CN1305150C - Modified graphite and its preparing method - Google Patents

Abstract

A modified graphite, comprising graphite core material particles and an amorphous carbon film layer coated on the surface thereof, wherein the crystallite layer spacing _d002 of the graphite core material particles is 0.335-0.340nm, and the ratio of the modified graphite The surface area is 1.3-4.2m ² /g, and the average particle size is 8-35μm. The preparation method of the modified graphite comprises the following steps: immersing the graphite core material particles in a solution of a polymer surface modifier, stirring, separating, sieving, and then solidifying and carbonizing. The modified graphite of the invention has excellent high current performance, high reversible specific capacity and long cycle life, and can meet the requirements of practical application; the preparation method has simple process, low cost and is easy for industrialized production.

Description

A kind of modified graphite and preparation method thereof

【Technical field】

The invention relates to a modified graphite and a preparation method thereof, more specifically to a modified graphite with excellent high-current performance and a preparation method thereof.

【Background technique】

With the rapid development of the electronics industry and the information industry, people have higher and higher requirements for the power supply of various electrical products. Lithium-ion secondary batteries have developed rapidly and are widely used in the past ten years due to their superior comprehensive performance.

As the functions of electrical products, especially portable electrical products, are becoming more and more complex, and the volume is becoming smaller and smaller, the requirements for the power supply are also increasing, which is reflected in: sufficiently high volumetric energy density, superior high-current performance, suitable cycle life and reliability. The safety performance, especially the requirements for volumetric energy density and high current performance.

In some occasions, modern portable products, electric tools, electric bicycles, electric vehicles and other electrical products have high requirements on power consumption per unit time, for example, mobile Internet access, multimedia and other functions of modern color-screen mobile phones, and notebook computers require long power consumption , the starting process of the powered car, and so on. In this case, for the lithium-ion secondary battery used as a power source, it belongs to a high-current discharge process. That is to say, in many cases, the high current performance of the lithium-ion secondary battery has been highly demanded in recent years.

The high-current performance of lithium-ion secondary batteries mainly depends on the positive and negative electrode materials, and the negative electrode active materials are of great significance in improving the high-current performance, which is mainly reflected in the conductivity of the material itself and its structural stability. .

At present, the most commonly used negative electrode active materials for lithium-ion secondary batteries are graphite materials, mainly because graphite has a lower discharge platform and better cycle stability. Among them, natural graphite with a higher degree of graphitization has a higher specific capacity, but since the microcrystalline structure is easily relaxed or even destroyed during the lithium intercalation/delithiation process, its cycle performance and high current performance are poor.

Patent CN1230159 "Graphite Particles and Lithium Secondary Battery Using Graphite Particles as Negative Electrode" invented a kind of graphite particles, and the lithium secondary batteries using the graphite particles have excellent fast charge and discharge performance and cycle performance. The feature of this graphite particle is that a plurality of flat-shaped particles are combined or bonded together, so that the oriented crystal planes of each flat-shaped particle are not parallel to each other, that is, the orientation of the graphite particles is reduced. In this way, the intercalation and deintercalation of Li ⁺ can be carried out more fully at a higher rate, so the obtained lithium secondary battery made of graphite has excellent high-current performance. However, the preparation method of graphite is actually an artificial graphite process, which requires graphitization at a temperature as high as 2800° C., which is costly and difficult to produce industrially.

【Content of invention】

An object of the present invention is to provide a modified graphite with excellent high current performance, higher reversible specific capacity and longer cycle life;

Another object of the present invention is to provide a method for preparing the above-mentioned modified graphite with simple process, low cost and easy industrial production.

Modified graphite of the present invention is realized by following technical scheme:

A modified graphite, comprising graphite core material particles and an amorphous carbon film layer coated on the surface thereof, wherein the crystallite layer spacing _d002 of the graphite core material particles is 0.335-0.340nm, and the ratio of the modified graphite The surface area is 1.3-4.2m ² /g, and the average particle size is 8-35μm.

Above-mentioned technical scheme can also be further improved as:

The interlayer spacing d ₀₀₂ of the modified graphite is 0.335-0.338 nm, the specific surface area is 1.8-3.5 m ² /g, and the average particle diameter is 10-20 μm.

The thickness of the amorphous carbon film layer is 0.05 μm˜1 μm.

The present invention prepares the method for above-mentioned modified graphite to realize by following technical scheme:

The preparation method of above-mentioned modified graphite, comprises the steps:

(1), dissolving the polymer surface modifier in a corresponding organic solvent to obtain a polymer surface modifier solution, the concentration of the polymer surface modifier solution is 90.9% to 100% of the concentration of the polymer surface modifier saturated solution %;

(2), immerse the graphite core material particles in the polymer surface modifier solution obtained above, carry out according to the ratio of graphite 1kg, polymer surface modifier solution 1.5L～3L, stir at a speed of 100～2000rpm, stir The time is 0.5-10 hours, so that the surface modifier fully contacts and adheres to the graphite core material particles;

(3), the graphite core material particles are separated from the polymer surface modifier solution, and the residual solvent on the graphite core material particles is dried and sieved;

(4), under protective atmosphere, the dry graphite obtained is respectively solidified and carbonized to obtain surface-modified graphite;

Wherein, the graphite core material particles can be natural graphite or artificial graphite, with an average particle size of 7-35 μm;

The polymer surface modifier can be one or more of coal pitch, coal tar, petroleum pitch, petroleum tar, benzene, naphthalene, benzene-naphthalene copolymer, petroleum wax, and petroleum resin;

The organic solvent can be one of acetone, absolute ethanol, N-methylpyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane;

Curing is carried out at 200-600°C, the heating rate is 0.5-35°C/min, and the holding time is 0.2-12 hours;

The carbonization is carried out at 750-1300°C, the heating rate is 0.1-30°C/min, and the holding time is 1-24 hours.

Above-mentioned technical scheme can also be further improved as:

The curing is carried out at 300-500° C., and the holding time is 0.5-3 hours.

During the curing process, the heating rate is 5-20° C./min.

The carbonization is carried out at 800-1200° C., and the holding time is 2-10 hours.

During the carbonization process, the heating rate is 3-20° C./min.

During the carbonization process in the step (4), the cooling rate is 5-15° C./minute.

The modified graphite of the present invention has the advantages of excellent high current performance, high reversible specific capacity and long cycle life.

The preparation method of the modified graphite of the present invention has the advantages of simple process, low cost and easy industrial production.

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

【Description of drawings】

Fig. 1 is the SEM figure of the raw material graphite that embodiment 1 adopts

Fig. 2 is the SEM picture of the modified graphite that embodiment 1 makes

Fig. 3 is the DSC figure of the modified graphite that embodiment 1 makes

【Detailed ways】

A modified graphite provided by the present invention includes graphite core material particles and an amorphous carbon film layer coated on the surface thereof, wherein the distance d ₀₀₂ of crystallite layers is 0.335-0.340nm, and the specific surface area is 1.3-4.2m ² / g, the average particle size is 8-30 μm.

Wherein, the average particle size is: D ₅₀ obtained by measuring with a laser scattering particle size analyzer;

Crystallite layer spacing d ₀₀₂ : measured by X-ray diffraction analyzer;

The specific surface area is measured by the BET single-point measurement method of the N2 displacement method.

By coating the surface of graphite core material particles with an amorphous carbon film layer, on the one hand, the surface morphology of graphite particles can be improved, the specific surface area can be reduced, and the compatibility between graphite and electrolyte can be improved, thereby improving the first charge and discharge efficiency; It can make up for the structural defects of the edge part of the microcrystalline structure of graphite core material particles, reduce the orientation of the microcrystals, stabilize the graphite microcrystals, and further improve its conductivity and uniformity of electron distribution, so that during the charge and discharge process, Li The electromigration rate of ⁺ and the stability of the anode structure are enhanced, leading to enhanced high-current performance.

In the present invention, the average particle diameter of the modified graphite is 8-30 μm, preferably 10-20 μm. If the average particle size of graphite is too small, the graphite powder will be too fine and the specific surface area will ^be too large, which will damage the reversible discharge capacity of the material; Intercalation and deintercalation are not suitable for negative electrode active materials of lithium-ion secondary batteries.

In the present invention, the interlayer distance d ₀₀₂ of the modified graphite is 0.335-0.340 nm, preferably 0.335-0.338 nm. The d ₀₀₂ of graphite mainly depends on the raw material - natural graphite or artificial graphite with a higher degree of graphitization, while the surface modification process has little effect on d ₀₀₂ . 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. Therefore, in order to ensure the discharge capacity of lithium-ion secondary batteries, natural graphite or artificial graphite with a high degree of graphitization should be selected as raw materials.

In the present invention, the specific surface area of the modified graphite is 1.3-4.2m ² /g, preferably 1.8-3.5m ² /g, and the modified graphite after surface modification effectively reduces the specific surface area of the raw graphite (ratio of raw graphite The surface area is about 5m ² /g), and 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 charging process of the lithium ion secondary battery, that is, it is directly related to the first charge and discharge efficiency and reversible discharge capacity of the battery. size. Therefore, from the perspective of ensuring the reversible specific capacity of the negative electrode material, the smaller the specific surface area, the better.

The thickness of the amorphous carbon film layer is 0.05 μm to 1 μm, and the thickness of the amorphous carbon film layer is calculated according to the coating amount of the polymer surface modifier on the surface of the graphite core material particles and the average particle diameter of the graphite core material particles Obtained; the DSC curve of modified graphite can be obtained by thermal analysis.

In the present invention, the modified graphite is mainly obtained by surface modification treatment of natural graphite or artificial graphite with a relatively high degree of graphitization. The preparation method includes the following steps: pre-preparing an organic solution of a polymer surface modifier with a certain concentration (generally saturated or close to saturation); immersing the graphite core material in it and stirring it, the stirring speed is 100-2000rpm, and the stirring time 0.5~10h; then separate (filter or centrifuge) the graphite in it, dry the solvent involved in it, and sieve; finally, the dry graphite obtained is solidified and carbonized under a protective atmosphere without crushing process The modified graphite of the present invention can be obtained.

In the preparation method of modified graphite of the present invention, described surface modifier is a kind of very high organic matter of carbon content, can be coal pitch, coal tar, petroleum pitch, petroleum tar, benzene, naphthalene, benzene-naphthalene copolymer, Petroleum wax, one or more of petroleum resins; the corresponding organic solvent can be one of acetone, absolute ethanol, N-methylpyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane.

In the preparation method of the modified graphite of the present invention, in the stirring treatment process, the surface modification effect is related to the stirring time. If the stirring time is too long, the thickness of the organic matter film adhering to the surface of the graphite core material particles will be too thick, which will have a certain impact on the performance of the modified graphite, such as reducing the first charge and discharge efficiency of the modified graphite; if the stirring time is too short, the The thickness of the organic film adhered to the surface of the graphite core material particles is too thin, and the distribution of the organic film on the surface of the graphite core material particles is uneven, which has a certain impact on the performance of the modified graphite.

In the preparation method of the modified graphite of the present invention, the curing process is to heat up to the temperature required for curing at a heating rate of 0.5-35°C/min, preferably 5-20°C/min; the curing temperature is 200-600°C °C, more preferably 300-500 °C, and the holding time is 0.2-12 hours, more preferably 0.5-3 hours. After the curing is completed, the temperature is raised to the temperature required for carbonization at a heating rate of 0.1-30°C/min (more preferably 3-20°C/min). The carbonization temperature is 750-1300°C, preferably 800-1200°C. The time is 1 to 24 hours, more preferably 2 to 10 hours. After carbonization, the temperature can be lowered at a rate of 1-20°C/min, more preferably 5-15°C/min, or naturally.

If the carbonization temperature is too low, such as lower than 750°C, the carbonization of the organic layer on the surface of the graphite core material particles is not sufficient to form a stable and dense carbon film layer, or even a microporous structure with a large specific surface area, which is not conducive to the surface morphology. If the carbonization temperature is too high, such as higher than 1200 ° C, the improvement of the effect will not be obvious by increasing the temperature, but the energy consumption will increase, which is uneconomical; the holding time should not be too short, otherwise the carbonization will not be sufficient and a stable carbonization cannot be formed. Dense carbon film layer; if the time is too long, the energy consumption will increase, which is uneconomical.

In the preparation method of modified graphite of the present invention, solidification and carbonization can adopt box-type resistance furnace, tube furnace, push-pull tunnel furnace, rotary tunnel furnace, etc., as long as it can reach the required temperature and can be sealed to pass the protective gas, The protective atmosphere can be one or a combination of argon, helium, and nitrogen.

In order to test the electrochemical performance of the modified graphite preparation method of the present invention and the modified graphite prepared by the method, add binder and deionized water to the modified graphite of the present invention and stir, coat and dry to obtain lithium ion For the negative electrode sheet of the secondary battery, the negative electrode sheet prepared above, the positive electrode made of LiCoO ₂ and the corresponding conductive agent, binder, and the corresponding electrolyte are made into a lithium-ion secondary battery according to the existing process, and the relevant performance is carried out. test.

【Example 1】

Weigh 8 g of petroleum tar, dissolve it in carbon tetrachloride, and prepare 200 ml of a 4% surface modifier solution for later use. Weigh 100g of dry natural graphite, dip it in the organic solution of the surface modifier, and stir it at a speed of 300rpm for 1 hour, so that a thin layer of surface modification film is formed on the surface of the graphite particles. Then filter to obtain graphite, dry, and pass through a 300-mesh sieve. Put the sieved graphite into a closed tube-type high-temperature furnace, pass high-purity N ₂ at a flow rate of 10 liters/min, raise the temperature to 400°C at a rate of 15°C/min, keep it for 1 hour, and then heat it at 10°C/min The heating rate was raised to 1000° C. for 3 hours, and the temperature was naturally lowered to room temperature to obtain surface-modified graphite. The average particle diameter of the graphite represented by D ₅₀ is 13.8 μm, the distance between crystallite layers d ₀₀₂ is 0.3365, and the specific surface area is 2.8 m ² /g.

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]

In this example, the surface modification agent is 5% coal tar pitch in tetrahydrofuran solution, except that, other processes are consistent with Example 1. The average particle diameter D ₅₀ and the distance between crystallite layers d ₀₀₂ of the modified graphite obtained in this example are basically the same as those in Example 1.

[Example 3]

In this embodiment, the surface modification agent adopts 3% petroleum asphalt in chloroform solution, except that, other processes are consistent with embodiment 1. The average particle diameter D ₅₀ and the distance between crystallite layers d ₀₀₂ of the modified graphite obtained in this example are basically the same as those in Example 1.

【Example 4】

In this example, the carbonization temperature is 800° C., except that, other processes are consistent with Example 1. The average particle diameter D ₅₀ and the distance between crystallite layers d ₀₀₂ of the modified graphite obtained in this example are basically the same as those in Example 1.

【Example 5】

In this example, the carbonization temperature is 1200° C., except that, other processes are consistent with Example 1. The average particle diameter D ₅₀ and the distance between crystallite layers d ₀₀₂ of the modified graphite obtained in this example are basically the same as those in Example 1.

[Example 6]

In this embodiment, natural graphite with a smaller average particle size is used as a raw material, except that, other processes are consistent with Embodiment 1. The average particle size D ₅₀ of the modified graphite obtained in this example is 8.2 μm, and the distance between crystallite layers d ₀₀₂ is basically the same as that in Example 1.

[Example 7]

In this embodiment, natural graphite with a relatively large average particle size is used as a raw material, except that other processes are consistent with Embodiment 1. The average particle size D ₅₀ of the modified graphite obtained in this example is 35.0 μm, and the interlayer spacing d ₀₀₂ of the crystallites is basically the same as that in Example 1.

[Comparative Example 1]

Directly use the natural graphite raw material in Example 1 as the negative electrode active material to make a battery, except that other processes are consistent with Example 1.

【Comparative example 2】

In this embodiment, the carbonization temperature is 700° C., and the holding time is 5 hours. Other processes are consistent with Embodiment 1.

【Performance Testing】

Specific surface area: measured by the BET single-point measurement method of the N2 displacement method.

The conditions of the thermal analysis test are as follows:

Instrument model: NETSCH STA449C

Method: Synchronous thermal analysis, ie DSC-TG

Sample weight: graphite 6.824mg

Crucible material: AL ₂ O ₃

Furnace atmosphere: air, flow rate is 25Nml/min

Heating process: starting from room temperature, the heating rate is 10°C/min, and the temperature is raised to 1000°C.

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

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

In high current 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.

Reversible specific capacity: first charge to 4.2V with a current of 0.1C, and then discharge from 4.2V to 3.0V with a current of 0.1C for the first time discharge capacity/mass of negative active material.

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 above performance test results are shown in the table below: serial number Specific surface area/m ² /g High current performance Reversible specific capacity/mAh/g cycle life C _3C /C _0.5C C _2C /C _0.5C Example 1 2.8 86.2 96.4 340 280 Example 2 1.8 86.1 96.9 351 312 Example 3 2.6 82.6 96.4 338 264 Example 4 3.5 80.5 95.8 330 320 Example 5 2.3 88.4 96.7 350 277 Example 6 4.2 82.8 95.1 332 320 Example 7 1.5 80.2 94.6 340 245 Comparative example 1 5.7 57.4 88.1 337 76 Comparative example 2 4.8 72.2 94.5 332 253

As can be seen from the above table, the specific surface area of the modified graphite in Examples 1-7 is smaller than that of the graphite in Comparative Examples 1-2; the batteries made in Examples 1-7 have excellent high-current performance, higher reversible specific capacity and long cycle life.

Fig. 1 and Fig. 2 are respectively the SEM figure of raw material graphite and modified graphite in embodiment 1, obtain by scanning electron microscope (using equipment is the JSM-5160 model of JEOL company), as can be seen from the figure, the modified graphite The shape is potato-like or spherical.

Figure 3 shows the thermal analysis results of the modified graphite obtained in Implementation 1, where the peak at 592.1°C corresponds to the amorphous carbon film layer; the peak at 821.2°C corresponds to the graphite core material; in general, the peak at 500-650°C The peak corresponds to the amorphous carbon film layer, and 750-850°C corresponds to the graphite core material.

The modified graphite of the present invention has excellent high-current performance, and has great application potential and advantages in many occasions that require high fast charging and discharging. At the same time, the negative electrode active material maintains a high reversible specific capacity, and has a relatively Long cycle life, stable and reliable, can meet the requirements of practical applications. The preparation method of the modified graphite of the present invention has the advantages of simple process, low cost and easy industrial production.

Claims (9)

1, a kind of modified graphite is characterized in that: described modified graphite comprises graphite core material granule and surface coated amorphous carbon rete thereof, wherein, and the microcrystalline coating interplanar distance d of graphite core material granule ₀₀₂Be 0.335～0.340nm, the specific area of described modified graphite is 1.3～4.2m ²/ g, average grain diameter is 8～35 μ m.

2, modified graphite as claimed in claim 1 is characterized in that: the microcrystalline coating interplanar distance d of described modified graphite ₀₀₂Be 0.335～0.338nm, specific area is 1.8～3.5m ²/ g, average grain diameter is 10～20 μ m.

3, modified graphite as claimed in claim 1 is characterized in that: the thickness of described amorphous carbon rete is 0.05 μ m～1 μ m.

4, a kind of preparation method as claim 1 or 2 or 3 described modified graphites comprises the steps:

(1), the polymer surfaces dressing agent is dissolved in the corresponding organic solvent, obtain polymer surfaces dressing agent solution, described polymer surfaces dressing agent solution concentration is 90.9%～100% of this polymer surfaces dressing agent saturated solution concentration;

(2), the graphite core material granule is immersed in the above-mentioned polymer surfaces dressing agent solution that obtains, ratio according to graphite 1kg, polymer surfaces dressing agent solution 1.5L～3L is carried out, speed with 100～2000rpm stirs, mixing time is 0.5～10h, coating material is fully contacted with the graphite core material granule and sticks thereon;

(3), graphite is separated from polymer surfaces dressing agent solution, and the residual solvent on the oven dry graphite core material granule, sieve;

(4), under protective atmosphere, the above-mentioned dry graphite that obtains is cured respectively and carbonization treatment, obtain surface modified graphite;

Wherein, the graphite core material granule is native graphite or Delanium, and average grain diameter is 7～35 μ m;

The polymer surfaces dressing agent is coal tar pitch, coal tar, petroleum asphalt, petroleum tar, benzene, naphthalene, benzene naphthalene copolymer, pertroleum wax, one or more in the Petropols;

Organic solvent is a kind of in acetone, absolute ethyl alcohol, N-methyl pyrrolidone, chloroform, oxolane, carbon tetrachloride, the cyclohexane;

Curing is to carry out under 200～600 ℃, and heating rate is 0.5～35 ℃/minute, and temperature retention time is 0.2～12 hour;

Carbonization is to carry out under 750～1300 ℃, and heating rate is 0.1～30 ℃/minute, and temperature retention time is 1～24 hour.

5, the preparation method of modified graphite as claimed in claim 4 is characterized in that: described curing is to carry out under 300～500 ℃, and temperature retention time is 0.5～3 hour.

6, the preparation method of modified graphite as claimed in claim 4 is characterized in that: in the described solidification process, heating rate is 5～20 ℃/minute.

7, the preparation method of modified graphite as claimed in claim 4 is characterized in that: described carbonization is to carry out under 800～1200 ℃, and temperature retention time is 2～10 hours.

8, the preparation method of modified graphite as claimed in claim 4 is characterized in that: in the described carbonisation, heating rate is 3～20 ℃/minute.

9, the preparation method of modified graphite as claimed in claim 4 is characterized in that: in the described carbonisation, rate of temperature fall is 5～15 ℃/minute.

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