Silicon-graphite composite material, preparation method, application and lithium ion battery thereof

Abstract

The application belongs to the field of negative electrode materials, and particularly relates to a silicon-graphite composite material, a preparation method, application and a lithium ion battery, wherein the preparation method comprises the following steps: mixing silicon powder, graphite and a composite functional additive, and then performing ball milling modification after slurry treatment to obtain primary composite particles; the composite functional additive comprises an additive A, an additive B and an additive C; the primary composite particles and a carbon source liquid are combined, and then spray drying and heat treatment or direct spray pyrolysis are performed to obtain secondary composite particles; the secondary composite particles obtained in step 2 are modified in a surface modification liquid, and then low-temperature annealing is performed at 150-300 DEG C to obtain the silicon-graphite composite material; the surface modification liquid is an organic solution in which an electrically conductive lithium salt and an additive D are dissolved; and based on the combined control of the preparation process and structure, the silicon-based negative electrode material with super-long cycle life and excellent low-temperature fast charging performance can be obtained.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material technology, specifically to the field of silicon-graphite composite anode materials. Background Technology

[0002] Silicon-based anodes have attracted much attention due to their extremely high theoretical specific capacity (4200 mAh / g), but their huge volume expansion (~300%) and poor intrinsic conductivity severely limit cycle life and fast charging performance. Especially in low-temperature environments (such as below -20°C), the deterioration of interface dynamics leads to a sharp decline in performance.

[0003] To address this issue, existing technologies have proposed several improvement schemes, such as silicon nano-sizing, carbon coating, and elemental doping. For example, Chinese patent document CN106784755A discloses a graphite / silicon / graphene composite material, which is a three-layer sandwich structure consisting of graphite, nano-silicon, and graphene from the inside out; the weight ratio of the three layers is 80-90% graphite, 1-5% nano-silicon, and 5-19% graphene. Another example is Chinese patent document CN112186159A, which discloses a method for preparing a nitrogen-doped modified silicon / graphite / graphene composite material. This method involves first preparing modified nano-silicon powder, then using the modified nano-silicon powder to prepare a modified nano-silicon / graphite composite material, while simultaneously preparing a viscous solution of graphene oxide, and finally obtaining the nitrogen-doped modified silicon / graphite / graphene composite material. Chinese patent document CN110635115A discloses a nano-silicon / graphite phase carbon nitride / graphite composite anode material, comprising a graphite matrix, nano-silicon particles dispersed in the graphite matrix, and graphite phase carbon nitride coated on the surface of the nano-silicon particles.

[0004] Existing silicon-graphite modification schemes can solve certain aspects of the material's performance, but they are still difficult to systematically address issues such as the difficulty in simultaneously achieving low-temperature and fast-charging properties. Summary of the Invention

[0005] To address the problems existing in the prior art, the primary objective of this invention is to provide a method for preparing silicon-graphite composite materials, aiming to provide a composite anode material that can be mass-produced and achieves both low-temperature and fast-charging performance.

[0006] The second objective of this invention is to provide the silicon-graphite composite material prepared by the aforementioned method and its application in lithium secondary batteries.

[0007] A third objective of this invention is to provide a lithium-ion battery comprising the aforementioned silicon-graphite composite material.

[0008] A method for preparing a silicon-graphite composite material, comprising the following steps:

[0009] Step 1:

[0010] Silicon powder, graphite, and composite functional additives are mixed and slurried, then ball-milled and modified to obtain primary composite particles;

[0011] The aforementioned composite functional additive includes additive A, additive B, and additive C;

[0012] Wherein, additive A includes at least one of long-chain alkylamines and ionic liquids; additive B includes at least one of elemental substances, alloys, oxides, sulfides, and phosphides of at least one of Bi, In, Ga, and Sn; and additive C includes at least one polymer of polyvinyl alcohol, polypyrrole, polythiophene, or polyaniline.

[0013] Step 2:

[0014] Secondary composite particles are obtained by spray drying and heat treatment after combining primary composite particles with a carbon source liquid phase, or by directly spraying pyrolysis.

[0015] Step 3:

[0016] The secondary composite particles obtained in step 2 are modified in a surface modification liquid and then annealed at a low temperature of 150~300℃ to obtain the silicon-graphite composite material.

[0017] The surface-modifying liquid is an organic solution containing a conductive lithium salt and additive D; the additive D comprises C2~C 10 Fluorinated ethers, C3~C 10 At least one of the fluorinated esters and LiPO2F2.

[0018] This invention innovatively modifies graphite and silicon beforehand using a special composite functional additive through ball milling. This achieves the joint modification of silicon amorphization into a graphite layer structure. Subsequently, a second coating is performed to obtain secondary composite particles with carbon media filling the spaces between the primary particles. These secondary composite particles are then further modified in the surface modification liquid to obtain the silicon-graphite composite material. Based on the combined control of the preparation process and structure, this invention achieves synergy, improving the material's mechanical stability, electronic / ionic conductivity, and low-temperature interface kinetics, ultimately yielding a silicon-based anode material with ultra-long cycle life and excellent low-temperature fast-charging performance.

[0019] In this invention, in step 1, the silicon powder can be micron-sized silicon powder;

[0020] The graphite mentioned can be natural flake graphite;

[0021] In this invention, in the additive A of the composite functional additive, the long-chain alkylamine can be, for example, C. 10 ~C 20Alkylamines. The ionic liquid is, for example, an imidazolium ionic liquid and other quaternary ammonium salt type ionic liquids; for example, it can be 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide salt or 1-butyl-3-methylpyrrolidine bis(fluorosulfonyl)imide salt.

[0022] Preferably, the additive A comprises an ionic liquid. Studies have shown that using an ionic liquid as additive A can synergistically enhance the long-cycle and ultra-low temperature performance of the prepared material by combining it with other components and processes.

[0023] In this invention, the auxiliary agent B contains at least one of SnS2, Bi2S3, Ga2S3, In2S3, and SnP3.

[0024] Furthermore, the additive B also includes auxiliary additive B, which includes lithium phosphate. Preferred combinations of additive B, in conjunction with other components and processes, can further enhance the long-cycle and ultra-low temperature stability of the silicon-graphite composite material. When auxiliary additive B is added to additive B, its content in additive B can be 10-60%; more specifically, it can be 10-30%.

[0025] Additive C is polyacrylonitrile.

[0026] The weight ratio of additive A, additive B, and additive C in the composite functional additive is 5~30:0.5~5:2~20; preferably 15~25:1~2:5~10:

[0027] The weight ratio of silicon powder, graphite, and composite functional additives is 1:0.5~2:0.001~0.2; it can be further expressed as 1:0.8~1.2:0.05~0.1.

[0028] In this invention, the rotational speed of the ball mill can be 200~600 rpm.

[0029] Preferably, the ball milling modification process is a gradient ball milling modification process, which includes a high-speed modification process at a rotation speed of 400-600 rpm and a low-speed modification process at a rotation speed of 200-300 rpm. This preferred high-speed and low-speed combined gradient ball milling process can be combined with composite additives to further synergistically enhance the optimized construction of the primary particulate physicochemical structure of graphite and silicon, contributing to further strengthening the long-cycle and ultra-low temperature stability of silicon-graphite composite materials.

[0030] The high-speed modification process takes 10-50 hours, or even 25-35 hours; the low-speed modification process takes 5-25 hours, or even 8-12 hours.

[0031] In this invention, primary particles and a carbon source are combined and subjected to spray-heat treatment or spray pyrolysis treatment. This allows the construction of a secondary particulate material in which the primary particles are dispersed in a carbon matrix. This is beneficial for synergistically improving the interfacial stability and conductive network of the material, and improving the low-temperature and fast-charging performance of the material.

[0032] Preferably, in step 2, the carbon source includes one or more of the following: phenolic resin, polyacrylonitrile, polyimide, polyvinylidene chloride, lignin, cellulose, chitosan, starch, sodium alginate, coal tar pitch, and petroleum asphalt.

[0033] The weight ratio of silicon, graphite, and carbon source in the primary composite particles is 100:5~15; it can be further 100:8~12.

[0034] The solvents used in the liquid-phase recombination process include one or more of the following: dimethylformamide, dimethylacetamide, hexamethylphosphoric triamine, water, ethanol, and isopropanol.

[0035] The heat treatment temperature is 500~600℃;

[0036] Step 3: The heat treatment is carried out in an inert atmosphere, wherein the inert gas is, for example, at least one of nitrogen or rare gases.

[0037] Preferably, the heat treatment process is carried out under positive pressure. The positive pressure is, for example, 2 to 5 times the atmospheric pressure.

[0038] The heat treatment time is, for example, 0.5 to 4 hours, or even 1 to 2 hours.

[0039] In step 2, spray pyrolysis can also be used directly, where the spray pyrolysis temperature is 500~600℃.

[0040] In this invention, the auxiliary agent D includes at least one of Formula 1 and Formula 2;

[0041] Formula 1;

[0042] Formula 2;

[0043] In Formula 1, R1 is H, F, or a C1-C3 alkyl group; R2 is a fluorine-substituted C1-C4 alkyl group; and R3 is a C1-C4 alkyl group or a fluorine-substituted C1-C4 alkyl group.

[0044] Preferably, additive D includes additive D1 and additive D2, wherein additive D1 is at least one of formula 1 and formula 2, and additive D2 is LiPO2F2. The molar ratio of additive D1 to additive D2 can be 1:0.5~2. Studies have shown that the preferred additive D can further modify the surface of the material, which helps to further improve the long-cycle and ultra-low temperature stability of the silicon-graphite composite material.

[0045] Conductive lithium salts include one or more of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluorooxalate borate (LiDFOB);

[0046] The solvent in the organic solution includes at least one of ether solvents and ester solvents;

[0047] In the organic solution, the concentration of the conductive lithium salt is 0.1~1M, and more preferably 0.2~0.5M, and the concentration of the additive D is 0.01~0.5M, and more preferably 0.1~0.3M;

[0048] Preferably, in step 3, the modification process is carried out under positive pressure, which is 2 to 10 atmospheres; more preferably, it can be 3 to 6 atmospheres. This preferred positive pressure modification process helps to further improve the long-cycle and cryogenic stability of the silicon-graphite composite material.

[0049] In this invention, the low-temperature annealing process is carried out under a protective atmosphere, which includes at least one of nitrogen and rare gases.

[0050] The annealing temperature is 200~250℃.

[0051] Preferably, the low-temperature annealing time is 2 to 6 hours.

[0052] The present invention also provides a silicon-graphite composite material prepared by the preparation method described above.

[0053] In this invention, thanks to the preparation method described above, the prepared material can be endowed with special physicochemical characteristics, thus giving the material excellent rate capability, low temperature performance, and long cycle performance.

[0054] The present invention also provides the application of the aforementioned silicon-graphite composite material as a negative electrode active material for the preparation of lithium secondary batteries.

[0055] The present invention also provides a lithium secondary battery, wherein the negative electrode comprises the aforementioned silicon-graphite composite material.

[0056] The lithium secondary battery of the present invention, apart from the silicon-graphite composite material described in the present invention, may contain other known components and structural parts.

[0057] Beneficial effects

[0058] This invention innovatively modifies graphite and silicon beforehand using a special composite functional additive through ball milling. This achieves the joint modification of silicon amorphization into a graphite layer structure. Subsequently, a second coating is performed to obtain secondary composite particles with carbon media filling the spaces between the primary particles. These secondary composite particles are then further modified in the surface modification liquid to obtain the silicon-graphite composite material. Based on the combined control of the preparation process and structure, this invention achieves synergy, improving the material's mechanical stability, electronic / ionic conductivity, and low-temperature interface kinetics, ultimately yielding a silicon-based anode material with ultra-long cycle life and excellent low-temperature fast-charging performance.

[0059] The present invention demonstrates that optimizing the composition of additives A to C, the high-speed-low-speed two-stage ball milling, the positive pressure heat treatment, additive D, and the positive pressure liquid phase modification helps to further optimize the physicochemical and interfacial structures of the core-intermediate layer and the surface layer, and helps to further enhance the long-range cycling and ultra-low temperature stability of the material. Attached Figure Description

[0060] Figure 1 The image shows a SEM image of the silicon-graphite composite anode material obtained in Example 1.

[0061] Figure 2 The image shows the XRD pattern of the silicon-graphite composite anode material obtained in Example 1.

[0062] Figure 3 The first charge-discharge curve at 0.2C is shown for the silicon-graphite composite anode material obtained in Example 1.

[0063] Figure 4 This is a long-cycle curve of the silicon-graphite composite anode material obtained in Example 1 after 2000 cycles at 1C and 25°C. Detailed Implementation

[0064] Example 1

[0065] Step 1: Preparation of primary composite particles

[0066] First, materials were prepared according to a weight ratio of 1:1.0:0.05 for micron-sized silica powder (D50 of 4.8µm), natural flake graphite (D50 of 3.2µm), and composite functional additives. The composite functional additives consisted of additive A (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imine salt, [BMIM][TFSI]), additive B (a mixture of tin disulfide SnS2 and lithium phosphate Li3PO4 in a weight ratio of 3:1), and additive C (polyacrylonitrile, PAN), with a weight ratio of 20:1.5:10. All the above materials were placed in the grinding jar of a planetary ball mill and subjected to two-stage high-energy ball milling under argon protection. In the first stage, ball milling was carried out at a high speed of 520 rpm for 30 hours. In the second stage, the speed was reduced to 250 rpm and ball milling continued for 15 hours to obtain a slurry-like primary composite particle mixture.

[0067] Step 2: Preparation of secondary composite particles

[0068] The primary composite particle mixture obtained in step 1 was dispersed in a carbon source solution prepared by dissolving phenolic resin in a mixed solvent of ethanol and N-methylpyrrolidone (volume ratio 1:1), wherein the concentration of phenolic resin was 8 wt%; the weight ratio of silicon, graphite, and phenolic resin in the primary composite particles was 100:8. After stirring and ultrasonic dispersion to form a uniform slurry, spray drying was performed with an inlet temperature of 280℃ and an outlet temperature of 120℃ to obtain precursor microspheres. Subsequently, the precursor was placed in a tube furnace and heated to 550℃ (heat treatment) at a heating rate of 5℃ / min under a nitrogen atmosphere, and held at this temperature for 2 hours. After furnace cooling, secondary composite particles completely coated with a carbon layer were obtained.

[0069] Step 3: Surface pre-modification and low-temperature annealing

[0070] Preparation of surface modification solution: Lithium difluorosulfonyl imide (LiFSI) was dissolved to a concentration of 0.5 M using ethylene glycol dimethyl ether (DME) as solvent, and Formula 1A (Formula 1 with R1 as H) was added to a concentration of 0.1 M to obtain the modification solution.

[0071] The secondary composite particles obtained in step 2 were placed in a high-pressure reactor, and the modified liquid (liquid-to-solid ratio 12 mL / g) was added. After sealing, argon gas was introduced into the reactor to a pressure of 0.8 MPa, and the mixture was kept at room temperature for 1 hour for liquid-phase impregnation modification. After the treatment, the pressure was released, and solid-liquid separation was performed. The resulting solid was vacuum dried at 80°C for 6 hours. Finally, the dried material was placed in an argon-protected tube furnace and heated to 200°C at a rate of 2°C / min, and held at that temperature for 3 hours for low-temperature annealing. After annealing, the material was cooled to room temperature to obtain the final silicon-graphite composite anode material.

[0072] Example 2

[0073] Compared with Example 1, the only difference is that the composite functional additives were changed, and the experimental groups were as follows:

[0074] Example 2A: Additive A in the composite functional additive was replaced with octadecylamine; the amount of additive A and other operating conditions were the same as in Example 1;

[0075] Example 2B: Additive A in the composite functional additive was replaced with 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide salt ([EMIM][FSI]); the amount of additive A and other operating conditions were the same as in Example 1;

[0076] Example 2C: Additive B in the composite functional additive was replaced with bismuth oxide (Bi2O3); the amount of additive B and other operating conditions were the same as in Example 1;

[0077] Example 2D: Replace additive B in the composite functional additive with SnS2; the dosage of additive B and other operating conditions are the same as in Example 1;

[0078] Example 2E: Additive C in the composite functional additive was replaced with polyaniline (PANI); the amount of additive C and other operating conditions were the same as in Example 1;

[0079] Example 2F: Additive C in the composite functional additive was replaced with a mixture of PAN and PANI (weight ratio 7:3); the amount of additive C and other operating conditions were the same as in Example 1;

[0080] Example 2G: In the composite functional additive, additive A is [EMIM][FSI], additive B is a mixture of indium sulfide (In2S3) and lithium phosphate (Li3PO4) (weight ratio 1:1); additive C is polypyrrole; the ratio of additive A, additive B and additive C is 15:1:5, and the weight ratio of silicon, graphite and composite additive is 1:0.8:0.1; other operating conditions are the same as in Example 1;

[0081] Example 2H: Single-stage ball milling was performed, meaning the second ball mill operated at the same speed as the first, for a total of 45 hours at 520 rpm. All other operating conditions were the same as in Example 1.

[0082] Example 3

[0083] Compared to Example 1, the only difference is that the conditions in step 2 are changed; the experimental group is as follows:

[0084] Example 3A: The carbon source was sucrose, which was used at 10% of the total weight of silicon and graphite in the primary composite particle mixture. The heat treatment temperature was 580°C and the time was 1 hour.

[0085] Example 3B: The heat treatment process is carried out under positive pressure, which is 3 times the atmospheric pressure. Other operations and parameters are the same as in Example 1.

[0086] Example 4

[0087] Compared to Example 1, the only difference is that the conditions in step 3 are changed; the experimental group is as follows:

[0088] Example 4A: In the surface modifier, the auxiliary agent D is of formula 2A ( Other operations and parameters are the same as in Example 1;

[0089] Example 4B: The surface modification solution used was a composite system of 0.5 M LiFSI + 0.05 M LiPO2F2 + 0.05 M Formula 1A (solvent DME); other operations and parameters were the same as in Example 1;

[0090] Example 4C: The lithium salt used was 0.3 M lithium difluorooxalate borate (LiDFOB), the additive D was 0.3 M formula 1A, the annealing temperature was increased to 250°C, the annealing time was shortened to 2 hours, and other operations and parameters were the same as in Example 1.

[0091] Example 4D: The liquid phase impregnation modification process was carried out under atmospheric pressure, and other operations and parameters were the same as in Example 1.

[0092] Comparative Example 1

[0093] Compared with Example 1, the only difference is that ethanol is used to replace auxiliary agent A in the composite functional auxiliary agent; the total amount of auxiliary agent and other conditions are the same as in Example 1.

[0094] Comparative Example 2

[0095] Compared with Example 1, the only difference is that additive B is missing in the composite functional additive; the total amount of additives and other conditions are the same as in Example 1.

[0096] Comparative Example 3

[0097] Compared with Example 1, the only difference is that Li3PO4 is used to replace additive B in the composite functional additive; the total amount of additives and other conditions are the same as in Example 1.

[0098] Comparative Example 4

[0099] Compared with Example 1, the only difference is that in the compound functional additive, glucose is used to replace additive C in an equal amount; the total amount of additives and other conditions are the same as in Example 1.

[0100] Comparative Example 5

[0101] Compared with Example 1, the only difference is that the operating conditions of Step 1 and Step 2 are combined. That is, the carbon source solution of Step 2 is added to Step 1, and the slurry obtained after ball milling is directly treated under spray-heat treatment conditions (the conditions of spray and heat treatment are the same as in Step 2). Then, Step 3 is directly performed. All other operations and parameters are the same as in Example 1.

[0102] Comparative Example 6

[0103] Compared to Example 1, the only difference is that step 3 is omitted. All other operations and parameters are the same as in Example 1.

[0104] Comparative Example 7

[0105] Compared with Example 1, the only difference is that in step 3, no additive D was added; all other operations and parameters are the same as in Example 1.

[0106] Comparative Example 8

[0107] Compared with Example 1, the only difference is that in step 3, comparative formula A is used. Replace additive D, and all other operations and parameters are the same as in Example 1.

[0108] test

[0109] Electrode sheet preparation: The active material (the material finally obtained in each case), Super P conductive carbon black and CMC / SBR mixed binder (the weight ratio of CMC / SBR in the mixed binder is 1:1) are mixed in deionized water at a mass ratio of 8:1:1 and ground and stirred for a long time to form a uniform slurry; then the slurry is coated on copper foil and dried by forced air and vacuum to obtain the electrode sheet. Battery assembly was completed inside an argon-filled glove box: using the prepared electrode as the working electrode, a lithium metal sheet as the counter / reference electrode, a Celgard 2400 separator, and adding electrolyte (electrolyte A for room temperature testing, which is a mixed solution of LiPF6, VC, and a mixed solvent, wherein the concentration of LiPF6 is 1M, VC is 2% of the electrolyte weight, and the mixed solvent includes EC / EMC with a volume ratio of 3:7; electrolyte B for low temperature testing, which is a mixed solution of LiFSI and a mixed solvent, wherein the concentration of LiFSI is 1M, and the mixed solvent includes FEC / DEC / TTE with a volume ratio of 2:6:2); finally, it was packaged into a coin cell and left to stand for 4-8 hours.

[0110] All performance tests were conducted on a battery testing system with a temperature control accuracy of ±0.1℃. The initial coulombic efficiency and long-cycle stability tests were performed at 25℃ with a voltage window of 0.01~1.2 V. The system first underwent 5 activation cycles at 0.1C, followed by 2000 cycles at 1C to determine its long-cycle performance at high rates. Furthermore, its rate performance was determined based on the ratio of 5C capacity to 0.1C capacity.

[0111] The low-temperature performance evaluation first involves testing the battery at 25°C at a 0.2C rate to obtain the room temperature baseline capacity. Then, the battery is transferred to a high and low temperature chamber and kept at a constant temperature of -20°C or -40°C for more than 4 hours until the internal and external temperatures of the battery are completely balanced. After that, the battery is charged and discharged at a 0.2C rate, and its capacity retention rate relative to room temperature is calculated.

[0112] The results for each case are shown in Table 1:

[0113]

[0114] This invention achieves precise "core-shell-interface" synergistic construction at the microstructure level through a three-step interconnected process design. First, step 1 (ball milling modification) is not a simple mechanical mixing, but rather, with the synergistic effect of composite functional additives (additive A / additive B / additive C), the amorphization of silicon particles, interlayer slippage, exfoliation, and modification of graphite are simultaneously realized in a high-energy mechatronics process, constructing a core with stable and excellent interfacial transport channels. Subsequently, step 2 (spray composite) precisely composites the above-mentioned "activated" primary composite particles with a carbon source, and through spray drying / pyrolysis, not only is a complete and uniform carbon coating layer (shell) formed on the outside, but the carbon medium is also filled between the primary particles, constructing a supportive three-dimensional multi-level carbon skeleton, thereby providing a stable mechanical support and electronic conductivity network for buffering volume expansion. Finally, in step 3 (surface modification), the conductive lithium salt and additive D in the surface modification solution can penetrate into the carbon layer formed in step 2 and the interface constructed in step 1, undergoing an in-situ reaction under low-temperature annealing. This achieves "refinement" and "lithiation" of the existing interface, and pre-generates a stable, LiF-rich solid-state electrolyte (SEI) film. This progressive process of "core modification - structural encapsulation - interface refinement" ensures that the chemical output of each step lays the foundation for the functional realization of the next step, ultimately forming a synergistic protection system in the material consisting of an internal fast ion channel, a strong and tough carbon framework in the middle, and an external stable SEI. This system fundamentally and synergistically improves the material's mechanical stability, electronic / ionic conductivity, and interfacial lithium-ion transport kinetics, thereby jointly enabling the final composite material to have an ultra-long cycle life and excellent low-temperature fast-charging performance.

[0115] As can be seen from Examples 1 and 2, the combined additive B, combined additive C, or gradient mechanical action can further enhance the long-cycle stability and ultra-low temperature stability of the material.

[0116] As can be seen from Examples 1 and 3, positive pressure heat treatment can further enhance the long-cycle stability and ultra-low temperature stability of materials.

[0117] As can be seen from Examples 1 and 4, the use of the combined additive D or positive pressure liquid phase modification described in this invention can further enhance the long-cycle stability and ultra-low temperature stability of the material.

[0118] As demonstrated in Example 1 and Comparative Examples 1-8, this invention innovatively modifies graphite and silicon beforehand using a special composite functional additive through ball milling. This achieves the joint modification of silicon amorphization into a graphite layer structure. Subsequently, a second coating is performed to obtain secondary composite particles with carbon media filling the spaces between the primary particles. These secondary composite particles are then further modified in the surface modification liquid to obtain the silicon-graphite composite material. Based on the combined control of the aforementioned preparation process and structure, this invention achieves synergy, improving the material's mechanical stability, electronic / ionic conductivity, and low-temperature interface kinetics, ultimately yielding a silicon-based anode material with ultra-long cycle life and excellent low-temperature fast-charging performance.

Claims

1. A method for preparing a silicon-graphite composite material, characterized in that the steps include... include: Step 1: Silicon powder, graphite, and composite functional additives are mixed and slurried, then ball-milled and modified to obtain primary composite particles; The aforementioned composite functional additive includes additive A, additive B, and additive C; Among them, additive A includes at least one of long-chain alkylamines and ionic liquids; additive B includes at least one of the elements selected from Bi, In, Ga, and Sn, as well as an alloy, oxide, sulfide, and phosphide. Additive C includes at least one polymer selected from polyvinyl alcohol, polypyrrole, polythiophene, or polyaniline; or, additive C is polyacrylonitrile; or, additive C is a mixture of polyacrylonitrile and polyaniline. Step 2: Secondary composite particles are obtained by spray drying and heat treatment after combining primary composite particles with a carbon source liquid phase, or by directly spraying pyrolysis. Step 3: The secondary composite particles obtained in step 2 are modified in a surface modification liquid and then annealed at a low temperature of 150~300℃ to obtain the silicon-graphite composite material. The surface-modifying liquid is an organic solution containing a conductive lithium salt and additive D; the additive D comprises C2~C 10 Fluorinated ethers, C3~C 10 At least one of the fluorinated esters and LiPO2F2.

2. The method for preparing the silicon-graphite composite material as described in claim 1, characterized in that, In step 1, the silicon powder is micron-sized silicon powder; The graphite mentioned is natural flake graphite; The auxiliary agent A comprises an ionic liquid; Additive B contains at least one of SnS2, Bi2S3, Ga2S3, In2S3, and SnP3; The aforementioned adjuvant B also includes auxiliary additive B, which includes lithium phosphate.

3. The method for preparing the silicon-graphite composite material as described in claim 1 or 2, characterized in that, In composite functional additives, the weight ratio of additive A, additive B and additive C is 5~30:0.5~5:2~20; The weight ratio of silicon powder, graphite, and composite functional additives is 1:0.5~2:0.001~0.

2.

4. The method for preparing the silicon-graphite composite material as described in claim 1, characterized in that, The ball milling modification process is a gradient ball milling modification process, which includes a high-speed modification process with a rotation speed of 400~600 rpm and a low-speed modification process with a rotation speed of 200~300 rpm. The high-speed modification process takes 10-50 hours, while the low-speed modification process takes 5-25 hours.

5. The method for preparing the silicon-graphite composite material as described in claim 1, characterized in that, In step 2, the carbon source includes one or more of the following: phenolic resin, polyacrylonitrile, polyimide, polyvinylidene chloride, lignin, cellulose, chitosan, starch, sodium alginate, coal tar pitch, and petroleum asphalt. The weight ratio of silicon, graphite, and carbon source in the primary composite particles is 100:5~15. The solvents used in the liquid-phase recombination process include one or more of the following: dimethylformamide, dimethylacetamide, hexamethylphosphoric triamine, water, ethanol, and isopropanol. The heat treatment temperature is 500~600℃; The heat treatment process is carried out under positive pressure; The temperature of spray pyrolysis is 500~600℃.

6. The method for preparing the silicon-graphite composite material as described in claim 1, characterized in that, Additive D includes at least one of Formula 1 and Formula 2; Formula 1 Formula 2 In Formula 1, R1 is H, F, or a C1-C3 alkyl group; R2 is a fluorine-substituted C1-C4 alkyl group; and R3 is a C1-C4 alkyl group or a fluorine-substituted C1-C4 alkyl group. Conductive lithium salts include one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, or lithium difluorooxalateborate. The solvent in the organic solution includes at least one of ether solvents and ester solvents; In the organic solution, the concentration of conductive lithium salt is 0.1~1M, and the concentration of additive D is 0.01~0.5M.

7. The method for preparing the silicon-graphite composite material as described in claim 6, characterized in that, The modification process is carried out under positive pressure, which is 2 to 10 atmospheres.

8. A silicon-graphite composite material prepared by the preparation method according to any one of claims 1 to 7.

9. An application of the silicon-graphite composite material according to claim 8, characterized in that, It is used as a negative electrode active material in the preparation of lithium secondary batteries.

10. A lithium secondary battery, characterized in that, Its negative electrode comprises the silicon-graphite composite material as described in claim 8.

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