Button battery preparation method of silicon / graphite / carbon composite material lithium ion battery negative electrode

Abstract

The invention discloses a preparation method of a button battery of a silicon / graphite / carbon composite material lithium ion battery negative electrode. Nano silicon powder, a carbon source and expanded graphite are used as raw materials, slurry is prepared from the prepared powder material, a conductive agent and a binder through high-temperature carbonization coating in an inert atmosphere, a copper foil is coated with the slurry, the copper foil is placed in a vacuum glove box after being subjected to tabletting treatment, a metal lithium sheet is selected as a counter electrode, and the lithium ion battery is obtained. The button cell is assembled in a vacuum glove box by taking a lithium ion battery as a cathode, a polypropylene film as a diaphragm and 1mol / L of LiPF6 / EC, EMC, DEC ethylene carbonate, methyl vinyl carbonate and diethyl carbonate in a volume ratio of 1: 1: 1 as an electrolyte. According to the prepared lithium ion battery, the safety problem caused by pulverization of a traditional pure silicon negative electrode material is reduced, the volume expansion effect generally existing in the lithium-silicon battery is relieved, the chemical stability of the lithium battery is remarkably improved due to introduction of carbon, and an experimental basis is provided for further practical application of the lithium-silicon battery.

Description

Technical Field

[0001] The invention belongs to the field of battery manufacturing and relates to a method for preparing a button cell of a lithium ion battery negative electrode of a silicon / graphite / carbon composite material. Background Art

[0002] After 1970, silicon batteries made of metallic lithium as negative electrode materials were commercialized, and ultra-high capacity lithium-silicon batteries were used in all walks of life. The reason for limiting their further development is that the volume of pure silicon negative electrode materials will become several times the original during the process of lithium insertion and extraction. However, in actual use, it is found that when metallic lithium is used as a negative electrode material for charging and discharging, lithium dendrites are easily generated to pierce the battery's diaphragm, causing the positive and negative electrodes to contact and form a short circuit, thereby causing an explosion. In order to solve the poor cycle performance of lithium batteries, scientists use it as a stepping stone to develop lithium-ion batteries. With the development of science and technology and the in-depth research, lithium-ion batteries have been widely used today. Compared with previous lithium batteries, it has the characteristics of relatively safe charging and discharging, excellent cycle performance, high specific capacity, stable operating voltage, long service life and low pollution. It is one of the most promising chemical power sources at present.

[0003] In recent years, the most commercially promising lithium-ion batteries are made of carbon materials as negative electrode materials. With the growing needs of people, this type of lithium-ion battery can no longer meet them. Silicon has a theoretical lithium storage capacity of 4200mAh / g and abundant mineral resources, so it has attracted great attention. Silicon materials are easy to pulverize at high temperatures. People usually nano-size silicon powder or use carbon sources to dope and coat it to alleviate the volume expansion effect of silicon, while also increasing the conductivity of silicon materials. However, the addition of amorphous carbon alone is not enough to support the structure. Some materials with stable structure and certain flexibility need to be added to better buffer the expansion of silicon. Summary of the invention

[0004] 1. Technical problems to be solved: How to improve the stability of silicon-containing batteries.

[0005] 2. Technical solution: The purpose of the present invention is to improve the deficiencies of the prior art and to design and prepare a spherical silicon / graphite / carbon nanocomposite material with a built-in porous carbon skeleton network structure, nano-silicon particles embedded therein, and a surface carbon shell coated therein, so as to prepare a nano-silicon-based negative electrode material with high specific capacity and stable electrochemical performance.

[0006] The object of the present invention is achieved by the following method: a method for preparing a button cell of a lithium ion battery negative electrode of a silicon / graphite / carbon composite material, comprising the following steps: Step S01: Place the silicon / graphite / carbon composite material and the conductive agent in a magnetic stirrer and stir them in a dry powder state for 1 to 2 hours. At the same time, weigh a certain mass of the binder and completely dissolve it in an N-methylpyrrolidone (NMP) solution to prepare a binder solution with a concentration of 10wt%.

[0007] Step S02: The two solutions obtained in step S01 are uniformly mixed and stirred for 2 h to 5 h, and NMP is added during the stirring process to adjust the viscosity to obtain a slurry.

[0008] Step S03: The slurry in step S02 is evenly coated on the copper foil cleaned with ethanol, and then dried in a forced air drying oven at 80°C for 2-4 hours, and finally dried in a vacuum drying oven at 120°C for 12-18 hours to obtain an electrode.

[0009] Step S04: Punch the electrode sheet obtained in step S03 into a circular electrode sheet with a diameter of 13 mm, and compact the electrode sheet under a certain pressure under a tablet press, weigh it, and after weighing, put the electrode sheet into a vacuum drying oven for secondary drying to obtain a silicon / graphite / carbon composite electrode material.

[0010] Step S05: The silicon / graphite / carbon composite electrode material obtained in step S04 is placed in a glove box filled with argon protection, a metal round lithium sheet with a diameter of 16 mm is used as the counter electrode, a Celgard2325 polypropylene film is selected as the diaphragm, and 1 mol / L LiPF6 / EC:EMC:DEC (ethylene carbonate (EC): methyl vinyl carbonate (EMC): diethyl carbonate (DEC) with a volume ratio of 1:1:1) is selected as the electrolyte, and a button battery is assembled in a vacuum glove box.

[0011] In step S01, the method for preparing the silicon / graphite / carbon composite material is as follows: Step S21: a solution is prepared by mixing a certain mass ratio of nano silicon powder and carbon source (15 wt%~30 wt%:35 wt%~65wt%) with distilled water, and 3 wt%~10 wt% of cetyltrimethylammonium bromide (CTAB) of the nano silicon powder is added, and ultrasonic dispersion is performed.

[0012] Step S22: Weigh a corresponding amount of expanded graphite (10 wt% to 50 wt%), use ethanol as a solvent, prepare a graphite solution, ultrasonicate for 1 h, then pour it into the solution of step (1), and stir evenly.

[0013] Step S23: Transfer the mixed solution in step S22 to a heated magnetic stirrer, stir at 60°C for 1 hour, then heat to 90°C and stir until the solvent evaporates completely to obtain a mixed solid, which is then placed in a forced air drying oven and dried at 105°C for 24-36 hours to obtain a solid block.

[0014] Step S24: Transfer the solid block of step S03 to a porcelain boat, put it into a tubular furnace and introduce an inert atmosphere for heat treatment at a heating rate of 2-10°C / min, pre-sinter at 200-300°C for 1-3h, then heat to 550-650°C at the same heating rate, and keep warm for 2-5h to obtain a silicon / graphite / carbon composite material.

[0015] In step S21, the carbon source is one or a mixture of sucrose, starch, and cellulose.

[0016] In step S24, the inert atmosphere is argon or nitrogen.

[0017] In step S01, the conductive agent is SP-Li.

[0018] In step S01, the binder is polyvinylidene fluoride or aqueous carboxymethyl cellulose or styrene-butadiene rubber.

[0019] 3. Beneficial effects: The present invention prepares a button battery with silicon / graphite / carbon (Si / G / C) as the negative electrode and a metal lithium sheet as the positive electrode. By using nano silicon powder coated with a carbon source, the volume expansion effect of silicon can be alleviated while increasing the conductivity of the silicon material. However, the addition of amorphous carbon alone is not enough to support the structure, and some materials with stable structure and certain flexibility need to be added so that the expansion of silicon can be better buffered. Expanded graphite has high electronic conductivity, large lithium ion diffusion coefficient and unique lamellar structure, which is conducive to the repeated insertion and extraction of lithium ions, and improves the electrochemical stability and specific capacity of the battery.

[0020] Compared with the prior art, the present invention prepares a spherical silicon / graphite / carbon nanocomposite material with a built-in porous carbon skeleton network structure, nano-silicon particles embedded therein, and a surface carbon shell layer coated, so that the volume expansion effect of silicon can be alleviated while increasing the conductivity of the silicon material; the present invention prepares a nano-silicon-based negative electrode material with high specific capacity and stable electrochemical performance; the present invention has low cost, simple and convenient operation, and provides an experimental basis for further research and practical application of lithium-silicon batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the first charge and discharge capacity graph of five Si / G / C composite batteries.

[0022] Figure 2 This is a graph of the 50-cycle reversible specific capacity of batteries prepared with five Si / G / C composite materials.

[0023] Figure 3This is the AC impedance spectra of batteries made of five Si / G / C composite materials after 50 cycles.

[0024] Figure 4 This is the SEM of Si / G / C composite material No. 3. DETAILED DESCRIPTION

[0025] Example 1 Mix 0.27g of nano-silicon powder and 0.63g of sucrose with distilled water to prepare a solution, add 0.1g of CTAB, and disperse it ultrasonically. Weigh 0.1g of expanded graphite and use ethanol as a solvent to prepare a graphite solution and ultrasonicate for 1h. After the two solutions are mixed evenly, transfer them to a heated magnetic stirrer, stir at 60°C for 1h, then heat to 90°C and stir until the solvent evaporates to obtain a mixed solid, which is placed in a forced air drying oven and dried at 105°C for 28h to obtain a solid block. The obtained solid block was transferred to a porcelain boat, placed in a tube furnace and introduced into an argon atmosphere for heat treatment at a heating rate of 2°C / min, pre-calcined at 200°C for 1h, then heated to 550°C at the same heating rate and kept at that temperature for 2h to obtain a silicon / graphite / carbon composite material (Si / G / C). The obtained Si / G / C composite material and conductive agent were weighed in a certain proportion, placed in a magnetic stirrer and stirred in a dry powder state for 1h. At the same time, a certain mass of binder polyvinylidene fluoride (PVDF) was weighed. VDF) was completely dissolved in N-methylpyrrolidone (NMP) solution to prepare a binder solution with a concentration of 10wt%. The mixed powder and the binder were evenly mixed and stirred for 2h to obtain a slurry. The slurry was evenly coated on a copper foil cleaned with ethanol, then dried in a blast drying oven at 80℃ for 2h, and finally dried in a vacuum drying oven at 120℃ for 12h. It was punched into a circular electrode sheet with a diameter of 13mm, and the electrode sheet was compacted at a certain pressure under a tablet press and weighed. After weighing, the electrode pieces were placed in a vacuum drying oven for secondary drying to obtain Si / G / C composite electrode materials. The obtained Si / G / C electrode materials were placed in a glove box filled with argon protection, and a metal round lithium sheet with a diameter of 16 mm was used as the counter electrode. Celgard2325 polypropylene film was selected as the diaphragm, and 1 mol / L LiPF6 / EC:EMC:DEC (ethylene carbonate (EC): methyl vinyl carbonate (EMC): diethyl carbonate (DEC) with a volume ratio of 1:1:1) was selected as the electrolyte, and button batteries were assembled in the vacuum glove box.

[0026] See attached figure, Figure 1Sample No. 1 shows the first charge and discharge capacity diagram of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 1 of the present invention. It can be seen that the potential slope platform of the first discharge curve of the battery assembled from the five samples is between 0.1V and 1V. This is caused by the SEI film formed on the surface of the composite material during the discharge process. The potential platform between 0.01 and 0.1V is the formation of an alloy phase between silicon and lithium. In the charging curve, 0.01V to 0.2V is mainly the delithiation process of graphite, and the charging process between 0.4V and 0.8V corresponds to the delithiation potential of silicon, and the charging platform appears at around 0.5V. The lithium-ion battery prepared in Example 1 has the highest first discharge specific capacity, reaching 1194mAh / g, but its first coulombic efficiency is the lowest, at 62.40%, and as the silicon content decreases, its first coulombic efficiency becomes higher and higher; Figure 2 Sample No. 1 shows the reversible specific capacity of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 1 of the invention after 50 cycles. It is found that after 50 cycles, the reversible specific capacity of the battery prepared in Example 1 is 385 mAh / g, and the capacity retention rate is 51.58%; Figure 3 Sample No. 1 shows the AC impedance spectrum of the lithium ion battery of the Si / G / C composite negative electrode material obtained in Example 1 of the present invention after 50 cycles. The AC impedance spectra of the five batteries are composed of a semicircular high-frequency area and a linear low-frequency area, and the shape is basically the same. The charge transfer impedance (Rct) of the material can be known from the diameter of the semicircular arc in the high-frequency area, and the diffusion ability of lithium ions in the solid phase during the charge and discharge process can be known from the slope of the line in the low-frequency area. It can be seen from the figure that the order of the charge transfer impedance values ​​of the five batteries from small to large is No. 3 < No. 5 < No. 4 < No. 2 < No. 1. The reason why the impedance value of battery No. 1 is the largest is that the silicon content in the composite material is the highest, and the conductivity of silicon is poor. At the same time, after 50 cycles, the electrode material structure also collapses, resulting in a larger impedance value; while the composite material with a large amount of graphite and carbon added is stable in structure, and both graphite and carbon have good conductivity, which can enhance the conductivity of the composite material to a certain extent and promote the transfer of charge.

[0027] Example 2 Mix 0.24 g of nano-silicon powder and 0.56 g of sucrose with distilled water to prepare a solution, add 0.06 g of CTAB, and disperse it ultrasonically. Weigh 0.2 g of expanded graphite and use ethanol as a solvent to prepare a graphite solution and ultrasonicate for 1 hour. After the two solutions are mixed evenly, transfer them to a heated magnetic stirrer, stir at 60°C for 1 hour, then heat to 90°C and stir until the solvent evaporates to obtain a mixed solid, which is placed in a forced air drying oven and dried at 105°C for 28 hours to obtain a solid block. The obtained solid block was transferred to a porcelain boat, placed in a tubular furnace and introduced into a nitrogen atmosphere for heat treatment at a heating rate of 5°C / min, pre-calcined at 250°C for 2h, then heated to 600°C at the same heating rate and kept warm for 3h to obtain a silicon / graphite / carbon composite material (Si / G / C). The obtained Si / G / C composite material and conductive agent were weighed in a certain proportion, placed in a magnetic stirrer and stirred in a dry powder state for 2h. At the same time, a certain mass of binder polyvinylidene fluoride (PVDF) was weighed. VDF) was completely dissolved in N-methylpyrrolidone (NMP) solution to prepare a binder solution with a concentration of 10wt%. The mixed powder and the binder were evenly mixed and stirred for 3 hours to obtain a slurry. The slurry was evenly coated on a copper foil cleaned with ethanol, then dried in a blast drying oven at 80°C for 3 hours, and finally dried in a vacuum drying oven at 120°C for 16 hours. It was punched into a circular electrode sheet with a diameter of 13mm, and the electrode sheet was compacted at a certain pressure under a tablet press and weighed. After weighing, the electrode pieces were placed in a vacuum drying oven for secondary drying to obtain Si / G / C composite electrode materials. The obtained Si / G / C electrode materials were placed in a glove box filled with argon protection, and a metal round lithium sheet with a diameter of 16 mm was used as the counter electrode. Celgard2325 polypropylene film was selected as the diaphragm, and 1 mol / L LiPF6 / EC:EMC:DEC (ethylene carbonate (EC): methyl vinyl carbonate (EMC): diethyl carbonate (DEC) with a volume ratio of 1:1:1) was selected as the electrolyte, and button batteries were assembled in the vacuum glove box.

[0028] See attached figure, Figure 1 Sample No. 2 shows the first charge and discharge capacity of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 2 of the present invention. The first discharge capacity of the battery prepared in Example 2 is 1024 mAh / g, and the first coulombic efficiency is improved to 65.33%; Figure 2 Sample No. 2 shows the reversible specific capacity of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 2 of the invention after 50 cycles. It is found that after 50 cycles, the reversible specific capacity of the battery prepared in Example 2 is 435 mAh / g, and the capacity retention rate is 66.02%; Figure 3Sample No. 2 shows the AC impedance spectrum of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 2 of the present invention after 50 cycles. The charge transfer impedance values ​​of the five batteries are in the order of No. 3 < No. 5 < No. 4 < No. 2 < No. 1 from small to large.

[0029] Example 3 Mix 0.21 nanometer silicon powder and 0.49g cellulose with distilled water to prepare a solution, add 0.05g CTAB, ultrasonically disperse, weigh 0.3g expanded graphite, use ethanol as solvent, prepare a graphite solution and ultrasonicate for 1h. After the two solutions are mixed evenly, transfer them to a heated magnetic stirrer, stir at 60°C for 1h, then heat to 90°C and stir until the solvent evaporates to obtain a mixed solid, which is placed in a forced air drying oven and dried at 105°C for 36h to obtain a solid block. The obtained solid block was transferred to a porcelain boat, placed in a tubular furnace and introduced into an argon atmosphere for heat treatment at a heating rate of 7°C / min, pre-calcined at 280°C for 3h, then heated to 650°C at the same heating rate and kept warm for 5h to obtain a silicon / graphite / carbon composite material (Si / G / C). The obtained Si / G / C composite material and conductive agent were weighed in a certain proportion, placed in a magnetic stirrer and stirred in a dry powder state for 2h, and a certain mass of binder aqueous carboxymethyl cellulose was weighed at the same time. (CMC) was completely dissolved in N-methylpyrrolidone (NMP) solution to prepare a binder solution with a concentration of 10wt%. The mixed powder and the binder were evenly mixed and stirred for 5 hours to obtain a slurry. The slurry was evenly coated on a copper foil cleaned with ethanol, and then dried in a blast drying oven at 80°C for 3 hours, and finally dried in a vacuum drying oven at 120°C for 18 hours. It was punched into a circular electrode sheet with a diameter of 13mm, and the electrode sheet was compacted at a certain pressure under a tablet press and weighed. After weighing, the electrode pieces were placed in a vacuum drying oven for secondary drying to obtain Si / G / C composite electrode materials. The obtained Si / G / C electrode materials were placed in a glove box filled with argon protection, and a metal round lithium sheet with a diameter of 16 mm was used as the counter electrode. Celgard2325 polypropylene film was selected as the diaphragm, and 1 mol / L LiPF6 / EC:EMC:DEC (ethylene carbonate (EC): methyl vinyl carbonate (EMC): diethyl carbonate (DEC) with a volume ratio of 1:1:1) was selected as the electrolyte, and button batteries were assembled in the vacuum glove box.

[0030] See attached figure, Figure 1 Sample No. 3 shows the first charge and discharge capacity of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 3 of the present invention. The first discharge capacity of the battery prepared in Example 3 is 917 mAh / g, and the first coulombic efficiency is 67.07%; Figure 2Sample No. 3 shows the 50-cycle reversible specific capacity diagram of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 3 of the invention. After 50 charge and discharge cycles, the reversible specific capacity of the battery prepared in Example 3 reaches the maximum value of 495 mAh / g, and the capacity retention rate is increased to 80.05%, which is a relatively ideal silicon-based negative electrode material; Figure 3 Sample No. 3 shows the AC impedance spectrum of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 3 of the present invention after 50 cycles. The order of the charge transfer impedance values ​​of the five batteries from small to large is No. 3 < No. 5 < No. 4 < No. 2 < No. 1; Figure 4 The SEM image of the Si / G / C composite material obtained by Example 3 of the present invention is shown in the sample No. 3. The main structure of the composite material is graphite, and the particle size is about several microns, which is an irregular block structure. Since the expanded graphite with a porous structure is used, the silicon carbon after pyrolysis is uniformly embedded in the pores of the expanded graphite. Since the texture of the expanded graphite is softer and has a certain pore structure, the volume expansion in the charge and discharge process can be alleviated after silicon is embedded therein, and it can keep in contact with the electrolyte for a long time. At the same time, graphite is a good conductor material, and silicon is a semiconductor with poor conductivity, so graphite can also enhance the electrical conductivity of the composite material. The lamellar block structure of graphite is also very stable, which can support the spatial structure of the entire composite material, prevent collapse, and effectively improve the cyclic stability of the composite material.

[0031] Example 4 Mix 0.18 nanometer silicon powder and 0.42g starch with distilled water to prepare a solution, add 0.03g CTAB, ultrasonically disperse, weigh 0.4g expanded graphite, use ethanol as solvent, prepare a graphite solution and ultrasonicate for 1h. After the two solutions are mixed evenly, transfer them to a heated magnetic stirrer, stir at 60°C for 1h, then heat to 90°C and stir until the solvent evaporates to obtain a mixed solid, which is placed in a forced air drying oven and dried at 105°C for 36h to obtain a solid block. The obtained solid block was transferred to a porcelain boat, placed in a tubular furnace and introduced into a nitrogen atmosphere for heat treatment at a heating rate of 10°C / min, pre-calcined at 300°C for 3h, then heated to 650°C at the same heating rate and kept warm for 5h to obtain a silicon / graphite / carbon composite material (Si / G / C). The obtained Si / G / C composite material and conductive agent were weighed in a certain proportion, placed in a magnetic stirrer and stirred in a dry powder state for 2h. At the same time, a certain mass of binder styrene-butadiene rubber (SBR) was weighed. BR), completely dissolved it in N-methylpyrrolidone (NMP) solution to prepare a binder solution with a concentration of 10wt%, the mixed powder and the binder were uniformly mixed and stirred for 5h to prepare a slurry, the slurry was uniformly coated on the copper foil cleaned with ethanol, then dried in a blast drying oven at 80℃ for 4h, and finally dried in a vacuum drying oven at 120℃ for 14h, punched it into a circular electrode sheet with a diameter of 13mm, and the electrode sheet was compacted at a certain pressure under a tablet press and weighed. After weighing, the electrode pieces were placed in a vacuum drying oven for secondary drying to obtain Si / G / C composite electrode materials. The obtained Si / G / C electrode materials were placed in a glove box filled with argon protection, and a metal round lithium sheet with a diameter of 16 mm was used as the counter electrode. Celgard2325 polypropylene film was selected as the diaphragm, and 1 mol / L LiPF6 / EC:EMC:DEC (ethylene carbonate (EC): methyl vinyl carbonate (EMC): diethyl carbonate (DEC) with a volume ratio of 1:1:1) was selected as the electrolyte, and button batteries were assembled in the vacuum glove box.

[0032] See attached figure, Figure 1 Sample No. 4 shows the first charge and discharge capacity of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 4 of the present invention. The first discharge capacity of the battery prepared in Example 4 is 802 mAh / g, and the first coulombic efficiency is 68.08%. Figure 2 Sample No. 4 shows the 50-cycle reversible specific capacity diagram of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Inventive Example 4. After 50 charge and discharge cycles, the battery specific capacity remains at 495 mAh / g, and the capacity retention rate is 82.23%; Figure 3Sample No. 4 shows the AC impedance spectrum of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 4 of the present invention after 50 cycles. The charge transfer impedance values ​​of the five batteries are in the order of No. 3 < No. 5 < No. 4 < No. 2 < No. 1 from small to large.

[0033] Example 5 Mix 0.15g of nano-silicon powder and 0.35g of sucrose with distilled water to prepare a solution, add 0.03g of CTAB containing nano-silicon powder, ultrasonically disperse, weigh 0.5g of expanded graphite (10 wt%~50 wt%), use ethanol as a solvent, prepare a graphite solution and ultrasonicate for 1h. After the two solutions are mixed evenly, transfer them to a heated magnetic stirrer, stir at 60°C for 1h, then heat to 90°C and stir until the solvent evaporates to obtain a mixed solid, which is placed in a forced air drying oven and dried at 105°C for 24h to obtain a solid block. The obtained solid block was transferred to a porcelain boat, placed in a tube furnace and introduced into an inert atmosphere for heat treatment at a heating rate of 6°C / min, pre-calcined at 200°C for 3h, then heated to 550°C at the same heating rate and kept warm for 4h to obtain a silicon / graphite / carbon composite material (Si / G / C). The obtained Si / G / C composite material and conductive agent were weighed in a certain proportion, placed in a magnetic stirrer and stirred in a dry powder state for 2h. At the same time, a certain mass of binder styrene-butadiene rubber (SBR) was weighed. BR), completely dissolved it in N-methylpyrrolidone (NMP) solution to prepare a binder solution with a concentration of 10wt%, the mixed powder and the binder were evenly mixed and stirred for 2h to prepare a slurry, the slurry was evenly coated on the copper foil cleaned with ethanol, then dried in a blast drying oven at 80℃ for 2h, and finally dried in a vacuum drying oven at 120℃ for 16h, punched it into a circular electrode sheet with a diameter of 13mm, and the electrode sheet was compacted at a certain pressure under a tablet press and weighed. After weighing, the electrode pieces were placed in a vacuum drying oven for secondary drying to obtain Si / G / C composite electrode materials. The obtained Si / G / C electrode materials were placed in a glove box filled with argon protection, and a metal round lithium sheet with a diameter of 16 mm was used as the counter electrode. Celgard2325 polypropylene film was selected as the diaphragm, and 1 mol / L LiPF6 / EC:EMC:DEC (ethylene carbonate (EC): methyl vinyl carbonate (EMC): diethyl carbonate (DEC) with a volume ratio of 1:1:1) was selected as the electrolyte, and button batteries were assembled in the vacuum glove box.

[0034] See attached figure, Figure 1 Sample No. 5 shows the first charge and discharge capacity of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 5 of the present invention. The first discharge capacity of the battery prepared in Example 5 is 698 mAh / g, and the first coulombic efficiency is increased to 72.21%; Figure 2Sample No. 5 shows the reversible specific capacity of the lithium-ion battery of Si / G / C composite negative electrode material prepared in Example 5 of the invention after 50 cycles. After 50 charge and discharge cycles, the reversible specific capacity of the battery prepared in Example 5 is 420mAh / g, and the capacity retention rate is 83.33%. The highest capacity retention rate is in Example 5. The reason for this phenomenon is the addition of expanded graphite. Due to the pore structure on the surface of expanded graphite, nano silicon powder can be well dispersed on the surface of graphite during stirring. At the same time, the amorphous carbon pyrolyzed at high temperature is also well coated on the surface of graphite and silicon powder, providing sufficient buffer space for the volume expansion of silicon, so it has good cycle stability. Figure 3 Sample No. 5 shows the AC impedance spectrum of the lithium-ion battery of the Si / G / C composite negative electrode material prepared in Example 5 of the present invention after 50 cycles. The charge transfer impedance values ​​of the five batteries are in the order of No. 3 < No. 5 < No. 4 < No. 2 < No. 1 from small to large.

Claims

1. A method for preparing a button cell for a lithium-ion battery negative electrode of a silicon / graphite / carbon composite material, The following steps are involved: Step S01: Put the silicon / graphite / carbon composite material and the conductive agent into a magnetic stirrer and stir them in a dry powder state for 1 to 2 hours, and at the same time weigh a certain mass of the binder and completely dissolve it in an N-methylpyrrolidone (NMP) solution to prepare a binder solution with a concentration of 10wt%; Step S02: the two solutions obtained in step S01 are uniformly mixed and stirred for 2 h to 5 h, and NMP is added during the stirring process to adjust the viscosity to obtain a slurry; Step S03: evenly coating the slurry in step S02 on the copper foil cleaned with ethanol, then drying in a forced air drying oven at 80° C. for 2 to 4 hours, and finally drying in a vacuum drying oven at 120° C. for 12 to 18 hours to obtain an electrode; Step S04: punching the electrode sheet obtained in step S03 into a circular electrode sheet with a diameter of 13 mm, and compacting the electrode sheet under a certain pressure in a tablet press, weighing, and after weighing, placing the electrode sheet in a vacuum drying oven for secondary drying to obtain a silicon / graphite / carbon composite electrode material; Step S05: The silicon / graphite / carbon composite electrode material obtained in step S04 is placed in a glove box filled with argon protection, a metal round lithium sheet with a diameter of 16 mm is used as the counter electrode, a Celgard2325 polypropylene film is selected as the diaphragm, and 1 mol / L LiPF6 / EC:EMC:DEC (ethylene carbonate (EC): methyl vinyl carbonate (EMC): diethyl carbonate (DEC) with a volume ratio of 1:1:1) is selected as the electrolyte, and a button battery is assembled in a vacuum glove box.

2. The method for preparing a button cell of a lithium ion battery negative electrode of a silicon / graphite / carbon composite material as claimed in claim 1, Features: In step S01, the method for preparing the silicon / graphite / carbon composite material is as follows: Step S21: mixing a certain mass ratio of nano silicon powder and carbon source 15 wt%~30 wt%:35 wt%~65 wt% with distilled water to prepare a solution, adding 3 wt%~10 wt% of cetyltrimethylammonium bromide (CTAB) of nano silicon powder, and ultrasonically dispersing; Step S22: Weigh a corresponding amount of expanded graphite (10 wt% to 50 wt%), use ethanol as a solvent, prepare a graphite solution, ultrasonicate for 1 h, pour it into the solution of step (1), and stir evenly; Step S23: transferring the mixed solution in step S22 to a heated magnetic stirrer, stirring at 60°C for 1 hour, and then heating to 90°C and stirring until the solvent evaporates completely to obtain a mixed solid, which is then placed in a forced air drying oven and dried at 105°C for 24-36 hours to obtain a solid block; Step S24: Transfer the solid block of step S03 to a porcelain boat, put it into a tubular furnace and introduce an inert atmosphere for heat treatment at a heating rate of 2-10°C / min, pre-sinter at 200-300°C for 1-3h, then heat to 550-650°C at the same heating rate, and keep warm for 2-5h to obtain a silicon / graphite / carbon composite material.

3. The method for preparing a button cell of a lithium ion battery negative electrode of a silicon / graphite / carbon composite material as claimed in claim 2, Features: In step S21, the carbon source is one or a mixture of sucrose, starch, and cellulose.

4. The method for preparing a button cell of a lithium ion battery negative electrode of a silicon / graphite / carbon composite material as claimed in claim 2, Features: In step S24, the inert atmosphere is argon or nitrogen.

5. The method for preparing a button cell of a lithium ion battery negative electrode of a silicon / graphite / carbon composite material as claimed in claim 1, Features: In step S01, the conductive agent is SP-Li.

6. The method for preparing a button cell of a lithium ion battery negative electrode of a silicon / graphite / carbon composite material as claimed in claim 1, Features: In step S01, the binder is polyvinylidene fluoride or aqueous carboxymethyl cellulose or styrene-butadiene rubber.

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