Low-grade flake graphite-based silicon-carbon composite material, and preparation method and application thereof

Abstract

The present application belongs to the technical field of lithium ion batteries, and particularly relates to a low-grade flake graphite-based silicon-carbon composite material, a preparation method and application thereof. The preparation method of the low-grade flake graphite-based silicon-carbon composite material comprises the following steps: (1) performing step-by-step crushing on low-grade natural flake graphite raw ore to obtain flake graphite FG meeting the feeding particle size, and performing rough flotation on the flake graphite FG to obtain flake graphite rough concentrate PG; (2) performing regrinding on the flake graphite rough concentrate PG, and then performing re-flotation according to step (1); and repeating step (2) until flake graphite concentrate PG-X is obtained; (3) performing magnesium hot reduction on the flake graphite concentrate PG-X to obtain an intermediate product; and (4) performing acid treatment and water washing on the intermediate product to remove impurities. The silicon-carbon composite material is helpful to inhibit the expansion of silicon, and realizes high lithium storage characteristics and excellent cycle stability of the silicon-carbon composite material.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a low-grade flake graphite-based silicon-carbon composite material, its preparation method, and its application. Background Technology

[0002] With the global energy structure transformation, lithium-ion batteries, as efficient energy storage carriers, occupy a core position in electric vehicles, portable electronic devices, and large-scale energy storage systems. Since their commercialization in 1991, traditional commercial graphite anode materials have consistently dominated the anode market due to their stable electrochemical performance, excellent conductivity, mature manufacturing processes, and low cost. In recent years, with the increasing demands for driving range from new energy vehicles and the rising energy density requirements of energy storage systems, achieving performance breakthroughs based on existing graphite anode systems has become a focus of attention for both academia and industry.

[0003] Silicon is widely recognized as a candidate for next-generation high-energy-density anodes due to its high theoretical specific capacity (4200 mAh / g). However, the significant volume changes and poor conductivity during lithiation and delithiation of silicon-based anodes hinder their practical application. Combining silicon with carbon materials can enhance conductivity while mitigating volume expansion. Graphite, a carbon material with good mechanical strength and excellent conductivity, has reached its capacity limit when used alone as an anode material in lithium-ion batteries. Therefore, combining highly conductive graphite with silicon to form a uniform and stable composite, fully leveraging the synergistic effect of silicon and graphite, is considered an effective method to alleviate silicon volume expansion and improve conductivity.

[0004] For example, Chinese patent document CN117096329A discloses a method for preparing a low-cost silicon-graphite anode active material and its application. This method uses a two-stage ball milling process to dope modified porous silicon onto sheet graphite. Chinese patent document CN112125304A discloses a metal oxide-modified micro / nano silicon-graphite composite anode material and its preparation method. This method uses simple liquid-phase stirring and sintering to prepare micro / nano silicon-graphite composite lithium storage material. Chinese patent document CN117525350A discloses a silicon / graphite anode material and its preparation method. This method uses silicon source gas pyrolysis to deposit nano-silicon between the spread graphite sheets.

[0005] The aforementioned methods for preparing silicon-graphite composite anodes involve simple mixing of silicon and graphite through mechanical ball milling, but the interface between silicon and graphite is not tightly bonded, resulting in generally unsatisfactory cycle stability. Liquid-phase composite methods achieve uniform dispersion in a liquid environment, but typically require the introduction of a third phase, such as amorphous carbon, as a "binder" to promote tight bonding between silicon and graphite. Vapor deposition can achieve highly uniform silicon distribution on the graphite surface and precise control of the deposition amount, but its high equipment cost and the typically expensive precursor gases limit its practical application. Therefore, how to achieve simple and effective control of the silicon-graphite interface has become the primary scientific problem in the current research and application of silicon-graphite composite anode materials.

[0006] Therefore, there is a need to provide an improved technical solution that addresses the shortcomings of the existing technology. Summary of the Invention

[0007] The purpose of this invention is to provide a low-grade flake graphite-based silicon-carbon composite material, its preparation method, and its application, so as to help solve or improve the problems of high cost of silicon-carbon composite material preparation process or poor cycle performance caused by insufficient bonding between silicon and graphite interface in the composite material under the existing technology.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing low-grade flake graphite-based silicon-carbon composite material, comprising the following steps: (1) performing stepwise crushing of low-grade natural flake graphite ore to obtain flake graphite FG conforming to the feed particle size, and performing flake graphite FG flotation roughing to obtain flake graphite rough concentrate PG; (2) re-grinding the flake graphite rough concentrate PG and then performing re-flotation treatment according to step (1); repeating step (2) until obtaining (3) Using the flake graphite concentrate PG-X as a precursor, the flake graphite concentrate PG-X is subjected to magnesothermic reduction to obtain an intermediate product; (4) The intermediate product is subjected to acid treatment and water washing to remove impurities, thereby obtaining the low-grade flake graphite-based silicon-carbon composite material; the fixed carbon content of the flake graphite concentrate PG-X is 36%-75%; the fixed carbon content of the low-grade natural flake graphite ore is less than that of the flake graphite concentrate PG-X.

[0009] Preferably, in step (1), the particle size of the flake graphite FG obtained by crushing that meets the feed particle size is ≤75μm; in step (1), the flotation roughing includes the following steps: the flake graphite FG that meets the feed particle size is made into a slurry, the slurry is placed in a single-cell flotation machine for flotation roughing, and conditioning agent, inhibitor, capture agent and frother are added to the slurry in sequence, and after aeration, the upper foam layer is scraped off to obtain flake graphite rough concentrate PG.

[0010] Preferably, in step (1), the amount of flake graphite (FG) is measured in tons (t), and the amounts of the modifier, inhibitor, trapping agent, and frother are measured in grams (g); the modifier is lime, and the ratio of the modifier to flake graphite (FG) is 550-650 g / t; the inhibitor is water glass, and the ratio of the inhibitor to flake graphite (FG) is 550-650 g / t; the trapping agent is kerosene, and the ratio of the trapping agent to flake graphite (FG) is 395-415 g / t; the frother is pine oil, and the ratio of the frother to flake graphite (FG) is 95-105 g / t; in step (2), the re-grinding method is ball milling, the ball milling speed is 450-550 r / min, and the ball milling time is 1.5-2.5 h.

[0011] Preferably, before step (2), the process further includes a step of treating the flake graphite concentrate PG-X with hydrochloric acid solution.

[0012] Preferably, the concentration of the hydrochloric acid solution is 1-2 mol / L, and the hydrochloric acid solution is mixed with flake graphite concentrate PG-X and then treated under water bath heating conditions at 75-85℃ for 1.5-2.5h.

[0013] Preferably, in step (3), the flake graphite concentrate PG-X is mixed evenly with magnesium powder and heat removal agent, and then heated to 600-800℃ and kept at that temperature for 6-10h under an argon atmosphere to carry out magnesothermic reduction.

[0014] Preferably, in step (3), the heat scavenging agent is NaCl, and the mass ratio of flake graphite concentrate PG-X to magnesium powder and NaCl is 1:(0.2-0.7):3.

[0015] Preferably, in step (4), hydrochloric acid solution is used for acid treatment, and the concentration of the hydrochloric acid solution is 1-2 mol / L; the acid treatment is carried out at 75-85℃ for 2-4 hours.

[0016] The present invention also provides a low-grade flake graphite-based silicon-carbon composite material, which adopts the following technical solution: a low-grade flake graphite-based silicon-carbon composite material, wherein the low-grade flake graphite-based silicon-carbon composite material is prepared by the method described above.

[0017] The present invention also provides a negative electrode material, which adopts the following technical solution: a negative electrode material, wherein the components of the negative electrode material include the low-grade flake graphite-based silicon-carbon composite material as described above.

[0018] The present invention also provides a lithium-ion battery, which adopts the following technical solution: a lithium-ion battery, wherein the lithium-ion battery adopts the low-grade flake graphite-based silicon-carbon composite material as described above or the negative electrode material as described above.

[0019] Beneficial effects: The preparation method of the low-grade flake graphite-based silicon-carbon composite material of the present invention firstly achieves directional control of graphite and silicon components in flotation graphite concentrate by adjusting the flotation process to obtain a natural silicon-rich graphite precursor with close bonding between silicon and graphite; then, the silicon-rich components in the flotation graphite concentrate are reduced to elemental silicon by a molten salt-assisted magnesothermic reduction process to prepare a graphite-based silicon-carbon composite material that can be applied to lithium-ion batteries, thus broadening the preparation route of silicon-graphite composite materials.

[0020] In the low-grade flake graphite-based silicon-carbon composite material of the present invention, as the ratio of silicon and graphite intergrowth increases, the expansion of silicon can be significantly suppressed, thereby achieving high lithium storage characteristics and excellent cycle stability of the silicon-carbon composite material. Attached Figure Description

[0021] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein: Figure 1 Process flow diagram for preparing silicon-carbon composite materials by molten salt-assisted magnesiothermal reduction of low-grade natural flake graphite; Figure 2 The image is a scanning electron microscope image of the flake graphite concentrate PG obtained by step (2) in Example 1; Figure 3 The images shown are cross-sectional scanning electron microscope images of the Si-Graphite-1 electrodes before and after charge-discharge cycles obtained in Example 1. Figure 4 The images shown are cross-sectional scanning electron microscope images of the Si-Graphite-2 electrodes before and after charge-discharge cycles obtained in Example 2. Figure 5 The image is a scanning electron microscope image of PG-2 flake graphite concentrate obtained by step (4) in Example 3; Figure 6 The image shows the XRD pattern of Si-Graphite-3 obtained in Example 3. Figure 7 The images shown are the scanning electron microscope (SEM) image and EDS spectrum of Si-Graphite-3 obtained in Example 3. Figure 8 The graphs show the first three constant current charge-discharge curves of Si-Graphite-3 obtained in Example 3; Figure 9 The images show cross-sectional scanning electron microscope (SEM) images of the Si-Graphite-3 electrodes obtained in Example 3 before and after charge-discharge cycles. Figure 10 The images show cross-sectional scanning electron microscope (SEM) images of the Si-Graphite-4 electrodes obtained in Example 4 before and after charge-discharge cycles. Figure 11 The image shows cross-sectional scanning electron microscope (SEM) images of the Si-Graphite-5 electrodes obtained in Example 5 before and after charge-discharge cycles. Figure 12 The image is a scanning electron microscope image of PG-5 flake graphite concentrate obtained by step (7) in Example 6; Figure 13 The image shows cross-sectional scanning electron microscope (SEM) images of the Si-Graphite-6 electrodes obtained in Example 6 before and after charge-discharge cycles. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0023] The present invention will now be described in detail with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0024] This invention addresses the problems of high cost in the preparation process of silicon-carbon composite materials or poor cycle performance caused by insufficient bonding between silicon and graphite in the composite material under existing technologies, and provides a method for preparing low-grade flake graphite-based silicon-carbon composite materials.

[0025] The preparation method of the low-grade flake graphite-based silicon-carbon composite material of the present invention includes the following steps: (1) the low-grade natural flake graphite ore is subjected to stepwise crushing to obtain flake graphite FG that meets the feed particle size, and the flake graphite FG is subjected to flotation roughing to obtain flake graphite rough concentrate PG; (2) the flake graphite rough concentrate PG is ground again and then subjected to flotation treatment again according to step (1); step (2) is repeated until flake graphite concentrate PG-X is obtained; (3) the flake graphite concentrate PG-X is used as a precursor and subjected to magnesothermic reduction to obtain an intermediate product; (4) the intermediate product is subjected to acid treatment and water washing to remove impurities, and the low-grade flake graphite-based silicon-carbon composite material is obtained; the fixed carbon content of flake graphite concentrate PG-X is 36%-75%; the fixed carbon content of low-grade natural flake graphite ore is less than that of flake graphite concentrate PG-X. Among them, "step-by-step crushing" refers to gradually reducing the particle size (for example, first crushing into larger particle sizes, and then gradually using more refined crushing methods).

[0026] This invention removes easily dissociable impurities from low-grade natural flake graphite by crushing and flotation, obtaining a silicon-rich graphite precursor (flake graphite concentrate PG-X) in which silica impurities are tightly bonded to graphite flakes. By adjusting the preliminary purification process of flotation, the silicon and graphite components in the flotation graphite concentrate can be directionally controlled. Natural graphite-based silicon-carbon composite materials with different silicon to graphite ratios are prepared by using a magnesothermic reduction process. The method described in this invention is simple, obtaining a naturally occurring silicon-rich and graphite-rich precursor through a simple flotation process, thus reducing production costs. In the prepared low-grade flake graphite-based silicon-carbon composite material, silicon and graphite exhibit a tightly intergrowth state. Specifically, as the ratio of silicon to graphite intergrowth increases (this ratio varies with the number of flotation cycles. The composition of the products changes continuously with different grinding and flotation cycles. The flotation products can be divided into silicon impurities, silicon-graphite intergrowths, and pure graphite; in the first few grinding and flotation cycles, there are more silicon impurities, fewer silicon-graphite intergrowths, and less pure graphite; as the number of grinding and flotation cycles increases, silicon impurities decrease, silicon-graphite intergrowths increase, and pure graphite decreases; as the number of grinding and flotation cycles increases again, silicon impurities decrease, silicon-graphite intergrowths decrease, and pure graphite increases), silicon expansion can be significantly suppressed, achieving high lithium storage characteristics and excellent cycle stability in the silicon-carbon composite material.

[0027] Preferably, when the fixed carbon content of flake graphite concentrate PG-X is 36%-75%, the silica content is 12.2%-34.8%.

[0028] Preferably, the fixed carbon content of the flake graphite concentrate PG-X is 58%-68% (e.g., 58%, 60%, 62%, 64%, 66% or 68%).

[0029] In a preferred embodiment of the preparation method of the low-grade flake graphite-based silicon-carbon composite material of the present invention, in step (1), the particle size of the flake graphite FG obtained by crushing that meets the feed particle size is ≤75μm (can pass through a 200-mesh sieve); in step (1), the flotation roughing includes the following steps: S1, the flake graphite FG that meets the feed particle size is made into a slurry, the slurry is placed in a single-cell flotation machine for flotation, and conditioning agent, inhibitor, capture agent and frother are added to the slurry in sequence, and after aeration, the upper foam layer is scraped off to obtain flake graphite rough concentrate PG.

[0030] Preferably, in step (1), the amount of flake graphite (FG) is measured in tons (t), and the amounts of modifier, inhibitor, trapping agent, and foaming agent are measured in grams (g). The modifier is lime, and the ratio of modifier to flake graphite (FG) is 550-650 g / t (i.e., 550-650 g of modifier corresponds to 1 ton of flake graphite (FG); for example, 550 g / t, 580 g / t, 610 g / t, 630 g / t, or 650 g / t). The inhibitor is water glass, and the ratio of inhibitor to flake graphite FG is 550-650 g / t (i.e., the amount of inhibitor corresponding to 1 ton of flake graphite FG is 550-650 g; for example, 550 g / t, 570 g / t, 590 g / t, 620 g / t, or 650 g / t); the capture agent is kerosene, and the ratio of capture agent to flake graphite FG is 395-415 g / t (i.e., 1 ton of flake graphite FG corresponds to 550-650 g / t of water glass; for example, 550 g / t, 570 g / t, 590 g / t, 620 g / t, or 650 g / t of water glass ... The dosage of the capture agent corresponding to flake graphite FG is 395-415g; for example, 395g / t, 400g / t, 405g / t, 410g / t, or 415g / t; the foaming agent is pine oil, and the ratio of foaming agent to flake graphite FG is 95-105g / t (i.e., the dosage of foaming agent corresponding to 1 ton of flake graphite FG is 95-105g; for example, 95g / t, 98g / t, 100g / t). t, 103g / t or 105g / t); in step (2), the re-grinding method is ball milling, the ball milling speed is 450-550r / min (e.g. 450r / min, 480r / min, 500r / min, 520r / min or 550r / min), and the ball milling time is 1.5-2.5h (e.g. 1.5h, 1.8h, 2h, 2.2h or 2.5h).

[0031] Preferably, the crushing method in step (1) is cascade crushing; including the following steps: first, primary crushing is carried out by a hammer crusher; then further crushing is carried out by a disc mill and deep crushing is carried out by a ball mill.

[0032] In a preferred embodiment of the preparation method of the low-grade flake graphite-based silicon-carbon composite material of the present invention, before step (2), a step of treating the flake graphite concentrate PG-X with hydrochloric acid solution is also included. Treating the flake graphite concentrate PG-X with hydrochloric acid solution helps to remove impurities in the flake graphite concentrate PG-X (this step is mainly used to remove impurities such as Al2O3, Fe2O3, CaO and MgO in the flake graphite concentrate PG-X).

[0033] Preferably, the concentration of the hydrochloric acid solution is 1-2 mol / L (e.g., 1 mol / L, 1.3 mol / L, 1.6 mol / L, 1.8 mol / L or 2 mol / L). After mixing the hydrochloric acid solution with flake graphite concentrate PG-X, the mixture is treated under a water bath heating condition of 75-85℃ (e.g., 75℃, 78℃, 80℃, 82℃ or 85℃) for 1.5-2.5 h (e.g., 1.5 h, 1.8 h, 2 h, 2.2 h or 2.5 h). Subsequently, solid-liquid separation, washing, and drying are performed.

[0034] In a preferred embodiment of the preparation method of the low-grade flake graphite-based silicon-carbon composite material of the present invention, in step (3), the flake graphite concentrate PG-X is mixed evenly with magnesium powder and heat scavenging agent, and then heated to 600-800℃ (e.g., 600℃, 650℃, 700℃, 750℃ or 800℃) under an argon atmosphere for magnesium thermal reduction, and the holding time is 6-10h (e.g. 6h, 7h, 8h, 9h or 10h).

[0035] In a preferred embodiment of the preparation method of the low-grade flake graphite-based silicon-carbon composite material of the present invention, in step (3), the heat scavenger is NaCl, and the mass ratio of flake graphite concentrate PG-X to magnesium powder and NaCl is 1:(0.2-0.7):3 (for example, 1:0.2:3, 1:0.3:3, 1:0.4:3, 1:0.5:3, 1:0.6:3 or 1:0.7:3).

[0036] In a preferred embodiment of the preparation method of the low-grade flake graphite-based silicon-carbon composite material of the present invention, in step (4), an acid treatment is performed using a hydrochloric acid solution with a concentration of 1-2 mol / L (e.g., 1 mol / L, 1.3 mol / L, 1.6 mol / L, 1.8 mol / L or 2 mol / L); the acid treatment is performed at 75-85℃ (e.g., 75℃, 78℃, 80℃, 82℃ or 85℃) for 2-4 h (e.g., 2 h, 2.5 h, 3 h, 3.5 h or 4 h).

[0037] Preferably, in step (3), the drying method is vacuum drying; the drying temperature is 80-120℃ (e.g., 80℃, 90℃, 100℃, 110℃ or 120℃), and the drying time is 8-12h (e.g., 8h, 9h, 10h, 11h or 12h).

[0038] This invention proposes a silicon-carbon composite material, which is prepared by the method described above in the embodiments of this invention.

[0039] This invention proposes a negative electrode material, wherein the components of the negative electrode material in the embodiments of this invention include the silicon-carbon composite material as described above.

[0040] The present invention also proposes a battery, wherein the battery of the present invention uses the negative electrode material as described above.

[0041] The preparation method of the low-grade flake graphite-based silicon-carbon composite material of the present invention, the silicon-carbon composite material and its application are described in detail below through specific embodiments.

[0042] The main raw materials used in the following examples are sourced from: low-grade natural flake graphite, which was obtained from Xingsheng Graphite Co., Ltd. in Boli County, Heilongjiang Province, with a fixed carbon content of 6.23 wt%.

[0043] Example 1 The preparation method of the low-grade flake graphite-based silicon-carbon composite material in this embodiment includes the following steps: (1) Take natural flake graphite ore, perform primary crushing by hammer crusher, further crush by disc mill, and finally put it into ball mill for deep crushing. Take flake graphite with a particle size of less than 75μm and record it as FG (as flake graphite that meets the feed particle size). (2) Take 200g of flake graphite (FG) that meets the feed particle size to prepare a slurry with a mass concentration of 10%, place it in a single-cell flotation machine for flotation, and after stirring for 2 minutes, add the modifier (0.12g lime) and the inhibitor (0.12g water glass solution; the density of the water glass solution is 1.10-1.55g / cm³) to the slurry in sequence. 3 The collector (kerosene 0.081g) and foaming agent (pine oil 0.02g) were added. After aeration for 2 minutes, the upper foam layer was scraped off to obtain flake graphite crude concentrate (PG), with a fixed carbon content of 22% (the fixed carbon content of natural flake graphite was determined by an indirect method, and the specific determination steps strictly followed the method specified in national standard GB3521-2023; the indirect calculation formula was: fixed carbon = 100% - ash content - volatile matter, which yielded the fixed carbon content value of the sample to be tested). (3) The flake graphite concentrate PG was treated with 1.5 mol / L HCl solution to remove impurities (the two were mixed and treated under water bath heating at 80℃ for 2 h; then filtered, washed and dried) to obtain sample c; (4) Weigh sample c (using sample c as a silicon-rich graphite-rich precursor), magnesium powder and NaCl (heat scavenger) at a mass ratio of 1:0.7:3. Grind and mix them evenly in a glove box filled with Ar and then transfer them to a stainless steel reactor. Place the high-pressure reactor in a vertical furnace (molten salt assists in magnesium thermal reduction), and heat it to 650°C at a heating rate of 5°C / min under an Ar atmosphere. Hold it for 6 hours and cool it to room temperature to obtain the intermediate product (i.e., the mixed product after magnesium thermal reduction). (5) The intermediate product was treated with 1.5 mol / L HCl solution (treated for 3 h in a water bath at 80 °C) to remove the byproducts generated during the molten salt-assisted magnesium thermal reduction process. After washing and filtering, it was vacuum dried at 80 °C for 10 h (vacuum degree -0.1 MPa) to obtain the low-grade flake graphite-based silicon-carbon composite material of this embodiment (i.e., graphite-based silicon-carbon composite material for lithium-ion batteries), denoted as Si-Graphite-1.

[0044] The morphology and structure of the flake graphite rough concentrate were characterized using a Compact Merlin field emission scanning electron microscope manufactured by ZEISS GmbH, Germany. The scanning electron microscope image of the flake graphite rough concentrate PG (obtained after step (2)) in this embodiment is shown below. Figure 2 As shown in the figure, silicon components dominate the impurities in PG flake graphite concentrate.

[0045] The cross-section of the Si-Graphite-1 electrode before and after charge-discharge cycles was observed using a Compact Merlin field emission scanning electron microscope manufactured by ZEISS GmbH, Germany. The results are as follows: Figure 3 As shown.

[0046] When the Si-Graphite-1 prepared in this embodiment is used as a negative electrode material for lithium-ion batteries, the electrode thickness before cycling is about 14.6 μm. The electrode structure is dense and tightly attached to the copper current collector. After charge-discharge cycles (charge-discharge cycles are performed at 50 mA / g, and the number of charge-discharge cycles is 300), the electrode exhibits severe expansion, with the thickness becoming 27.0 μm, the electrode expansion amount being 12.4 μm, and the expansion rate being 84.9%.

[0047] The lithium-ion battery anode material was prepared using the following steps: Si-Graphite-1, conductive carbon black, and sodium alginate prepared in this example were weighed in a mass ratio of 70:15:15. Si-Graphite-1 and conductive carbon black were thoroughly mixed in a mortar and then added to sodium alginate pre-dissolved in deionized water to form a slurry. The slurry was coated onto a 0.2 mm copper foil and dried in a vacuum oven at 100°C for 10 hours. After drying, it was cut into 14 mm diameter discs as the working electrode. A circular lithium metal sheet was used as the control electrode, a Celgard 2400 polypropylene membrane as the separator, and a 1 mol / L LiPF6 electrolyte. An additional 10 wt.% FEC was added to the solvent (volume ratio EC:DEC = 1:1) as a protective agent. The assembly process of the CR2016 button-type half-cell was carried out in an argon-filled glove box (H2O and O2 levels were both below 0.1 ppm). Constant current charge-discharge (GCD) tests were performed using the Newwell test system (BTS-4000).

[0048] Example 2 The preparation method of the low-grade flake graphite-based silicon-carbon composite material in this embodiment includes the following steps (refer to...). Figure 1 ): (1) Take natural flake graphite ore, perform primary crushing by hammer crusher, further crush by disc mill, and finally put it into ball mill for deep crushing. Take flake graphite with a particle size of less than 75μm and record it as FG (as flake graphite that meets the feed particle size). (2) Take 200g of flake graphite FG that meets the feed particle size to make a slurry with a mass concentration of 10%, place it in a single cell flotation machine for flotation roughing process, stir for 2 minutes and then add the modifier (0.12g lime), inhibitor (0.12g water glass), collector (0.081g kerosene) and frother (0.02g pine oil) to the slurry in sequence. After aeration for 2 minutes, scrape off the upper foam layer to obtain flake graphite rough concentrate (PG); (3) The flake graphite concentrate (PG) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 1 (PG-1), and its fixed carbon content was 36% (testing method is the same as in Example 1). (4) The PG-1 flake graphite concentrate was treated with 1.5 mol / L HCl solution to remove impurities (the two were mixed and treated under water bath heating at 80℃ for 2 h; then filtered, washed and dried) to obtain sample c; (5) Weigh sample c (as a silicon-rich graphite-rich precursor), magnesium powder and NaCl in a mass ratio of 1:0.6:3. Grind and mix them evenly in a glove box filled with Ar and then transfer them to a stainless steel reactor. Place the high-pressure reactor in a vertical furnace (molten salt-assisted magnesium thermal reduction), heat it to 650°C at a heating rate of 5°C / min under an Ar atmosphere, hold for 6 hours, and cool to room temperature to obtain the intermediate product (i.e., the mixed product after magnesium thermal reduction). (6) The intermediate product was acid-treated with 1.5 mol / L HCl solution (the two were mixed and treated under water bath heating at 80°C for 3 h) to remove the byproducts generated during the molten salt-assisted magnesium thermal reduction process. After washing with water and filtering, it was vacuum dried at 80°C for 10 h (vacuum degree -0.1 MPa) to obtain the low-grade flake graphite-based silicon-carbon composite material of this embodiment (i.e., graphite-based silicon-carbon composite material for lithium-ion batteries), denoted as Si-Graphite-2.

[0049] The cross-section of the Si-Graphite-2 electrode before and after charge-discharge cycles was observed using a Compact Merlin field emission scanning electron microscope manufactured by ZEISS GmbH, Germany. The results are as follows: Figure 4 As shown. When the Si-Graphite-2 prepared in this embodiment is used as the negative electrode material for lithium-ion batteries (the fabrication method is the same as in Example 1), the electrode thickness before cycling is about 10.8 μm. The electrode structure is dense and tightly attached to the copper current collector. After charge-discharge cycles (charge-discharge cycles are performed at 50 mA / g, and the number of charge-discharge cycles is 300), the electrode exhibits severe expansion, with the thickness becoming 18.9 μm, the electrode expansion amount being 8.1 μm, and the expansion rate being 75.0%.

[0050] Example 3 The preparation method of the low-grade flake graphite-based silicon-carbon composite material in this embodiment (refer to...) Figure 1 ), including the following steps: (1) Take natural flake graphite ore, perform primary crushing by hammer crusher, further crush by disc mill, and finally put it into ball mill for deep crushing. Take flake graphite ore with a particle size of less than 75 μm and record it as FG (as flake graphite that meets the feed particle size). (2) Take 200g of flake graphite FG that meets the feed particle size to make a slurry with a mass concentration of 10%, place it in a single cell flotation machine for flotation, stir for 2 minutes and then add the modifier (0.12g lime), inhibitor (0.12g water glass), collector (0.081g kerosene) and frother (0.02g pine oil) to the slurry in sequence. After aeration for 2 minutes, scrape off the upper foam layer to obtain flake graphite rough concentrate (PG); (3) The flake graphite rough concentrate (PG) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 1 (PG-1). (4) The flake graphite concentrate 1 (PG-1) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 2 (PG-2), and its fixed carbon content was 58%. (5) Flake graphite concentrate 2 (PG-2) was treated with 1.5 mol / L HCl solution to remove impurities (the two were mixed and treated under water bath heating at 80℃ for 2 h; then filtered, washed and dried) to obtain sample c; (6) Weigh sample c (as a silicon-rich graphite-rich precursor), magnesium powder and NaCl in a mass ratio of 1:0.4:3. Grind and mix them evenly in a glove box filled with Ar and then transfer them to a stainless steel reactor. Place the high-pressure reactor in a vertical furnace (molten salt-assisted magnesium thermal reduction), heat it to 650°C at a heating rate of 5°C / min under an Ar atmosphere, hold it for 6 hours, and cool it to room temperature to obtain the intermediate product (i.e., the mixed product after magnesium thermal reduction). (7) The intermediate product was acid-treated with 1.5 M HCl solution (the two were mixed and treated under water bath heating at 80°C for 3 h) to remove the byproducts generated during the molten salt-assisted magnesium thermal reduction process. After washing with water and filtering, it was vacuum dried at 80°C for 10 h (vacuum degree -0.1 MPa) to obtain the low-grade flake graphite-based silicon-carbon composite material of this embodiment (i.e., graphite-based silicon-carbon composite material for lithium-ion batteries), denoted as Si-Graphite-3.

[0051] The scanning electron microscope image of the flake graphite concentrate 2 (PG-2) obtained by step (4) in this embodiment is shown below. Figure 5 As shown in the figure, silicon components and graphite intergrowths dominate in flake graphite concentrate 2.

[0052] The crystal structure of the silicon-carbon composite material (Si-Graphite-3) was characterized using a Shimadzu Smart Lab X-ray diffractometer (Japan). The results are as follows: Figure 6 As shown, diffraction peaks representing graphite appear at 2θ values ​​of 26.6, 42.5, 44.7, 50.8, 54.8, 60.0, 71.7, and 77.7°, while five diffraction peaks at 2θ values ​​of 28.4, 47.3, 56.1, 69.1, and 76.4° correspond to the (111), (220), (311), (400), and (331) crystal planes of cubic silicon, respectively.

[0053] Figure 7 The images show the scanning electron microscope (SEM) image and EDS spectrum of Si-Graphite-3. As can be seen from the figures, the prepared Si-Graphite-3 has a graphite layer as the substrate, with silicon particle aggregates tightly encapsulated within it. When Si-Graphite-3 is used as a negative electrode material for lithium-ion batteries (prepared using the same method as in Example 1), the initial charge specific capacity at a current density of 50 mA / g is (…). Figure 8 The values ​​are 756 mAh / g and the initial coulombic efficiency is 82.0%; Figure 9As shown, the thickness of the Si-Graphite-3 negative electrode before cycling is about 10.3 μm. After charge-discharge cycling (charge-discharge cycling at 50 mA / g for 300 cycles), the electrode thickness becomes 16.7 μm, the electrode expansion is 6.4 μm, and the expansion rate is 62.1%.

[0054] Example 4 The preparation method of the low-grade flake graphite-based silicon-carbon composite material in this embodiment (refer to...) Figure 1 ), including the following steps: (1) Take natural flake graphite ore, perform primary crushing by hammer crusher, further crush by disc mill, and finally put it into ball mill for deep crushing. Take flake graphite ore with a particle size of less than 75 μm and record it as FG (as flake graphite that meets the feed particle size). (2) Take 200g of flake graphite FG that meets the feed particle size to make a slurry with a mass concentration of 10%, place it in a single cell flotation machine for flotation, stir for 2 minutes and then add the modifier (0.12g lime), inhibitor (0.12g water glass), collector (0.081g kerosene) and frother (0.02g pine oil) to the slurry in sequence. After aeration for 2 minutes, scrape off the upper foam layer to obtain flake graphite rough concentrate (PG); (3) The flake graphite rough concentrate (PG) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 1 (PG-1). (4) The flake graphite concentrate 1 (PG-1) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 2 (PG-2). (5) The flake graphite concentrate 2 (PG-2) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The resulting product was recorded as flake graphite concentrate 3 (PG-3), and its fixed carbon content was 68% (testing method is the same as in Example 1). (6) Flake graphite concentrate 3 (PG-3) was treated with 1.5 mol / L HCl solution to remove impurities (the two were mixed and treated under water bath heating at 80℃ for 2 h; then filtered, washed and dried) to obtain sample c; (7) Weigh sample c (as a silicon-rich graphite-rich precursor), magnesium powder and NaCl in a mass ratio of 1:0.3:3. Grind and mix them evenly in a glove box filled with Ar and then transfer them to a stainless steel reactor. Place the high-pressure reactor in a vertical furnace (molten salt-assisted magnesium thermal reduction), heat it to 650°C at a heating rate of 5°C / min under an Ar atmosphere, hold for 6 hours, and cool to room temperature to obtain the intermediate product. (8) The intermediate product was treated with 1.5 M HCl solution (the two were mixed and treated under water bath heating at 80°C for 3 h) to remove the byproducts generated during the molten salt-assisted magnesium thermal reduction process. After washing with water and filtering, it was vacuum dried at 80°C for 10 h (vacuum degree -0.1 MPa) to obtain the low-grade flake graphite-based silicon-carbon composite material of this embodiment (i.e., graphite-based silicon-carbon composite material for lithium-ion batteries), denoted as Si-Graphite-4.

[0055] like Figure 10 As shown, when the Si-Graphite-4 prepared in this embodiment is used as a negative electrode material for lithium-ion batteries (the preparation method is the same as in Example 1), the electrode thickness is about 9.8 μm before the first cycle. After charge-discharge cycles (charge-discharge cycles are performed at 50 mA / g, and the number of charge-discharge cycles is 300), the electrode thickness becomes 16.5 μm, the electrode expansion is 6.7 μm, and the expansion rate is 68.4%.

[0056] Example 5 The preparation method of the low-grade flake graphite-based silicon-carbon composite material in this embodiment includes the following steps: (1) Take natural flake graphite ore, perform primary crushing by hammer crusher, further crush by disc mill, and finally put it into ball mill for deep crushing. Take flake graphite ore with a particle size of less than 75 μm and record it as FG (as flake graphite that meets the feed particle size). (2) Take 200g of flake graphite FG that meets the feed particle size to make a slurry with a mass concentration of 10%, place it in a single cell flotation machine for flotation, stir for 2 minutes and then add the modifier (0.12g lime), inhibitor (0.12g water glass), collector (0.081g kerosene) and frother (0.02g pine oil) to the slurry in sequence. After aeration for 2 minutes, scrape off the upper foam layer to obtain flake graphite rough concentrate (PG); (3) The flake graphite rough concentrate (PG) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 1 (PG-1). (4) The flake graphite concentrate 1 (PG-1) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 2 (PG-2). (5) The flake graphite concentrate 2 (PG-2) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 3 (PG-3). (6) The flake graphite concentrate 3 (PG-3) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 4 (PG-4), with a fixed carbon content of 75%. (7) Flake graphite concentrate 4 (PG-4) was treated with 1.5 mol / L HCl solution to remove impurities (the two were mixed and treated under water bath heating at 80℃ for 2 h; then filtered, washed and dried) to obtain sample c; (8) Weigh sample c (as a silicon-rich graphite-rich precursor), magnesium powder and NaCl at a mass ratio of 1:0.25:3. Grind and mix them evenly in a glove box filled with Ar and then transfer them to a stainless steel reactor. Place the high-pressure reactor in a vertical furnace (molten salt-assisted magnesium thermal reduction), heat it to 650°C at a heating rate of 5°C / min under an Ar atmosphere, hold it for 6 hours, and cool it to room temperature to obtain the intermediate product (i.e., the mixed product after magnesium thermal reduction). (9) The intermediate product was treated with 1.5 mol / L HCl solution (the two were mixed and treated under water bath heating at 80°C for 3 h) to remove the byproducts generated during the molten salt-assisted magnesium thermal reduction process. After washing with water and filtering, it was vacuum dried at 80°C for 10 h (vacuum degree -0.1 MPa) to obtain the low-grade flake graphite-based silicon-carbon composite material of this embodiment (i.e., graphite-based silicon-carbon composite material for lithium-ion batteries), denoted as Si-Graphite-5.

[0057] like Figure 11 As shown, when the Si-Graphite-5 prepared in this embodiment is used as a negative electrode material for lithium-ion batteries (the preparation method is the same as in Example 1), the electrode thickness is about 10.5 μm before cycling. After charge-discharge cycles (charge-discharge cycles are performed at 50 mA / g, and the number of charge-discharge cycles is 300), the electrode thickness becomes 18.2 μm, the electrode expansion is 7.7 μm, and the expansion rate is 73.3%.

[0058] Example 6 The preparation method of the low-grade flake graphite-based silicon-carbon composite material in this embodiment includes the following steps: (1) Take natural flake graphite ore, perform primary crushing by hammer crusher, further crush by disc mill, and finally put it into ball mill for deep crushing. Take flake graphite ore with a particle size of less than 75μm and record it as FG (as flake graphite that meets the feed particle size). (2) Take 200g of flake graphite FG that meets the feed particle size to make a slurry with a mass concentration of 10%, place it in a single cell flotation machine for flotation, stir for 2 minutes and then add the modifier (0.12g lime), inhibitor (0.12g water glass), collector (0.081g kerosene) and frother (0.02g pine oil) to the slurry in sequence. After aeration for 2 minutes, scrape off the upper foam layer to obtain flake graphite rough concentrate (PG); (3) The flake graphite rough concentrate (PG) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 1 (PG-1). (4) The flake graphite concentrate 1 (PG-1) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 2 (PG-2). (5) The flake graphite concentrate 2 (PG-2) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 3 (PG-3). (6) The flake graphite concentrate 3 (PG-3) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 4 (PG-4). (7) The flake graphite concentrate 4 (PG-4) was re-ground by ball mill (mechanical ball milling, ball milling speed 500 r / min, ball milling time 2 h), and the above flotation process was repeated. The product obtained was recorded as flake graphite concentrate 5 (PG-5), with a fixed carbon content of 81%. (8) Flake graphite concentrate 5 (PG-5) was treated with 1.5M HCl solution to remove impurities (the two were mixed and treated under water bath heating at 80℃ for 2h; then filtered, washed and dried) to obtain sample c; (9) Weigh sample c (as a silicon-rich graphite-rich precursor), magnesium powder and NaCl in a mass ratio of 1:0.2:3. Grind and mix them evenly in a glove box filled with Ar and then transfer them to a stainless steel reactor. Place the high-pressure reactor in a vertical furnace (molten salt-assisted magnesium thermal reduction), heat it to 650°C at a heating rate of 5°C / min under an Ar atmosphere, hold it for 6 hours, and cool it to room temperature to obtain the intermediate product (i.e., the mixed product after magnesium thermal reduction). (10) The intermediate product was treated with 1.5 M HCl solution (the two were mixed and treated under water bath heating at 80°C for 3 h) to remove the byproducts generated during the molten salt-assisted magnesium thermal reduction process. After washing with water and filtering, it was vacuum dried at 80°C for 10 h (vacuum degree -0.1 MPa) to obtain the low-grade flake graphite-based silicon-carbon composite material of this embodiment (i.e., graphite-based silicon-carbon composite material for lithium-ion batteries), denoted as Si-Graphite-6.

[0059] The scanning electron microscope image of the flake graphite concentrate 5 (PG-5 obtained by step (7)) selected in this embodiment is shown below. Figure 12 As shown in the figure, graphite components dominate in precursor flake graphite concentrate 5. Figure 13 As shown, when the Si-Graphite-6 prepared in this embodiment is used as a negative electrode material for lithium-ion batteries (the preparation method is the same as in Example 1), the electrode thickness is about 11.4 μm before cycling. After charge-discharge cycles (charge-discharge cycles are performed at 50 mA / g, and the number of charge-discharge cycles is 300), the electrode thickness becomes 20.1 μm, the electrode expansion is 8.7 μm, and the expansion rate is 76.3%.

[0060] Comparative Example 1 The only difference between this comparative example and Example 5 is that: this comparative example directly selects 87.8% high-purity flake graphite and 12.2% silicon dioxide as raw materials, and obtains a silicon dioxide-graphite composite material precursor by mechanical ball milling. The silicon dioxide-graphite composite material, magnesium powder and NaCl are weighed at a mass ratio of 1:0.25:3, ground and mixed evenly in a glove box filled with Ar, and then transferred to a stainless steel reactor. The autoclave is placed in a vertical furnace, and heated to 650°C at a heating rate of 5°C / min under an Ar atmosphere, held for 6 hours, and cooled to room temperature to obtain an intermediate product. The intermediate product is treated with 1.5 M HCl solution (the two are mixed and treated under 80°C water bath heating for 3 hours) to remove the byproducts generated during the molten salt-assisted magnesium thermal reduction process. After washing with water and filtering, it is vacuum dried at 80°C for 10 hours (vacuum degree -0.1MPa) to obtain the silicon-carbon composite material of this comparative example. All other aspects are consistent with Example 5.

[0061] The cross-section of the electrode in this comparative example before and after charge-discharge cycles was observed using a Compact Merlin field emission scanning electron microscope manufactured by ZEISS GmbH, Germany, and the volume expansion rate before and after charge-discharge was calculated; the test results are shown in Table 1 below.

[0062] Comparative Example 2 The only difference between this comparative example and Example 6 is that: this comparative example directly selects 90.8% high-purity flake graphite and 9.2% silicon dioxide as raw materials, and obtains a silicon dioxide-graphite composite material precursor by mechanical ball milling. The silicon dioxide-graphite composite material, magnesium powder and NaCl are weighed at a mass ratio of 1:0.2:3, ground and mixed evenly in a glove box filled with Ar, and then transferred to a stainless steel reactor. The autoclave is placed in a vertical furnace, and heated to 650°C at a heating rate of 5°C / min under an Ar atmosphere, held for 6 hours, and cooled to room temperature to obtain an intermediate product. The intermediate product is treated with 1.5 M HCl solution (the two are mixed and treated under 80°C water bath heating for 3 hours) to remove the byproducts generated during the molten salt-assisted magnesium thermal reduction process. After washing with water and filtering, it is vacuum dried at 80°C for 10 hours to obtain the silicon-carbon composite material of this comparative example. All other aspects are consistent with Example 6.

[0063] The cross-section of the electrode in this comparative example before and after charge-discharge cycles was observed using a Compact Merlin field emission scanning electron microscope manufactured by ZEISS GmbH, Germany, and the volume expansion rate before and after charge-discharge was calculated; the test results are shown in Table 1 below.

[0064] Experimental Example 1. The volume expansion rate results of each embodiment obtained from the above tests are summarized in Table 1 below: Table 1

[0065] 2. Electrochemical performance testing: The initial charge specific capacity, initial coulombic efficiency, and capacity retention after 300 cycles were tested at 50 mA / g. The test results are shown in Table 2 below. Table 2

[0066] In summary, this invention focuses on low-grade natural flake graphite. By adjusting the preliminary flotation purification process, the silicon and graphite components in the flotation graphite concentrate can be directionally controlled. Furthermore, a molten salt-assisted magnesiothermal reduction process can be used to prepare natural graphite-based silicon-carbon composite materials with different silicon-to-graphite ratios. In these composites, silicon and graphite exhibit a tightly intergrowth state. Moreover, as the proportion of silicon and graphite intergrowths in the prepared composites increases, the volume expansion of silicon can be significantly suppressed, resulting in high lithium storage characteristics and excellent cycle stability in the silicon-carbon composite materials.

[0067] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a low-grade flake graphite-based silicon-carbon composite material, characterized in that, Includes the following steps: (1) Low-grade natural flake graphite ore is subjected to step-by-step crushing to obtain flake graphite FG that meets the feed particle size, and the flake graphite FG is subjected to flotation roughing to obtain flake graphite rough concentrate PG. (2) After re-grinding the flake graphite rough concentrate PG, perform flotation treatment again according to step (1); repeat step (2) until flake graphite concentrate PG-X is obtained; (3) Using the flake graphite concentrate PG-X as a precursor, the flake graphite concentrate PG-X is subjected to magnesothermic reduction to obtain an intermediate product; (4) The intermediate product is subjected to acid treatment and water washing to remove impurities, thereby obtaining the low-grade flake graphite-based silicon-carbon composite material. The fixed carbon content of the flake graphite concentrate PG-X is 36%-75%; the fixed carbon content of the low-grade natural flake graphite ore is less than that of the flake graphite concentrate PG-X.

2. The method for preparing low-grade flake graphite-based silicon-carbon composite material as described in claim 1, characterized in that, In step (1), the particle size of the flake graphite (FG) obtained by crushing that meets the feed particle size is ≤75μm; In step (1), the flotation roughing includes the following steps: the flake graphite FG that meets the feed particle size is made into a slurry, the slurry is placed in a single-cell flotation machine for flotation roughing, and conditioning agent, inhibitor, capture agent and frother are added to the slurry in sequence. After aeration, the upper foam layer is scraped off to obtain flake graphite rough concentrate PG.

3. The method for preparing low-grade flake graphite-based silicon-carbon composite material as described in claim 2, characterized in that, In step (1), the amount of flake graphite FG used is in tons (t), and the amount of modifier, inhibitor, trapping agent and foaming agent used is in grams (g). The modifier is lime, and the ratio of the modifier to flake graphite (FG) is 550-650 g / t. The inhibitor is water glass, and the ratio of the inhibitor to flake graphite (FG) is 550-650 g / t. The capturing agent is kerosene, and the ratio of the capturing agent to flake graphite (FG) is 395-415 g / t. The foaming agent is pine oil, and the ratio of the foaming agent to flake graphite (FG) is 95-105 g / t. In step (2), the re-grinding method is ball milling, with a ball milling speed of 450-550 r / min and a ball milling time of 1.5-2.5 h.

4. The method for preparing low-grade flake graphite-based silicon-carbon composite material as described in claim 1, characterized in that, Before step (2), the process also includes treating the flake graphite concentrate PG-X with hydrochloric acid solution; Preferably, the concentration of the hydrochloric acid solution is 1-2 mol / L, and the hydrochloric acid solution is mixed with flake graphite concentrate PG-X and then treated under water bath heating conditions at 75-85℃ for 1.5-2.5h.

5. The method for preparing low-grade flake graphite-based silicon-carbon composite material as described in claim 1, characterized in that, In step (3), the flake graphite concentrate PG-X is mixed evenly with magnesium powder and heat removal agent, and then heated to 600-800℃ and kept at that temperature for 6-10h under an argon atmosphere for magnesium thermal reduction.

6. The method for preparing low-grade flake graphite-based silicon-carbon composite material as described in claim 5, characterized in that, In step (3), the heat removal agent is NaCl, and the mass ratio of flake graphite concentrate PG-X to magnesium powder and NaCl is 1:(0.2-0.7):

3.

7. The method for preparing low-grade flake graphite-based silicon-carbon composite material as described in claim 5, characterized in that, In step (4), hydrochloric acid solution is used for acid treatment, and the concentration of the hydrochloric acid solution is 1-2 mol / L; The acid treatment is carried out at 75-85℃ for 2-4 hours.

8. A low-grade flake graphite-based silicon-carbon composite material, characterized in that, The low-grade flake graphite-based silicon-carbon composite material is prepared by the method described in any one of claims 1-7.

9. A negative electrode material, characterized in that, The composition of the negative electrode material includes the low-grade flake graphite-based silicon-carbon composite material as described in claim 8.

10. A lithium-ion battery, characterized in that, The lithium-ion battery uses the low-grade flake graphite-based silicon-carbon composite material as described in claim 8 or the negative electrode material as described in claim 9.

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