CN1635650A - A kind of negative electrode graphite material of lithium ion secondary battery and preparation method thereof - Google Patents

Abstract

This invention refers to the negative graphite material of lithium ion secondary cell and preparation thereof, said graphite is made from natural scale graphite by mechanical treatment which can disintegrate graphite to the required grain size range and remove the edges and corners to obtain smooth spherical or potato shape graphite grain with greater than 0.2 of sphericity index I110/I004 obtained by wide angle X ray diffraction. The lithium ion secondary cell using said graphite has as negative active material has high first charge/discharge efficiency, fine high speed charge/discharge and cycle performance.

Description

A kind of negative electrode graphite material of lithium ion secondary battery and preparation method thereof

【Technical field】

The invention relates to a negative electrode graphite material of a lithium ion secondary battery and a preparation method of the graphite material.

【Background technique】

Lithium-ion secondary batteries are widely used due to their high voltage, light weight, good safety performance, no memory effect, long cycle life and no environmental pollution. With the development of portable electrical products, people have higher and higher requirements for the batteries used in them, which are mainly reflected in small size, long power consumption time, high-rate charge and discharge performance, and long service life. Especially the power batteries required by electric bicycles, electric vehicles and common electric tools, which have been vigorously developed in the past two years, have high requirements for high-rate charging and discharging performance.

Graphite is the most commonly used negative electrode active material in lithium-ion secondary batteries. It has a theoretical capacity of 372mAh/g, and has the advantages of low discharge platform and good reversibility of lithium intercalation/delithiation. However, due to the heat treatment process and other reasons, the general artificial graphite has a low degree of graphitization and only has a capacity of about 250-300mAh/g. The degree of graphitization of natural graphite is relatively high, which can reach a capacity of 300-350mAh/g, or even higher, but the first charge and discharge efficiency is not very ideal if the flake-like natural graphite itself is directly used, that is, the reversible capacity that can be released is often not enough. It meets the requirements, and the high-rate charge-discharge performance and cycle performance are poor, which limits the practical application.

Therefore, the demand pressure of high capacity forces people to seek new materials on the one hand and strive to improve the defects of natural graphite on the other hand. The research and development of new materials often requires a lot of investment and a long process, and it is difficult to meet the current urgent needs for a while. Therefore, many engineers and technicians have proposed many methods for modifying graphite, including oxidation, mechanical treatment, coating, doping and so on.

Patent CN1241824A provides a modification method of mechanical treatment. It molds or extrudes domestic high-purity graphite powder, then performs secondary graphitization, and finally performs secondary graphitization, and finally performs mechanical crushing in different ways on the molded body obtained from the secondary graphitization treatment, so that A graphite powder with a large number of defects inside and outside the particles is obtained, and the graphite powder has excellent Li ⁺ intercalation and extraction properties. The average specific capacity of the first eleven discharges of the charge and discharge cycle is as high as 341mAh/g. However, the process of this method is complicated, and the obtained graphite particles have a large number of pores (microcracks) inside, so they are fluffy, uneven in appearance, large in specific surface area, and the charging capacity cannot be fully released. The charging and discharging efficiency and cycle performance still need to be improved.

【Content of invention】

The purpose of the present invention is to overcome the disadvantages of insufficient discharge of charge capacity, low charge-discharge efficiency and low cycle performance caused by fluffy graphite particles and uneven appearance, so as to provide a high-spheroidizer with high initial charging capacity. Spherical or potato-shaped graphite with high discharge efficiency and excellent high-rate charge-discharge and cycle performance.

Another object of the present invention is to provide a method for preparing the negative electrode spherical graphite of the lithium ion secondary battery.

The purpose of the present invention is achieved through the following technical solutions:

A negative electrode graphite for a lithium ion secondary battery, wherein the sphericity index I ₁₁₀ /I ₀₀₄ diffraction peak intensity ratio of the negative electrode graphite measured by a wide-angle X-ray diffraction analyzer is above 0.2.

A method for preparing lithium-ion secondary battery cathode graphite, comprising mixing flake-shaped natural graphite with an average particle size of 20-50 μm and a solvent to form a slurry with a solid content of 10-50% and placing it in a media mill containing media balls , add 0-5% dispersant, stir, then take out, dry, crush, and classify.

The mass ratio of the slurry to the medium ball is 1:4˜1:12.

The filling rate of the slurry is 30-70%.

The rotating speed of the ball mill is 80-500 rpm, more preferably 200-350 rpm.

The ball milling time is 30-150 minutes.

The solvent can be absolute ethanol, acetone, deionized water.

Described dispersant can be Tween, Span, sodium alkylsulfonate.

The dielectric balls can be one or more of chromium steel balls, zirconia balls, and agate balls, and can be mixed with multiple media balls of different sizes.

Compared with the prior art, the present invention has the following outstanding advantages, this is by placing the natural flake graphite in a media mill and rotating at a certain speed, and there is a certain distance between the medium ball and the graphite particles and between the graphite particles. Compared with the original flake graphite, the obtained graphite not only adjusts the particle size distribution of graphite, but also greatly improves the appearance of graphite particles. Most of the edges and corners of the original scales are removed, and the degree of graphite spheroidization is high, forming a potato-like or spherical particle state, which greatly reduces the orientation of graphite particles. Structurally, this is beneficial for Li ⁺ to be fully intercalated and deintercalated from all directions, with uniform electron distribution, high initial charge-discharge efficiency, and excellent high-rate charge-discharge and cycle performance.

【Description of drawings】

Fig. 1 is the morphology of embodiment 1 raw material natural flake graphite

Fig. 2 is the morphology of the spherical graphite that embodiment 1 makes

【Detailed ways】

The present invention will be described in more detail below,

In the present invention, the raw material used is flaky natural graphite. In order to ensure that spherical graphite in a suitable particle size range is obtained, the average particle size of the raw material is 20 to 50 μm. By adjusting the process parameters of mechanical treatment such as rotating speed and time, it can be obtained Products ranging from 10 to 30 μm. The particle size distribution of the product (spherical graphite) cannot be too small, if it is too small, the graphite powder is too fine, the specific surface area increases, the irreversible capacity consumed by the formation of the SEI film increases, and the first charge and discharge efficiency decreases; it cannot be too large, such as If it is too large, the edge of the graphite particle is far away from the center, and the steric hindrance increases, which is not conducive to the sufficient and rapid intercalation and deintercalation of Li ⁺ , and is not suitable as a negative electrode material for batteries.

In the present invention, the d ₀₀₂ of the raw materials used is 0.336-0.338 nm. Generally speaking, the closer d ₀₀₂ is to the ideal graphite value of 0.3354nm, the higher the degree of graphitization, and the higher the degree of graphitization of graphite, the higher its reversible specific capacity. When selecting natural graphite raw materials, natural graphite with a higher degree of graphitization should be selected, that is, the smaller the d _002, the better. In the present invention, the crystallite layer spacing d ₀₀₂ of the obtained spheroidized graphite is 0.336-0.338 nm, which is determined by the raw material, and the mechanical treatment process does not change the size of d ₀₀₂ .

In the present invention, the spheroidization process can also reduce the specific surface area of graphite to a certain extent, by spheroidizing flake graphite particles into spherical or potato-shaped, the surface is smooth, and the specific surface area of the product is 1.0-5.5m ² /g , more preferably 1.0 to 2.6 m ² /g. The size of the specific surface area directly affects the size of the irreversible capacity consumed by the formation of the SEI film during the first charge of the lithium-ion secondary battery, that is, it is directly related to the first charge and discharge efficiency of the battery and the size of the reversible discharge capacity. Therefore, from the perspective of ensuring the reversible specific capacity of the negative electrode material, the smaller the specific surface area, the better.

In the present invention, spheroidized graphite is obtained by performing a mechanical treatment on natural flake graphite in a suitable media mill. The media mill requires strong tangential force and weak normal force, and is easy to realize flaky graphite. Grinding is accompanied by self-grinding between particles. In graphite crystal, the layers are combined by weak van der Waals force. When a large normal impact force acts vertically on the surface of the crystallite, it shows a kind of toughness. If the normal force is too large, then As the vibration of graphite particles transmits destructive force in all directions, graphite particles are easily fragmented and the crystal structure is severely damaged. However, when another parallel tangential frictional force acts on the surface of the scales, misalignment and sliding between layers are prone to occur, thereby achieving grinding. At the same time, in the grinding process, the participation of medium balls and the frequent collision, friction and self-grinding between graphite particles under the action of input energy, thus removing many original edges and corners, and gradually becoming Sleek to achieve ballization. In the present invention, the media mill may specifically be a stirred mill, a planetary ball mill, or the like.

In the preparation method of the present invention, the purpose of the solvent and the dispersant is to disperse the graphite, so as to prevent the graphite particles from agglomerating and slipping, moving synchronously with the medium ball, and the relative displacement is small, which affects the spheroidization effect. There are no requirements for solvents and dispersants, and the solvents used can be absolute ethanol, acetone, deionized water, etc., and the dispersants used can be Tween, Span, sodium alkyl sulfonate series.

In the preparation method of the present invention, the solid content of the slurry is 10 to 50%, the dispersant is 0 to 5%, and the mass ratio of the slurry to the medium ball is 1:4 to 1:12 (this is also related to the specific gravity of the medium ball), The filling rate is 30-70%. The said medium balls can be chrome steel balls, zirconia balls, agate balls, etc., in order to improve the effect, a variety of different sizes of medium balls can be mixed.

In the preparation method of the present invention, the stirring speed should be controlled within a certain range. If the speed is too slow, the medium balls will basically roll and squeeze in the original position, and the order of each other will not change much; if the speed is too fast, the medium balls will move synchronously along the inner wall of the container in an orderly manner under the action of centrifugal force. There is almost no relative displacement between each other; if the stirring speed is appropriate, the extrusion between the medium balls will be intensified, and some small balls will jump out of the original position for parabolic motion after being hit, and then fall to collide with other small balls, and the order of each other will be disordered. Simultaneously collide and rub graphite particles. To achieve the ideal movement stage of the medium ball, the stirring speed is also related to the diameter of the mill. Taking Φ200 as an example, the stirring speed is preferably 80-500rpm, and 200-350rpm is the better range.

In the preparation method of the present invention, the processing time directly affects the particle size distribution and spheroidization effect of the obtained product. The specific processing time is related to other process parameters, and needs to be explored according to the equipment size, filling rate, pulp-ball ratio, and rotating speed. In the present invention, the treatment time may be 30-150 minutes.

A graphite material for lithium-ion secondary batteries prepared by this method, the graphite particles are a spherical graphite in appearance, the median diameter measured by a laser scattering particle size analyzer is 10-30 μm, and the wide-angle X-ray diffraction analysis The sphericity index I ₁₀₀ /I ₀₀₄ obtained by the instrument is above 0.2, the spacing d ₀₀₂ of the crystallite layer is 0.336-0.338nm, and the specific surface area measured by the BET single-point measurement method replaced by N ₂ is 1.0-5.5m ² /g.

The spheroidized graphite obtained above is used as the negative electrode active material and assembled into a lithium-ion secondary battery. The first charge and discharge efficiency is improved, the high-rate charge and discharge performance is excellent, and the cycle life is long.

Below in conjunction with example the present invention will be further described.

【Example 1】

A Φ200 stirred mill is selected as the media mill, and the media balls are chromium steel balls, which are respectively combined in three ratios of Φ6, Φ8, and Φ10 in a ratio of 4:2:1. Take 500g of raw graphite (average particle size: 31.5 μm), and use deionized water as a solvent to prepare a slurry with a solid content of 45%, and add 1% Tween 65 in addition. The filling rate is 60%, the pulp-ball ratio is 1:7 (mass ratio), the rotation speed is 300 rpm, and the stirring treatment time is 60 minutes. Finally, it was taken out, dried, crushed, and classified to obtain spherical graphite with an average particle size of 17.8 μm (as shown in Figure 2).

Use the graphite as the negative electrode active material to assemble a lithium-ion secondary battery, adopt the positive electrode sheet whose active material is LiCoO ₂ , the electrolyte salt is LiPF ₆ , and the electrolyte solvent is a mixture of ethylene carbonate, ethylene carbonate, and diethyl carbonate. Organic solvent, the concentration is 1 mol/liter, and the separator paper is composite separator paper made of polyethylene and polypropylene.

[Example 2]

A Φ200 stirred mill is selected as the media mill, and the media balls are chromium steel balls, which are respectively combined in three ratios of Φ6, Φ8, and Φ10 in a ratio of 4:2:1. Get 500g of raw graphite (average particle diameter 22.5 μm), the solvent is absolute ethanol, adjust into slurry, solid content 25%, add 0.5% sodium alkyl sulfonate in addition. The filling rate is 50%, the pulp-ball ratio is 1:5 (mass ratio), the rotation speed is 200 rpm, and the stirring treatment time is 40 minutes. Finally, it was taken out, dried, crushed, and classified to obtain spherical graphite with an average particle size of 16.6 μm.

Use the graphite as the negative electrode active material to assemble a lithium-ion secondary battery, adopt the positive electrode sheet whose active material is LiCoO ₂ , the electrolyte salt is LiPF ₆ , and the electrolyte solvent is a mixture of ethylene carbonate, ethylene carbonate, and diethyl carbonate. Organic solvent, the concentration is 1 mol/liter, and the separator paper is composite separator paper made of polyethylene and polypropylene.

[Example 3]

A Φ200 stirred mill is selected as the media mill, and the media balls are chromium steel balls, which are respectively combined in three ratios of Φ6, Φ8, and Φ10 in a ratio of 4:2:1. Take 500g of raw graphite (average particle size: 40.5 μm), and use deionized water as a solvent to prepare a slurry with a solid content of 35%, and add 2% Tween 65 in addition. The filling rate is 40%, the pulp-ball ratio is 1:9 (mass ratio), the rotating speed is 400 rpm, and the stirring treatment time is 100 min. Finally, it was taken out, dried, crushed, and classified to obtain spherical graphite with an average particle size of 20.5 μm.

Use the graphite as the negative electrode active material to assemble a lithium-ion secondary battery, adopt the positive electrode sheet whose active material is LiCoO ₂ , the electrolyte salt is LiPF ₆ , and the electrolyte solvent is a mixture of ethylene carbonate, ethylene carbonate, and diethyl carbonate. Organic solvent, the concentration is 1 mol/liter, and the separator paper is composite separator paper made of polyethylene and polypropylene.

【Comparative example】

A lithium-ion secondary battery was assembled directly using natural flake graphite with an average particle size of 18 μm as the negative electrode active material, except that other processes were consistent with the examples.

Battery Characteristic Test

The specific surface area of the negative electrode active material: obtained by the BET single-point measurement method of the _N2 replacement method.

Carry out performance test to the battery of embodiment and comparative example, as follows:

The specific capacity of the first charge: the charge capacity of the first charge to 4.2V at a current of 0.1C/mass of the negative electrode active material.

First discharge specific capacity: the discharge capacity of the first discharge from 4.2V to 3.0V at a current of 0.1C/mass of the negative electrode active material.

First charge and discharge efficiency=(first discharge capacity/first charge capacity)×100%.

In the high-rate charge and discharge performance, C _3C /C _0.5C : the ratio of the discharge capacity discharged from 4.2V to 3.0V at a current of 3C to the discharge capacity discharged from 4.2V to 3.0V at a current of 0.5C.

In the high-rate charge and discharge performance, C _2C /C _0.5C : the ratio of the discharge capacity discharged from 4.2V to 3.0V with a current of 2C to the discharge capacity discharged from 4.2V to 3.0V with a current of 0.5C.

Cycle life: charging with 1C current to 4.2V and then discharging with 1C current to 3.0V is called a cycle, so repeated, the discharge capacity obtained is the capacity of this cycle. In the present invention, the cycle life refers to the number of cycles when the discharge capacity reaches 80% of the first discharge capacity.

The test results are shown in the table below: serial number Sphericity Index I ₁₁₀ /I ₀₀₄ First charge and discharge efficiency/% High rate charge and discharge performance cycle life C _3C /C _0.5C C _2C /C _0.5C Example 1 0.32 89 68.4 92.6 220 Example 2 0.29 91 65.5 89.8 209 Example 3 0.41 86 67.7 91.4 215 comparative example 0.11 76 48.7 81.5 69

It can be seen from the above table that compared with the natural flake graphite, the spheroidized graphite prepared by the present invention greatly improves the initial charge and discharge efficiency, high rate charge and discharge performance and cycle life, which can meet the requirements of practical applications.

Claims (10)

1. A negative electrode graphite for a lithium ion secondary battery, characterized in that, the sphericity index I ₁₁₀ /I ₀₀₄ diffraction peak intensity ratio of the described negative electrode graphite measured by a wide-angle X-ray diffraction analyzer is more than 0.2. 2. A preparation method for lithium-ion secondary battery negative electrode graphite, characterized in that the flaky natural graphite with an average particle diameter of 20 to 50 μm is mixed with a solvent to form a slurry with a solid content of 10 to 50% and placed in a medium containing In the ball media mill, add 0-5% dispersant, stir, then take out, dry, crush and classify. 3. The preparation method of negative electrode spherical graphite of lithium ion secondary battery according to claim 2, characterized in that the mass ratio of the slurry to the dielectric ball is 1:4 to 1:12. 4. The preparation method of negative electrode spherical graphite of lithium ion secondary battery according to claim 2, characterized in that the filling rate of the slurry is 30% to 70%. 5. The preparation method of negative electrode spherical graphite of lithium ion secondary battery according to claim 2, characterized in that the rotating speed of the ball mill is 80～500rpm. 6 . The method for preparing spherical graphite in the negative electrode of lithium ion secondary battery according to claim 2 , characterized in that the rotational speed of the ball mill is 200 to 350 rpm. 7 . 7 . The method for preparing spherical graphite in the negative electrode of lithium ion secondary battery according to claim 2 , characterized in that the ball milling time is 30 to 150 min. 8. the preparation method of lithium ion secondary battery negative electrode spherical graphite according to claim 2 is characterized in that described solvent can be dehydrated alcohol, acetone, deionized water. 9. the preparation method of lithium ion secondary battery negative electrode spherical graphite according to claim 2 is characterized in that described dispersant can be Tween, Span, sodium alkyl sulfonate. 10. The preparation method of lithium-ion secondary battery negative electrode spherical graphite according to claim 2, characterized in that said dielectric balls can be one or more of chromium steel balls, zirconia balls, and agate balls.

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