Silicon composite negative electrode material, negative electrode sheet, and battery

Abstract

A silicon composite negative electrode material, a negative electrode sheet, and a battery. The silicon composite negative electrode material comprises an inner core (1), an intermediate layer (2), and an outer shell (3) in sequence, wherein the inner core (1) is silicon particles; the intermediate layer (2) is a fluorine-doped carbon layer; the outer shell (3) is a conductive polymer layer; and the mass proportion of fluorine in the silicon composite negative electrode material is 0.4-1.5%. Coating the surfaces of the silicon particles with the fluorine-doped carbon layer can effectively alleviate mechanical stress generated by volume expansion accompanying the intercalation of lithium into silicon particles, thereby improving the structural stability of the silicon composite negative electrode material. Moreover, fluorine doped in the carbon layer can form LiF with lithium ions, resulting in the formation of a stable SEI film rich in LiF on the negative electrode side. Further coating the surface of the fluorine-doped carbon layer with the conductive polymer layer can further optimize the conductivity and structural stability of the silicon composite negative electrode material, thereby ensuring that a battery containing the silicon composite negative electrode material has good cycling performance and high-rate fast charging performance.

Description

A silicon composite anode material, anode sheet, and battery

[0001] This application claims priority to Chinese Patent Application No. 2024110535834, filed with the Chinese Patent Office on August 1, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium battery technology, and in particular to a silicon composite anode material, anode sheet, and battery. Background Technology

[0003] Among the anode materials that are key components of lithium-ion batteries, commercially available graphite-based carbon materials have reached their capacity ceiling (theoretical capacity 372 mAh / g), while silicon-oxygen and silicon-carbon composite materials with higher energy density are the preferred choice for the next generation of lithium-ion battery anodes.

[0004] However, the large-scale application of silicon anodes currently faces many challenges. Firstly, silicon exhibits a severe volume expansion effect of nearly 300% after complete lithium intercalation; even with only one lithium atom intercalated to form LiSi, the silicon cell volume expands by 60.2%. This generates significant stress on the silicon surface, leading to electrode structure damage and the pulverization of the active silicon material. Another major reason limiting the practical application of silicon anodes is the repeated formation of the solid electrolyte interphase (SEI) layer. Severe volume changes continuously expose new silicon surfaces, resulting in the continuous formation of new SEIs. Ultimately, a thicker SEI film leads to excessive lithium-ion consumption and exacerbates poor electrical contact. Secondly, silicon is a semiconductor with an electronic conductivity (10⁻⁶ Ω·cm). -3 ~10 -2 μS·cm -1 ) and ion diffusion coefficient (10 -14 ~10 -13 S·cm -1 The efficiency of lithium ions is much lower than that of graphite, resulting in greater irreversibility during lithium-ion intercalation and reducing the initial coulombic efficiency (ICE).

[0005] Currently, the main technical means to address the problems of low conductivity and severe volume expansion effect of silicon in lithium storage are as follows: ① Reducing the size of silicon to the nanoscale below the critical value can prevent crack propagation and improve stability and cycle life; ② Combining nano-silicon with carbon avoids direct contact between the silicon core and the electrolyte, reduces lithium loss caused by the first lithiation reaction, significantly improves the cycle stability of silicon and increases the volumetric energy density of the material. Technical issues

[0006] Although nano-silicon has a positive effect on optimizing parameters such as mass-to-volume energy density, coulombic efficiency, electrode areal density, and electrode thickness of commercial lithium-ion batteries, it is extremely expensive, has unavoidable side reactions, and has low volumetric energy density. Technical solutions

[0007] In a first aspect, this application provides a silicon composite anode material, characterized in that it comprises a core, an intermediate layer, and an outer shell in sequence; the core is a silicon particle, the intermediate layer is a fluorine-doped carbon layer, and the outer shell is a conductive polymer layer; the mass percentage of fluorine in the silicon composite anode material is 0.4~1.5%.

[0008] Secondly, this application provides a negative electrode sheet comprising the aforementioned silicon negative electrode material.

[0009] Thirdly, this application provides a battery including the aforementioned negative electrode sheet. Beneficial effects

[0010] (1) In the silicon composite anode material provided in this application, firstly, a fluorine-doped carbon layer is coated on the surface of silicon particles. On the one hand, the fluorine-doped carbon layer can effectively buffer the volume expansion of lithium intercalation in the silicon anode. On the other hand, the fluorine-doped carbon layer, similar to other ordinary carbon layers, has abundant pores, which can provide abundant lithium-ion transport channels and optimize lithium-ion transport efficiency. In addition, after fluorine is doped into the carbon layer, it can form LiF with lithium ions during charge-discharge cycles. LiF is part of the SEI film and can greatly improve the stability of the SEI film. Therefore, a stable SEI film rich in LiF can be formed on the anode side, which is beneficial to optimizing the stability of the anode side and reducing the side reactions on the anode side. The improved stability of the SEI film can further alleviate the volume expansion of silicon particles, thus further improving the structural stability of the silicon composite anode material. Moreover, a stable SEI film is more conducive to the rapid insertion and extraction of lithium ions and will not consume too much active lithium, which is beneficial to improving ionic conductivity, optimizing rate performance, and improving the utilization rate of active materials.

[0011] (2) A conductive polymer layer is further coated on the surface of the fluorine-doped carbon layer, which can further optimize the conductivity and structural stability of the silicon composite material. During the battery charge-discharge cycle, as the number of charge-discharge cycles increases, the volume expansion stress effect of silicon becomes larger and larger. The ability of the fluorine-doped carbon layer to resist the volume expansion stress effect of silicon is limited, and some cracks may occur, or the coating of the fluorine-doped carbon layer on the surface of silicon particles may be incomplete. All of these will lead to a deterioration in the structural stability of the silicon material. However, by further coating the fluorine-doped carbon layer with a conductive polymer layer, the incomplete coating on the surface of silicon particles can be reduced. It can also work together with the fluorine-doped carbon layer to maintain the structural stability of the silicon composite anode material even when the number of charge-discharge cycles increases and the volume expansion effect of silicon becomes larger and larger. This makes the silicon composite anode material exhibit better cycle performance and rate performance. At the same time, the conductive polymer layer has excellent conductivity and corrosion resistance, which not only helps to improve the conductivity of silicon material, but also can slow down the degree of corrosion of silicon material by electrolyte to a greater extent, reduce electrolyte side reactions, optimize the service life of silicon composite anode material, and improve its cycle performance and rate performance.

[0012] (3) This application further limits the mass ratio of fluorine in the fluorine-doped carbon layer in the silicon composite anode material, and the mass ratio of the conductive polymer in the silicon composite anode material. First, controlling the mass ratio of fluorine is to ensure that a certain amount of fluorine forms LiF with lithium, so as to effectively improve the stability of the SEI film. However, if the amount of fluorine doping is too large, the conductivity of the silicon material will decrease, which is also not conducive to the electrochemical performance of the silicon material.

[0013] (4) The negative electrode sheet prepared using the above-mentioned silicon composite negative electrode material has good structural stability and conductivity. After multiple charge-discharge cycles, there are no obvious structural defects or cracks on its surface.

[0014] (5) The battery prepared using the above-mentioned negative electrode sheet has excellent structural stability and conductivity of active material in the negative electrode. It can effectively alleviate the volume expansion of silicon material in charge and discharge cycle, greatly optimizing the cycle performance and high-rate fast charging performance of the battery. Attached Figure Description

[0015] Figure 1 is a schematic diagram of the silicon composite anode material in this application.

[0016] The attached diagram is labeled as follows: 1-kernel, 2-intermediate layer, 3-shell. Embodiments of the present invention

[0017] In some embodiments, the conductive polymer accounts for 1-4% of the mass of the silicon composite anode material. Controlling the mass percentage of the conductive polymer is to ensure that a certain thickness of conductive polymer can further protect the silicon material. However, if the mass percentage of the conductive polymer is too high, it will affect the capacity of the silicon material and is also detrimental to the cycle performance and high-rate fast charging performance of the silicon material.

[0018] In some implementations, the D50 of the silicon particles is 500-700 nm. The D50 of the silicon particles should not be too large, as this would result in excessively large particle sizes in the silicon composite anode material formed after coating with a fluorine-doped carbon layer and a conductive polymer layer. This would hinder electrolyte wetting, slow down electron transport, and degrade the cycle performance of the silicon composite anode material. Conversely, the D50 of the silicon particles should not be too small, as this would also lead to excessively small particle sizes in the final silicon composite anode material. Smaller particles experience more severe expansion and contraction, resulting in greater capacity loss. Furthermore, excessively small silicon particles can cause electrode structure instability, and may even lead to silicon particle recombination and deformation, affecting electrode performance and lifespan. Moreover, an excessively small D50 is also detrimental to coating, significantly increasing the difficulty of coating and making it prone to uneven coating. This also fails to effectively improve the structural stability, cycle performance, and high-rate fast-charging performance of the silicon material.

[0019] In some embodiments, the thickness of the fluorine-doped carbon layer is 3-20 nm, and the thickness of the conductive polymer layer is 3-60 nm. If the fluorine-doped carbon layer is too thin, the fluorine content is too low, resulting in insufficient LiF formation, which is detrimental to improving the stability of the SEI film, increasing side reactions, and also hindering the improvement of the silicon material's cycle performance and high-rate fast-charging performance. If the fluorine-doped carbon layer is too thick, the fluorine content is too high, reducing the conductivity and capacity utilization of the silicon material, also negatively impacting its cycle performance or high-rate fast-charging performance. Meanwhile, in this application, the conductive polymer layer is also set to be relatively thin. Firstly, because the silicon particle surface is already coated with a fluorine-doped carbon layer, a thin conductive polymer layer is sufficient to provide significant further protection for the silicon material. Secondly, this allows the electrolyte to fully wet the fluorine-doped carbon layer and the silicon particle core, ensuring high lithium-ion transport efficiency while minimizing capacity loss in the silicon material. This allows the conductive polymer to effectively improve the conductivity and structural stability of the silicon material while fully ensuring its capacity utilization, further enhancing the electrochemical performance of the silicon composite anode material.

[0020] In some embodiments, the conductive polymer includes at least one of polyaniline and poly(4-benzenesulfonic acid).

[0021] In some embodiments, the conductive polymer includes polyaniline and poly(4-benzenesulfonic acid); the mass ratio of polyaniline to poly(4-benzenesulfonic acid) is 3~6:1. Polyaniline has excellent corrosion resistance and conductivity, while poly(4-benzenesulfonic acid) also has excellent conductivity and can ionize active groups that release protons, thereby improving electrochemical activity. When these two are combined, they can exert a significant synergistic effect, not only further improving the conductivity and structural stability of silicon materials, but also further optimizing the electrochemical activity, cycle performance, and high-rate fast-charging performance of silicon materials.

[0022] In some embodiments, the preparation of the above-mentioned silicon composite anode material includes the following steps: S1. Under a protective atmosphere, a fluorocarbon source is placed upstream of the gas flow, and silicon particles are placed downstream of the gas flow. The mixture is calcined at 500-800°C for 2-8 hours to obtain fluorine-doped carbon-coated silicon particles; S2. A conductive polymer solution is prepared, and the fluorine-doped carbon-coated silicon particles are added to the conductive polymer solution. The mixture is stirred at 50-100°C for 6-10 hours. The resulting mixture is then spray-dried to obtain the silicon composite anode material. In method S1, a highly efficient gas-phase fluorination method is used to coat the surface of silicon particles with a high-defect amorphous carbon layer rich in fluorine to improve its structural and interfacial stability. Compared with traditional fluorination techniques using high-cost, high-toxicity fluorine sources such as XeF2 or F2, this strategy is simpler and less toxic. A layer of conductive polymer is then coated using spray drying. This method is simple to operate, and the resulting coating layer is uniform and dense, while also being tightly connected to the fluorine-doped carbon layer, forming a structurally stable multilayer core-shell structure.

[0023] In some embodiments, in S1, the feed ratio of silicon particles to fluorocarbon source is calculated with a Si:F molar ratio of 20 to 50:1.

[0024] In some embodiments, in S2, the conductive polymer solution contains 10-40% by mass; the mass ratio of fluorine-doped carbon-coated silicon particles to the conductive polymer solution is 7-11:1.

[0025] In some embodiments, in S2, when preparing the conductive polymer solution, the organic solvent used includes at least one of N,N-dimethylacetamide, dimethyl sulfoxide, toluene, and dichloromethane.

[0026] In some embodiments, during S2, the inlet temperature is 100~200°C and the outlet temperature is 60~90°C during the spray drying process.

[0027] Example 1

[0028] 1. Preparation of silicon composite anode materials

[0029] The silicon composite anode material of this embodiment was prepared according to the following steps:

[0030] S1. Polyvinylidene fluoride (PVDF) and silicon powder were placed in a tube furnace under argon protection, wherein the quartz boat containing PVDF was located upstream of the gas flow and the quartz boat containing silicon powder was located downstream of the gas flow. The mixture was calcined at 750°C for 5 h to obtain fluorine-doped carbon-coated silicon particles. The feed ratio of silicon particles to fluorine-carbon source was calculated with a Si:F molar ratio of 35:1. The D50 of the silicon powder particles was 600 nm.

[0031] S2. Prepare a conductive polymer solution, add fluorine-doped carbon-coated silicon particles to the conductive polymer solution, stir at 80°C for 8 hours, and spray-dry the resulting mixture to obtain a silicon composite anode material; the conductive polymer is polyaniline, the mass fraction of the conductive polymer in the conductive polymer solution is 25%, and the mass ratio of fluorine-doped carbon-coated silicon particles to the conductive polymer solution is 10:1; in the final silicon composite anode material, the mass percentage of fluorine is 0.9%; the mass percentage of conductive polymer in the silicon composite anode material is 2.5%; the thickness of the fluorine-doped carbon layer is 13~19 nm; and the thickness of the conductive polymer layer is 20~35 nm.

[0032] The structural schematic diagram of the silicon composite anode material prepared by the above steps is shown in Figure 1. Except for Comparative Examples 3 and 4, the structural schematic diagrams of the silicon composite anode materials in other embodiments and comparative examples are also shown in Figure 1. The silicon composite anode material in Comparative Example 3 does not contain an intermediate layer, and the silicon composite anode material in Comparative Example 4 does not contain a shell.

[0033] 2. Preparation of negative electrode and battery

[0034] (1) Preparation of negative electrode

[0035] The silicon anode material, conductive agent SP, conductive agent SWCNT and binder PAA prepared above are dissolved in a solvent at a mass percentage of 90:3:1:6 and mixed. The solid content is controlled at 20-50%. The mixture is coated on a copper foil current collector and vacuum dried to obtain the anode sheet.

[0036] (2) Battery preparation

[0037] The battery uses the above-mentioned negative electrode, a ternary positive electrode prepared by traditional mature process (the positive electrode active material is NCM811, and the mass ratio of NCM811, conductive SP and binder PVDF is 93:3:4), an electrolyte of 1 mol / L LiPF6 / EC+DMC+EMC (v / v=1:1:1) and 5% FEC, a PE+alumina separator, and a soft pack battery assembled using conventional production processes.

[0038] Example 2

[0039] 1. Preparation of silicon composite anode materials

[0040] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that, in S1, the feed ratio of silicon particles to fluorocarbon source is calculated with a Si:F molar ratio of 50:1, so that the mass percentage of fluorine in the final silicon composite anode material is 0.4%, and the thickness of the fluorine-doped carbon layer is 3~10 nm. The remaining operations are the same as in Example 1.

[0041] 2. Preparation of negative electrode and battery

[0042] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0043] Example 3

[0044] 1. Preparation of silicon composite anode materials

[0045] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that, in S1, the feed ratio of silicon particles to fluorocarbon source is calculated with a Si:F molar ratio of 20:1, so that the mass percentage of fluorine in the final silicon composite anode material is 1.5%, and the thickness of the fluorine-doped carbon layer is 22-30 nm. The remaining operations are the same as in Example 1.

[0046] 2. Preparation of negative electrode and battery

[0047] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0048] Example 4

[0049] 1. Preparation of silicon composite anode materials

[0050] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that, in S2, the mass ratio of fluorine-doped carbon-coated silicon particles to the conductive polymer solution is 12:1, so that the conductive polymer accounts for 0.5% of the mass of the final silicon composite anode material, and the thickness of the conductive polymer layer is 1-2 nm. The remaining operations are the same as in Example 1.

[0051] 2. Preparation of negative electrode and battery

[0052] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0053] Example 5

[0054] 1. Preparation of silicon composite anode materials

[0055] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that, in S2, the mass ratio of fluorine-doped carbon-coated silicon particles to the conductive polymer solution is 6:1, so that the conductive polymer accounts for 5% of the mass of the final silicon composite anode material, and the thickness of the conductive polymer layer is 65-80 nm. The remaining operations are the same as in Example 1.

[0056] 2. Preparation of negative electrode and battery

[0057] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0058] Example 6

[0059] 1. Preparation of silicon composite anode materials

[0060] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that the silicon powder particles used in S1 have a D50 of 450 nm. The remaining operations are the same as in Example 1.

[0061] 2. Preparation of negative electrode and battery

[0062] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0063] Example 7

[0064] 1. Preparation of silicon composite anode materials

[0065] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that the silicon powder particles used in S1 have a D50 of 750 nm. The remaining operations are the same as in Example 1.

[0066] 2. Preparation of negative electrode and battery

[0067] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0068] Example 8

[0069] 1. Preparation of silicon composite anode materials

[0070] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that the conductive polymer used in S2 is poly(4-benzenesulfonic acid). The remaining operations are the same as in Example 1.

[0071] 2. Preparation of negative electrode and battery

[0072] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0073] Example 9

[0074] 1. Preparation of silicon composite anode materials

[0075] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that, in step S2, the conductive polymer used is polyaniline + poly(4-benzenesulfonic acid), and the mass ratio of polyaniline to poly(4-benzenesulfonic acid) is 4.5:1. The remaining operations are the same as in Example 1.

[0076] 2. Preparation of negative electrode and battery

[0077] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0078] Example 10

[0079] 1. Preparation of silicon composite anode materials

[0080] The preparation of the silicon composite anode material in this embodiment differs from that in Example 1 in that, in step S2, the conductive polymer used is polyaniline + poly(4-benzenesulfonic acid), and the mass ratio of polyaniline to poly(4-benzenesulfonic acid) is 2:1. The remaining operations are the same as in Example 1.

[0081] 2. Preparation of negative electrode and battery

[0082] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0083] Comparative Example 1

[0084] 1. Preparation of silicon composite anode materials

[0085] The preparation of this comparative silicon composite anode material differs from that of Example 1 in that, in S1, the feed ratio of silicon particles to fluorocarbon source is calculated with a Si:F molar ratio of 65:1, so that the mass percentage of fluorine in the final silicon composite anode material is 0.2%, and the thickness of the fluorine-doped carbon layer is 1-2 nm. The remaining operations are the same as in Example 1.

[0086] 2. Preparation of negative electrode and battery

[0087] The preparation of the negative electrode and battery in this comparative example is the same as in Example 1.

[0088] Comparative Example 2

[0089] 1. Preparation of silicon composite anode materials

[0090] The preparation of this comparative silicon composite anode material differs from that of Example 1 in that, in S1, the feed ratio of silicon particles to fluorocarbon source is calculated with a Si:F molar ratio of 16:1, so that the mass percentage of fluorine in the final silicon composite anode material is 1.8%, and the thickness of the fluorine-doped carbon layer is 23-27 nm. The remaining operations are the same as in Example 1.

[0091] 2. Preparation of negative electrode and battery

[0092] The preparation of the negative electrode and battery in this comparative example is the same as in Example 1.

[0093] Comparative Example 3

[0094] 1. Preparation of silicon composite anode materials

[0095] The preparation of this comparative silicon composite anode material differs from that of Example 1 in that the silicon particles are not coated with a fluorine-doped carbon layer, i.e., step S1 is omitted. Instead, the silicon powder in step S1 is directly mixed with the conductive polymer solution and then spray-dried. The remaining operations are the same as in Example 1.

[0096] 2. Preparation of negative electrode and battery

[0097] The preparation of the negative electrode and battery in this comparative example is the same as in Example 1.

[0098] Comparative Example 4

[0099] 1. Preparation of silicon composite anode materials

[0100] The preparation of this comparative silicon composite anode material differs from that of Example 1 in that the fluorine-doped carbon-coated silicon particles obtained in S1 are not coated with a conductive polymer layer, i.e., step S2 is omitted, and the fluorine-doped carbon-coated silicon particles obtained in S1 are directly used as the silicon composite anode material. The remaining operations are the same as in Example 1.

[0101] 2. Preparation of negative electrode and battery

[0102] The preparation of the negative electrode and battery in this comparative example is the same as in Example 1.

[0103] Comparative Example 5

[0104] 1. Preparation of silicon composite anode materials

[0105] In preparing this comparative silicon anode material, the difference from Example 1 is that the fluorine-doped carbon-coated silicon particles obtained in S1 are coated with a carbon shell. Specifically, the operation in S2 is modified as follows: the fluorine-doped carbon-coated silicon particles are introduced into a fluidized bed furnace, heated to 850°C under an inert atmosphere, and then ethylene, the carbon source gas, is introduced and held at this temperature for 4 hours. The carbon source gas is then turned off, and the temperature is lowered to room temperature to obtain the silicon composite anode material. Furthermore, in the above process, the flow rate of the carbon source gas ethylene is controlled at 50 L / min, and the amount of fluorine-doped carbon-coated silicon particles used is the same as in Example 1, ensuring that the shell thickness of the obtained silicon composite anode material is consistent with that of Example 1.

[0106] The remaining operations are the same as in Example 1.

[0107] 2. Preparation of negative electrode and battery

[0108] The preparation of the negative electrode and battery in this comparative example is the same as in Example 1.

[0109] Test case

[0110] 1. Experimental Construction Method

[0111] Electrochemical performance tests were conducted on the batteries prepared in all the above examples and comparative examples. Specifically, the tests were performed at room temperature (25°C) using the LAND battery testing system from Wuhan Jinno Electronics Co., Ltd., with the charge / discharge voltage limited to 2.5V~4.2V. The calculation methods or test conditions for the first-time efficiency (first charge / discharge efficiency), cycle performance, and rate performance are as follows:

[0112] (1) Initial efficiency: Total capacity of battery during the first discharge at 0.33C / Total capacity of battery during the first charge at 0.33C

[0113] (2) Capacity retention rate after 500 cycles at room temperature 2C / 2C

[0114] At 25℃, the battery was charged at 2C constant current and constant voltage to 4.2V, cut off at 0.05C, and left to stand for 10 minutes. Then, it was discharged at 2C constant current to 2.5V and left to stand for 10 minutes. The battery was cycled for 500 cycles, and the capacity retention rate of the battery after 500 cycles of 2C / 2C was recorded.

[0115] (3) Performance at 6°C in ambient temperature

[0116] 6C rate charging: At 25℃, discharge at 1C constant current to 2.5V, let stand for 10 minutes, charge at 6C constant current and constant voltage to 4.2V, cut off at 0.05C, record the battery constant current charging capacity, constant current and constant voltage charging total capacity and the highest temperature during fast charging. Constant current charging ratio = constant current charging capacity / constant current and constant voltage charging total capacity.

[0117] 2. Experimental Results

[0118] The test results of the first-cycle efficiency (first charge-discharge efficiency), cycle performance, and rate performance of the batteries prepared in all the above embodiments and comparative examples are shown in Table 1.

[0119] Table 1. Performance test results of the batteries prepared in the Examples and Comparative Examples

[0120] Group First-time efficiency (%) Capacity retention after 500 cycles at room temperature 2C / 2C (%) 6C rate charge - constant current charge ratio (%) 6C rate charge - maximum temperature (°C) Example 1 83.2 81.9 78.5 38 Example 2 81.1 79.8 75.5 41 Example 3 80.3 78.2 76.4 40 Example 4 81.8 75.1 72.3 45 Example 5 82.0 76.6 74.5 42 Example 6 82.1 79.0 73.5 4 Example 779.777.474.842 Example 881.478.574.244 Example 984.282.879.337 Example 1083.582.278.740 Comparative Example 172.473.263.852 Comparative Example 271.571.063.252 Comparative Example 367.568.460.955 Comparative Example 472.373.163.951 Comparative Example 569.570.161.254

[0121] As shown in Table 1, the battery prepared using the silicon composite anode material provided in this application has excellent structural stability, which can effectively alleviate the volume expansion stress effect of silicon material. At the same time, the silicon composite anode material also has excellent conductivity, which makes the battery prepared by it have high initial efficiency. It still has a high capacity retention rate after multiple high rate (2C) charge-discharge cycles, and also has a high charge-constant current charge ratio at higher rates (6C). Meanwhile, the battery temperature rise is not significant. For details, please refer to Examples 1 to 10.

[0122] In Comparative Examples 1 and 2, the mass percentages of fluorine in the silicon composite anode materials are 0.2% and 1.8%, respectively, which is too low and too high. The low fluorine content prevents the formation of sufficient LiF, reducing the stability of the SEI film. Furthermore, the low fluorine content also indicates a thinner fluorine-doped carbon layer, which has limited effect on mitigating the silicon volume expansion effect. Therefore, the battery in Comparative Example 1 experiences a significant decrease in initial efficiency, room temperature cycle performance, high-rate charging performance, and high-rate charging thermal stability. Conversely, the high fluorine content results in a thicker fluorine-doped carbon layer, affecting the overall conductivity of the silicon composite anode material and negatively impacting its performance. This also leads to a substantial decrease in the relevant performance characteristics of the battery in Comparative Example 2.

[0123] In the silicon composite anode material of Comparative Example 3, the silicon particles are not coated with a fluorine-doped carbon layer, and the volume expansion effect of the silicon particles cannot be effectively alleviated. As a result, the silicon composite anode material is prone to structural breakage and collapse during charge-discharge cycles, which ultimately leads to a significant decline in the performance of the battery in all aspects.

[0124] In the silicon composite anode material of Comparative Example 4, the fluorine-doped carbon-coated silicon particles obtained in S1 are not coated with a conductive polymer layer. This results in one less layer of protection, an increase in side reactions, and a decrease in the conductivity of the material. Therefore, it is not conducive to improving the performance of the silicon composite anode material, and ultimately leads to a decline in the performance of the battery in multiple aspects.

[0125] In the silicon composite anode material of Comparative Example 5, replacing the outer conductive polymer layer with a carbon layer and coating another carbon layer on the outside of the fluorine-doped carbon layer not only did not significantly improve the performance of the silicon composite anode material, but also made the carbon layer too thick because the carbon layer is similar to the fluorine-doped carbon layer. This affected the lithium-ion transport and conductivity, and ultimately led to a significant decline in the performance of the battery in all aspects.

[0126] Further comparing Examples 1 and 2 / 3, compared to Example 1, the mass percentage of fluorine in the silicon composite anode material in Examples 2 and 3 is relatively lower and higher, respectively. This means the fluorine-doped carbon layer is also relatively thinner and thicker, respectively. This results in a certain degree of reduction in the initial efficiency, room temperature cycle performance, high-rate charging performance, and high-rate charging thermal stability of the batteries in Examples 2 and 3. This indicates that the fluorine content has a significant impact on the performance of the silicon composite anode material, and further limiting the fluorine content is more conducive to further improving the performance of the silicon composite anode material and further optimizing the battery performance.

[0127] Comparing Examples 1 and 4-5, the mass percentage of conductive polymer in the silicon composite anode material in Examples 4 and 5 was too low and too high, respectively. Too little conductive polymer results in a thinner outer shell, failing to provide adequate protection for the internal materials, increasing side reactions, and thus degrading the battery's performance in various aspects. Conversely, too much conductive polymer also affects the capacity utilization of the silicon material, negatively impacting its cycle performance and high-rate fast-charging capabilities, leading to a decline in overall battery performance.

[0128] Comparing Examples 1 and 6-7, the silicon particles before coating in Examples 6 and 7 were too small and too large, respectively. This would result in the silicon composite anode material being too small and too large in the final product. Since both small and large silicon composite anode materials are not conducive to taking into account the various performance aspects of silicon composite anode materials, they will also have a certain impact on the performance of the battery prepared with them.

[0129] Comparing Examples 1 and 8, 9, and 10, even when using other conductive polymers as the outer shell, the silicon composite anode material still exhibits good structural stability and conductivity. This is evident from the high initial efficiency, good room-temperature cycling performance, and high-rate performance of the batteries prepared using these silicon composite anode materials. Further comparison revealed that when the conductive polymer is a combination of polyaniline and poly(4-benzenesulfonic acid), the batteries prepared with the silicon composite anode material exhibit better electrochemical performance. Furthermore, it was found that within a specific mass ratio range, the combination of polyaniline and poly(4-benzenesulfonic acid) is more beneficial for improving the electrochemical performance of the silicon composite anode material, thus further optimizing battery performance.

Claims

1. A silicon composite negative electrode material, comprising, in sequence, a core, an intermediate layer, and a shell; the core comprises silicon particles, the intermediate layer comprises a fluorine-doped carbon layer, and the shell comprises a conductive polymer layer; a mass percentage of fluorine in the silicon composite negative electrode material is 0.4-1.5%.

2. The silicon composite negative electrode material of claim 1, wherein: a mass percentage of the conductive polymer in the silicon composite negative electrode material is 1-4%.

3. The silicon composite negative material of claim 1, wherein: D50 of the silicon particles is 500-700 nm.

4. The silicon composite negative material of claim 1, wherein: a thickness of the fluorine-doped carbon layer is 3-20 nm, and a thickness of the conductive polymer layer is 3-60 nm.

5. The silicon composite negative material of claim 1, wherein: the conductive polymer comprises at least one of polyaniline and poly(4-phenyl sulfonic acid).

6. The silicon composite negative material of claim 5, wherein: the conductive polymer comprises the polyaniline and the poly(4-phenyl sulfonic acid). a mass ratio of the polyaniline to the poly(4-phenyl sulfonic acid) is 3-6:

1. 7.The silicon composite negative electrode material of claim 1, wherein a preparation method thereof comprises the following steps: S1.under a protective atmosphere, placing a fluorocarbon source upstream of a gas flow and placing the silicon particles downstream of the gas flow, and calcining at 500-800℃ for 2-8 h to obtain fluorine-doped carbon-coated silicon particles; S2.configuring a conductive polymer solution, adding the fluorine-doped carbon-coated silicon particles into the conductive polymer solution, stirring at 50-100℃ for 6-10 h, and spray drying the obtained mixture to obtain the silicon composite negative electrode material.

8. The silicon composite negative material of claim 7, wherein: in the S1, a feeding ratio of the silicon particles to the fluorocarbon source is calculated according to a Si:F molar ratio of 20-50:

1. 9.A negative electrode sheet, comprising the silicon composite negative electrode material of any one of claims 1-8. 10.A battery, comprising the negative electrode sheet of claim 9.

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