Negative electrode active material for lithium ion secondary battery and lithium ion secondary battery

Abstract

To provide a negative electrode active material for a lithium ion secondary battery having excellent rate characteristics and cycle characteristics, and a lithium ion secondary battery SOLUTION: A negative electrode active material for lithium ion secondary batteries, comprising graphite-based carbonaceous particles, wherein equivalent Young's moduli of the graphite-based carbonaceous particles measured by the following method are 2.0 to 8.0 0GPa on average, and standard deviations of the equivalent Young's moduli are 1.9 or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Lithium-ion secondary batteries are primarily used as power sources for portable devices, electric vehicles, and the like. Lithium-ion secondary batteries are required to have excellent rate and cycle characteristics. Rate characteristics refer to the battery's charging and discharging capabilities, and are evaluated herein as the ratio (2C / 0.2C) of the charging and discharging current to the battery capacity (C). Cycle characteristics refer to the degree of deterioration of battery capacity due to charging and discharging, and are evaluated herein as the battery capacity retention rate (%) after 200 cycles of 1.0C charging and discharging at 25°C.

[0003] In general, lithium ion secondary batteries use a lithium transition metal composite oxide such as lithium cobalt oxide as the positive electrode active material, and graphite-based carbonaceous particles as the negative electrode active material.

[0004] Graphite is classified into natural graphite and artificial graphite. Natural graphite has the advantage of being inexpensive, but has the disadvantage of being easily broken by rolling during electrode production. For example, Patent Document 1 proposes a method of coating the surface of spherically processed natural graphite with artificial carbon. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-340232 Summary of the Invention [Problem to be solved by the invention]

[0006] The characteristics of batteries using graphite-based carbonaceous particles as anode active materials are affected by the porous structure of the graphite-based carbonaceous particles and the contact area between the particles. The smaller the porosity of the graphite-based carbonaceous particles, the greater the capacity and the greater the contact area between the particles. This improves output characteristics, but the reactivity deep inside the electrode tends to decrease. For this reason, it has been common to adjust battery characteristics by adjusting the particle size and porosity of the graphite-based carbonaceous particles. However, simply adjusting the particle size and porosity of the graphite-based carbonaceous particles can sometimes result in degradation of battery characteristics, particularly cycle characteristics.

[0007] The present inventors have an object to provide a negative electrode active material for a lithium ion secondary battery and a lithium ion secondary battery having excellent rate characteristics and cycle characteristics. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to achieve the above object, and as a result have obtained the following findings.

[0009] Even when graphite-based carbonaceous particles have the same particle size and porosity, the cycle characteristics of batteries using these particles as negative electrode active materials have been significantly reduced. The present inventors hypothesized that this was due to differences in the mechanical strength of each particle, and investigated a method for measuring the mechanical strength of each particle using a micro-compression tester.

[0010] The inventors have found that the equivalent Young's modulus of graphite-based carbonaceous particles can be calculated by conducting a microcompression test under predetermined conditions and organizing the results based on Hertz's contact theory. Based on the equivalent Young's modulus thus obtained, the inventors have calculated the rate and cycle characteristics of the battery. They have found that adjusting the equivalent Young's modulus to a predetermined range makes it possible to obtain a negative electrode active material for a lithium ion secondary battery having excellent rate and cycle characteristics. This finding has led to the completion of the present invention.

[0011] The present invention is summarized as follows.

[0012] Contains graphite-based carbonaceous particles, The equivalent Young's modulus of the graphite-based carbonaceous particles measured by the following method is 2.0 to 8.0 GPa on average, and the standard deviation of the equivalent Young's modulus is 1.9 or less. Negative electrode active material for lithium-ion secondary batteries. <Method for measuring equivalent Young's modulus> The graphite-based carbonaceous particles are placed on the stage of a microcompression tester. The graphite-based carbonaceous particles are then observed under a microscope, and the radius R of each particle is calculated from the average value of the maximum diameters in the horizontal and vertical directions of the observation field. Next, using a flat indenter, a compression test is performed on any particle whose radius R is within the range of D50 / 2±2% under conditions of a maximum test force of 490 mN and a loading rate of 4.8 mN / sec. The compression load W, the radius R of each particle, and the displacement δ in the compression direction are substituted into the following equation to determine the equivalent Young's modulus E. This procedure is performed on any 10 graphite-based carbonaceous particles, and the average value is calculated.

number

[0013] According to the present invention, it is possible to obtain a negative electrode active material for a lithium ion secondary battery having excellent rate characteristics and cycle characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0014] [Negative electrode active material] The negative electrode active material for a lithium ion secondary battery according to this embodiment contains graphite-based carbonaceous particles. The graphite-based carbonaceous particles include graphite particles such as artificial graphite and natural graphite. In particular, the natural graphite-based carbonaceous particles preferably include graphite particles and an amorphous carbon layer covering at least a portion of the surface of the graphite particles. The content of the amorphous carbon layer is preferably 5% or less.

[0015] The average lattice spacing (d 002) is more preferably less than 0.0.336. In addition, in the Raman spectrum obtained by Raman spectroscopy, -1 The intensity of the peak in the region (G band) from 1350 to 1370 cm -1 The ratio of the intensities of the peaks present in this region (D band) is preferably less than 1.0, and more preferably less than 0.5.

[0016] The equivalent Young's modulus of the graphite-based carbonaceous particles in the negative electrode active material for a lithium ion secondary battery according to this embodiment is 2.0 to 8.0 GPa on average. The equivalent Young's modulus is determined as follows.

[0017] <Method for measuring equivalent Young's modulus> Graphite-based carbonaceous particles are placed on the stage of a microcompression tester (Shimadzu Corporation MCT-W510). Next, the particles are observed under a microscope, and the radius R of each particle is calculated from the average value of the maximum diameters in the horizontal and vertical directions of the observation field. Next, using a flat indenter, a compression test is performed on any particle whose radius R is within the range of the average particle diameter D50 / 2±2% under conditions of a maximum test force of 490 mN and a loading speed of 4.8 mN / sec. The compression load W, the radius R of each particle, and the displacement δ in the compression direction at this time are substituted into the following equation to determine the equivalent Young's modulus E. As a solution, the measurement data is converted into (σ / 2) 3 / 2 is on the horizontal axis, and W / R 1 / 2 The equivalent Young's modulus E can be obtained by plotting the linear regression line with the vertical axis and multiplying the slope of the linear regression line by 3 / 4. This procedure is carried out for any 10 graphite-based carbonaceous particles, and the average value is calculated.

[0018] Furthermore, since Young's modulus is a mechanical property value in the elastic region and Hertz's contact theory also holds in the elastic region, the slope of the plot above must be determined from the linear change region. As the amount of displacement increases, the curve deviates from the linear elastic deformation region and reaches a curved plastic deformation region. It is important to note that the equivalent Young's modulus E is determined from the linear deformation region.

number

[0019] As mentioned above, battery characteristics are typically adjusted by adjusting the particle size and porosity of graphite-based carbonaceous particles. However, simply adjusting the particle size and porosity of graphite-based carbonaceous particles can sometimes result in deterioration of cycle characteristics. The inventors focused on the mechanical strength of the graphite-based carbonaceous particles that constitute the negative electrode of a lithium-ion secondary battery as the cause of the deterioration. They measured the equivalent Young's modulus of individual particles extracted from graphite-based carbonaceous particles, fabricated lithium-ion secondary batteries using graphite-based carbonaceous particles with various equivalent Young's moduli as the negative electrode active material, and conducted experiments to measure the cycle characteristics. They found that lithium-ion secondary batteries using graphite-based carbonaceous particles with an equivalent Young's modulus of 2.0 GPa or more and a standard deviation of the equivalent Young's modulus of 1.9 or less as the negative electrode active material did not exhibit deterioration of cycle characteristics.

[0020] During the charge and discharge of a lithium-ion secondary battery, the graphite-based carbonaceous particles repeatedly expand and contract. It is believed that when graphite-based carbonaceous particles have a too small equivalent Young's modulus, the particles are prematurely destroyed by this repeated stress, resulting in deterioration of cycle characteristics. A larger equivalent Young's modulus is advantageous for cycle characteristics, but it also increases resistance to volumetric changes during charge and discharge, increasing reaction overvoltage and deteriorating rate characteristics. For this reason, the equivalent Young's modulus of graphite-based carbonaceous particles is set to an average of 8.0 GPa or less.

[0021] The lower limit of the average equivalent Young's modulus of the graphite-based carbonaceous particles is preferably 3.0 GPa, more preferably 4.0 GPa, and the upper limit of the average equivalent Young's modulus of the graphite-based carbonaceous particles is preferably 7.0 GPa, more preferably 6.0 GPa.

[0022] The standard deviation of the equivalent Young's modulus of the graphite-based carbonaceous particles is preferably 1.6 or less. This is because if particles with a small equivalent Young's modulus are included in some parts, the cycle characteristics are likely to deteriorate. The smaller the standard deviation of the equivalent Young's modulus, the better, but the lower limit due to manufacturing constraints is 0.6, and the practical lower limit is 1.0.

[0023] There are no particular restrictions on the particle size of the graphite-based carbonaceous particles, but if the particle size is too large, the rate characteristics are likely to deteriorate, and if the particle size is too small, the cycle characteristics are likely to deteriorate. Therefore, the average particle size of the graphite-based carbonaceous particles should be in the range of 3.0 to 18.0 μm. The preferred lower limit is 5.0 μm. The preferred upper limit is 15.0 μm, and more preferably 10.0 μm. The particle size is the average value measured by adding a few drops of dispersant to a particle size distribution analyzer, SALD2300, manufactured by Shimadzu Corporation.

[0024] [Method of manufacturing negative electrode active material] The method for producing the negative electrode active material includes the steps of flotation, spheroidization, acid treatment, carbon coating, and removal of foreign matter.

[0025] (flotation) Flotation is a process in which graphite ore is extracted from mines with graphite veins by open-cut or underground mining, and then separated and dried using underwater flotation to obtain flake-shaped graphite (called flake graphite).

[0026] (spheroidization) Spheroidization is a process in which flake graphite obtained by flotation is crushed and molded in a spheroidization line. Spheroidization equipment can be combined in series or parallel as needed, with 10 to 30 stages. After obtaining the desired spheroidized product, it is classified to obtain spherical graphite with the desired shape and particle size. Because this process affects the equivalent Young's modulus of the graphite particles, it is necessary to perform crushing, molding, and classification under conditions that optimize the rotation speed of the spheroidization line, the residence time in the spheroidization equipment, and the number of stages in series.

[0027] (acid treatment) The acid treatment is a process in which the obtained spheroidized graphite is heated and pickled, then separated using a dehydration device (filter press or centrifuge), and repeatedly washed in a water tank to remove impurities contained in the spheroidized graphite. The purpose of the acid treatment is to remove impurities contained in natural graphite derived from mines (earth-derived), as well as impurities introduced by the machine during spheroidization. The pickled spheroidized graphite is dehydrated and dried to a predetermined moisture content, then magnetically separated and sieved. If the carbon concentration of the spheroidized graphite (in the graphite raw material) after this process is too low, the desired equivalent Young's modulus cannot be obtained. Therefore, the carbon concentration of the spheroidized graphite must be 99.9% or more by mass. Preferably, it is 99.97% or more.

[0028] (carbon coating) Carbon coating is a process in which the surface of the obtained spheroidized graphite is coated with pitch (amorphous carbonaceous raw material) adjusted to a predetermined particle size to produce carbon-coated graphite particles. For example, coal tar pitch is used as the pitch. The softening point of the pitch is, for example, 150°C to 400°C, and more preferably, 200°C to 300°C. Furthermore, the pitch used is sufficiently pulverized using a jet mill or the like, and the particle size distribution D50 of the pitch is preferably 1 / 3 or less of the particle size distribution D50 of the spheroidized graphite, and more preferably, the particle size distribution D50 of the pitch is 1 / 5 or less.

[0029] Pitch is added to the spheroidized graphite taking into consideration the residual carbon rate. The spheroidized graphite and pitch are mixed in a vertical mixer (such as a Nauta mixer) until uniform. Because there is a risk of foreign matter being mixed in during mixing, it is preferable to apply a resin lining to the sliding parts of the mixer to reduce the risk of contamination. The mixed spheroidized graphite and pitch are heat-treated using a roller hearth kiln or the like. The heat treatment is preferably performed in a non-oxidizing atmosphere, such as steaming or in an inert atmosphere, preferably a nitrogen atmosphere, at a temperature at which the pitch is appropriately carbonized. Hereinafter, this heat treatment in a non-oxidizing atmosphere will be simply referred to as "heat treatment."

[0030] In this process, the mixing ratio of the spheroidized graphite and pitch, the mixing state, and the heat treatment conditions affect the equivalent Young's modulus of the graphite particles. Specifically, the mixing ratio of pitch is set to 1% by mass to 6% by mass with respect to the spheroidized graphite. The mixing state of the spheroidized graphite and pitch is checked at regular intervals to check the particle size distribution, and the mixture is mixed until it becomes uniform.

[0031] The heat treatment temperature is usually as low as possible, for example, around 800°C, to reduce the heat treatment power cost. However, if the heat treatment temperature is too low, a sufficient equivalent Young's modulus cannot be obtained, which deteriorates the cycle characteristics of a battery using such graphite-based carbon particles as a negative electrode active material. For this reason, the heat treatment temperature is set to 900°C or higher. On the other hand, the higher the heat treatment temperature, the higher the equivalent Young's modulus can be, improving the cycle characteristics. However, if the heat treatment temperature is too high, the equivalent Young's modulus becomes too high, which deteriorates the rate characteristics of a battery using such graphite-based carbon particles as a negative electrode active material. For this reason, the heat treatment temperature is set to 1200°C or lower. Therefore, the heat treatment temperature is set in the range of 900 to 1200°C.

[0032] If the holding time at 900°C or higher is too short, the variation in the equivalent Young's modulus will increase, leading to a large standard deviation, which will degrade the cycle characteristics of the battery when used as the negative electrode active material. For this reason, the holding time at 900°C or higher should be at least one hour.

[0033] On the other hand, the higher the maximum temperature during heat treatment, the higher the equivalent Young's modulus can be, improving cycle characteristics but degrading rate characteristics, so it is best to set the maximum temperature according to the desired battery characteristics. If the holding time at the maximum temperature is too short, the variation in the equivalent Young's modulus increases, the standard deviation increases, and when used as a negative electrode active material in a battery, the cycle characteristics of the battery deteriorate. Therefore, it is preferable to hold the holding time at the maximum temperature for 1 hour or more.

[0034] (Foreign matter removal) The removal of foreign matter is a process in which the shape and particle size of the carbon-coated graphite particles are adjusted and foreign matter, such as coarse graphite and metal powder derived from the machinery used in each process, is removed by repeatedly using a sieving machine to remove coarse foreign matter and magnetic foreign matter.

[0035] [Lithium-ion secondary battery] The lithium ion secondary battery according to this embodiment includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode uses the negative electrode active material according to this embodiment.

[0036] (Negative electrode) There are no restrictions on the material of the negative electrode current collector as long as it does not form an alloy with lithium and has low electrical resistance. For example, it may be a sheet or foil made of a metal such as copper, nickel, titanium, or stainless steel. Although copper foil can be used, copper foil is generally used. The negative electrode active material layer contains, for example, the negative electrode active material according to this embodiment and a binder.

[0037] (positive electrode) The positive electrode current collector can be made of any material as long as it does not dissolve at the potential of the positive electrode and has low electrical resistance, but aluminum foil is generally used. The positive electrode active material layer includes, for example, a positive electrode active material, a conductive material, and a binder. The positive electrode active material may be a known positive electrode active material used in lithium ion secondary batteries, such as a lithium transition metal composite oxide having a layered structure or a spinel structure, or a lithium transition metal composite phosphate compound having an olivine structure. The conductive material may be, for example, a carbon material such as acetylene black (AB). The binder may be, for example, polyvinylidene fluoride (PVDF).

[0038] (separator) There are no restrictions on the separator, and for example, a porous sheet made of resin such as polyethylene (PE) or polypropylene (PP), or a nonwoven fabric made of cellulose or resin fibers can be used.

[0039] (non-aqueous electrolyte) There are no limitations on the non-aqueous electrolyte, and for example, a non-aqueous electrolyte solution containing a supporting salt in an organic solvent (nonaqueous solvent) is used. For example, aprotic solvents such as carbonates, esters, and ethers are used as the non-aqueous solvent. One or more of these non-aqueous solvents may be included. For example, lithium salts such as LiPF6, LiBF4, and LiClO4 are used as the supporting salt.

[0040] [Method of manufacturing lithium-ion secondary batteries] (Negative electrode) There are no restrictions on the method for manufacturing the negative electrode, and a commonly used method may be applied. For example, a negative electrode active material is mixed with a binder and a solvent to prepare a slurry, and the resulting slurry is applied to a negative electrode current collector and dried to prepare a negative electrode laminate. The negative electrode laminate is then compressed to prepare a negative electrode.

[0041] (positive electrode) There are no restrictions on the method for manufacturing the positive electrode, and any commonly used method may be used. For example, a positive electrode active material is mixed with a conductive material, a binder, and a solvent to prepare a slurry, which is then applied to a positive electrode current collector and dried to prepare a positive electrode laminate. The positive electrode laminate is then compressed to prepare the positive electrode.

[0042] (battery) There are no restrictions on the structure, shape, or size of the battery, and any commonly manufactured battery may be used, such as a cylindrical battery, a prismatic battery, or a pouch-shaped battery having a wound electrode structure, or a prismatic battery, a pouch-shaped battery, or a coin-shaped battery having a stacked electrode structure. [Example]

[0043] Negative electrode active materials with various equivalent Young's moduli were prepared according to the manufacturing conditions shown in Table 1. Cylindrical lithium-ion secondary batteries (18 mm diameter, 65 mm height) were fabricated using a negative electrode made from the obtained negative electrode active material, a positive electrode with lithium cobalt oxide as the active material, a PP / PE / PP three-layer separator, and a 1M LiPF6 / EC:DEC (1:1 volume ratio) non-aqueous electrolyte, and the cycle and rate characteristics were investigated. The equivalent Young's modulus, cycle and rate characteristics of the negative electrode active materials are shown in Table 2. The cycle and rate characteristics were measured using the following methods.

[0044] (Cycle characteristics) The resulting battery was maintained at an ambient temperature of 25°C and repeatedly charged and discharged 200 times at 1.0 C. The first discharge capacity and the 200th discharge capacity were measured, and the retention rate (%) of the 200th discharge capacity relative to the first discharge capacity was measured. A cycle characteristic of 75% or more is considered to be in the good range. A cycle characteristic of 80% or more is preferred, and 85% or more is more preferred.

[0045] (rate characteristics) The obtained secondary battery was kept at an ambient temperature of 25°C, and the retention rate (%) of the discharge capacity when discharged at 2.0 C relative to the discharge capacity when discharged at 0.2 C was measured. A rate characteristic of 80% or more is considered to be in the good range. A rate characteristic of 83% or more is preferable, and 85% or more is more preferable.

[0046] [Table 1]

[0047] [Table 2]

[0048] As shown in Table 2, Examples 1 to 20, whose average equivalent Young's modulus was within the range of 2 to 8 GPa, were excellent in cycle characteristics and rate characteristics. Comparative Examples 1 to 8, whose average equivalent Young's modulus was outside the range of 2 to 8 GPa, were inferior in either cycle characteristics or rate characteristics. [Industrial Applicability]

[0049] According to the present invention, it is possible to obtain a negative electrode active material for a lithium ion secondary battery having excellent rate characteristics and cycle characteristics.

Claims

1. Contains graphite-based carbonaceous particles, the equivalent Young's modulus of the graphite-based carbonaceous particles, measured by the following method, is 2.0 to 8.0 GPa on average, and the standard deviation of the equivalent Young's modulus is 1.9 or less; Negative electrode active material for lithium-ion secondary batteries. <Method for measuring equivalent Young's modulus> The graphite-based carbonaceous particles are placed on the stage of a microcompression tester. Next, the graphite-based carbonaceous particles are observed under a microscope, and the radius R of each particle is calculated from the average value of the maximum diameters in the horizontal and vertical directions of the observation field. Next, using a flat indenter, a compression test is performed on any particle whose radius R is in the range of D50 / 2±2% under conditions of a maximum test force of 490 mN and a loading rate of 4.8 mN / sec, and the equivalent Young's modulus E is calculated by substituting the compression load W, the radius R of each particle, and the displacement δ in the compression direction into the following formula. This operation is performed on any 10 graphite-based carbonaceous particles, and the average value is calculated. [Equation 1]

2. The graphite-based carbonaceous particles include graphite particles and an amorphous carbon layer that covers at least a portion of the surface of the graphite particles. The negative electrode active material for a lithium ion secondary battery according to claim 1 .

3. The standard deviation of the equivalent Young's modulus is 1.6 or less. The negative electrode active material for a lithium ion secondary battery according to claim 1 or 2.

4. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, The negative electrode is a negative electrode using the negative electrode active material according to claim 1 or 2. Lithium-ion secondary battery.

5. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, The negative electrode is a negative electrode using the negative electrode active material according to claim 3. Lithium-ion secondary battery.

Images