Silicon-based negative electrode material, negative electrode sheet, and battery

Abstract

The present application provides a silicon-based negative electrode material, a negative electrode sheet, and a battery. The silicon-based negative electrode material comprises a core and a shell covering the core, the core comprises non-metal-doped silicon, and the shell comprises a conductive metal carbon layer; in the non-metal-doped silicon, the non-metal comprises B; and in the conductive metal carbon layer, the conductive metal carbon comprises at least one of Ti3C2(OH)2 and Ti3AlC2.

Description

Silicon-based negative electrode material, negative electrode sheet and battery

[0001] The present application claims priority to the Chinese patent application No. 2024110534367 filed on August 01, 2024 with the China Patent Office, the whole content of the above application is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium batteries, in particular to a silicon-based negative electrode material, a negative electrode sheet and a battery. BACKGROUND

[0003] Silicon as a negative electrode material has great potential in secondary batteries, it has the advantages of green, non-toxic and harmless, and abundant reserves on earth. Especially for lithium-ion batteries, the room temperature theoretical capacity of silicon negative electrode is as high as 3579 mA·h / g, and the working potential is low, which is considered as one of the ideal negative electrodes. However, the large-scale application of silicon negative electrode in lithium-ion batteries still faces several key challenges. Among them, the biggest challenge affecting the industrialization of silicon negative electrode is the large volume change of silicon during the cycle process. Compared with the volume change of only 12% of graphite negative electrode, the volume change of silicon negative electrode is as high as 300%, which means that during the charge and discharge cycle, the silicon negative electrode will experience a huge expansion and contraction, leading to its serious crushing and pulverization, so that the silicon particles lose electrical contact with the current collector, affecting the battery performance.

[0004] In the past few decades, people have dealt with the defects of silicon electrode through methods such as nanocrystallization, silicon-carbon composite and alloying. TECHNICAL PROBLEM

[0005] Although the methods such as nanocrystallization, silicon-carbon composite and alloying effectively improve the volume expansion effect of silicon material and improve the electrochemical performance, the cost is high or a part of the specific capacity of silicon is sacrificed. TECHNICAL SOLUTION

[0006] In a first aspect, the present application provides a silicon-based negative electrode material, comprising a core and a shell covering the core, the core comprises metal-doped silicon, and the shell comprises a conductive metal carbon layer; in the non-metal-doped silicon, the non-metal comprises B; in the conductive metal carbon layer, the conductive metal carbon comprises at least one of Ti3C2(OH)2 and Ti3AlC2.

[0007] In a second aspect, the present application provides a negative electrode sheet comprising the above-mentioned silicon negative electrode material.

[0008] In a third aspect, the present application provides a battery comprising the above-mentioned negative electrode sheet. ADVANTAGEOUS EFFECTS

[0009] (1) The silicon-based negative electrode material provided by the application is a core-shell structure, wherein the inner core is B-doped silicon. The non-metallic element B doped in the silicon lattice can increase the active sites and lithium storage performance of the silicon material, so that the doped silicon becomes p-type silicon, thereby significantly improving the electrical conductivity of the silicon, and effectively improving the rate performance and electrochemical cycle stability of the silicon-based negative electrode material. Moreover, the non-metallic element B can form a "buffer framework" in the inner core, thereby effectively relieving the volume expansion stress and greatly improving the structural stability of the silicon-based negative electrode material, thereby improving the cycle stability and high-rate fast-charging performance of the silicon-based negative electrode material. The use of conductive metal carbon as the shell of the silicon-based negative electrode material can further improve the structural stability of the silicon-carbon negative electrode and optimize the rate performance. This is because, first, the conductive metal carbon has good electrical conductivity, which can enhance the Li + shuttling at the interface between the electrode material and the electrolyte, has good rate performance. Second, the conductive metal carbon can form metal-Si and C-Si with silicon, and the dual action of metal-Si and C-Si can improve the tight connection between the non-metallic doped silicon inner core and the conductive metal carbon layer, which is beneficial to further improve the material stability and better inhibit the volume expansion of silicon. Third, the conductive metal carbon itself has excellent mechanical properties, good thermodynamic properties and corrosion resistance, so it can reduce the side reactions on the surface of the silicon negative electrode and better protect the non-metallic doped silicon inner core, thereby further improving the structural stability of the silicon-based negative electrode material and optimizing the rate performance.

[0010] (2) The negative electrode sheet prepared by using the silicon-based negative electrode material improved by the application has excellent structural stability and electrical conductivity. The particle size of the silicon-based negative electrode material used can balance the processability, electrolyte wettability and particle dispersion of the electrode sheet. Therefore, the negative electrode sheet prepared thereby also has excellent structural stability and electrical conductivity, and has no obvious cracks or structural defects on the surface after multiple charge and discharge cycles, thus having good cycle stability and high-rate fast-charging performance.

[0011] (3) The battery prepared from the above negative electrode sheet has good structural stability and electrical conductivity, and thus the battery prepared from the above negative electrode sheet has good room temperature cycle stability, high initial efficiency and good high-rate fast-charging performance. BRIEF DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a structural schematic diagram of the silicon negative electrode material in the application.

[0013] The reference signs are 1-inner core, 2-shell. EMBODIMENTS OF THE INVENTION

[0014] In some embodiments, the mass percentage of B in the silicon-based negative electrode material is 0.5-2.0%. If the proportion of non-metallic elements is too low, the conductivity of the silicon-based negative electrode material cannot be effectively improved, and the buffer framework formed in the non-metallic doped silicon core cannot effectively resist the volume expansion stress of silicon, which will reduce the conductivity and structural stability of the silicon-based negative electrode material. If the proportion of non-metallic elements is too high, the proportion of silicon will be relatively low, which will reduce the specific capacity of the silicon-based negative electrode material and is also not conducive to the cycle performance and high-rate charging performance of the silicon-based negative electrode material.

[0015] In some embodiments, the mass percentage of the conductive metal carbon in the silicon-based negative electrode material is 2-5%. Controlling the mass percentage of the conductive metal carbon in the silicon-based negative electrode material within a certain range can ensure that the outer layer of the conductive metal carbon has a certain thickness, which can effectively protect the non-metallic doped silicon and improve the conductivity of the silicon-based negative electrode material, and also can ensure that the overall capacity of the silicon-based negative electrode material is not too low.

[0016] In some embodiments, the D50 of the silicon-based negative electrode material is 0.5-1.5 μm. The particle size of the silicon-based negative electrode material has a significant influence on its electrochemical performance. If the D50 of the silicon-based negative electrode material is too small, it is easy to aggregate and the overall structural stability is poor, which is not conducive to ensuring the processing of the electrode sheet and is also not conducive to the performance and service life of the electrode sheet. At the same time, a small D50 and a large specific surface area result in more severe expansion and contraction, and greater capacity loss. If the D50 of the silicon-based negative electrode material is too large, the specific surface area is small, the electrolyte wettability is poor, and the conductivity and lithium ion deintercalation ability are poor, which will also deteriorate the cycle performance and high-rate charging performance of the silicon-based negative electrode material. Moreover, a large D50 results in large volume expansion stress of silicon, and the silicon-based negative electrode material has limited ability to relieve the volume expansion stress, which cannot relieve the volume expansion stress of silicon to a greater extent and is easy to cause the structure of the silicon-based negative electrode material to break, thereby deteriorating the cycle stability of the silicon-based negative electrode material.

[0017] In some embodiments, the thickness of the conductive metal carbon layer is 5-80 nm. If the thickness of the conductive metal carbon layer is too thin, it cannot effectively protect the non-metallic doped silicon core, increase the interface side reaction, and deteriorate the stability of the silicon-based negative electrode material, and also cannot further improve the conductivity of the silicon-based composite material. If the thickness of the conductive metal carbon layer is too thick, it will reduce the surface porosity, which is not conducive to the efficient transmission of lithium ions, and will also reduce the overall capacity of the silicon-based negative electrode material and is not conducive to the performance of the silicon-based material.

[0018] In some embodiments, the preparation of the above-mentioned silicon-based negative electrode material comprises the following steps: S1. mixing silicon powder and B oxide, heat treating the obtained mixture at 900-1200°C for 2-5h under a protective atmosphere to obtain a calcined product; S2. removing silicon oxide from the calcined product to obtain non-metal doped silicon; S3. preparing a conductive metal carbon solution, dispersing the non-metal doped silicon in the conductive metal carbon solution, stirring at 30-100°C for 4-8h, and then spray drying the obtained mixture to obtain the silicon-based negative electrode material. In the calcination process in S1, the non-metal oxide is reduced to non-metallic elements, and the silicon is oxidized to silicon oxide. After removing the silicon oxide by S2, the non-metal doped silicon is obtained. The above-mentioned reaction steps enable the non-metal elements to be doped into the silicon lattice, greatly increasing the number of majority carriers, i.e. free electrons (or holes) in the lattice, and the more majority carriers, the stronger the conductivity of the impurity semiconductor, thus greatly enhancing the conductivity of the silicon-based negative electrode material.

[0019] In some embodiments, in S1, the feeding amount of silicon powder and B oxide is calculated according to the molar ratio of Si:B of (400-600):(2-8).

[0020] In some embodiments, in S1, the D50 of the silicon powder particles is 0.5-1μm.

[0021] In some embodiments, in S1, the temperature is raised to 900-1200°C at a temperature raising rate of 4°C / min during the heat treatment.

[0022] In some embodiments, in S1, the flow rate of the protective atmosphere is 3L / min during the heat treatment; the protective atmosphere comprises nitrogen.

[0023] In some embodiments, in S1, the B oxide comprises B2O3.

[0024] In some embodiments, in S2, hydrofluoric acid is used to remove the silicon oxide from the calcined product.

[0025] In some embodiments, in S2, the specific operation of using hydrofluoric acid to remove the silicon oxide from the calcined product is as follows: after the calcined product obtained in S1 is cooled to room temperature, it is added to hydrofluoric acid with a concentration of 3-6%, stirred into a mud slurry at 65-75°C, and then transferred into a vacuum oven at 70-100°C overnight; the operation of S1 is repeated three times, i.e. the acid washing with hydrofluoric acid is repeated three times.

[0026] In some embodiments, in S3, the mass fraction of the conductive metal carbon in the conductive metal carbon solution is 8-35%.

[0027] In some embodiments, in S3, the mass ratio of the non-metal doped silicon to the conductive metal carbon solution is 2-12:1.

[0028] In some embodiments, in S3, the solvent used for preparing the conductive metal carbon solution comprises at least one of ethanol, water.

[0029] Example 1

[0030] 1. Preparation of silicon-based negative electrode material

[0031] The silicon-based negative electrode material of the present example is prepared according to the following steps:

[0032] S1. Mix silicon powder and B2O3, heat treat the obtained mixture to 1150°C at a temperature rising rate of 4°C / min under a nitrogen atmosphere with a flow rate of 3 L / min for 3.5 h to obtain a calcined product; the D50 of the silicon powder particles is 0.7; the feeding amount of silicon powder and B2O3 is calculated according to the molar ratio of Si:B of 500:5;

[0033] S2. After the calcined product obtained in S1 is cooled to room temperature, it is added to hydrofluoric acid with a concentration of 3-6%, stirred into a mud-like state at 70°C, and then transferred to a vacuum oven at 85°C overnight; after repeating the acid washing with hydrofluoric acid three times and drying, non-metal doped silicon is obtained.

[0034] S3. Prepare a conductive metal carbon solution, disperse the non-metal doped silicon in the conductive metal carbon solution, stir at 60°C for 6 h, and then spray dry the obtained mixed system to obtain a silicon-based negative electrode material; the conductive metal carbon is Ti3C2(OH)2; the mass fraction of the conductive metal carbon in the conductive metal carbon solution is 20wt%, the mass ratio of the non-metal doped silicon to the conductive metal carbon solution is 5:1, the D50 of the final silicon negative electrode material is 1.0 (μm, the conductive metal carbon shell thickness is 30-50 nm, and the mass proportion of B in the silicon-based negative electrode material is 1.3% respectively.

[0035] The structure of the silicon-based negative electrode material prepared by the above steps is shown in FIG. 1. The structure of the silicon negative electrode material in the other examples and comparative examples is also shown in FIG. 1, except for Comparative Example 2, which has only a core.

[0036] 2. Preparation of negative electrode sheet and battery

[0037] (1) Preparation of negative electrode sheet

[0038] The silicon negative electrode material prepared above, the conductive agent SP, the conductive agent SWCNT and the binder PAA are dissolved in a solvent in a mass percentage of 90:3:1:6, the solid content is controlled to be 20-50%, and the mixture is coated on a copper foil current collector, vacuum dried to obtain a negative electrode sheet.

[0039] (2) Preparation of the battery

[0040] The above negative electrode sheet, a ternary positive electrode sheet (the positive electrode active material is NCM811, the mass ratio of NCM811, conductive SP and binder PVDF is 93:3:4, 1 mol / L LiPF6 / EC+DMC+EMC (v / v=1:1:1) electrolyte, PE+aluminum oxide separator, and the shell is assembled into a soft package battery by using a conventional production process.

[0041] Example 2

[0042] 1. Preparation of the silicon-based negative electrode material

[0043] In the preparation of the silicon-based negative electrode material in this example, the difference from Example 1 is that in S3, the conductive metal carbon is Ti3AlC2. The rest of the operations are consistent with Example 1.

[0044] 2. Preparation of the negative electrode sheet and the battery

[0045] The preparation of the negative electrode sheet and the battery in this example is consistent with Example 1.

[0046] Example 3

[0047] 1. Preparation of the silicon-based negative electrode material

[0048] In the preparation of the silicon-based negative electrode material in this example, the difference from Example 1 is that in S1, the amount of B2O3 is too small, specifically, the amount of silicon powder and B2O3 is calculated based on the molar ratio of Si:B as 500:2, so that the mass percentage of B in the silicon-based negative electrode material is 0.5% respectively. The rest of the operations are consistent with Example 1.

[0049] 2. Preparation of the negative electrode sheet and the battery

[0050] The preparation of the negative electrode sheet and the battery in this example is consistent with Example 1.

[0051] Example 4

[0052] 1. Preparation of the silicon-based negative electrode material

[0053] In the preparation of the silicon-based negative electrode material in this example, the difference from Example 1 is that in S1, the amount of B2O3 is too large, specifically, the amount of silicon powder and B2O3 is calculated based on the molar ratio of Si:B as 550:8. The rest of the operations are consistent with Example 1.

[0054] 2. Preparation of the negative electrode sheet and the battery

[0055] The preparation of the negative electrode sheet and the battery in this example is consistent with Example 1.

[0056] Example 5

[0057] 1. Preparation of silicon-based negative electrode material

[0058] In the preparation of the silicon-based negative electrode material in this example, the difference from Example 1 is that in S1, the amount of B2O3 input is too small, specifically, the amount of silicon powder and B2O3 is calculated based on the molar ratio of Si:B of 600:1.5, so that the final mass percentage of B in the silicon-based negative electrode material is 0.05% respectively. The rest of the operation is consistent with Example 1.

[0059] 2. Preparation of negative electrode sheet and battery

[0060] The preparation of the negative electrode sheet and battery in this example is consistent with Example 1.

[0061] Example 6

[0062] 1. Preparation of silicon-based negative electrode material

[0063] In the preparation of the silicon-based negative electrode material in this example, the difference from Example 1 is that in S1, the amount of B2O3 input is too small, specifically, the amount of silicon powder and B2O3 is calculated based on the molar ratio of Si:B of 600:15, so that the final mass percentage of B in the silicon-based negative electrode material is 2.3% respectively. The rest of the operation is consistent with Example 1.

[0064] 2. Preparation of negative electrode sheet and battery

[0065] The preparation of the negative electrode sheet and battery in this example is consistent with Example 1.

[0066] Example 7

[0067] 1. Preparation of silicon-based negative electrode material

[0068] In the preparation of the silicon-based negative electrode material in this example, the difference from Example 1 is that in S3, the mass fraction of conductive metal carbon in the conductive metal carbon solution is 13wt%, so that the thickness of the conductive metal carbon shell in the final silicon negative electrode material is 5-15nm. The rest of the operation is consistent with Example 1.

[0069] 2. Preparation of negative electrode sheet and battery

[0070] The preparation of the negative electrode sheet and battery in this example is consistent with Example 1.

[0071] Example 8

[0072] 1. Preparation of silicon-based negative electrode material

[0073] The preparation of the silicon-based negative electrode material in the present embodiment is different from that in Embodiment 1 in that, in S3, the mass fraction of the conductive metal carbon in the conductive metal carbon solution is 35 wt%, so that the thickness of the conductive metal carbon shell in the final silicon negative electrode material is 65-80 nm. The remaining operations are consistent with those in Embodiment 1.

[0074] 2. Preparation of negative electrode sheet and battery

[0075] The preparation of the negative electrode sheet and battery in the present embodiment is consistent with that in Embodiment 1.

[0076] Embodiment 9

[0077] 1. Preparation of silicon-based negative electrode material

[0078] The preparation of the silicon-based negative electrode material in the present embodiment is different from that in Embodiment 1 in that, in S3, the mass fraction of the conductive metal carbon in the conductive metal carbon solution is 45 wt% during the spray drying process, so that the thickness of the conductive metal carbon shell in the final silicon negative electrode material is 85-100 nm. The remaining operations are consistent with those in Embodiment 1.

[0079] 2. Preparation of negative electrode sheet and battery

[0080] The preparation of the negative electrode sheet and battery in the present embodiment is consistent with that in Embodiment 1.

[0081] Embodiment 10

[0082] 1. Preparation of silicon-based negative electrode material

[0083] The preparation of the silicon-based negative electrode material in the present embodiment is different from that in Embodiment 1 in that, in S1, the D50 of the silicon powder particles is 0.3 μm, so that the D50 of the final silicon negative electrode material is 0.4 μm. The remaining operations are consistent with those in Embodiment 1.

[0084] 2. Preparation of negative electrode sheet and battery

[0085] The preparation of the negative electrode sheet and battery in the present embodiment is consistent with that in Embodiment 1.

[0086] Embodiment 11

[0087] 1. Preparation of silicon-based negative electrode material

[0088] The preparation of the silicon-based negative electrode material in the present embodiment is different from that in Embodiment 1 in that, in S1, the D50 of the silicon powder particles is 1.6 μm, so that the D50 of the final silicon negative electrode material is 1.8 μm. The remaining operations are consistent with those in Embodiment 1.

[0089] 2. Preparation of negative electrode sheet and battery

[0090] The preparation of the negative electrode sheet and battery in the present embodiment is consistent with that in Embodiment 1.

[0091] Comparative Example 1

[0092] 1. Preparation of silicon-based negative electrode material

[0093] In the preparation of the silicon-based negative electrode material of the present comparative example, different from Example 1 is that the steps of S1-S2 are not performed, and in S3, the silicon particles are directly mixed with the conductive polymer solution and spray-dried.

[0094] 2. Preparation of negative electrode sheet and battery

[0095] The preparation of the negative electrode sheet and battery in the present comparative example is consistent with Example 1.

[0096] Comparative Example 2

[0097] 1. Preparation of silicon-based negative electrode material

[0098] In the preparation of the silicon-based negative electrode material of the present comparative example, different from Example 1 is that the step of S3 is not performed, and the non-metallic doped silicon prepared in S3 is directly used as the silicon negative electrode material.

[0099] 2. Preparation of negative electrode sheet and battery

[0100] The preparation of the negative electrode sheet and battery in the present comparative example is consistent with Example 1.

[0101] Comparative Example 3

[0102] 1. Preparation of silicon-based negative electrode material

[0103] In the preparation of the silicon-based negative electrode material of the present comparative example, different from Example 1 is that the non-metallic doped silicon obtained in S3 is coated with a carbon shell, i.e. the operation of S3 is changed as follows: the non-metallic doped silicon is passed into a fluidized bed type atmosphere furnace, heated to 700°C under an inert atmosphere, carbon source gas ethylene is introduced, and the temperature is maintained for 6h, then the carbon source gas is turned off, and the temperature is lowered to room temperature to obtain the silicon negative electrode material. And in the above process, the flow rate of the carbon source gas ethylene is controlled to be 50L / min, the amount of the non-metallic doped silicon is consistent with Example 1, the D50, shell thickness, and mass ratio of non-metallic B in the silicon-based negative electrode material are consistent with Example 1. The remaining operations are consistent with Example 1.

[0104] 2. Preparation of negative electrode sheet and battery

[0105] The preparation of the negative electrode sheet and battery in the present comparative example is consistent with Example 1.

[0106] Comparative Example 4

[0107] 1. Preparation of silicon-based negative electrode material

[0108] In the preparation of the silicon-based negative electrode material of the present comparative example, the B oxide obtained in S1 is replaced by arsenic pentoxide (As2O5), which is different from Example 1. The rest of the operation is consistent with Example 1.

[0109] 2. Preparation of negative electrode sheet and battery

[0110] The preparation of the negative electrode sheet and battery in the present comparative example is consistent with Example 1.

[0111] Test example

[0112] 1. Experimental construction

[0113] The batteries prepared in all the above examples and comparative examples were tested for relevant electrochemical performance. The tests were carried out on a LAND battery test system from Wuhan Jinuo Electronics Co., Ltd. at room temperature (25°C), and the charge and discharge voltage was limited to 2.5V-4.2V. The calculation method or test conditions for the first efficiency (first charge and discharge efficiency), cycle performance and rate performance are as follows:

[0114] (1) First efficiency: total capacity of 0.33C first discharge of the battery / total capacity of 0.33C first charge of the battery

[0115] (2) Capacity retention rate at room temperature 2C / 2C for 500 cycles

[0116] At 25°C, 2C constant current and constant voltage charging to 4.2V, 0.05C cutoff, standing for 10min, 2C constant current discharging to 2.5V, standing for 10min, cycling for 500 cycles, recording the capacity retention rate of the battery at 2C / 2C for 500 cycles.

[0117] (3) Rate performance at room temperature 6C

[0118] 6C rate charging: at 25°C, 1C constant current discharging to 2.5V, standing for 10min, 6C constant current and constant voltage charging to 4.2V, 0.05C cutoff, recording the constant current charging capacity, constant current and constant voltage charging total capacity and the highest temperature in the fast charging process, constant current charging ratio = constant current charging capacity / constant current and constant voltage charging total capacity.

[0119] 2. Experimental results

[0120] The first efficiency (first charge and discharge efficiency), cycle performance and rate performance test results of the batteries prepared in all the above examples and comparative examples are shown in Table 1.

[0121] Table 1. Test results of the relevant performance of the batteries prepared in the examples and comparative examples

[0122] Group Initial efficiency, room temperature 2C / 2C cycle 500 cycles capacity retention rate (%) 6C rate charge-constant current charge ratio (%) 6C rate charge-highest temperature (°C) Example 1 82.9 85.8 80.1 33 Example 2 82.2 83.7 78.2 34 Example 3 81.6 83.5 77.3 35 Example 4 82.0 81.6 74.9 37 Example 5 80.1 82.2 75.8 36 Example 6 80.8 84.1 76.5 35 Example 7 81.1 83.6 78.0 34 Example 8 81.5 81.2 74.8 37 Example 9 82.3 81.9 74.1 38 Example 10 80.0 83.6 78.2 35 Example 11 82.1 83.0 77.8 35 Comparative Example 1 71.2 72.8 55.9 54 Comparative Example 2 73.5 75.1 60.4 47 Comparative Example 3 76.2 76.8 62.4 45 Comparative Example 4 73.7 74.3 61.2 45

[0123] As can be seen from Table 1, the battery prepared by using the silicon-based negative electrode material provided in the present application has excellent conductivity of the silicon-based negative electrode material, and the silicon-based negative electrode material itself has stable structure, and can effectively alleviate the volume expansion stress of silicon in the process of charge and discharge cycle, thereby optimizing the initial efficiency, room temperature cycle performance and high rate fast charging performance of the battery. For details, refer to Examples 1-11.

[0124] In the silicon-based negative electrode material in Comparative Example 1, the non-metallic B is not doped in the core, so the conductivity of silicon cannot be effectively improved, and the "buffer skeleton" effect of the non-metallic element is also lost, resulting in a decrease in the conductivity and structural stability of the silicon-based negative electrode material, thereby causing a decrease in the initial efficiency, room temperature cycle performance, high rate fast charging performance, and high rate fast charging thermal stability of the battery.

[0125] In the silicon-based negative electrode material in Comparative Example 2, only the non-metallic doped silicon core is present, and there is no coated conductive metal carbon layer, so the protection effect on the core is decreased, the core is directly contacted with the electrolyte, and the degree of side reaction is increased, which will cause a decrease in the structural stability of the silicon-based negative electrode material, and at the same time, the overall volume expansion alleviation performance of the silicon-based negative electrode material is also decreased due to the absence of the outer coated conductive metal carbon layer. Thus, the battery prepared by using the silicon-based negative electrode material has a decrease in the initial efficiency, room temperature cycle performance, high rate fast charging performance, and high rate fast charging thermal stability.

[0126] In the silicon-based negative electrode material in Comparative Example 3, the shell is a carbon layer, and compared with the conductive metal carbon layer, the carbon layer has a decreased tightness in connection with the non-metallic doped silicon core due to the absence of the metal-Si effect, which will cause a decrease in the overall structural stability of the silicon-based negative electrode material in the process of charge and discharge cycle, and at the same time, the material capacity will be decreased compared with the conductive metal carbon layer. The above reasons cause a decrease in the performance of the battery prepared by using the silicon-based negative electrode material.

[0127] In the silicon-based negative electrode material in Comparative Example 4, the doped non-metallic element is arsenic As, which is limited for further improving the conductivity of silicon and affects the capacity of the silicon material, thereby causing the decline of the various performances of the battery prepared from the silicon-based negative electrode material.

[0128] Further observation of Example 1 and Example 2 shows that when the type of conductive metal carbon is changed, the silicon-based negative electrode material still has good conductivity and structural stability, which can be seen from the high initial efficiency and good room temperature cycle performance and high rate performance of the battery prepared from the silicon-based negative electrode material.

[0129] Comparing Example 1 and Examples 3-6, it can be found that the doping amount of non-metallic elements such as B has a certain influence on the performance of the silicon-based negative electrode material, thereby affecting the electrochemical performance of the battery. The non-metallic element B in Examples 5 and 6 is 0.05% and 2.3% respectively, which is too little and too much compared with Examples 1, 3-4. Too little non-metallic element cannot effectively improve the conductivity of silicon, and has limited effect on relieving the volume expansion of silicon, thereby causing the decline of the various performances of the battery. Too much non-metallic element will affect the capacity of silicon, and is also not conducive to the performance of the silicon-based negative electrode material, which eventually causes the decline of the various performances of the battery.

[0130] Comparing Example 1 and Examples 7-9, it can be found that when the thickness of the conductive metal carbon shell is changed, it will also have a significant influence on the structural stability or conductivity of the silicon-based negative electrode material, which is specifically manifested in the influence on the electrochemical performance of the battery. For example, in Example 8, the thickness of the conductive metal carbon shell is too thick, which will reduce the surface porosity, not conducive to the efficient transmission of lithium ions, and also reduce the overall capacity of the silicon-based negative electrode material, which is not conducive to the performance of the silicon-based material, thereby causing the decline of the various performances of the battery in Example 8. Similarly, the conductive metal carbon shell is too thin, which is also proportional to the overall performance of the silicon-based negative electrode material. Because the thickness of the conductive metal carbon shell is too thin, it cannot effectively protect the non-metallic doped silicon core, increase the interface side reaction, and deteriorate the stability of the silicon-based negative electrode material, and also cannot further improve the conductivity of the silicon-based composite material. At the same time, the conductive metal carbon shell with a thin coating is also difficult, which is easy to cause uneven coating, and cannot play the performance of the conductive metal carbon coating, and even lead to the easy peeling of the conductive metal carbon coating, which deteriorates the structural stability of the silicon-based negative electrode material.

[0131] Comparing Comparative Example 1 and Examples 10-11, it can be found that the D50 of the finally prepared silicon-based negative electrode material also has an impact on the electrochemical performance of the final battery. This is because, if the D50 is too small, it is easy to aggregate, and the overall structural stability is poor, which is not conducive to ensuring the processing of the electrode sheet, and is also not conducive to the performance and life of the electrode sheet, resulting in a decline in various aspects of the battery performance. If the D50 is too large, the volume expansion stress of silicon is large, and the ability of the silicon-based negative electrode material to relieve the volume expansion stress is limited, and it also cannot relieve the volume expansion stress of silicon to a greater extent, resulting in a decline in the structural stability of the silicon-based negative electrode material, and also resulting in a decline in various aspects of the battery performance.

Claims

A silicon-based negative electrode material, comprising an inner core and a shell covering the inner core, wherein the inner core comprises non-metallic doped silicon, and the shell comprises a conductive metal carbon layer. In the non-metallic doped silicon, the non-metallic element comprises B. In the conductive metal carbon layer, the conductive metal carbon comprises at least one of Ti3C2(OH)2 and Ti3AlC2. The silicon-based negative electrode material as claimed in claim 1, wherein: The mass percentage of B in the silicon-based negative electrode material is 0.5-2.0%. The silicon-based negative electrode material as claimed in claim 1, wherein: The mass percentage of the conductive metal carbon in the silicon-based negative electrode material is 2-5%. The silicon-based negative electrode material as claimed in claim 1, wherein: The D50 of the silicon-based negative electrode material is 0.5-1.5 μm. The silicon-based negative electrode material as claimed in claim 1, wherein: The thickness of the conductive metal carbon layer is 5-80 nm. The silicon-based negative electrode material of claim 1 is prepared by the following steps: S1. Mixing silicon powder and B oxide, and heat treating the mixture obtained at 900-1200 °C for 2-5 h under a protective atmosphere to obtain a calcined product; S2. Removing silicon oxide from the calcined product to obtain the non-metallic doped silicon; S3. Preparing a conductive metal carbon solution, dispersing the non-metallic doped silicon in the conductive metal carbon solution, stirring at 30-100 °C for 4-8 h, and then spray drying the mixture obtained to obtain the silicon-based negative electrode material. The silicon-based negative electrode material as claimed in claim 6, wherein: In S1, the feeding amount of the silicon powder and the B oxide is calculated based on the molar ratio of Si:B of (400-600):(2-8). The silicon-based negative electrode material as claimed in claim 6, wherein: In S3, the mass ratio of the non-metallic doped silicon to the conductive metal carbon solution is 2-12:

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

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