Lithium-ion cell composite carbon negative polar material and preparing method

[0025] The preparation method of the composite carbon negative electrode material for lithium ion batteries of the present invention includes the following steps: 1. A graphite powder with a particle size of 0.3 to 30 μm and a fixed carbon content of ≥90.0%, and a binder with a weight of 1-40% of the graphite powder, Mixing and granulating, adding graphite powder and graphitization catalyst with 0.01-10% of binder weight at the same time; 2. Extruding the mixed pellets into a shape with a density of 1.3-1.9g/cm after molding 3 3. Carry out carbonization or graphitization of the molding material, heat it in a protective gas from 450°C to 3000°C, keep it for 1 to 10 hours, and then drop it to room temperature in a protective atmosphere; 4. Crush the above-mentioned composite carbon material; 3. Crush the above-mentioned composite carbon material; 5. Carry out purification treatment.

[0026] The graphite micropowder in the preparation method of the lithium ion battery composite carbon negative electrode material of the present invention is a mixture of any one, two or three of natural flake graphite, microcrystalline graphite and artificial graphite; the mixing ratio of the graphite micropowder is natural graphite micropowder :Artificial graphite powder=0~100:100~0. That is, natural graphite powder or artificial graphite powder can be used, or the two can be mixed in a certain ratio. The particle size of the micropowder used is smaller than the particle size of the prepared composite carbon material.

[0027] The binder is a mixture of any one, two or three of pitch, tar and resin; the graphitization catalyst is a simple substance of boron, silicon, beryllium, aluminum, titanium, nickel, and cobalt or a compound thereof;

[0028] The protective gas is nitrogen, argon, vacuum or reducing gas, the heat treatment adopts tunnel kiln or graphitization furnace well known to those skilled in the art, and the purification adopts acid purification method, oxidation method and complexation method purification treatment process.

[0029] Such as figure 1 As shown, the graphite micropowder of the present invention has irregular shapes such as lumps and flakes before treatment; after bonding, granulation and heat treatment, it is spherical or quasi-spherical, such as figure 2 As shown, the specific surface area of ​​the material is reduced, the tap density is increased, and the crystal structure of graphite has also undergone major changes, such as image 3 As shown, before graphitization, the graphite powder of the present invention has diffraction peaks of diamond-shaped graphite crystals at diffraction angles of 43.3 degrees and 46.0 degrees, which are a mixture of hexagonal phase and rhombic phase; Figure 4 After the heat treatment of the composite carbon anode material for lithium ion batteries, the diamond-shaped phase diffraction peak disappears, and the diamond-shaped phase is transformed into a more stable hexagonal phase.

[0030] Such as figure 2 As shown, the composite carbon negative electrode material for lithium ion batteries of the present invention is composed of a plurality of flake graphite particles combined or combined to form secondary particles. The layers of graphite crystallites in the particles are randomly arranged with each other to form each crystal plane. Non-parallel, non-directional arrangement of microstructures. The above-mentioned flake graphite particles refer to particles having a long axis and one short axis in the shape of the particles, that is, the shape of the particles is not ideally spherical. Including irregular shapes such as scales, flakes, and columns. Each of the foregoing secondary particles contains a plurality of graphite crystallites, and these graphite crystallites are randomly arranged with each other, so that the graphite layers inside the overall secondary graphite particles are not arranged parallel to each other.

[0031] The above-mentioned "multiple flake graphite particles are combined or combined to form secondary particles", "combined" refers to the state in which graphite particles are bonded together by an intermediate medium such as a binder; "combination" refers to particles The state where the particles are gathered together due to factors such as shape, intermolecular force and surface tension, and the particles are not combined through an intermediate medium. Obviously, from the point of view of mechanical strength, the combined particles are the better choice.

[0032] The average particle size of a single graphite particle is 0.3-30μm, which is less than 2/3 of the size of the secondary particle composed of a single graphite particle, and the number of graphite particles combined with each other is more than 3; in order to combine the single particles together The size of the secondary particles of the composition should not be too large to affect the charge and discharge performance, and the number of graphite particles combined with each other should be less than 2175. The above-mentioned average particle size is measured using the British Malvern Mastersizer 2000 laser particle size analyzer.

[0033] The composite carbon negative electrode material particles of the lithium ion battery of the present invention have an aspect ratio of 1.5-4. The aspect ratio is defined by the following method. Through the graphite particles magnified under a microscope, if the long axis size of a single particle is a and the short axis size is b, then the aspect ratio is a/b. The present invention uses JEOL JSM- 6380LV scanning electron microscope, measured by Millitrac particle image analyzer in the United States. When the aspect ratio is less than 1.5, the contact area between the particles is reduced, which reduces the conductivity; when the aspect ratio of the particles is greater than 4, the degree of bonding between the particles is reduced, which will also deteriorate the rapid charging of the material. Discharge performance.

[0034] The composite carbon negative electrode material of the lithium ion battery of the present invention also has a 1.0-6.0 m 2 The specific surface area per g is measured by the BET method of nitrogen adsorption. By adjusting the specific surface area of ​​the material in an appropriate range, the rapid charge and discharge performance and cycle performance of the battery can be improved, and the irreversible capacity during the first charge and discharge process can be reduced. The specific surface area is greater than 6.0m 2 /g, the irreversible capacity loss during the first cycle is increased, and the processing performance of the material is deteriorated. A large amount of binder needs to be added during the preparation of the negative electrode; and the specific surface area is less than 1.0m 2 /g, the fast charge-discharge performance and cycle performance of the battery will be adversely affected.

[0035] The composite carbon negative electrode material of the lithium ion battery of the present invention has a graphite crystal layer spacing d 002 In the range of 0.335nm~0.338nm, it is measured by the wide-angle diffraction method of powder XRD. When the layer spacing of graphite crystals is greater than 0.338 nm, the degree of graphitization is reduced, reducing the charge and discharge capacity of the negative electrode material.

[0036] The composite carbon negative electrode material of the lithium ion battery of the present invention has a crystal size Lc in the c-axis direction of the graphite crystals of 50-150 nm, a crystal size La in the direction of the crystal basal plane of 50-100 nm, and the graphite crystallite size in the c-axis The length measured in the direction, if the crystallite size is less than 50nm, the probability of occurrence of defects in the lattice structure of the graphite crystal increases, which has an adverse effect on increasing the insertion capacity of lithium ions in it. The crystallite size La is greater than 100 nm, which affects the diffusion rate of lithium ions in the graphite crystal, which is not conducive to the high current charge and discharge of the battery. Lc and La are measured by powder X-ray wide-angle diffraction method.

[0037] The average particle diameter of the graphite powder within the scope of the present invention is the value at the 50% integration of the volume cumulative distribution curve of the particle diameter measured by the laser diffraction/scattering method. If the average particle diameter is less than 5 μm, the specific surface area of ​​the graphite powder increases, thereby reducing the charge and discharge coulombic efficiency. For the average particle diameter greater than 60 μm, it may take a long time for lithium ions to diffuse therein, thereby affecting the discharge performance, especially the high current performance or the low temperature discharge performance. Therefore, the average particle diameter of the composite graphite powder of the present invention is preferably 5-60 μm. In addition, graphite powder preferably does not contain coarse particles larger than 75 μm that adversely affect high current or low-temperature discharge performance, nor does it contain fine particles smaller than 5 μm that are not conducive to improving the initial charge and discharge efficiency. In addition, if graphite powder containing coarse particles is used as the negative electrode material to make the negative plate and then pack it into the battery case, concentrated stress is likely to be applied to the coarse particles, which may pierce the separator and cause an internal short circuit between the positive and negative electrodes. This problem is more likely to occur for graphite powders with wider irregular shapes. If the average particle diameter of the graphite powder is greater than 60 μm, the possibility of including irregularly shaped particles increases.

[0038] The composite carbon negative electrode material of the lithium ion battery of the present invention has an average particle diameter of 5-60 μm, which is measured by a British Malvern Mastersizer 2000 laser particle size analyzer.

[0039] The composite carbon negative electrode material particles of the lithium ion battery of the present invention also have a certain number of micropores inside. Based on the weight of the graphite particles, the volume of the pores with a size of 10nm~10μm is 0.5~2.2cm 3 /g, measured using the Quantachrome Poremaster 60 mercury porosimeter from the United States. Using such graphite particles as a negative electrode material, the volume expansion and contraction caused during repeated charging and discharging are alleviated and absorbed by the micropores contained inside, thereby inhibiting the structural damage of the electrode and improving the cycle performance. When the pore volume is less than 0.5cm 3 /g, the cycle performance of the material deteriorates; the pore volume is greater than 2.2cm 3 /g, which reduces the volume specific capacity of the material.

[0040] The above-mentioned secondary graphite particles composed of multiple graphite particles are used as anode materials for lithium-ion batteries, and it is not easy to cause the c-axis of the graphite crystals to be aligned parallel to the direction of the current collector, and the resistance of lithium ions to enter and exit the graphite layer is reduced. Therefore, the rapid charge-discharge performance and cycle performance of lithium-ion batteries are improved.

[0041] As a graphitization catalyst, elemental elements such as boron, silicon, beryllium, aluminum, titanium, nickel, and cobalt or their compounds can be used. The average particle size of the graphitization catalyst is below 60μm. When the particle size of the catalyst is too large, graphitization The growth can not proceed uniformly, which affects the full play of the discharge capacity of the finished negative electrode material. If the amount of catalyst added is less than 0.01%, measured by the weight percentage of the catalyst element, the growth of graphite crystals is restricted, and the porosity of the finished product is too small, which will affect the discharge capacity and cycle performance of the battery; if the amount of catalyst added is greater than 10%, measured as the weight percentage of the catalyst element, the processing performance of the material will become poor, and the porosity of the finished product will be too large, which will affect the volume specific capacity of the material. The function of the catalyst is to reduce the energy required for the graphitization process and accelerate the graphitization process. At the same time, the catalyst is discharged from the graphite particles during the graphitization process to form micropores, thereby giving the negative electrode material of the present invention better performance. Among these graphitization catalysts, carbides and oxides of boron and silicon are better choices.

[0042] The method of mixing and granulating one or more graphite powders with the binder is not particularly limited. Any known mixing and granulating equipment can be used. The preferred process is performed below the softening point of the binder. For example, when the binder used is asphalt and tar, the temperature for mixing and granulation is selected at 70-300℃, and when the binder is resin, the temperature for mixing and granulating is selected at 20-100℃.

[0043] Subsequently, the above-mentioned mixture is carbonized or graphitized. If necessary, the mixture is pressed into a desired shape before the carbonization or graphitization treatment. The compression molding method is not particularly limited, and any pressure processing method for making graphite electrodes is acceptable. Use, such as compression molding, vibration molding, etc. After molding, its density is 1.3~1.9g/cm 3. If the density after molding is less than 1.3g/cm 3 , The strength of the molded body after molding is poor, which affects the normal progress of the later graphitization process; if the density after molding is greater than 1.9g/cm 3 , It increases the difficulty of forming, and the later crushing process is difficult to carry out. The above-mentioned compression molding process is not necessary. If the material is not subjected to graphitization treatment, it can be formed without compression molding. At this time, it is only necessary to pulverize the mixture and adjust the particle size to the required value. The carbonization or graphitization is carried out in a non-oxidizing atmosphere, for example, in a nitrogen, argon, vacuum or reducing atmosphere. The carbonization temperature is 450°C to 1500°C, and the graphitization temperature is 2200°C to 3000°C. If the graphitization temperature is lower than 2200°C, graphite crystals cannot grow sufficiently, and part of the graphitization catalyst remains in the graphite, which will affect the charge and discharge capacity; if the graphitization temperature is too high, the manufacturing cost will increase, which will also cause graphite The sublimation affects the yield.

[0044] The composite carbon negative electrode material of the present invention may not be subjected to graphitization treatment, but must be subjected to low-temperature carbonization treatment. Without high-temperature graphitization treatment, no graphitization catalyst needs to be added in the aforementioned mixing and granulation stage.

[0045] In the subsequent crushing process, a negative electrode material with a reasonable particle size distribution is obtained. The crushing process is not particularly limited, and high-pressure pulverizers, rod-type mechanical crushers, low-speed impact spheroidizing crushers, and jet vortex crushing can be used. Commonly-used grinding equipment such as pulverizer, ultra-fine pulverizer, ultra-micro ball mill, internal grading impact type micro-powder pulverizer or pendulum mill.

[0046] In Example 1, 100 parts by weight of natural graphite powder with an average particle size of 5μm and a carbon content of 93.6% were added to 20 parts by weight of petroleum pitch, 10 parts by weight of coal tar, and 10 parts of silicon carbide (the amount of addition of Si equivalent to 5.3wt%) The parts by weight are mixed and granulated at a temperature of 150°C and pressed into cylindrical graphite blocks with a pressing density of 1.9g/cm 3 , Graphitized at a temperature of 2800°C for 4 hours, then cooled to room temperature, and pulverized with a high-pressure mill to obtain composite carbon particles with an average particle size of 20μm. Such as figure 2 As shown, it can be seen from the scanning electron microscope photo of the composite carbon particles that the composite carbon particles are secondary particles formed by combining multiple graphite powders. This is measured by the JEOLJSM-6380LV scanning electron microscope and the Millitrac particle image analyzer in the United States. The aspect ratio of the composite carbon material is 1.6, and the graphite crystal d is obtained by the powder X-ray wide-angle diffraction method. 002 0.3356nm, average crystal size La=80nm, Lc=100nm, measured by BET method, specific surface area is 2.6m 2 /g, the tap density of the powder measured by Quantachrome AutoTap tap density meter is 1.02g/cm 3 , Use Quantachrome PoreMaster 60 mercury porosimeter to measure the internal pore volume and pore size distribution of the powder, and the pore volume with a size of 10nm~10μm is 0.8cm 3 /g.

[0047] To evaluate the preparation of the battery, the following method was used to prepare 053048A square lithium ion battery. The lithium ion battery composite carbon anode material prepared in the step of Example 1 was used with the binder styrene-butadiene rubber latex SBR, suspending agent carboxymethyl cellulose CMC, Conductive carbon black Super-P is mixed according to the weight ratio of 95:2.5:1.5:1, add an appropriate amount of pure water as a dispersant to adjust the slurry, uniformly coat it on the copper foil, vacuum dry and roll to form a negative electrode sheet; use LiCoO 2It is the positive electrode active material, mixed with the binder polyvinylidene fluoride PVDF and the conductive agent Super-P in a weight ratio of 94:3:3, and an appropriate amount of N-methylpyrrolidone NMP is added as a dispersant to prepare a slurry. Coated on aluminum foil, vacuum dried and rolled to prepare a positive electrode sheet; use 1mol/LLiPF 6 The three-component mixed solvent EC:DMC:EMC=1:1:1, the v/v solution is the electrolyte, the polypropylene microporous membrane is the diaphragm, and the battery is assembled. The cycle performance test uses a current of 300mA for a constant current charge and discharge experiment, and the charge and discharge voltage is limited to 4.2 ~ 3.0 volts; the fast charge and discharge test uses a current of 500 mA, 750 mA, and 900 mA, respectively. The test results are shown in Table 1.

[0048] In Example 2, 50 parts by weight of natural graphite powder with an average particle size of 10 μm and a carbon content of 96.4% and 50 parts by weight of artificial graphite powder with the same particle size and carbon content were mixed uniformly, 10 parts by weight of petroleum pitch and 20 parts by weight of coal tar were added. , 17 parts by weight of boron carbide (10wt% equivalent to B) is mixed and granulated at a temperature of 150°C, and compressed into cylindrical graphite blocks with a density of 1.65g/cm 3 Graphitized at 3000°C for 1 hour, then cooled to room temperature, and pulverized with a low-speed impact spheroidizing mill to obtain composite carbon particles with an average particle size of 20 μm. From the scanning electron micrograph of the composite carbon particles, it can be seen that the composite carbon particles are secondary particles combined with a number of graphite micropowders. The length-to-diameter ratio of the composite carbon material is 3.6 by the analysis and test of the image analyzer. D of graphite crystal obtained by wide-angle diffraction method 002 0.3368nm, average crystal size La=50nm, Lc=150nm, measured by BET method, specific surface area is 4.6m 2 /g. The tap density of the powder measured by Quantachrome AutoTap tap density meter is 0.98g/cm 3 , Use Quantachrome PoreMaster 60 mercury porosimeter to measure the internal pore volume and pore size distribution of the powder, and the pore volume with a size of 10nm~10μm is 1.1cm 3 /g.

[0049] The battery was prepared by the same method as in Example 1, and the electrochemical performance test was performed. The results are shown in Table 1.