Preparation method and application from graphite ore to graphite / silicon composite material

Abstract

The invention belongs to the technical field of lithium ion battery materials, and relates to a preparation method and application from graphite ore to a graphite / silicon composite material, the preparation method comprises the following steps: step 1, adding ethanol into the graphite ore, grinding, carrying out solid-liquid separation, and freeze-drying to obtain a powder sample; 2, alloying the powder sample and magnesium powder to obtain a reaction product I; 3, dealloying the reaction product I to obtain a reaction product II; 4, hydrochloric acid is added into the second reaction product, solid-liquid separation and vacuum freeze drying are conducted, and the graphite / silicon composite material is obtained. According to the graphite / silicon composite material in which the silicon nanoparticles are randomly distributed in the graphite matrix, the silicon nanoparticles are always firmly fixed in the graphite matrix and cannot be separated from the graphite along with the increase of the cycle index, and the graphite matrix serving as a buffer layer can accommodate the volume expansion of silicon and has excellent electrochemical stability; wide application prospects are realized in the aspect of lithium ion batteries; the synthesis is simple and convenient, the production cost is low, and the environmental pollution is small.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and relates to a preparation method and application of a graphite / silicon composite material from graphite ore. Background Art

[0002] In recent years, with the continuous growth of the global demand for renewable energy and electric vehicles, the demand for high-energy-density batteries has also increased. As a traditional anode material, the specific capacity of graphite has approached the theoretical specific capacity limit of 372 mAh / g, which limits the development of high-performance lithium-ion batteries. Silicon-based materials are regarded as key materials for improving the energy density of batteries due to their ultra-high theoretical specific capacity (4200 mAh / g), low lithium insertion potential, etc. However, silicon-based anode materials have problems such as low conductivity, severe volume expansion during lithium insertion, and unstable interfaces in practical applications, which can lead to the destruction of the electrode material structure and the attenuation of battery performance, restricting their commercial applications.

[0003] To solve these problems, people have been working hard to construct composite materials by reducing the particle size of silicon and using conductive materials to improve the electrochemical performance of silicon-based anode materials. Among many matrix materials, graphite is sought after due to its good conductivity and small volume expansion. Chinese Patent CN116845225A discloses a preparation method of a nano-silicon / graphene lithium-ion battery anode material, which requires treating silicon nanoparticles with a mixed acid of hydrofluoric acid and nitric acid to obtain a Si-H surface, then performing a hydrosilylation reaction on the surface-treated silicon nanoparticles with various carbon-containing organic compounds containing double bonds or triple bonds, and finally mixing graphene with the silicon nanoparticles after the hydrosilylation reaction and drying to obtain the nano-silicon / graphene anode material.

[0004] Currently, graphite / silicon composite materials are mainly prepared by methods such as high-energy mechanical ball milling, spray drying, chemical vapor deposition, and liquid solidification. By high-energy ball milling, silicon nanoparticles are uniformly distributed on the surface of graphite, but there are problems of damaged composite material structure and additional side reactions; by spray drying to prepare graphite / silicon composite materials, they have a uniform particle size distribution and a high packing density, but the spray drying equipment has limited control over particle size; by chemical vapor deposition, a silicon layer is deposited on the surface of graphite to form a core-shell structure, but the chemical vapor deposition method has high requirements for equipment and needs to be carried out at high temperatures; the liquid solidification method is a method of obtaining a composite material by evaporating a uniform solution composed of graphite, silicon nanoparticles, binder, and conductive agent. The synthesis method is simple and can be mass-produced. However, the mixing order of materials and the particle size of materials have a huge impact on the morphology and electrochemical performance of the composite material, and further optimization is still needed.

[0005] The graphite / silicon composites prepared by the above methods are obtained by separately combining the graphite matrix and silicon. During the cycling process, the silicon particles will undergo a huge volume expansion, which will cause cracks and pulverization inside the silicon particles. Moreover, with the increase in the number of cycles, the binding force between the silicon particles and the graphite weakens, and eventually the silicon particles may fall off from the composite material. In addition, the lithium storage mechanism of silicon is of the alloy type, while that of graphite is of the insertion type. This difference will lead to uneven stress distribution inside the composite material and poor interfacial stability, thus making it easy for silicon to fall off during the cycling process.

[0006] Therefore, a preparation method of a graphite / silicon composite material is needed, in which silicon nanoparticles can always be firmly fixed in graphite, will not separate from graphite with the increase in the number of cycles, and has excellent electrochemical stability. Summary of the Invention

[0007] Aiming at the deficiencies of the above-mentioned existing technologies, the present invention provides a preparation method and application of a graphite / silicon composite material from graphite ore. The specific technical solutions are as follows: The first object of the present invention is to provide a preparation method of a graphite / silicon composite material from graphite ore, including the following steps: Step 1: Add graphite ore into a ball milling tank filled with ethanol, place the ball milling tank in a ball mill, and ball mill at a rotation speed of 300 - 400 r / min for 2 - 3 h; perform solid-liquid separation, and place the solid in a freeze dryer for freeze drying to obtain a powder sample; Step 2: Grind the powder sample and magnesium powder evenly into a powder, pour it into a reaction kettle, then place the reaction kettle in an inert atmosphere tube furnace, heat it to 550 - 750 °C, keep it warm for 5 - 7 h for alloying, take it out after cooling with the furnace, and obtain the first reaction product. During the alloying process, keep the inert gas flowing in; Step 3: Grind the first reaction product thoroughly into a uniform powder, add it into a crucible, then place the crucible in an inert atmosphere tube furnace, heat it to 750 - 780 °C and start to introduce ammonia gas, keep it warm for 5 - 7 h for dealloying, stop introducing ammonia gas after the heat preservation ends, take it out after cooling with the furnace, and obtain the second reaction product. During the dealloying process, keep the inert gas flowing in; Step 4: Grind the second reaction product into a powder, add it into a hydrochloric acid solution, stir at 50 - 80 °C for 6 - 8 h, perform solid-liquid separation, and perform vacuum freeze drying on the solid to obtain a graphite / silicon composite material.

[0008] The content of silicon dioxide in the graphite ore of the present invention is 5 wt.% - 15 wt.%, the fixed carbon content is 70 wt.% - 80 wt.%, and the rest are impurities; the content of silicon in the graphite / silicon composite material is 3 wt.% - 10 wt.%, and the balance is carbon.

[0009] The present invention provides a method for preparing graphite / silicon composite materials from graphite ore. By using magnesium powder to undergo a magnesiothermic reaction with impurities in the graphite ore, magnesium oxide that can be removed by hydrochloric acid is generated, thereby improving the purity of the graphite / silicon composite materials. Taking silicon dioxide in the graphite ore as a silicon source, after uniformly mixing the graphite ore and magnesium powder, an alloying reaction is carried out under an inert atmosphere, and then the obtained powder is subjected to a dealloying reaction under an ammonia atmosphere. Then, by pickling, the silicon dioxide in the graphite ore is in-situ transformed into silicon nanoparticles, obtaining a graphite / silicon composite material with silicon nanoparticles randomly distributed in the graphite matrix. The silicon nanoparticles are always firmly fixed in the graphite matrix and will not separate from the graphite with the increase in the number of cycles. Moreover, the graphite matrix can accommodate the volume expansion of silicon as a buffer layer. Therefore, the synthesized graphite / silicon composite material has excellent electrochemical stability and broad application prospects in lithium-ion batteries.

[0010] Further, in step 1, the mass ratio of graphite ore to grinding balls in the ball mill is 1:8 - 12; the solid-liquid ratio of graphite ore to ethanol is 1 g:30 - 50 ml, preferably 1 g:40 ml.

[0011] Further, the solid-liquid separation in step 1 is carried out by gradient centrifugation. The gradient centrifugation is to first centrifuge at a centrifugal speed of 200 - 600 r / min for 5 - 15 min; then take the supernatant and continue centrifuging at a centrifugal speed of 800 - 1400 r / min for 5 - 15 min, take the precipitate, and pour off the supernatant.

[0012] Further, in step 2, the mass ratio of the powder sample to magnesium powder is greater than 2:1 and less than 6:1, preferably 4.27:1.

[0013] Further, the inert gas in steps 2 and 3 is argon or nitrogen.

[0014] Further, the heating rate of the inert atmosphere tube furnace in steps 2 and 3 is 2 - 20 °C / min, preferably 5 °C / min.

[0015] Generally, the heating rate of a conventional experimental furnace is controlled within 10 °C / min to protect the resistance wire of the tube atmosphere furnace and achieve precise control of the temperature of the tube atmosphere furnace.

[0016] Further, the mass fraction of hydrochloric acid in step 4 is 5 - 15%, preferably 15%.

[0017] The second object of the present invention is to provide the application of the graphite / silicon composite material prepared by the above preparation method in the negative electrode material of a lithium-ion battery.

[0018] The beneficial effects of the present invention are as follows: The present invention provides a method for preparing graphite / silicon composite materials from graphite ore, using silicon dioxide in the graphite ore as a silicon source, and obtaining the graphite / silicon composite materials through a process route of alloying, dealloying, and pickling. The nanoparticles of the composite material are randomly distributed inside the graphite particles. The graphite / silicon composite material can be used to prepare the anode material of a long-life and high-energy-density lithium-ion battery, and has good cycle stability, thereby solving the technical problems such as the complex preparation process and high cost of the existing graphite / silicon composite materials.

[0019] The preparation method of the present invention is simple in synthesis, low in production cost, and small in environmental pollution. It can be widely applied to the preparation of high-performance graphite / silicon composite materials, and can also be extended to other graphite ore materials, which helps to improve the energy density of lithium-ion batteries and promote the development of the next generation of lithium-ion batteries. Brief Description of the Drawings

[0020] Figure 1 XRD pattern of the first reaction product prepared in Example 1 of the present invention.

[0021] Figure 2 XRD pattern of the second reaction product prepared in Example 1 of the present invention.

[0022] Figure 3 XRD pattern of the graphite / silicon composite material prepared in Example 1 of the present invention.

[0023] Figure 4 Scanning electron microscope image of the graphite / silicon composite material prepared in Example 1 of the present invention.

[0024] Figure 5 Transmission electron microscope image of the graphite / silicon composite material prepared in Example 1 of the present invention.

[0025] Figure 6 First charge-discharge curve of the graphite / silicon composite material prepared in Example 1 of the present invention.

[0026] Figure 7 Electrochemical cycling performance of the graphite / silicon composite material prepared in Example 1 of the present invention.

[0027] Figure 8 Electrochemical cycling stability of the graphite / silicon composite material obtained in Example 2 of the present invention.

[0028] Figure 9 Electrochemical cycling stability of the graphite / silicon composite material obtained in Example 3 of the present invention. Detailed Description of the Invention

[0029] The principles and features of the present invention will be described below in conjunction with embodiments. The examples cited are only used to explain the present invention and are not intended to limit the scope of the present invention. Embodiment

[0030] A preparation method from graphite ore to graphite / silicon composite material includes the following steps: Step 1: Add 10 g of graphite ore into a ball milling tank filled with 400 ml of ethanol, with the mass ratio of material to ball being 1:12. Place the ball milling tank in a planetary ball mill and ball mill at a speed of 400 r / min for 2 h. Then, perform gradient centrifugation on the liquid in the ball milling tank. First, centrifuge at a speed of 400 r / min for 10 min, take the supernatant and continue centrifuging, discarding the precipitate. Then, centrifuge at 1200 r / min for 10 min, take the precipitate, and pour off the supernatant. Place the solid in a freeze dryer and perform freeze drying at -40 °C for 12 h to obtain a powder sample.

[0031] Step 2: Mix the powder sample in Step 1 and magnesium powder in a mass ratio of 4.27:1, grind the powder evenly and pour it into a reaction kettle. Then, place the reaction kettle in a tubular furnace under an argon atmosphere, heat it to 650 °C, keep it warm for 6 h for alloying, take it out after cooling with the furnace, and obtain the first reaction product. Keep the argon flowing during the alloying process. Selecting the mass ratio of the powder sample to magnesium powder as 4.27:1 is to enable the magnesium powder to react with silicon dioxide in the powder sample to generate magnesium silicide, and at the same time, the magnesium powder can undergo a magnesiothermic reaction with the impurities in the graphite ore to generate magnesium oxide that can be removed with hydrochloric acid later.

[0032] Step 3: Grind the first reaction product obtained in Step 2 sufficiently until it becomes a uniform powder, add it to a crucible, then place the crucible in a tubular furnace under an argon atmosphere, start passing ammonia gas when heating to 780 °C, keep it warm for 6 h for dealloying, stop passing ammonia gas after the insulation ends, take it out after cooling with the furnace, and obtain the second reaction product. Keep the argon flowing during the dealloying process.

[0033] Step 4: Grind the second reaction product in Step 3 into powder, add it to a 15wt.% hydrochloric acid solution, heat and stir it in a water bath at 60 °C for 6 h, perform solid-liquid separation on it by suction filtration, and perform vacuum freeze drying on the solid. The temperature of freeze drying is -40 °C and the time is 12 h to obtain the graphite / silicon composite material.

[0034] From Figure 1 The XRD pattern of the first reaction product prepared in Example 1 shows that the main phases of the first reaction product prepared in this example are graphite, magnesium silicide, magnesium oxide, and some other impurities.

[0035] From Figure 2From the XRD pattern of the reaction product II prepared in Example 1, it can be seen that the main phases of the reaction product II prepared in this example are graphite, magnesium cyanamide, magnesium oxide, silicon and some other impurities.

[0036] From Figure 3 From the XRD pattern of the graphite / silicon composite material prepared in Example 1, it can be seen that the phases of the graphite / silicon composite material prepared in this example are only silicon and graphite, indicating that other impurities have been removed during the hydrochloric acid pickling process.

[0037] From Figure 4 From the scanning electron microscope image of the graphite / silicon composite material prepared in Example 1, it can be seen that the graphite / silicon composite material prepared in this example is a particulate material in the 1 - 10 micron size range.

[0038] From Figure 5 From the transmission electron microscope image of the graphite / silicon composite material prepared in Example 1, it can be seen that in the graphite / silicon composite material prepared in this example, silicon is randomly distributed in the graphite matrix in the form of nanoparticles.

[0039] Figure 6 The first charge - discharge curve of the graphite / silicon composite material prepared in Example 1 shows that during the charging process, a charging plateau of silicon can be observed at about 0.4 V.

[0040] Figure 7 The electrochemical cycling performance graph of the graphite / silicon composite material prepared in Example 1 shows that for the electrochemical cycling performance of the graphite / silicon composite material at a current density of 0.5 C (1 C = 500 mAh / g), after 400 cycles, it still has a capacity of 375 mAh / g, with a capacity retention rate of 93.1%, indicating excellent cycling stability. Example

[0041] Step 1: Add 20 g of graphite ore into a ball - milling tank filled with 800 ml of ethanol. The mass ratio of the material to the balls is 1:10. Place the ball - milling tank in a planetary ball mill and ball - mill at a speed of 300 r / min for 2 h. Then perform gradient centrifugation on the liquid in the ball - milling tank. The gradient centrifugation speed is 300 r / min for 15 min. Take the supernatant and continue centrifugation, discarding the precipitate. Then centrifuge at 900 r / min for 15 min, take the precipitate, and pour off the supernatant. Place the solid in a freeze - dryer and perform freeze - drying at - 40 °C for 12 h to obtain a powder sample.

[0042] Step 2: Mix the powder sample and magnesium powder in Step 1 at a mass ratio of 3.13:1, grind them evenly and pour them into a reaction kettle. Then place the reaction kettle in a tubular furnace under an argon atmosphere, heat it up to 550 °C, keep it warm for 7 h for alloying, take it out after cooling with the furnace, and obtain the first reaction product. Keep the argon flowing during the alloying process.

[0043] Step 3: Grind the first reaction product obtained in Step 2 sufficiently until it becomes a uniform powder, add it to a crucible, then place the crucible in a tubular furnace under an argon atmosphere, heat it up to 780 °C and start introducing ammonia gas, keep it warm for 7 h for dealloying, stop introducing ammonia gas after the heat preservation ends, take it out after cooling with the furnace, and obtain the second reaction product. Keep the argon flowing during the dealloying process.

[0044] Step 4: Grind the second reaction product in Step 3 into powder, add it to a 10 wt.% hydrochloric acid solution, heat and stir it in a water bath at 80 °C for 7 h, perform solid-liquid separation on it by filtration, place the solid in a freeze dryer and freeze-dry it at -40 °C for 12 h to obtain the graphite / silicon composite material.

[0045] Figure 8 It is the electrochemical cycling performance diagram of the graphite / silicon composite material prepared in Example 2. It can be seen that the electrochemical cycling performance of the graphite / silicon composite material at a current density of 0.5 C still has a capacity of 318 mAh / g after 400 cycles, and the capacity retention rate is 90.3%. The cycling stability performance is excellent, and the battery capacity is lower than that of Example 1. Example

[0046] Step 1: Add 5 g of graphite ore to a ball milling tank filled with 200 ml of ethanol, with a material-to-ball mass ratio of 1:8. Place the ball milling tank in a planetary ball mill and ball mill at a speed of 300 r / min for 3 h; then perform gradient centrifugation on the liquid in the ball milling tank. The speed of the gradient centrifugation is 500 r / min for 5 min, take the supernatant and continue centrifuging, discard the precipitate; then centrifuge at 1300 r / min for 5 min, take the precipitate, and pour off the supernatant. Place the solid in a freeze dryer for freeze drying. The temperature of the freeze drying is -40 °C and the time is 12 h to obtain a powder sample.

[0047] Step 2: Mix the powder sample and magnesium powder in Step 1 at a mass ratio of 2.13:1, grind them evenly and pour them into a reaction kettle. Then place the reaction kettle in a tubular furnace under an argon atmosphere, heat it up to 750 °C, keep it warm for 5 h for alloying, take it out after cooling with the furnace, and obtain the first reaction product. Keep the argon flowing during the alloying process.

[0048] Step 3: Thoroughly grind the reaction product 1 obtained in Step 2, add it to a crucible, then place the crucible in a tubular furnace under an argon atmosphere, heat up to 780 °C, start introducing ammonia gas, keep the temperature for 5 h for dealloying, stop introducing ammonia gas after the heat preservation ends, take it out after cooling with the furnace, and obtain reaction product 2. Keep introducing argon gas during the dealloying process.

[0049] Step 4: Grind the reaction product 2 in Step 3 into powder, add it to a 5wt.% hydrochloric acid solution, heat and stir in a water bath at 50 °C for 8 h, carry out solid-liquid separation by suction filtration, vacuum freeze-dry the solid, with the freeze-drying temperature at -40 °C for 12 h, and obtain the graphite / silicon composite material.

[0050] Figure 9 It is the electrochemical cycling performance diagram of the graphite / silicon composite material prepared in Example 3. It can be seen that the electrochemical cycling performance of the graphite / silicon composite material at a current density of 0.5 C still has a capacity of 297 mAh / g after 400 cycles, with a capacity retention rate of 83.2%. The cycling stability performance is worse compared with that of Example 1 and Example 2, and the battery capacity is lower.

Claims

1. A preparation method from graphite ore to graphite / silicon composite material, characterized in that, It includes the following steps: Step 1: Add graphite ore into a ball-milling tank filled with ethanol. Place the ball-milling tank in a ball mill and ball-mill at a rotation speed of 300 - 400 r / min for 2 - 3 h; perform solid-liquid separation, and place the solid in a freeze dryer for freeze-drying to obtain a powder sample; Step 2: Grind the powder sample and magnesium powder into a uniform powder, then pour it into a reaction kettle. Then place the reaction kettle in an inert atmosphere tube furnace, heat it to 550 - 750 °C, keep it warm for 5 - 7 h for alloying, take it out after cooling with the furnace, and obtain the first reaction product. Keep the inert gas flowing during the alloying process; Step 3: Grind the first reaction product thoroughly into a uniform powder, add it to a crucible, then place the crucible in an inert atmosphere tube furnace, heat it to 750 - 780 °C and start introducing ammonia gas, keep it warm for 5 - 7 h for dealloying, stop introducing ammonia gas after the heat preservation ends, take it out after cooling with the furnace, and obtain the second reaction product. Keep the inert gas flowing during the dealloying process; Step 4: Grind the second reaction product into powder, add it to a hydrochloric acid solution, stir at 50 - 80 °C for 6 - 8 h, perform solid-liquid separation, and perform vacuum freeze-drying on the solid to obtain a graphite / silicon composite material.

2. The preparation method from graphite ore to graphite / silicon composite material according to claim 1, wherein The content of silicon dioxide in the graphite ore is 5wt.% - 15wt.%, the fixed carbon content is 70wt.% - 80wt.%, and the rest are impurities.

3. The preparation method of the graphite / silicon composite material from graphite ore according to claim 2, characterized in that, In Step 1, the mass ratio of graphite ore to grinding balls in the ball-milling tank is 1:8 - 12, and the solid-liquid ratio of graphite ore to ethanol is 1:30 - 50.

4. The preparation method from graphite ore to graphite / silicon composite material according to claim 1, characterized in that, The solid-liquid separation in Step 1 adopts gradient centrifugation. The gradient centrifugation is to first centrifuge at a centrifugation speed of 200 - 600 r / min for 5 - 15 min; then take the supernatant and continue centrifuging, centrifuge at a centrifugation speed of 800 - 1200 r / min for 5 - 15 min, take the precipitate, and pour off the supernatant.

5. The preparation method of the graphite / silicon composite material from graphite ore according to claim 1, characterized in that, In Step 2, the mass ratio of the powder sample to magnesium powder is greater than 2:1 and less than 6:

1.

6. The preparation method from graphite ore to graphite / silicon composite material according to claim 5, characterized in that, In Step 2, the mass ratio of the powder sample to magnesium powder is 4.27:

1.

7. The preparation method of the graphite / silicon composite material from graphite ore according to claim 3, characterized in that, The inert gas in Step 2 and Step 3 is argon or nitrogen.

8. The preparation method of the graphite / silicon composite material from graphite ore according to claim 7, characterized in that, The heating rate of the inert atmosphere tube furnace in Step 2 and Step 3 is 2 - 20 °C / min.

9. The preparation method from graphite ore to graphite / silicon composite material according to claim 1, wherein The mass fraction of hydrochloric acid in Step 4 is 5% - 15%.

10. Application of the graphite / silicon composite material prepared by the preparation method of the graphite / silicon composite material from graphite ore as described in any one of claims 1 - 9 in the negative electrode material of a lithium-ion battery.

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