Fast-charging silicon negative electrode material, and preparation method therefor and use thereof

Abstract

A fast-charging silicon negative electrode material, and a preparation method therefor and the use thereof. The fast-charging silicon negative electrode material comprises silicon-containing particles, a first coating layer coating the outside of the silicon-containing particles and a second coating layer coating the outside of the first coating layer, wherein the first coating layer comprises fluorine, and the second coating layer comprises phosphorus.

Description

A fast-charging silicon negative electrode material and a preparation method and application thereof

[0001] The present application claims priority to the Chinese patent application No. 2024110987738 filed on August 9, 2024 with the China Patent Office, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium batteries, in particular to a fast-charging silicon negative electrode material and a preparation method and application thereof. BACKGROUND

[0003] In the development process of new energy vehicles, in order to alleviate the problem of long charging time, developing fast-charging technology has become one of the main directions of the industry. Under this background, using high power to charge the vehicle power battery has become a trend in market application, and among them, the high rate and high performance power battery research has become a key element to promote the development of the industry because the high rate charging method can further reduce the required charging time.

[0004] Compared with many industries, it is found that the charging rate of small power batteries in the consumer field can often reach more than 10C, while in the automotive field, the charging rate of power batteries is mainly 1C. Currently, commercial fast charging is mainly concentrated in the graphite system. TECHNICAL PROBLEM

[0005] (1) Due to the anisotropy of the graphite layer and the narrow interlayer spacing, lithium ions can only move parallelly between the graphite layers, and cannot move vertically, which reduces the diffusion coefficient of lithium ions; (2) When lithium ions are inserted into the graphite layer, they enter from the edge of the layered structure, which makes the diffusion path of lithium ions longer, and the longer diffusion path reduces the rate performance of lithium ion batteries; (3) The graphite layers are connected by weak van der Waals forces, resulting in poor stability of the graphite structure. During the process of lithium ion insertion into graphite, solvent molecules are inserted, causing the exfoliation of the graphite layers; (4) Under fast-charging conditions, the lithium insertion potential of the graphite layer will approach the lithium deposition potential (lithium precipitation effect), thereby reducing the cycle performance of the fast-charging lithium battery. TECHNICAL SOLUTION

[0006] In a first aspect, the present application provides a fast-charging silicon negative electrode material, which adopts the following technical solution:

[0007] A fast-charging silicon negative electrode material, comprising silicon-containing particles, a first coating layer coated outside the silicon-containing particles, and a second coating layer coated outside the first coating layer; the first coating layer comprises fluorine elements, and the second coating layer comprises phosphorus elements.

[0008] In a second aspect, the present application provides a preparation method of a fast-charging silicon negative electrode material, which adopts the following technical solution:

[0009] A preparation method of a fast-charging silicon negative electrode material, comprising the following steps:

[0010] S1, mixing fluoride and a first organic solvent to obtain a fluoride-containing dispersion liquid; mixing a phosphorus-containing compound and a second organic solvent to obtain a phosphorus-containing dispersion liquid;

[0011] S2, mixing the silicon-containing particles and the fluoride-containing dispersion liquid to obtain a first mixed liquid, and stirring to obtain a first mixed liquid; drying the first mixed liquid to obtain a first coated product;

[0012] S3, mixing the first coated product and the phosphorus-containing dispersion liquid to obtain a second mixed liquid, and drying the second mixed liquid to obtain a second coated product, i.e. the fast-charging silicon negative electrode material.

[0013] In a third aspect, the application provides a negative electrode sheet, which adopts the following technical solution:

[0014] A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer comprises the fast-charging silicon negative electrode material as described above.

[0015] In a fourth aspect, the application provides a lithium ion battery, which adopts the following technical solution:

[0016] A lithium ion battery comprises a positive electrode, a separator, an electrolyte and a negative electrode sheet as described above. Advantages

[0017] First, in the present application, silicon-containing particles are used as the main body of the silicon negative electrode material, which has a high carrier concentration and mobility, and in the case of achieving high lithium storage, lithium ions have excellent diffusion rate, thereby improving the conductivity of the silicon negative electrode material. The high-silicon negative electrode material of the present application has a high energy density, which can improve the fast-charging performance of the lithium battery while also improving the long-lasting performance of the lithium battery.

[0018] Second, by doping fluorine elements in the first coating layer and doping phosphorus elements in the second coating layer, on the one hand, the first coating layer is in contact with the silicon-containing particles, wherein the doped fluorine elements can react with the hydroxyl groups on the surface of the silicon-based material to form hydrogen bonds. The presence of hydrogen bonds can improve the structural stability and flexibility of the negative electrode surface SEI film, thereby improving the conductivity of the silicon negative electrode material. In particular, the silicon negative electrode material can more stably withstand the volume change of the silicon-containing particles during the lithium ion deintercalation process, thereby achieving better fast-charging cycle stability performance. On the other hand, the second coating layer can absorb HF in the electrolyte, which also helps to isolate the silicon-containing particles from the electrolyte and improve the stability of the silicon negative electrode material.

[0019] Thirdly, the double-layer coating structure formed by the second coating layer and the first coating layer can improve the coating completeness of the silicon negative electrode material, avoid the contact between the coated silicon-containing particles and the electrolyte, thereby inhibiting the formation of side reactions and resistive surface films, and help to improve the structural stability of the fast-charging silicon negative electrode material. Fourthly, during the charging and discharging process, the fast-charging lithium battery necessarily generates more heat compared to the ordinary lithium battery, and the first coating layer and the second coating layer can decompose to generate fluorine radicals and phosphorus radicals at a certain temperature, thereby improving the thermal stability and flame retardant performance of the lithium battery, and helping to improve the fast-charging safety of the lithium battery. Embodiments of the present application

[0020] In some embodiments, the ratio of the mass of silicon element in the silicon-containing particles to the mass of fluorine element in the first coating layer is 5-20:1.

[0021] The silicon-containing particles will expand during the charging and discharging process as lithium ions are deintercalated. Generally, the higher the silicon content, the more significant the expansion effect of the silicon-containing particles. By controlling the ratio of the mass of silicon element in the silicon-containing particles to the mass of fluorine element in the first coating layer, the content of fluorine element is controlled within a reasonable range, which helps to form an SEI film with a certain degree of flexibility, thereby sufficiently relieving the volume expansion of the silicon-containing particles with the above-mentioned silicon content during the charging and discharging process, and further improving the stability of the coating structure, which helps to improve the fast-charging cycle performance of the lithium battery.

[0022] In some embodiments, the ratio of the mass of fluorine element in the first coating layer to the mass of phosphorus element in the second coating layer is 1-3:1.

[0023] By controlling the mass of fluorine element in the first coating layer and the mass of phosphorus element in the second coating layer, on the one hand, it helps to improve the conductivity of the coating layer, and on the other hand, the combination of fluorine element in the first coating layer and phosphorus element in the second coating layer can form a synergistic effect, significantly improving the thermal stability and flame retardant performance of the fast-charging silicon negative electrode material, improving the structural stability of the silicon negative electrode material, and helping to improve the fast-charging performance of the lithium battery.

[0024] In some embodiments, the particle size D50 of the fast-charging silicon negative electrode material is 5-10 μm.

[0025] Controlling the particle size D50 of the silicon-containing particles within the above range helps to improve the coating effect of the first coating layer and the second coating layer. When the particle size D50 of the silicon-containing particles is too small, the coating effect of the first coating layer and the second coating layer is poor, which is not conducive to the improvement of the fast-charging performance of the silicon negative electrode material. When the particle size D50 of the silicon-containing particles is too large, the coating thickness of the first coating layer and the second coating layer will increase to a certain extent, which will lead to an increase in ion transmission distance and interface reaction polarization, thereby reducing the rate performance of the lithium battery.

[0026] In some embodiments, the first coating layer has a thickness of 5-15 nm, and / or the second coating layer has a thickness of 2-10 nm.

[0027] By controlling the thickness of the first coating layer and the second coating layer, the first coating layer and the second coating layer cooperate to form a good ion passage, which is conducive to the migration and diffusion of lithium ions in the charging and discharging process, can keep the migration coefficient of lithium ions in a good range, and helps to improve the rate performance of the lithium battery.

[0028] In some embodiments, the silicon-containing particles include at least one of silicon monoxide, silicon carbon, and silicon particles.

[0029] In some embodiments, in the first coating layer, the fluoride includes at least one of CeF3, AlF3, MgF2, and LaF3.

[0030] In some embodiments, in the second coating layer, the phosphorus-containing compound includes at least one of lithium phosphate and calcium phosphate.

[0031] In a second aspect, the present application provides a preparation method of a fast-charging silicon negative electrode material, which adopts the following technical scheme:

[0032] A preparation method of a fast-charging silicon negative electrode material, including the following steps:

[0033] S1, mixing fluoride and a first organic solvent to obtain a fluoride-containing dispersion liquid; mixing a phosphorus-containing compound and a second organic solvent to obtain a phosphorus-containing dispersion liquid;

[0034] S2, mixing the silicon-containing particles and the fluoride-containing dispersion liquid, and stirring to obtain a first mixed liquid; drying the first mixed liquid to obtain a first coating product;

[0035] S3, mixing the first coating product and the phosphorus-containing dispersion liquid to obtain a second mixed liquid; drying the second mixed liquid to obtain a second coating product, i.e., the fast-charging silicon negative electrode material.

[0036] In some embodiments, in the S2, the temperature during stirring is 30-100℃, and the stirring time is 5-10h; the drying process is spray drying, the inlet temperature of the spray drying is 100-200℃, and the outlet temperature is 60-90℃; in the S3, the drying process is spray drying, the inlet temperature of the spray drying is 100-200℃, and the outlet temperature is 60-90℃.

[0037] In some embodiments, the first organic solvent and the second organic solvent are independently selected from at least one of N,N-dimethylformamide, dimethyl sulfoxide, toluene, and dichloromethane.

[0038] Example 1

[0039] 1. Preparation of fast-charging silicon negative electrode material

[0040] S1, mix the fluorine source (CeF3) with the first organic solvent (N,N- dimethylformamide) to obtain a fluorine-containing dispersion liquid; mix the phosphorus source (lithium phosphate) with the second organic solvent (N,N- dimethylformamide) to obtain a phosphorus-containing dispersion liquid;

[0041] S2, mix the silicon-containing particles (silicon monoxide) with the above-mentioned fluorine-containing dispersion liquid, and stir at 80°C for 6h to obtain a first mixed liquid, and spray dry the above-mentioned first mixed liquid (the inlet temperature is 150°C, and the outlet temperature is 80°C) to obtain a first coated product;

[0042] S3, mix the above-mentioned first coated product with the above-mentioned phosphorus-containing dispersion liquid to obtain a second mixed liquid, and spray dry the above-mentioned second mixed liquid (the inlet temperature is 150°C, and the outlet temperature is 70°C) to obtain a second coated product, i.e. the fast-charging silicon negative electrode material.

[0043] The fast-charging silicon negative electrode material comprises silicon-containing particles with a particle size D50 of 7μm, a first coating layer with a thickness of 10nm coated on the silicon-containing particles, and a second coating layer with a thickness of 5nm coated on the first coating layer; the first coating layer comprises fluorine elements, and the second coating layer comprises phosphorus elements; the ratio of the mass of silicon elements in the silicon-containing particles to the mass of fluorine elements in the first coating layer is 10:1, and the ratio of the mass of fluorine elements in the first coating layer to the mass of phosphorus elements in the second coating layer is 2:1.

[0044] 2. Preparation of positive electrode sheet

[0045] Mix and stir the ternary material positive electrode active material (NCM811), polyvinylidene fluoride (PVDF) and conductive carbon black (SP) in a mass ratio of 94:3:3 to obtain a positive electrode slurry, then coat the positive electrode slurry on an aluminum foil through a coating process, and obtain a positive electrode sheet after drying and cold pressing.

[0046] 3. Preparation of negative electrode sheet

[0047] Dissolve the above-mentioned fast-charging silicon negative electrode material, conductive agent SP, conductive agent SWCNT and binder PAA in a solvent in a mass percentage of 80:9:1:10, control the solid content to be 30%, coat on a copper foil current collector, vacuum dry, and obtain a negative electrode sheet.

[0048] 4. Preparation of lithium battery

[0049] The soft package battery is assembled by using the above negative electrode sheet, the above positive electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (volume ratio of EC:DMC:EMC = 1:1:1) electrolyte, PE+aluminum oxide separator, and a conventional production process is used for the shell.

[0050] Example 2

[0051] 1. Preparation of fast-charging silicon negative electrode material

[0052] S1, a fluorine source (AlF3, MgF2, LaF3) is mixed with a first organic solvent (dimethyl sulfoxide) to obtain a fluorine-containing dispersion liquid; a phosphorus source (calcium phosphate) is mixed with a second organic solvent (dimethyl sulfoxide) to obtain a phosphorus-containing dispersion liquid;

[0053] S2, the silicon-containing particles (silicon-carbon particles) are mixed with the above-mentioned fluorine-containing dispersion liquid, and stirred at 40℃ for 10h to obtain a first mixed liquid, and the first mixed liquid is spray dried (the inlet temperature is 100℃, and the outlet temperature is 60℃) to obtain a first coated product;

[0054] S3, the first coated product is mixed with the above-mentioned phosphorus-containing dispersion liquid to obtain a second mixed liquid, and the second mixed liquid is spray dried (the inlet temperature is 100℃, and the outlet temperature is 60℃) to obtain a second coated product, i.e. a fast-charging silicon negative electrode material.

[0055] The fast-charging silicon negative electrode material includes silicon-containing particles with a particle size D50 of 5μm, a first coating layer with a thickness of 5nm coated on the silicon-containing particles, and a second coating layer with a thickness of 2nm coated on the first coating layer; the first coating layer includes fluorine elements, and the second coating layer includes phosphorus elements; the mass ratio of silicon elements in the silicon-containing particles to fluorine elements in the first coating layer is 5:1, and the mass ratio of fluorine elements in the first coating layer to phosphorus elements in the second coating layer is 1:1.

[0056] 2. Preparation of positive electrode sheet

[0057] The ternary material positive electrode active material (NCM811), polyvinylidene fluoride (PVDF), and conductive carbon black (SP) are mixed and stirred uniformly at a mass ratio of 94:3:3 to obtain a positive electrode slurry, and then the positive electrode slurry is coated on an aluminum foil through a coating process, and after drying and cold pressing, a positive electrode sheet is obtained.

[0058] 3. Preparation of negative electrode sheet

[0059] The above-mentioned fast-charging silicon negative electrode material, conductive agent SP, conductive agent SWCNT, and binder PAA are dissolved in a solvent at a mass percentage of 80:9:1:10, the solid content is controlled at 30%, and the mixture is coated on a copper foil current collector, vacuum dried, and a negative electrode sheet is prepared.

[0060] 4. Lithium battery preparation

[0061] The soft package battery was assembled by using the above negative electrode sheet, the above positive electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (volume ratio of EC:DMC:EMC = 1:1:1) electrolyte, PE+aluminum oxide separator, and a conventional production process was used for the shell.

[0062] Example 3

[0063] 1. Preparation of fast-charging silicon negative electrode material

[0064] S1, mix the fluorine source (MgF2) with the first organic solvent (toluene) to obtain a fluorine-containing dispersion liquid; mix the phosphorus source (lithium phosphate) with the second organic solvent (toluene) to obtain a phosphorus-containing dispersion liquid;

[0065] S2, mix the silicon-containing particles (silicon-carbon particles and silicon particles with a mass ratio of 1:1) with the above fluorine-containing dispersion liquid, and stir at 100°C for 5h to obtain a first mixed liquid. The first mixed liquid is spray dried (inlet temperature 200°C, outlet temperature 90°C) to obtain a first coated product;

[0066] S3, mix the first coated product with the above phosphorus-containing dispersion liquid to obtain a second mixed liquid. The second mixed liquid is spray dried (inlet temperature 200°C, outlet temperature 80°C) to obtain a second coated product, which is a fast-charging silicon negative electrode material.

[0067] The fast-charging silicon negative electrode material includes silicon-containing particles with a particle size D50 of 10 μm, a first coating layer with a thickness of 15 nm coated on the silicon-containing particles, and a second coating layer with a thickness of 10 nm coated on the first coating layer; the first coating layer contains fluorine elements, and the second coating layer contains phosphorus elements; the mass ratio of silicon elements in the silicon-containing particles to fluorine elements in the first coating layer is 20:1, and the mass ratio of fluorine elements in the first coating layer to phosphorus elements in the second coating layer is 3:1.

[0068] 2. Preparation of positive electrode sheet

[0069] Mix and stir the ternary material positive electrode active material (NCM811), polyvinylidene fluoride (PVDF), and conductive carbon black (SP) in a mass ratio of 94:3:3 to obtain a positive electrode slurry. Then, the positive electrode slurry is coated on an aluminum foil through a coating process, and after drying and cold pressing, a positive electrode sheet is obtained.

[0070] 3. Preparation of negative electrode sheet

[0071] The above fast-charging silicon negative electrode material, conductive agent SP, conductive agent SWCNT and binder PAA are dissolved in a solvent in a mass percentage of 80:9:1:10, the solid content is controlled at 30%, and the negative electrode sheet is coated on a copper foil current collector, vacuum dried, and prepared.

[0072] 4. Preparation of lithium battery

[0073] The above negative electrode sheet, the above positive electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (volume ratio of EC:DMC:EMC=1:1:1) electrolyte, PE+aluminum oxide separator, and shell are assembled into a soft package battery by using a conventional production process.

[0074] Example 4

[0075] The difference between this embodiment and Example 1 is that the ratio of the mass of silicon element in the silicon-containing particles to the mass of fluorine element in the first coating layer is 3:1; other steps and parameter settings remain the same as those in Example 1.

[0076] Example 5

[0077] The difference between this embodiment and Example 1 is that the ratio of the mass of silicon element in the silicon-containing particles to the mass of fluorine element in the first coating layer is 22:1; other steps and parameter settings remain the same as those in Example 1.

[0078] Example 6

[0079] The difference between this embodiment and Example 1 is that the ratio of the mass of fluorine element in the first coating layer to the mass of phosphorus element in the second coating layer is 0.2:1; other steps and parameter settings remain the same as those in Example 1.

[0080] Example 7

[0081] The difference between this embodiment and Example 1 is that the ratio of the mass of fluorine element in the first coating layer to the mass of phosphorus element in the second coating layer is 4:1; other steps and parameter settings remain the same as those in Example 1.

[0082] Example 8

[0083] The difference between this embodiment and Example 1 is that the particle size D50 of the fast-charging silicon negative electrode material is 3 μm; other steps and parameter settings remain the same as those in Example 1.

[0084] Example 9

[0085] The difference between this embodiment and Example 1 is that the particle size D50 of the fast-charging silicon negative electrode material is 12 μm; other steps and parameter settings remain the same as those in Example 1.

[0086] Example 10

[0087] The difference between this embodiment and embodiment 1 is that the thickness of the first coating layer is 3 nm, and the thickness of the second coating layer is 15 nm; other steps and parameter settings remain consistent with embodiment 1.

[0088] Embodiment 11

[0089] The difference between this embodiment and embodiment 1 is that the thickness of the first coating layer is 17 nm, and the thickness of the second coating layer is 1 nm; other steps and parameter settings remain consistent with embodiment 1.

[0090] Comparative Example 1

[0091] The difference between this comparative example and embodiment 1 is that the lithium battery in this comparative example is a commercial fast-charging lithium battery. Specifically, in the commercial fast-charging lithium battery in this comparative example, the positive active material is NCM811, and the negative active material is graphite.

[0092] Comparative Example 2

[0093] The difference between this comparative example and embodiment 1 is that the fast-charging silicon negative electrode material does not include the first coating layer and the second coating layer; other steps and parameter settings remain consistent with embodiment 1.

[0094] Comparative Example 3

[0095] The difference between this comparative example and embodiment 1 is that the fast-charging silicon negative electrode material does not include the first coating layer; other steps and parameter settings remain consistent with embodiment 1.

[0096] Comparative Example 4

[0097] The difference between this comparative example and embodiment 1 is that the fast-charging silicon negative electrode material does not include the second coating layer; other steps and parameter settings remain consistent with embodiment 1.

[0098] Comparative Example 5

[0099] The difference between this comparative example and embodiment 1 is that the first coating layer in the fast-charging silicon negative electrode material contains phosphorus elements, and the second coating layer contains fluorine elements; other steps and parameter settings remain consistent with embodiment 1.

[0100] Test Method

[0101] I. Rate performance test

[0102] The rate performance of the lithium batteries of the above examples and comparative examples was tested. The specific test method was as follows: at 25°C, 1C constant current discharge to 2.5V, standing for 10 min, 6C constant current constant voltage charging to 4.2V, 0.05C cutoff, recording the constant current charging capacity, constant current constant voltage charging total capacity and the highest temperature in the fast charging process, constant current charging ratio = constant current charging capacity / constant current constant voltage charging total capacity x 100%; the test results were recorded in Table 1.

[0103] II. Cycle performance test

[0104] The cycle performance of the lithium batteries of the above examples and comparative examples was tested. The specific test method was as follows: at 25°C, 2C constant current constant voltage charging to 4.2V, 0.05C cutoff, standing for 10 min, 2C constant current discharge to 2.5V, standing for 10 min, cycling for 500 times, recording the capacity retention rate of the lithium battery in 2C / 2C cycle for 500 times; the test results were recorded in Table 1.

[0105] III. ARC test

[0106] The ARC test of the lithium batteries of the above examples and comparative examples was performed. The specific test method was as follows: starting the ARC adiabatic thermal runaway test (the test sample was heated from room temperature to 45±2°C in the cavity, after standing for 90 min, the battery temperature rise rate was detected, if the temperature rise was more than 0.2°C in 10 min (i.e. SHR>0.02°C / min), it was considered that the self-heating reaction occurred in the battery, and the adiabatic environment was maintained until the thermal runaway of the battery occurred; if the temperature rise was not more than 0.2°C in 10 min (i.e. SHR≤0.02°C / min), the next step temperature rise test was continued; each temperature step was 5°C, and the steps were repeated at each step, the ARC test temperature range was 45-300°C, the self-heating starting temperature was T1 (the temperature rise rate SHR>0.02°C / min), and the thermal runaway starting temperature was T2 (the temperature rise rate SHR>1°C / min); the test results were recorded in Table 1.

[0107] Note: self-heating temperature rise rate (Self-heating Rate, SHR)

[0108] IV. Energy density test of lithium battery

[0109] The lithium battery in the above examples and comparative examples was subjected to energy density test. The specific test method was as follows: the weight of the test battery was weighed and recorded as m; the battery was placed in a clamp, a force of 3000N was applied, the single battery was charged at a current of 0.33C to 4.2V, and then was left for 30min, discharged at a current of 0.33C to 2.5V, and then was left for 30min, and the above process was continuously cycled for 3 times; the discharge capacity (in Ah) and energy E (average value of three cycles) were calculated, and the discharge energy density was E / m (in Wh / kg); the test results were recorded in Table 1.

[0110] V. Element content test

[0111] The silicon element content, fluorine element content and phosphorus element content in the fast-charging silicon negative electrode material prepared in the above examples and comparative examples were tested, and the test method was as follows:

[0112] The ion grinder can perform plane grinding and cross-section cutting on the sample by using argon ions; by using the ion beam grinding (CP) test principle, the argon ions are used to cut the section, and then X-ray energy spectrum analysis (EDS) is performed, and the mass ratio of silicon element to fluorine element and the mass ratio of fluorine element to phosphorus element are calculated.

[0113] VI. Coating layer thickness test

[0114] The first coating layer and the second coating layer thickness of the fast-charging silicon negative electrode material prepared in the above examples and comparative examples were subjected to TEM test, and the test method was as follows:

[0115] (1) The silicon negative electrode material sample prepared in the above examples and comparative examples was dispersed by ultrasonic vibration, the soft agglomeration was removed, and then the sample suspension was prepared by dispersing in water or other solvents;

[0116] (2) The copper mesh covered with carbon film or other polymer film was taken, the required amount was taken from the prepared sample suspension or sucked by a dropper, dropped on the copper mesh, and then dried by filter paper or air-dried, and then placed in the sample table for test;

[0117] (3) The dispersed place was photographed, and the size of the silicon-containing particles with complete morphology was arbitrarily selected; after the first coating layer coated the silicon-containing particles, the first coating layer thickness h1 was measured by TEM test; after the second coating layer coated the first coating layer, the total thickness of the first coating layer and the second coating layer was measured by TEM test and recorded as h; the second coating layer thickness h2, and the calculation formula of the second coating layer thickness was: h2=h-h1.

[0118] VII. Fast-charging time test

[0119] The lithium battery in Example 1 and Comparative Example 1 above was subjected to a fast charging time test. The test method was as follows: 1C constant current discharge to 2.5V at 25°C, static for 10 min, 1C constant current charging to 10% SOC, static for 10 min, and the time for 7C constant current charging to 80% SOC was the fast charging time.

[0120] The fast charging time of the lithium battery in Example 1 was 6 min, and the fast charging time of the lithium battery in Comparative Example 1 was 30 min.

[0121] Table 1

[0122]

[0123] In combination with Examples 1-3, Comparative Examples 1-5 and Table 1, it can be seen that the fast charging silicon negative electrode material containing silicon particles and a first coating layer containing fluorine elements and a second coating layer containing phosphorus elements prepared in the present application has excellent initial efficiency, rate capability, cycle capacity retention rate and thermal stability. Moreover, compared with the fast charging time of a commercial fast charging lithium battery ≥ 30 min and the energy density generally < 300 Wh.kg -1 , the fast charging time of the fast charging lithium battery in the present application is only 6 min (charging interval: 10% SOC-80% SOC), and the energy density can be as high as 347.4 Wh.kg -1 .

[0124] This is because the fast charging silicon negative electrode material in the present application not only can improve the diffusion rate of lithium ions, but also the introduction of fluorine elements can improve the flexibility and stability of the SEI film, thereby relieving the volume change of the silicon-containing particles during fast charging and discharging, and improving the cycle stability of the silicon negative electrode material; the second coating layer can absorb HF in the electrolyte, in combination with the first coating layer, can isolate the silicon-containing particles from the electrolyte to the greatest extent, and improve the stability of the fast charging silicon negative electrode material; finally, the cooperation of fluorine elements and phosphorus elements helps to improve the stability of the fast charging silicon negative electrode material in a high temperature environment. The fast charging process will inevitably generate more heat, therefore, the cooperation of fluorine elements and phosphorus elements has a significant contribution to the high temperature stability and safety performance of the lithium battery.

[0125] In combination with Embodiment 1, Embodiments 4-5 and Table 1, it can be seen that when the mass ratio of the silicon element content to the fluorine element is too low, the rate performance of the lithium battery slightly decreases, and at this time, the effect of the fluorine element and the phosphorus element on the improvement of the high-temperature stability of the lithium battery slightly decreases. When the mass ratio of the silicon element content to the fluorine element content is too high, at this time, the SEI film is not enough to buffer the volume expansion of the silicon-containing particles in the fast charging process due to the too high silicon element content in the silicon-containing particles, which will affect the structure stability of the fast-charging silicon negative electrode material, and then manifest as the decrease of the initial efficiency, the rate performance of the lithium battery, and the stability of the cooperation of the second coating layer and the first coating layer will also be affected, and even seriously cause the silicon-containing particles to contact with the electrolyte, which is not conducive to the improvement of the cycle performance and the high-temperature performance of the lithium battery.

[0126] In combination with Embodiment 1, Embodiments 6-7 and Table 1, it can be seen that the mass ratio of the fluorine element to the phosphorus element is too high, which is not conducive to the improvement of the cycle performance, the rate performance and the high-temperature performance of the lithium battery; and when the mass ratio of the fluorine element to the phosphorus element is too low, it is not conducive to the improvement of the rate performance and the high-temperature performance of the lithium battery.

[0127] In combination with Embodiment 1, Embodiments 8-9 and Table 1, it can be seen that the particle size D50 of the silicon-containing particles is too large or too small, which will cause the coating effect of the first coating layer and the second coating layer on the silicon-containing particles, and then affect the rate performance of the lithium battery.

[0128] In combination with Embodiment 1, Embodiments 10-11 and Table 1, it can be seen that by controlling the thickness of the first coating layer and the second coating layer to be kept within a suitable range, the migration channel and the migration distance in the lithium ion charging and discharging process can be formed, which is conducive to the improvement of the rate performance of the lithium battery.

Claims

1. A fast-charging silicon negative electrode material, comprising silicon-containing particles, a first coating layer coated on the silicon-containing particles, and a second coating layer coated on the first coating layer. The first coating layer comprises fluorine elements, and the second coating layer comprises phosphorus elements.

2. The fast-charging silicon anode material of claim 1, wherein: The ratio of the mass of silicon elements in the silicon-containing particles to the mass of fluorine elements in the first coating layer is 5-20:

1.

3. The fast-charging silicon anode material of claim 1, wherein: The ratio of the mass of fluorine elements in the first coating layer to the mass of phosphorus elements in the second coating layer is 1-3:

1.

4. The fast-charging silicon anode material of claim 1, wherein: The particle size D50 of the fast-charging silicon negative electrode material is 5-10 μm.

5. The fast-charging silicon anode material of any one of claims 1-4, wherein: The thickness of the first coating layer is 5-15 nm, and / or the thickness of the second coating layer is 2-10 nm.

6. The fast-charging silicon anode material of claim 1, wherein: The silicon-containing particles comprise at least one of silicon monoxide, silicon carbon, and silicon particles.

7. The fast-charging silicon anode material of claim 1, wherein: In the first coating layer, the fluoride comprises at least one of CeF3, AlF3, MgF2, and LaF3.

8. The fast-charging silicon anode material of claim 1, wherein: In the second coating layer, the phosphorus-containing compound comprises at least one of lithium phosphate and calcium phosphate. 9.A method for preparing the fast-charging silicon negative electrode material according to any one of claims 1-8, comprising the following steps: S1.mixing fluoride with a first organic solvent to obtain a fluoride-containing dispersion liquid, and mixing a phosphorus-containing compound with a second organic solvent to obtain a phosphorus-containing dispersion liquid; S2.mixing the silicon-containing particles with the fluoride-containing dispersion liquid to obtain a first mixed liquid, stirring the first mixed liquid, and drying the first mixed liquid to obtain a first coating product; S3.mixing the first coating product with the phosphorus-containing dispersion liquid to obtain a second mixed liquid, and drying the second mixed liquid to obtain a second coating product, i.e., the fast-charging silicon negative electrode material. 10.A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer comprises the fast-charging silicon negative electrode material according to any one of claims 1-8. 11.A lithium ion battery, comprising a positive electrode, a separator, an electrolyte, and the negative electrode sheet according to claim 10.

Images