural graphite flakes (produced in Heilongjiang Province, China) with an average particle size of 50 μm were subjected to a spheronization treatment using a spheronization treatment device (Nara Kikai, Hybridization NHS-0) at a rotor peripheral speed of 85.0 m / min for 3.0 minutes to produce spherical natural graphite. The produced spherical natural graphite was subjected to cyclone classification using a cyclone classifier with a classification point set to 10 μm. The particles removed by cyclone classification were further sieved to remove particles of 1 μm or less, thereby obtaining by-product particles. 30 parts by mass of the obtained by-product particles were mixed with 70 parts by mass of the spherical natural graphite obtained by cyclone classification (main particles in Table 1) to obtain spherical natural graphite for use in the production of negative electrode material.

[0103] (Production of negative electrode material) A negative electrode material was produced in the same manner as in Example 1, except that the spherical natural graphite obtained in Example 2 was used instead of the spherical natural graphite obtained in Example 1.

[0104] [Examples 3, 4, 7, and 8] Spheroidized natural graphite was obtained in the same manner as in Example 1, except that the flake natural graphite used in Example 1 was changed to that shown in Table 1, or the spheroidization treatment conditions were changed to those shown in Table 1.

[0105] (Production of negative electrode material) A negative electrode material was produced in the same manner as in Example 1, except that the spherical natural graphite obtained in Example 1 was replaced with the spherical natural graphite obtained in Example 3, 4, 7 or 8.

[0106] [Examples 5, 6, and 9] Cyclone classification was carried out in the same manner as in Example 2 to obtain by-product particles, except that the flake natural graphite used in Example 2 was changed to that shown in Table 1, or the conditions for the spheroidizing treatment were changed to those shown in Table 1. The obtained by-product particles and the spheroidized natural graphite obtained by cyclone classification were mixed in the ratio shown in Table 1 to obtain spheroidized natural graphite to be used in producing a negative electrode material.

[0107] (Production of negative electrode material) A negative electrode material was produced in the same manner as in Example 2, except that the spherical natural graphite obtained in Example 2 was replaced with the spherical natural graphite obtained in Example 5, 6 or 9.

[0108] [Comparative Examples 1, 2, and 4] Spheroidized natural graphite was obtained in the same manner as in Example 1, except that the spheroidization treatment conditions in Example 1 were changed as shown in Table 1.

[0109] (Production of negative electrode material) A negative electrode material was produced in the same manner as in Example 1, except that the spherical natural graphite obtained in Comparative Example 1, 2 or 4 was used instead of the spherical natural graphite obtained in Example 1.

[0110] Comparative Example 3 Cyclone classification was performed in the same manner as in Example 2, except that the spheroidizing treatment conditions in Example 2 were changed as shown in Table 1, to obtain by-product particles. 30 parts by mass of the obtained by-product particles were mixed with 70 parts by mass of spheroidized natural graphite obtained by cyclone classification, to obtain spheroidized natural graphite to be used in producing a negative electrode material.

[0111] Table 1 shows the spheronization conditions, cyclone classification conditions, and by-product particle addition conditions in each example and comparative example.

[0112] For the negative electrode materials obtained in each example and comparative example, the tap density, D90 / D10, average particle diameter (50%D), average circularity, N2 specific surface area, average interplanar spacing, R value, exothermic peak, number and percentage of particles with a circle-equivalent diameter of 5 μm or less, number-based circle-equivalent diameter at the cumulative 99% of the particle diameter distribution limited to particle diameters of 5 μm or more, and circularity at the cumulative 1% of the circularity distribution limited to particle diameters of 5 μm or more were measured by the following methods. The results are shown in Tables 2 and 3. In Table 3, "the boundary value of circularity obtained from the measured value of the equivalent circle diameter and formula (a)" means the value on the right side of formula (a) when the number-based equivalent circle diameter at 99% of the cumulative total is substituted into formula (a).

[0113] [Measurement of tap density] Capacity 150cm 3 100 cm of sample powder was placed in a graduated flat-bottom test tube (KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.). 3 The graduated flat-bottomed test tube was then stopped up and the graduated flat-bottomed test tube was dropped 250 times from a height of 5 cm, after which the value calculated from the mass and volume of the sample powder was taken as the tapped density. The results are shown in Table 2.

[0114] [Measurement of average particle size (D50), D10, D90 and D90 / D10] 0.06 g of anode material and purified water containing 0.2% by mass of a surfactant (Liponol T / 15, Lion Corporation) were placed in a test tube (12 mm x 120 mm, Maruemu Corporation) and stirred for 20 seconds using a test tube mixer (Pasolina NS-80, AS ONE Corporation) to obtain a dispersion of the anode material. This liquid was then placed in the sample water tank of a laser diffraction particle size analyzer (SALD-3100, Shimadzu Corporation). The liquid was then circulated with a pump while applying ultrasound (pump flow rate was 65% from the maximum), and the amount of water was adjusted to achieve an absorbance of 0.10 to 0.15. The particle size distribution obtained was calculated as the mean particle size, taking the particle size at 50% volume (D50). Furthermore, the particle size distribution was calculated by dividing the particle size distribution by the particle size at 10% volume (D10) and the particle size distribution at 90% volume (D90). The D90 / D10 ratio was calculated from the particle size distribution at 90% volume (D90). The results are shown in Table 2.

[0115] [Measurement of average circularity and circularity at cumulative 1% of circularity distribution limited to particle diameters of 5 μm or more] The circularity of the negative electrode material was measured using a wet flow particle size and shape analyzer (FPIA-3000 manufactured by Malvern Instruments). The average circularity, which is the circularity based on the number of particles at 50% of the cumulative total, and the circularity at 1% of the cumulative total of the circularity distribution limited to particles with diameters of 5 μm or more were calculated. The measurement temperature was 25° C., the concentration of the measurement sample was 10 mass %, and the number of particles counted was 10000. Water was used as the dispersion solvent. As a pretreatment for this measurement, 0.06 g of carbon material and purified water containing 0.2% by mass of surfactant (trade name: Liponol T / 15, manufactured by Lion Corporation) were placed in a test tube (12 mm x 120 mm, manufactured by Maruemu Corporation) and stirred for 20 seconds using a test tube mixer (Pasolina NS-80, manufactured by AS ONE Corporation), followed by ultrasonic stirring for 1 minute. The ultrasonic cleaner used was a US102 manufactured by SND Corporation (high frequency output 100 W, oscillation frequency 38 kHz). The results are shown in Tables 2 and 3.

[0116] [Measurement of N2 specific surface area] The N2 specific surface area was calculated by the BET method using a single-point measurement of nitrogen adsorption at liquid nitrogen temperature (77 K) using a high-speed specific surface area / pore distribution analyzer (FlowSorb III, manufactured by Shimadzu Corporation). The results are shown in Table 2.

[0117] [Average interplanar spacing, R value and heat generation peak] The negative electrode materials obtained in each of the examples and comparative examples were measured for average interplanar spacing, R value, and exothermic peak using the methods described above. The results are shown in Table 2. In Table 2, the negative electrode materials obtained in each of the Examples and Comparative Examples had one exothermic peak in the DTA between 300°C and 1000°C.

[0118] [Measurement of the number and percentage of particles with a circular equivalent diameter of 5 μm or less] The number and proportion of particles with a circle-equivalent diameter of 5 μm or less in the negative electrode material were measured using a wet flow particle size and shape analyzer (FPIA-3000 manufactured by Malvern Instruments) under the same conditions as those for measuring the average circularity. Furthermore, the proportion of particles with a circle-equivalent diameter of 5 μm or less to the total measured count (10,000 in this example) was calculated from the number of particles with a circle-equivalent diameter of 5 μm or less to the total measured count. The results are shown in Table 3.

[0119] [Number of particles with equivalent circle diameter of 5 μm or less / N2 specific surface area] The ratio of the number of particles with a circle-equivalent diameter of 5 μm or less / the N2 specific surface area was calculated using the N2 specific surface area and the number of particles with a circle-equivalent diameter of 5 μm or less obtained as described above. The results are shown in Table 3.

[0120] [Number-based circular equivalent diameter (μm) for cumulative 99% of particle size distribution of 5 μm or more] The number-based equivalent circle diameter (μm) of the cumulative 99% of the particle size distribution, limited to particle sizes of 5 μm or more, was measured using a wet flow particle size and shape analyzer (FPIA-3000 manufactured by Malvern Instruments) under the same conditions as those for measuring the average circularity. The results are shown in Table 3.

[0121] [Calculation of the boundary value of circularity obtained from the measured value of the equivalent circle diameter and formula (a)] The boundary value of circularity was determined by substituting the number-based equivalent circle diameter (μm) for the cumulative 99% of the particle size distribution of 5 μm or more measured as described above into X in formula (a) to calculate the value of the right side of formula (a). The results are shown in Table 3.

[0122] [Difference between the circularity at cumulative 1% of the circularity distribution limited to particle sizes of 5 μm or more and the boundary value of circularity] The difference between the circularity at 1% of the cumulative circularity distribution limited to particle diameters of 5 μm or more measured as described above and the boundary value of circularity calculated as described above was determined. If the difference is 0 or greater, the condition of formula (a) is satisfied. The results are shown in Table 3.

[0123] [Table 1]

[0124] [Table 2]

[0125] [Table 3]

[0126] (Fabrication of lithium-ion secondary batteries for input / output characteristic evaluation) Using the negative electrode materials prepared in each of the Examples and Comparative Examples, lithium ion secondary batteries for evaluating input / output characteristics were prepared according to the following procedure. First, an aqueous solution (CMC concentration: 2% by mass) of CMC (carboxymethyl cellulose, manufactured by Daicel FineChem Co., Ltd., product number 2200) as a thickener was added to 98 parts by mass of the negative electrode material so that the solid content of CMC was 1 part by mass, and the mixture was kneaded for 10 minutes. Next, purified water was added so that the total solid content of the negative electrode material and CMC was 40% to 50% by mass, and the mixture was kneaded for 10 minutes. Next, an aqueous dispersion (SBR concentration: 40% by mass) of styrene-butadiene copolymer rubber SBR (BM400-B, Zeon Corporation) was added as a binder so that the solid content of SBR was 1 part by mass, and the mixture was mixed for 10 minutes to prepare a paste-like negative electrode material composition. Next, the negative electrode material composition was applied to an 11 μm-thick electrolytic copper foil at a coating amount per unit area of ​​10.0 mg / cm. 2 The negative electrode layer was formed by applying a coating of 1.65 g / cm using a comma coater with the clearance adjusted to the desired value. 3 The electrode density was adjusted to 14 mm. The electrolytic copper foil on which the negative electrode material layer was formed was punched into a disk shape with a diameter of 14 mm to prepare a sample electrode (negative electrode).

[0127] The fabricated sample electrode (negative electrode), separator, and counter electrode (positive electrode) were placed in a coin-type battery container in this order, and an electrolyte solution was poured into the container to fabricate a coin-type lithium-ion secondary battery. The electrolyte solution used was a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio of EC to EMC: 3:7), to which 0.5% by mass of vinylene carbonate (VC) was added relative to the total volume of the mixed solution, and LiPF6 was dissolved to a concentration of 1 mol / L. Metallic lithium was used as the counter electrode (positive electrode). A 20 μm-thick polyethylene microporous membrane was used as the separator. The fabricated lithium-ion secondary battery was used to evaluate its initial charge / discharge characteristics and input characteristics (DCR) using the following methods.

[0128] [Evaluation of input / output characteristics] (Evaluation of initial charge / discharge characteristics) The fabricated lithium-ion secondary battery was charged at a current of 0.2 C and a voltage of 0 V (V vs. Li / Li + ), and then constant voltage charging was performed at 0 V until the current value reached 0.02 C. The capacity at this time was defined as the initial charge capacity. Then, after a 30-minute rest, the current was 0.2 C and the voltage was 1.5 V (V vs. Li / Li + The capacity at this time was taken as the initial discharge capacity (discharge capacity in Table 4). The results are shown in Table 4. The unit of current value, "C," means "current value (A) / battery capacity (Ah)."

[0129] The direct current resistance (DCR) of the lithium ion secondary battery was measured to determine the power density of the battery. Specifically, the following procedure was performed. The results are also shown in Table 4.

[0130] (DCR measurement at 25°C) The lithium ion secondary battery was placed in a thermostatic chamber set at 25° C. and subjected to one cycle of charge and discharge under the conditions of charge: CC / CV 0.2C 0V 0.02C cut, discharge: CC 0.2C 1.5V cut. Next, constant current charging was performed at a current value of 0.2 C up to SOC 50%. Furthermore, the lithium-ion secondary battery was placed in a thermostatic chamber set at 25°C, and constant current charging was performed at 1C, 3C, and 5C for 10 seconds each, and the voltage drop (ΔV) at each constant current was measured, and the direct current resistance (DCR) was measured using the following formula. DCR [Ω] = {(3C voltage drop ΔV - 1C voltage drop ΔV) + (5C voltage drop ΔV - 3C voltage drop ΔV)} / 4

[0131] (Fabrication of lithium-ion secondary batteries for cycle performance evaluation) Using the negative electrode materials prepared in each of the Examples and Comparative Examples, lithium ion secondary batteries for evaluating cycle characteristics were prepared according to the following procedure. First, as a negative electrode material, 7 mass parts of carbon-coated SiO (heat treatment temperature: 1000°C, carbon coating amount: 5 mass %) were added to 93 mass parts of the negative electrode material prepared in each example and comparative example, and then mixed for 10 minutes to prepare a negative electrode active material. To 98 parts by mass of each of the prepared negative electrode active materials, an aqueous solution (CMC concentration: 2% by mass) of CMC (carboxymethyl cellulose, Daiichi Kogyo Seiyaku Co., Ltd., Cellogen WS-C) was added as a thickener so that the solid content of CMC was 1 part by mass, and the mixture was kneaded for 10 minutes. Next, purified water was added so that the total solid content of the negative electrode material and CMC was 40% to 50% by mass, and the mixture was kneaded for 10 minutes. Next, an aqueous dispersion (SBR concentration: 40% by mass) of SBR (BM400-B, Zeon Corporation) was added as a binder so that the solid content of SBR was 1 part by mass, and the mixture was mixed for 10 minutes to prepare a paste-like negative electrode material composition. Next, the negative electrode material composition was applied to an 11 μm-thick electrolytic copper foil in a coating amount per unit area of ​​10.0 mg / cm. 2 The negative electrode layer was formed by coating with a comma coater with the clearance adjusted so that the thickness was 1.65 g / cm. 3 The electrode density was adjusted to 4.0 cm x 3.0 cm by punching out the electrolytic copper foil on which the negative electrode material layer was formed to prepare a sample electrode (negative electrode). The positive electrode was made of NMC and cut out to the same area. A separator was placed between the negative and positive electrodes. The electrolyte used was a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio of EC to EMC was 3:7), to which 0.5 mass% of vinylene carbonate (VC) and 1.0 mass% of fluoroethylene carbonate (FEC) were added, with LiPF6 dissolved to a concentration of 1 mol / L.

[0132] [Evaluation of cycle characteristics] In each of the Examples and Comparative Examples, the lithium ion batteries prepared as described above were used to evaluate the cycle characteristics as follows. First, the battery was charged at a constant current of 0.2 C at 25°C with a cut-off voltage of 4.2 V, and then at a constant voltage of 0.02 C from the time the voltage reached 4.2 V. After a 30-minute rest, the battery was discharged twice at a constant current of 0.2 C at 25°C with a cut-off voltage of 2.7 V for aging. After the first charge / discharge cycle under the above charge / discharge conditions, the current was increased to 1.0 C and 99 cycles were performed, for a total of 100 cycles. The discharge capacity retention rate (%) after 100 cycles (cycle retention rate in Table 4) was calculated using the following formula. The results are shown in Table 4. Discharge capacity retention rate (%) = (discharge capacity after 100 cycles / discharge capacity at 1st cycle) x 100

[0133] [Evaluation of injectability] The negative electrode for the cycle characteristic evaluation was prepared and roll-pressed to a density of 1.65 g / cm. 3 The electrode density was adjusted to the desired value. The pressed negative electrode was punched out into a 16mm diameter circle and attached to a glass substrate with double-sided tape to create a flat, distortion-free electrode surface. 3 μL of PC (polycarbonate, manufactured by Kishida Chemical Co., Ltd.) was dropped onto the center of the circular electrode using a micropipette, and the time it took for the solution to penetrate was measured. Three identical electrodes were prepared, and the average of three measurements was calculated. The results are shown in Table 4.

[0134] (Fabrication of lithium-ion secondary batteries for electrode orientation evaluation) A negative electrode for a lithium ion secondary battery was produced in the same manner as in the above (Production of a lithium ion secondary battery for evaluation of input / output characteristics), and the electrode orientation was evaluated under the conditions shown below.

[0135] [Evaluation of electrode orientation] The resulting negative electrode for lithium ion secondary batteries was placed on a plate with a width of 5 cm and an area of ​​600 cm. 2The negative electrode was formed into a rectangular sheet, and the resulting molded product was pressed with a 4 t hydraulic pressure using a roll-type press. The linear pressure was 4 t / 5 cm = 0.8 t / cm. The pressed negative electrode was placed in an X-ray diffraction measurement cell, and an X-ray diffraction pattern was measured using CuKα radiation (2θ = 25.5° to 27.5°, 76.5° to 78.5°) under the conditions of a scan rate of 0.25° / min, a tube voltage of 40 kV, a tube current of 30 mA, a divergence slit of 1°, a scattering slit of 1°, and a receiving slit of 0.3 mm, using an X-ray diffraction measurement device (X-RAY DIFFRACTIOMETER MultiFlex manufactured by Rigaku Corporation). 002 ) and the peak intensity of the 110 diffraction line (I 110 ) is the ratio of I 002 / I 110 The results are shown in Table 4.

[0136] [High temperature storage maintenance rate] In step 1, a lithium ion secondary battery prepared in the same manner as described above (preparation of a lithium ion secondary battery for evaluation of input / output characteristics) was placed in a thermostatic chamber set at 25°C, and the current was set at 0.2 C and the voltage was set at 0 V (V vs. Li / Li + ), then constant voltage charging was performed at 0 V until the current value reached 0.02 C. After a 30-minute rest, the battery was charged at a current value of 0.2 C and a voltage of 1.5 V (V vs. Li / Li). + After repeating this charge / discharge twice, the current was 0.2 C and the voltage was 0 V (V vs. Li / Li). + ), and then constant voltage charging was performed at 0 V until the current value reached 0.02 C. The battery was then placed in a thermostatic chamber set at 60° C. and stored for 7 days. Then, in step 2, the lithium-ion secondary battery was placed in a thermostatic chamber set at 25°C and left for 60 minutes, and the voltage was measured at a current of 0.2 C and a voltage of 1.5 V (V vs. Li / Li + ) was discharged at a constant current. Then, charging and discharging were repeated once under the above conditions. This process of storing at 60°C (step 1) and checking the capacity after storage (step 2) was repeated a total of three times. The high-temperature storage retention rate was calculated using the following formula, and the arithmetic mean value was taken as the high-temperature storage property (%). The results are shown in Table 4. High temperature storage retention rate (%) = (first discharge capacity at 25°C after storage at 60°C for 21 days) / (second discharge capacity at 25°C before storage at 60°C) × 100

[0137] [Table 4]

[0138] Comparing Example 2 and Comparative Example 3, D50 is about 18 μm and N2 specific surface area is about 6 m 2 / g, but in Example 2, the DCR during charging was low and improved. Since Example 2 has a higher average circularity than Comparative Example 3, it is thought that the effect of the average circularity contributed to the better DCR results. However, when comparing Example 2 with Comparative Example 2, which has a higher average circularity and a smaller D50, it was shown that a small D50, a large N2 specific surface area, and a high average circularity alone are not enough to sufficiently improve input characteristics.

[0139] Our results show that increasing the number of particles with a circular equivalent diameter of 5 μm or less and adjusting the composition of the coarse and fine particles to achieve a specific D90 / D10 ratio can improve the input characteristics of lithium-ion secondary batteries. Increasing the number of particles with a circular equivalent diameter of 5 μm or less results in a large number of particles with short-range diffusion within the solid, and these particles also function as conductive materials between other particles. This is thought to result in dispersed current flow at the anode, reducing resistance and resulting in not only good input characteristics but also good cycle characteristics. Furthermore, increasing the number of particles with a circular equivalent diameter of 5 μm or less per unit N2 specific surface area (number of particles with a circular equivalent diameter of 5 μm or less / N2 specific surface area) improves conductivity and shortens the distance of diffusion within the solid without increasing the reaction area between the anode material and the electrolyte. This is thought to result in better cycle characteristics because electrolyte consumption is reduced compared to simply increasing the N2 specific surface area by increasing the number of particles with a circular equivalent diameter of 5 μm or less. When D90 / D10 is within a specific range, the tortuous path length, which is the distance from one surface to the other in the thickness direction through the voids inside the negative electrode material layer, does not become too long, and the liquid tends to circulate well. This makes it less likely that an area will occur where the electrolyte and the negative electrode material cannot come into contact, making it easier to maintain good cycle characteristics. On the other hand, when D90 / D10 exceeds a certain value, the large particle diameter particles increase the tortuous path length, resulting in poor liquid circulation. For example, in Comparative Example 3, the injection time is the longest compared to the Examples and Comparative Examples, and the electrode orientation is also the highest, which is thought to be why the tortuous path length is long. As a result, in Comparative Example 3, the number of particles with a circle-equivalent diameter of 5 μm or less / N2 specific surface area, which contributes to cycle performance, is larger than in Example 9, but the long injection time is thought to result in inferior cycle performance compared to Example 9.

[0140] 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. A negative electrode material for a lithium-ion secondary battery, comprising a carbon material that satisfies the following (1), (2), and (4): (1) The volume-based particle size D90 / D10 is greater than 2.0 and less than 4.

3. (2) The value N / S, which is the number of particles N having a circular equivalent diameter of 5 μm or less based on the number of particles measured (10,000 particles in total), divided by the specific surface area S determined by nitrogen adsorption measurement at 77 K, is 750 (particles·g / m2) or more. (4) When X is the number-based circle-equivalent diameter at 99% of the cumulative total of a particle size distribution limited to particle sizes of 5 μm or more, and Y is the circularity at 1% of the cumulative total of a circularity distribution limited to particle sizes of 5 μm or more, the following formula (a) is satisfied: Y ≧ 0.3×log 10 (X)+(0.5-0.3×log 10 6)...(a)

2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the carbon material satisfies the following (3): (3) The percentage of particles having a circular equivalent diameter of 5 μm or less based on the number of particles measured is 45% or more out of a total of 10,000 particles.

3. The negative electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein the carbon material satisfies the following (5): (5) Tap density is 0.80 g / cm ３ ~0.95g / cm ３ is.

4. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the carbon material satisfies the following (6): (6) The average circularity is 0.90 to 0.

93.

5. The average interplanar spacing d of the carbon material obtained by X-ray diffraction ００２ The negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 4, wherein the surface roughness is 3.34 Å to 3.38 Å.

6. The negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 5, wherein the R value of the carbon material measured by Raman spectroscopy is 0.1 to 0.

4.

7. The specific surface area of ​​the carbon material determined by nitrogen adsorption measurement at 77 K was 14 m ２ The negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 6, wherein the SiO2 content is 0.01g or less.

8. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the carbon material does not have two or more exothermic peaks in a temperature range of 300 ° C to 1000 ° C in a differential thermal analysis in an air stream.

9. A negative electrode for a lithium ion secondary battery, comprising: a negative electrode material layer containing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 8; and a current collector.

10. A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 9, a positive electrode, and an electrolyte solution.

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