A silicon-carbon composite negative electrode material with high initial efficiency and high capacity, a preparation method and application thereof

Abstract

This invention relates to a high-efficiency, high-capacity silicon-carbon composite anode material, its preparation method, and its application. The silicon-carbon composite anode material comprises: a phosphorus-doped silicon-carbon material, and a composite coating layer covering the outer surface of the phosphorus-doped silicon-carbon material. A Si-P-C bond structure exists between the phosphorus-doped silicon-carbon material and the composite coating layer. The phosphorus-doped silicon-carbon material comprises: a porous carbon matrix, and nano-silicon particles and phosphorus elements deposited in the pores of the porous carbon matrix. The composite coating layer comprises: a carbon coating layer, and C-F bonds formed in situ within the carbon coating layer. Applying the silicon-carbon composite anode material provided in this invention to lithium-ion batteries can improve the first-cycle coulombic efficiency and cycle stability of lithium-ion batteries, and reduce the expansion rate.

Description

Technical Field

[0001] This invention relates to the field of lithium battery materials technology, and in particular to a silicon-carbon composite anode material with high initial efficiency and high capacity, its preparation method, and its application. Background Technology

[0002] Silicon is currently the most promising non-carbon anode material for lithium-ion batteries to replace graphite, with a theoretical specific capacity as high as 4200 mAh / g (Li i 22 Si5 has a specific capacity ten times that of the theoretical graphite anode; its lithiation potential is very close to that of lithium metal, only 0.4V vs. Li / Li + This is highly beneficial for improving the energy density of full-cell batteries; at the same time, silicon is abundant, accounting for 27% to 28% of the Earth's total mass, and has low production costs and is environmentally friendly.

[0003] However, the volume effect of silicon anodes in practical applications often leads to problems such as electrode structure pulverization, rapid capacity decay, and low coulombic efficiency. Silicon-carbon composites are considered one of the effective ways to solve the problems of high volume expansion and easy pulverization of silicon anodes. Silicon-carbon composites are a new type of anode material with carbon as the dispersion matrix and silicon as the active material. Silicon provides lithium storage capacity, while carbon, as the dispersion matrix, buffers the volume changes of silicon particles during lithium insertion and extraction, maintaining the integrity of the electrode structure and maintaining the internal electrical contact of the electrode. The silicon-carbon composite structure has high capacity and good conductivity. The carbon layer reduces the direct contact between bare silicon and electrolyte, inhibits the repeated growth of the solid electrolyte interphase (SEI) film, thereby improving the first-cycle coulombic efficiency and cycle stability of the battery to a certain extent.

[0004] Compared to milled silicon-carbon and micron-sized silicon-carbon, chemical vapor deposition (CVD) silicon-carbon materials, which use porous carbon as a framework to support deposited silicon, offer advantages such as high initial efficiency, low expansion, and long cycle life. Currently, the initial coulombic efficiency of CVD silicon-carbon materials cannot meet the development requirements of high-energy-density lithium-ion batteries. Therefore, there is an urgent need to develop new material structure modification methods to improve the initial coulombic efficiency of CVD silicon-carbon materials, further enhancing the initial coulombic efficiency of full-cell batteries and the cell energy density.

[0005] Existing research mainly employs methods such as pre-lithiation and carbon matrix structure design modification to improve the first-cycle coulombic efficiency of silicon-carbon anode materials. However, there is still limited research on further improving the first-cycle coulombic efficiency and cycle rate performance of silicon-carbon materials by constructing a reasonable SE I layer through surface fluorination to reduce the consumption of active lithium during the formation of the SE I film in the first charge and discharge process.

[0006] Among them, Chinese invention patent CN115799474A (published on March 14, 2023) discloses a method for preparing and applying fluorinated silicon-carbon anode material. The method involves dispersing silicon powder in a polymer solution and then preparing an amorphous carbon-coated silicon anode material through high-temperature carbonization. Subsequently, the amorphous carbon-coated silicon anode material is co-heat-treated with polyvinylidene fluoride powder in an inert atmosphere to obtain the fluorinated silicon-carbon anode material. While the fluorinated silicon-carbon anode material prepared by this patent can improve the first-cycle coulombic efficiency and cycle performance of batteries to some extent, the improvement is limited and cannot yet meet application requirements. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by providing a high-efficiency, high-capacity silicon-carbon composite anode material, its preparation method, and its applications.

[0008] To achieve the above objectives, in a first aspect, embodiments of the present invention provide a high-efficiency, high-capacity silicon-carbon composite anode material, the silicon-carbon composite anode material comprising: a phosphorus-doped silicon-carbon material, and a composite coating layer covering the outer surface of the phosphorus-doped silicon-carbon material, wherein the phosphorus-doped silicon-carbon material and the composite coating layer have SiPC bonds.

[0009] The phosphorus-doped silicon-carbon material comprises: a porous carbon matrix, and nano-silicon particles and phosphorus elements deposited in the pores of the porous carbon matrix;

[0010] The composite coating layer includes: a carbon coating layer, and CF bonds formed in situ in the carbon coating layer.

[0011] Preferably, the average particle size D50 of the porous carbon matrix is ​​between 1 μm and 15 μm;

[0012] The specific surface area of ​​the porous carbon matrix is ​​800 m². 2 / g~2500m 2 Between / g;

[0013] The average pore size of the porous carbon matrix is ​​between 1 nm and 50 nm.

[0014] The porous carbon matrix has a pore volume of 0.8 cm³. 3 / g~1.5cm 3 / g.

[0015] Preferably, the percentage of the nano-silicon particles in the total mass of the silicon-carbon composite anode material is 50% to 65%.

[0016] The phosphorus element in the silicon-carbon composite anode material accounts for 0.5% to 10% of the total mass of the silicon-carbon composite anode material;

[0017] The percentage of fluorine in the silicon-carbon composite anode material is 0.5% to 10% of the total mass of the silicon-carbon composite anode material;

[0018] The thickness of the composite coating layer is 2nm to 50nm.

[0019] In a second aspect, embodiments of the present invention provide a method for preparing the high-efficiency, high-capacity silicon-carbon composite anode material described in the first aspect above, the preparation method comprising:

[0020] Step S1, preparing phosphorus-doped silicon-carbon material, includes: placing a porous carbon matrix in a reaction device, heating it in a protective gas environment, using the protective gas as a carrier gas to introduce a mixture of silicon source gas and phosphorus source gas into the reaction device, maintaining the temperature, so that silicon elements decomposed from the silicon source gas are deposited in the pores of the porous carbon matrix and grow into nano-silicon particles, while phosphorus elements decomposed from the phosphorus source gas are deposited in the pores of the porous carbon matrix, and the phosphorus elements form SiP bonds with some silicon elements to obtain phosphorus-doped silicon-carbon material;

[0021] Step S2: The phosphorus-doped silicon-carbon material is subjected to carbon coating treatment to form a carbon coating layer on the outer surface of the phosphorus-doped silicon-carbon material, thereby obtaining an intermediate material.

[0022] Step S3: Grind the intermediate material with cobalt trifluoride to coat the surface of the intermediate material with cobalt trifluoride, thereby obtaining the precursor material;

[0023] Step S4: The precursor material is heat-treated so that the fluorine gas generated by the thermal decomposition of cobalt trifluoride attacks the defect sites of the carbon coating layer, causing F atoms to occupy the positions of C atoms and form CF bonds. At the same time, the SiP bonds on the surface of the porous carbon matrix form SiPC bond structures in situ with the carbon in the carbon coating layer, thereby obtaining a silicon-carbon composite anode material with high initial efficiency and high capacity.

[0024] Preferably, the reaction equipment includes any one of a fluidized bed reactor, a rotary kiln, and a chemical vapor deposition furnace;

[0025] The protective gas includes nitrogen and / or argon; the flow rate of the protective gas is 5 L / min to 50 L / min.

[0026] The temperature for heating is 400℃ to 800℃, and the holding time is 3 hours to 8 hours.

[0027] Preferably, the average particle size D50 of the porous carbon matrix is ​​between 1 μm and 15 μm, and the specific surface area is 800 m². 2 / g~2500m 2The average pore size of the porous carbon matrix is ​​between 1 nm and 50 nm, and the pore volume is between 0.8 cm³ / g. 3 / g~1.5cm 3 / g;

[0028] The silicon source gas includes one or more of the following: silane, disilane, propane, and silicon tetrachloride; the flow rate of the silicon source gas is 5 L / min to 50 L / min.

[0029] The phosphorus source gas includes: phosphorus hydrogenation, or phosphorus-containing gas generated by heating a solid phosphorus source; the solid phosphorus source includes: phosphorus pentoxide, phosphate, alkyl phosphate ester or one or more; the flow rate of the phosphorus source gas is 1L / min to 5L / min.

[0030] Preferably, the carbon coating treatment includes any one of gas phase carbon coating, liquid phase carbon coating, or solid phase carbon coating;

[0031] The carbon source gas for the gaseous carbon coating includes one or more of the following: methane, ethane, propane, acetylene, propyne, butyne, propylene, and ethylene; the gaseous carbon coating temperature is 300℃~1000℃, and the deposition time is 1 hour~8 hours.

[0032] Preferably, the mass ratio of cobalt trifluoride to the intermediate material is 0.1:1 to 4:1;

[0033] The grinding equipment for the grinding process includes any one of the following: ball mill, horizontal mill, vertical mill, planetary mill, and sand mill; the rotation speed of the grinding equipment is 500 rpm to 2000 rpm; and the grinding time is 0.5 hours to 24 hours.

[0034] Preferably, the heat treatment includes: placing the precursor material in a high-temperature furnace under a protective atmosphere, heating it to 300°C to 450°C at a heating rate of 1°C / min to 5°C / min, holding it at that temperature for 2 hours to 10 hours, and then allowing it to cool naturally to room temperature; the protective atmosphere includes nitrogen and / or argon.

[0035] Thirdly, embodiments of the present invention provide a lithium-ion battery, the lithium-ion battery comprising the high-efficiency, high-capacity silicon-carbon composite anode material described in the first aspect.

[0036] This invention provides a method for preparing a high-efficiency, high-capacity silicon-carbon composite anode material. The method involves co-depositing silicon and phosphorus in the pores of a porous carbon matrix via chemical vapor deposition (CVD), followed by carbon coating to obtain an intermediate material—a phosphorus-doped silicon-carbon material with a carbon coating layer. This intermediate material is then ground with cobalt trifluoride (CTF) to coat its surface. Finally, heat treatment causes fluorine gas generated from the thermal decomposition of CTF to attack defects in the carbon coating layer, allowing F atoms to occupy C atom positions and form CF bonds. Simultaneously, Si-P bonds on the porous carbon matrix surface and carbon atoms in the carbon coating layer form in-situ Si-PC bonds, ultimately yielding a high-efficiency, high-capacity silicon-carbon composite anode material. This preparation method is relatively simple and has good application prospects.

[0037] The silicon-carbon composite anode material prepared by the above-described method in this invention, when applied to lithium-ion batteries, exhibits several advantages. The phosphorus doping in the material improves electronic conductivity, thereby increasing the first-cycle coulombic efficiency and specific capacity. The in-situ formed Si-PC bond structure enhances the coating strength of the carbon coating layer on the silicon-carbon composite particles, thus improving the material's ability to suppress silicon expansion, enhancing its stability, and reducing electrode expansion rate. Furthermore, the CF bonds formed on the material surface can generate LiF in situ during battery formation, resulting in a LiF-rich solid electrolyte layer (SEI). This effectively isolates the electrolyte, reducing the consumption of active lithium due to side reactions between the electrolyte and nano-silicon during SEI film formation. Additionally, the LiF-rich SEI layer acts as a stress-flexible buffer and facilitates solid-phase transport of Li. + The silicon-carbon composite anode material provided in this invention can improve the first-cycle coulombic efficiency and cycle stability of lithium-ion batteries, and reduce the expansion rate. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of a high-efficiency, high-capacity silicon-carbon composite anode material provided in an embodiment of the present invention.

[0039] Figure 2 A flowchart illustrating the preparation method of a high-efficiency, high-capacity silicon-carbon composite anode material provided in this embodiment of the invention. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0041] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0042] This invention provides a high-efficiency, high-capacity silicon-carbon composite anode material, comprising: a phosphorus-doped silicon-carbon material, and a composite coating layer covering the outer surface of the phosphorus-doped silicon-carbon material, wherein the phosphorus-doped silicon-carbon material and the composite coating layer have a Si-PC bond structure.

[0043] Among them, phosphorus-doped silicon-carbon materials include: a porous carbon matrix, and nano-silicon particles and phosphorus elements deposited in the pores of the porous carbon matrix.

[0044] The porous carbon matrix has an average particle size D50 between 1 μm and 15 μm and a specific surface area of ​​800 m². 2 / g~2500m 2 The average pore size of the porous carbon matrix is ​​between 1 nm and 50 nm, and the pore volume is between 0.8 cm³ / g. 3 / g~1.5cm 3 / g.

[0045] The composite coating layer includes a carbon coating layer and CF bonds formed in situ in the carbon coating layer; the thickness of the composite coating layer is between 2 nm and 50 nm, preferably 10 nm.

[0046] Phosphorus forms SiP bonds with nano-silicon particles, and the Si-P bonds on the pore surface form Si-PC bonds with the carbon elements in the carbon coating layer, thereby improving the bonding force between the carbon coating layer and the porous carbon matrix, which in turn enhances the ability to suppress silicon volume expansion.

[0047] In the high-efficiency, high-capacity silicon-carbon composite anode material provided in this embodiment of the invention, the mass percentage of nano-silicon particles in the silicon-carbon composite anode material is 50% to 65%; the mass percentage of phosphorus in the silicon-carbon composite anode material is 0.5% to 10%, preferably 0.5% to 5%; and the mass percentage of fluorine in the silicon-carbon composite anode material is 0.5% to 10%, preferably 0.5% to 5%.

[0048] A schematic diagram of the structure of the high-efficiency, high-capacity silicon-carbon composite anode material provided in this embodiment of the invention is shown below. Figure 1 As shown, nano-silicon particles and phosphorus are deposited in the pores of the porous carbon matrix. Phosphorus forms Si-P bonds with some of the nano-silicon particles. After the pores are filled with nano-silicon particles, the Si-P bonds on the pore surface form SiPC bonds with the carbon elements in the carbon coating layer in the composite coating layer, while some defect sites in the carbon coating layer are occupied in situ by F elements to form CF bonds.

[0049] This invention provides a method for preparing the above-mentioned high-efficiency, high-capacity silicon-carbon composite anode material, such as... Figure 2 As shown, the specific steps include:

[0050] Step S1, preparing phosphorus-doped silicon-carbon material, includes: placing a porous carbon matrix in a reaction device, heating it in a protective gas environment, using the protective gas as a carrier gas to bring a mixture of silicon source gas and phosphorus source gas into the reaction device, maintaining the temperature, so that the silicon element decomposed by the silicon source gas is deposited in the pores of the porous carbon matrix and grows into nano-silicon particles, while the phosphorus element decomposed by the phosphorus source gas is deposited in the pores of the porous carbon matrix, and the phosphorus element forms Si-P bonds with some silicon elements to obtain phosphorus-doped silicon-carbon material;

[0051] The reaction equipment includes any one of the following: fluidized bed, rotary furnace, and chemical vapor deposition furnace;

[0052] The protective gas includes nitrogen and / or argon; the flow rate of the protective gas is 5 L / min to 50 L / min.

[0053] The heating temperature is 400℃~800℃, preferably 500℃, and the holding time is 3 hours~8 hours;

[0054] The porous carbon matrix has an average particle size D50 between 1 μm and 15 μm and a specific surface area of ​​800 m². 2 / g~2500m 2 The average pore size of the porous carbon matrix is ​​between 1 nm and 50 nm, and the pore volume is between 0.8 cm³ / g. 3 / g~1.5cm 3 / g;

[0055] The silicon source gas includes one or more of the following: silane, silane, propane, and silicon tetrachloride; the flow rate of the silicon source gas is 5 L / min to 50 L / min.

[0056] The phosphorus source gas includes: phosphorus hydrogenation, or phosphorus-containing gas generated by heating a solid phosphorus source; the solid phosphorus source includes: phosphorus pentoxide, phosphate, alkyl phosphate ester or one or more; the flow rate of the phosphorus source gas is 1L / min to 5L / min.

[0057] Step S2: The phosphorus-doped silicon-carbon material is subjected to carbon coating treatment to form a carbon coating layer on the outer surface of the phosphorus-doped silicon-carbon material, thereby obtaining the intermediate material.

[0058] Among them, carbon coating treatment is a known method, including any one of gas phase carbon coating, liquid phase carbon coating or solid phase carbon coating;

[0059] The carbon source gases for gas-phase carbon coating include one or more of the following: methane, ethane, propane, acetylene, propyne, butyne, propylene, and ethylene; the gas-phase carbon coating temperature is 300℃~1000℃, and the deposition time is 1 hour~8 hours.

[0060] Step S3: Grind the intermediate material with cobalt trifluoride to coat the surface of the intermediate material with cobalt trifluoride, and obtain the precursor material.

[0061] The mass ratio of cobalt trifluoride to the intermediate material is 0.1:1 to 4:1, and can be any ratio within this range, such as 0.1:1, 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, etc., but is not limited to the listed ratios. Other unlisted ratios within this range are also applicable.

[0062] The grinding equipment for the grinding process includes any one of the following: ball mill, horizontal mill, vertical mill, planetary mill, and sand mill; the speed of the grinding equipment is 500 rpm to 2000 rpm; and the grinding time is 0.5 hours to 24 hours.

[0063] Step S4: Heat-treat the precursor material so that the fluorine gas generated by the thermal decomposition of cobalt trifluoride attacks the defect sites of the carbon coating layer, causing F atoms to occupy the positions of C atoms and form CF bonds. At the same time, the Si-P bonds on the surface of the porous carbon matrix form Si-PC bond structures with the carbon in the carbon coating layer in situ, thus obtaining a silicon-carbon composite anode material with high efficiency and high capacity.

[0064] The heat treatment includes: placing the precursor material in a high-temperature furnace under a protective atmosphere, heating it to 300℃~450℃ at a heating rate of 1℃ / min~5℃ / min, holding it at that temperature for 2 hours~10 hours, and then allowing it to cool naturally to room temperature.

[0065] The heat treatment temperature can be any temperature value within the range of 300℃ to 450℃, such as: 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, etc., but is not limited to the listed temperature values. Other unlisted temperatures within this temperature range are also applicable.

[0066] High-temperature furnaces include: box furnaces, tube furnaces, rotary furnaces, or fluidized beds; protective atmosphere gases include: nitrogen and / or argon.

[0067] The high-efficiency, high-capacity silicon-carbon composite anode material provided in this invention can be used as an anode active material to prepare anode sheets, which can be assembled into lithium-ion batteries.

[0068] To better understand the technical solution provided by this invention, the following uses several specific examples to illustrate the preparation process and characteristics of the high-efficiency, high-capacity silicon-carbon composite anode material of this invention.

[0069] Example 1

[0070] This embodiment provides a preparation process for a high-efficiency, high-capacity silicon-carbon composite anode material, the specific process of which is as follows.

[0071] (1) Preparation of phosphorus-doped silicon-carbon material includes: placing 5 kg of porous carbon matrix in the chamber of a chemical vapor deposition furnace, heating to 500 °C in a nitrogen atmosphere, and introducing a mixed gas of silane with a flow rate of 23 L / min and phosphine with a flow rate of 3 L / min into the chamber using nitrogen with a flow rate of 25 L / min as the carrier gas, and holding at the temperature for 5 hours for vapor deposition, so that the silicon element decomposed from silane is deposited in the pores of the porous carbon matrix and grows into nano-silicon particles, while the phosphorus element decomposed from phosphine is deposited in the pores of the porous carbon matrix, and the phosphorus element on the surface of the porous carbon matrix forms SiP bonds with some silicon elements to obtain phosphorus-doped silicon-carbon material.

[0072] The porous carbon matrix has an average particle size D50 of 8.5 μm and a specific surface area of ​​2000 m². 2 / g, the pore size is between 3nm and 10nm, and the pore volume is 1.4cm³. 3 / g. The carbon content of the phosphorus-doped silicon-carbon material prepared in this step was determined to be 35 wt% using a combustion method and a carbon-sulfur analyzer.

[0073] (2) The phosphorus-doped silicon-carbon material is coated with carbon in the gas phase. The specific process is as follows: Under a nitrogen atmosphere, the temperature of the chemical vapor deposition furnace is raised to 500°C. Acetylene with a flow rate of 6L / min is introduced into the chemical vapor deposition furnace by using nitrogen with a flow rate of 6L / min as the carrier gas. The furnace is kept at the temperature for 6 hours, so that the carbon elements decomposed by acetylene are deposited on the outer surface of the phosphorus-doped silicon-carbon material to form a carbon coating layer, thus obtaining the intermediate material.

[0074] (3) Cobalt trifluoride and intermediate material are placed in a ball mill at a mass ratio of 0.5:1 and ground at 800 rpm for 1 hour to coat the surface of the intermediate material with cobalt trifluoride, thus obtaining the precursor material.

[0075] (4) The precursor material is placed in a box furnace and heated to 400°C at a heating rate of 5°C / min under an argon atmosphere. It is then held at this temperature for 6 hours for heat treatment. This allows the fluorine gas generated by cobalt trifluoride to attack the defect sites of the carbon coating layer, causing F atoms to occupy the positions of C atoms and form CF bonds in situ. At the same time, the Si-P bonds on the surface of the porous carbon matrix and the carbon in the carbon coating layer form Si-PC bond structures in situ. After that, the material is allowed to cool naturally to room temperature to obtain a silicon-carbon composite anode material with high initial efficiency and high capacity.

[0076] The specific surface area of ​​the silicon-carbon composite anode material prepared in this embodiment was determined by nitrogen adsorption method.

[0077] The resistivity of the silicon-carbon composite anode material prepared in this embodiment was tested at 100 MPa using a four-probe resistivity meter.

[0078] The silicon-carbon composite anode material prepared in this embodiment was used to fabricate an electrode sheet and assembled into a coin cell for electrochemical testing, as detailed below:

[0079] Electrode preparation: Silicon-carbon composite negative electrode material, conductive carbon black, and binder were mixed uniformly at a mass ratio of 90:5:5, wherein the binder was sodium carboxymethyl cellulose and styrene-butadiene rubber at a mass ratio of 1:0.8. The mixture was prepared into a slurry at room temperature. The slurry was coated onto copper foil to a thickness of 220 μm and dried in a forced-air drying oven at 55°C for 2 hours. The coated electrode was then cut into circular electrodes with a diameter of 14 mm and dried under vacuum at 100°C for 8 hours. The dried electrode was then immediately transferred to a glove box filled with a high-purity argon atmosphere for later use.

[0080] Assembly of button half-cells: The assembly is carried out in a glove box filled with high-purity argon atmosphere. A lithium metal sheet is used as the counter electrode. A solution of LiPF6 with a molar concentration of 1 mol / L in ethylene carbonate (EC) / dimethyl carbonate (DMC) / diethyl carbonate (DEC) (where the volume ratio of EC, DMC, and DEC is 1:1:1) is used as the electrolyte. A polyethylene (PE) membrane is used as the separator. The cells are then assembled into a battery.

[0081] Coin cell testing: Constant current charge-discharge mode testing was performed using a charge-discharge apparatus. The discharge cutoff voltage was 0.005V, and the charge cutoff voltage was 2V. Routine charge-discharge tests were conducted at a current density of C / 10. The test data included the discharge specific capacity (lithium intercalation capacity), the charge specific capacity to 0.8V (referred to as 0.8V capacity), and the first-cycle coulombic efficiency (0.8V first-cycle efficiency).

[0082] The button cell expansion test measures the ratio of the thickness difference of the electrode before and after discharge to the electrode thickness before discharge. The electrode thickness before the cycle is measured using a micrometer and recorded as A1. After the battery is assembled and the first discharge cycle is tested, the battery is disassembled and the electrode thickness A2 is measured. The expansion rate of the button cell (abbreviated as button cell expansion rate) is calculated using the formula (A2-A1) / A1×100%.

[0083] The withstand voltage test evaluates the electrochemical performance of the sample electrodes before and after pressure testing, including the capacity and first-time efficiency after pressure testing. This involves applying pressure to the dried electrodes using a roller press before assembling and testing the cells. The withstand voltage test conditions are: applying pressure of 50 MPa for 5 minutes, followed by testing the performance of the coin cells.

[0084] The silicon-carbon composite anode material prepared in this embodiment was used to fabricate an electrode sheet and assemble it into a pouch cell for electrochemical testing, as detailed below:

[0085] The material prepared in this embodiment is used as a negative electrode material for lithium-ion batteries to prepare negative electrode sheets. The process is as follows: 90 wt% of silicon-carbon composite negative electrode material, 5 wt% of carbon black, 2 wt% of sodium cellulate, and 3 wt% of styrene-butadiene rubber are uniformly mixed in deionized water to form a battery slurry, which is then coated on copper foil, dried, and cut into 8*8 mm square sheets. After vacuum drying at 110°C for 12 hours, the sheets are transferred to an argon-filled glove box for later use.

[0086] The positive electrode preparation process is as follows: 95.2 wt% lithium cobalt oxide, 2.8 wt% conductive agent (Super P) and 2 wt% binder (PVDF) are mixed with an appropriate amount of NMP, ground into a slurry, coated on aluminum foil, dried, cut into 8*8 mm square sheets, and placed in an argon-filled glove box for later use.

[0087] Assembly of the full cell: A 5085 type small pouch cell was assembled using 11 negative electrode plates and 10 positive electrode plates. The electrolyte was LiPF6 with a molar concentration of 1 mol / L (the solvent was ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1). The separator was a Celgard 2500 separator, and assembly was performed using conventional methods.

[0088] Full battery testing process: Under the voltage conditions of charging cut-off voltage of 4.25V and discharging cut-off voltage of 2.75V, the capacity retention rate was tested after 100 cycles at a current density of 0.1C.

[0089] The test data for Example 1 above are detailed in Table 1.

[0090] Example 2

[0091] This embodiment provides a preparation process for a high-efficiency, high-capacity silicon-carbon composite anode material. The difference from Embodiment 1 is that the mass ratio of cobalt trifluoride to intermediate material in step (3) is 1:1. The other preparation processes are the same as in Embodiment 1.

[0092] The silicon-carbon composite anode material prepared in this embodiment was tested using the same method as in Example 1. The test data are detailed in Table 1.

[0093] Example 3

[0094] This embodiment provides a preparation process for a high-efficiency, high-capacity silicon-carbon composite anode material. The difference from Embodiment 1 is that the mass ratio of cobalt trifluoride to intermediate material in step (3) is 2:1. The other preparation processes are the same as in Embodiment 1.

[0095] The silicon-carbon composite anode material prepared in this embodiment was tested using the same method as in Example 1. The test data are detailed in Table 1.

[0096] Example 4

[0097] This embodiment provides a preparation process for a high-efficiency, high-capacity silicon-carbon composite anode material. The difference from Embodiment 1 is that the mass ratio of cobalt trifluoride to intermediate material in step (3) is 4:1. The other preparation processes are the same as in Embodiment 1.

[0098] The silicon-carbon composite anode material prepared in this embodiment was tested using the same method as in Example 1. The test data are detailed in Table 1.

[0099] Example 5

[0100] This embodiment provides a preparation process for a high-efficiency, high-capacity silicon-carbon composite anode material. The difference from Embodiment 1 is that the heat treatment time in step (4) is 3 hours, while the other preparation processes are the same as in Embodiment 1.

[0101] The silicon-carbon composite anode material prepared in this embodiment was tested using the same method as in Example 1. The test data are detailed in Table 1.

[0102] Example 6

[0103] This embodiment provides a preparation process for a high-efficiency, high-capacity silicon-carbon composite anode material. The difference from Embodiment 1 is that the heat treatment time in step (3) is 9 hours, while the other preparation processes are the same as in Embodiment 1.

[0104] The silicon-carbon composite anode material prepared in this embodiment was tested using the same method as in Example 1. The test data are detailed in Table 1.

[0105] To better illustrate the effects of the embodiments of the present invention, Comparative Example 1 is compared with the above embodiments.

[0106] Comparative Example 1

[0107] This comparative example uses the intermediate material prepared in step (2) of Example 1 as the reference material, namely the phosphorus-doped silicon-carbon material containing a carbon coating layer.

[0108] The comparative materials of this comparative example were tested using the same methods as in Example 1. The test data are detailed in Table 1.

[0109] Comparative Example 2

[0110] This comparative example provides a preparation process for a silicon-carbon anode material. Unlike Example 1, step (3) is omitted, and the material is not mixed with cobalt trifluoride. Consequently, no CF bonds are formed in the carbon coating layer during subsequent heat treatment. The specific process is as follows.

[0111] (1) The process for preparing phosphorus-doped silicon-carbon materials is the same as in Example 1.

[0112] (2) The preparation of intermediate materials is the same as in Example 1.

[0113] (3) The intermediate material is placed in a box furnace and heated to 400°C at a heating rate of 5°C / min under an argon atmosphere. It is then held at the temperature for 6 hours for heat treatment, so that the SiP bond on the surface of the porous carbon matrix and the carbon in the carbon coating layer form a SiPC bond structure in situ. After that, it is naturally cooled to room temperature to obtain the silicon-carbon anode material.

[0114] The silicon-carbon anode material prepared in this comparative example was tested using the same method as in Example 1. The test data are detailed in Table 1.

[0115] Comparative Example 3

[0116] This comparative example provides a preparation process for a silicon-carbon anode material. Unlike Example 1, it does not involve phosphorus doping and does not form a Si-PC bond structure. The specific process is as follows.

[0117] (1) Preparation of silicon-carbon materials, including: placing 5 kg of porous carbon matrix in the chamber of a chemical vapor deposition furnace, heating to 500 °C under nitrogen atmosphere, introducing silane at a flow rate of 23 L / min into the chamber using nitrogen at a flow rate of 25 L / min as the carrier gas, holding at the temperature for 5 hours for vapor deposition, so that silicon elements decomposed from silane are deposited in the pores of the porous carbon matrix and grow into nano-silicon particles, thus obtaining silicon-carbon materials. The average particle size D50 of the porous carbon matrix is ​​approximately 8.5 μm, and the specific surface area is approximately 2000 m². 2 / g, with an average pore size between 3nm and 10nm and a pore volume of 1.5cm³. 3 / g. The carbon content in the phosphorus-doped silicon-carbon material is 38% wt%.

[0118] (2) The silicon-carbon material is coated with carbon in the gas phase. The specific process is as follows: Under a nitrogen atmosphere, the temperature of the chemical vapor deposition furnace is raised to 500°C. Acetylene with a flow rate of 3L / min is introduced into the chemical vapor deposition furnace through nitrogen. The furnace is kept at the temperature for 6 hours, so that the carbon elements decomposed by acetylene are deposited on the outer surface of the silicon-carbon material to form a carbon coating layer, thus obtaining silicon-carbon composite particles.

[0119] (3) Cobalt trifluoride and silicon-carbon composite particles are placed in a ball mill at a mass ratio of 0.5:1 and ground at 800 rpm for 1 hour to coat the surface of the silicon-carbon composite particles with cobalt trifluoride, thus obtaining the precursor material.

[0120] (4) The precursor material is placed in a box furnace and heated to 400°C at a heating rate of 5°C / min under an argon atmosphere. It is then held at the temperature for 6 hours for heat treatment, so that the fluorine gas generated by cobalt trifluoride attacks the defect sites of the carbon coating layer, causing F atoms to occupy the positions of C atoms and form CF bonds in situ. After that, it is naturally cooled to room temperature to obtain silicon-carbon anode material.

[0121] The silicon-carbon anode material prepared in this comparative example was tested using the same method as in Example 1. The test data are detailed in Table 1.

[0122] Comparative Example 4

[0123] This comparative example provides a preparation process for a silicon-carbon anode material, which involves performing a gas-phase carbon coating treatment on the high-efficiency, high-capacity silicon-carbon composite anode material prepared in Example 1, and then coating the outer surface of the composite coating layer of the silicon-carbon composite anode material in Example 1 with another carbon layer.

[0124] The specific coating process is as follows: Under a nitrogen atmosphere, the silicon-carbon composite anode material prepared in Example 1 is placed in a chemical vapor deposition furnace and heated to 500°C. Acetylene with a flow rate of 3 L / min is introduced into the chemical vapor deposition furnace through nitrogen gas and kept at this temperature for 2 hours, so that the carbon elements decomposed by acetylene are deposited on the outer surface of the silicon-carbon composite anode material of Example 1 to form a carbon coating layer, thereby obtaining the silicon-carbon anode material.

[0125] The silicon-carbon anode material prepared in this comparative example was tested using the same method as in Example 1. The test data are detailed in Table 1.

[0126] Table 1 summarizes the test data for Examples 1-6 and Comparative Examples 1-4:

[0127]

[0128] Table 1

[0129] The test data in Table 1 shows that:

[0130] Comparison of Comparative Example 1 with Examples 1-6 shows that the first-cycle coulombic efficiency of the silicon-carbon anode material is improved, the electrode expansion rate is reduced, and the capacity retention rate after 100 cycles is increased after fluorination heat treatment. Fluorination heat treatment can form a LiF-rich SEI film on the surface of silicon-carbon material. The formation of LiF can reduce the thickness of the SEI film. The fluorine-containing components in the SEI layer can change the interface stability, improve the first-cycle coulombic efficiency and cycle structure stability of the nano-silicon-carbon material, and help reduce the volume expansion rate of the material during cycling.

[0131] The comparison of Examples 1-4 shows that the amount of cobalt trifluoride fluorinating agent used in the fluorination heat treatment process has a significant impact on the first-cycle coulombic efficiency and battery expansion of the silicon-carbon composite material. This is because when the amount of fluorinating agent is too low, the degree of fluorination of the surface carbon layer is low, the LiF content is too low when SE I is formed, and the improvement in the first-cycle efficiency of silicon-carbon is not significant. When the amount of fluorinating agent is too high, the fluorination heat treatment process will etch the nano-silicon particles in the silicon-carbon material, causing capacity loss of the material.

[0132] As can be seen from the comparison between Comparative Example 2 and Example 1, a single heat treatment cannot significantly improve the first-cycle coulombic efficiency and capacity retention rate after 100 cycles of silicon-carbon materials.

[0133] The comparison of Examples 1, 5 and 6 shows that extending the fluorination heat treatment time does not significantly improve the coulombic efficiency of silicon-carbon materials in the first week, but it does improve the capacity retention rate after 100 cycles. The optimal fluorination heat treatment time is 6 hours.

[0134] As can be seen from the comparison between Comparative Example 3 and Example 1, the Example further improved the conductivity, first-cycle coulombic efficiency and cycle capacity retention of silicon-carbon materials through Si and P co-deposition.

[0135] As can be seen from the comparison between Comparative Example 4 and Example 1, after fluorination heat treatment and acetylene coating in Comparative Example 4, the first-week coulombic efficiency, capacity retention rate and electrode expansion rate of silicon-carbon material are significantly reduced. This is because the fluorinated carbon layer on the surface is covered by the coated carbon layer, which is not conducive to the formation of the Li-F-rich SE I film and reduces the stability of the SE I structure.

[0136] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. 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 silicon-carbon composite negative electrode material with high initial efficiency and high capacity, characterized in that, The silicon-carbon composite anode material includes: a phosphorus-doped silicon-carbon material, and a composite coating layer covering the outer surface of the phosphorus-doped silicon-carbon material, wherein the phosphorus-doped silicon-carbon material and the composite coating layer have a Si-PC bond structure. The phosphorus-doped silicon-carbon material comprises: a porous carbon matrix, and nano-silicon particles and phosphorus elements deposited in the pores of the porous carbon matrix; The composite coating layer includes: a carbon coating layer, and CF bonds formed in situ in the carbon coating layer. 2.The silicon-carbon composite negative electrode material with high initial efficiency and high capacity according to claim 1, characterized in that, The average particle size D50 of the porous carbon matrix is ​​between 1 μm and 15 μm. The specific surface area of the porous carbon matrix is between 800 m 2 / g and 2500 m 2 / g. The average pore size of the porous carbon matrix is ​​between 1 nm and 50 nm. The porous carbon matrix has a pore volume of 0.8 cm 3 / g to 1.5 cm 3 / g. 3.The silicon-carbon composite negative electrode material with high initial efficiency and high capacity according to claim 1, characterized in that, The percentage of the nano-silicon particles in the total mass of the silicon-carbon composite anode material is 50% to 65%. The phosphorus element in the silicon-carbon composite anode material accounts for 0.5% to 10% of the total mass of the silicon-carbon composite anode material; The percentage of fluorine in the silicon-carbon composite anode material is 0.5% to 10% of the total mass of the silicon-carbon composite anode material; The thickness of the composite coating layer is 2nm to 50nm.

4. A method for preparing a high-efficiency, high-capacity silicon-carbon composite anode material as described in any one of claims 1-3, characterized in that, The preparation method includes: Step S1, preparing phosphorus-doped silicon-carbon material, includes: placing a porous carbon matrix in a reaction device, heating it in a protective gas environment, using the protective gas as a carrier gas to introduce a mixture of silicon source gas and phosphorus source gas into the reaction device, maintaining the temperature, so that silicon elements decomposed from the silicon source gas are deposited in the pores of the porous carbon matrix and grow into nano-silicon particles, while phosphorus elements decomposed from the phosphorus source gas are deposited in the pores of the porous carbon matrix, and the phosphorus elements form Si-P bonds with some silicon elements to obtain phosphorus-doped silicon-carbon material; Step S2: The phosphorus-doped silicon-carbon material is subjected to carbon coating treatment to form a carbon coating layer on the outer surface of the phosphorus-doped silicon-carbon material, thereby obtaining an intermediate material. Step S3: Grind the intermediate material with cobalt trifluoride to coat the surface of the intermediate material with cobalt trifluoride, thereby obtaining the precursor material; Step S4: The precursor material is heat-treated so that the fluorine gas generated by the thermal decomposition of cobalt trifluoride attacks the defect sites of the carbon coating layer, causing F atoms to occupy the positions of C atoms and form CF bonds. At the same time, the Si-P bonds on the surface of the porous carbon matrix form Si-PC bond structures with the carbon in the carbon coating layer in situ, thus obtaining a silicon-carbon composite anode material with high initial efficiency and high capacity.

5. The preparation method according to claim 4, characterized in that, The reaction equipment includes any one of the following: fluidized bed, rotary furnace, and chemical vapor deposition furnace; The protective gas includes nitrogen and / or argon; the flow rate of the protective gas is 5 L / min to 50 L / min. The temperature for heating is 400℃ to 800℃, and the holding time is 3 hours to 8 hours.

6. The preparation method according to claim 4, characterized in that, The average particle size D50 of the porous carbon matrix is between 1 μm and 15 μm, the specific surface area is between 800 m 2 / g and 2500 m 2 / g; the average pore size of the pores of the porous carbon matrix is between 1 nm and 50 nm, the pore volume is between 0.8 cm 3 / g and 1.5 cm 3 / g. The silicon source gas includes one or more of the following: silane, disilane, propane, and silicon tetrachloride; the flow rate of the silicon source gas is 5 L / min to 50 L / min. The phosphorus source gas includes: phosphorus hydrogenation, or phosphorus-containing gas generated by heating a solid phosphorus source; the solid phosphorus source includes: phosphorus pentoxide, phosphate, alkyl phosphate ester or one or more; the flow rate of the phosphorus source gas is 1L / min to 5L / min.

7. The preparation method according to claim 4, characterized in that, The carbon coating treatment includes any one of gas phase carbon coating, liquid phase carbon coating or solid phase carbon coating. The carbon source gas for the gaseous carbon coating includes one or more of the following: methane, ethane, propane, acetylene, propyne, butyne, propylene, and ethylene; the gaseous carbon coating temperature is 300℃~1000℃, and the deposition time is 1 hour~8 hours.

8. The preparation method according to claim 4, characterized in that, The mass ratio of cobalt trifluoride to the intermediate material is 0.1:1 to 4:1; The grinding equipment for the grinding process includes any one of the following: ball mill, horizontal mill, vertical mill, planetary mill, and sand mill; the rotation speed of the grinding equipment is 500 rpm to 2000 rpm; and the grinding time is 0.5 hours to 24 hours.

9. The preparation method according to claim 4, characterized in that, The heat treatment includes: placing the precursor material in a high-temperature furnace under a protective atmosphere, heating it to 300℃ to 450℃ at a heating rate of 1℃ / min to 5℃ / min, holding it at that temperature for 2 hours to 10 hours, and then allowing it to cool naturally to room temperature; the protective atmosphere includes nitrogen and / or argon.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the high-efficiency, high-capacity silicon-carbon composite anode material as described in any one of claims 1-3.

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