Silicon negative electrode material, negative electrode sheet, and battery

Abstract

The present application provides a silicon negative electrode material, a negative electrode, and a battery. The silicon negative electrode material comprises an inner core and an outer shell covering the inner core, wherein the inner core comprises metal-doped silicon, and the outer shell comprises a conductive polymer; the mass proportion of a metal element in the silicon negative electrode material is 0.3-1%; and the metal element comprises at least one of germanium and tin.

Description

Silicon negative electrode material, negative electrode sheet and battery

[0001] The present application claims priority to the Chinese patent application No. 2024110535694 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 negative electrode material, a negative electrode sheet and a battery. 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. Since high-rate charging can further reduce the required charging time, the research of high-rate and high-performance power batteries has become a key element to promote the development of the industry.

[0004] Silicon-based negative electrode is a new type of lithium battery negative electrode material, which has a higher theoretical specific capacity (up to 4200 mAh / g, more than 10 times that of graphite) than traditional graphite negative electrode. In addition, silicon-based negative electrode also has the advantages of low discharge platform, high charging and discharging efficiency, good safety, etc., which can improve the energy density and endurance of the battery, and is an ideal choice for the next generation of high-energy-density lithium batteries, so it has attracted much attention. Silicon-based negative electrode has volume effect during charging and discharging, which leads to electrode pulverization and capacity decay. In addition, the conductivity of silicon-based negative electrode is poor, and a conductive agent needs to be added to improve its electrochemical performance.

[0005] To cope with the volume change of silicon negative electrode during lithium intercalation and deintercalation, poor conductivity and formation of unstable SEI, people generally use silicon nanocrystallization or silicon coating methods to improve the cycle performance of silicon negative electrode material. On the one hand, nanosilicon can reduce the absolute volume change of silicon particles caused by lithium ion intercalation and reduce the internal stress of the composite material; on the other hand, coating a material with good conductivity on the surface of nanosilicon will improve the conductivity, while avoiding direct contact between silicon and electrolyte, thereby forming a stable SEI. In related technology, there is a nano-copper coated porous nanosilicon composite material, which uses chemical copper plating method to coat nano-copper on the porous nanosilicon particles prepared by etching silicon alloy with acid. TECHNICAL PROBLEM

[0006] The method of "coating nano-copper on porous nano-silicon particles prepared by etching silicon alloy with acid by using electroless copper plating" plates copper on the surface of silicon, but there are many copper particles on the surface of silicon, no copper film is formed, and it cannot effectively avoid the direct contact between silicon and electrolyte to form unstable SEI, and the copper formed by the method contains a large amount of cuprous oxide, which has a negative impact on the cycle performance of the material. Technical solutions

[0007] In a first aspect, the application provides a silicon negative electrode material, which comprises a core and a shell covering the core, the core comprising metal-doped silicon, and the shell comprising a conductive polymer; the mass percentage of metal elements in the silicon negative electrode material is 0.3-1%; the metal elements comprise at least one of germanium and tin.

[0008] In a second aspect, the application provides a negative electrode sheet comprising the above silicon negative electrode material.

[0009] In a third aspect, the application provides a battery comprising the above negative electrode sheet. Advantages

[0010] (1) The silicon anode material provided in this application has a metal-doped silicon core and a conductive polymer shell. Firstly, the germanium, tin, and lead metal elements selected in this application have high specific capacity for lithium storage, as well as high conductivity and fast lithium-ion mobility, thus effectively improving the rate performance and cycle performance of the silicon anode material. Secondly, after doping silicon with these metal elements, silicon and the doped metal elements react with Li at different initial potentials, resulting in volume changes in silicon and the doped metal elements at different times (germanium and tin also undergo certain volume changes during charge-discharge cycles, but these volume changes are not as significant as those in silicon). This allows the strain-stress during cycling to be gradually released, further improving cycle performance. Thirdly, when Li is embedded in one of the silicon or one of the doped metal elements, the other component can act as a rigid stress buffer layer to alleviate volume changes, thereby improving cycle performance. Furthermore, using conductive polymers as the outer shell of silicon anode materials offers several advantages. First, these polymers possess excellent conductivity, which further enhances the conductivity of the silicon anode material and optimizes rate performance. Second, conductive polymers exhibit good elasticity and flexibility, while metals and silicon generally have poor elasticity and flexibility. Using conductive polymers as the outer shell for metal-doped silicon improves the overall elasticity and flexibility of the silicon anode material. This provides a buffer space for the metal and silicon when the silicon anode material is subjected to external forces or cyclic stress, preventing damage to the metal-doped silicon core and improving the overall structural stability of the silicon anode material. This effectively mitigates the silicon volume expansion effect and optimizes the cycle performance and rate performance of the silicon anode material. Moreover, the conductive polymer shell also has excellent corrosion resistance, reducing the corrosive effect of the electrolyte on the silicon core and minimizing side reactions on the silicon anode side, further optimizing the cycle stability of the silicon-based anode.

[0011] (2) The anode sheet prepared by using the silicon anode material provided in this application has excellent structural stability in terms of particle size, as well as conductivity and lithium ion insertion / extraction capability. It is not easy to break due to silicon volume expansion effect, so the prepared electrode sheet also exhibits relatively excellent stability and conductivity and ion conduction performance. During the charge and discharge cycle, the electrode sheet surface is not prone to structural defects such as cracks, thus optimizing its cycle stability and fast charging performance.

[0012] (3) The battery prepared using the negative electrode sheet provided in this application has good structural stability and conductivity and ion conduction performance. At the same time, the silicon negative electrode material in the negative electrode sheet can effectively alleviate the volume expansion effect of silicon particles during the charge and discharge cycle, thus effectively improving the cycle stability and high rate (fast charge) performance of the battery. Attached Figure Description

[0013] Figure 1 is a schematic diagram of the silicon anode material in this application.

[0014] Reference signs are, 1 - core, 2 - shell.

[0015] Embodiments of the present application

[0016] In some embodiments, the D50 of the silicon negative electrode material is 0.3-3.5 μm. Controlling the D50 of the silicon negative electrode material within the above range can, while taking into account the capacity and cycle performance, avoid the silicon material expanding and shrinking too quickly, and ensure the stability of the electrode structure. Generally speaking, reducing the particle size of the silicon negative electrode material can effectively improve the cycle performance and capacity retention rate of the battery, reduce the volume and weight of the battery, and reduce the manufacturing cost. This is because reducing the size of the silicon particles can increase the specific surface area of the silicon material, increase the contact area with the electrolyte, slow down the volume expansion and contraction of the silicon material, thereby reducing the destruction of the electrode structure and the capacity loss. However, too small silicon particles can also cause some problems. First, too small silicon particles can cause the electrode structure to be unstable, and even cause silicon particles to recombine, deform, and other phenomena, affecting the performance and life of the electrode. Second, too small silicon particles can cause excessive capacity loss, because the volume of the silicon particles is small and the surface area is large, the expansion and contraction is more violent, and the capacity loss is also greater. Finally, too small silicon particles can cause the charge and discharge rate to slow down, because the transfer speed of the electric charge is limited by the diffusion speed on the surface of the silicon particles, affecting its performance under high-speed charge and discharge conditions. If the particle size is too large, the storage capacity of the material will be limited, because the contact area between large particles is reduced and the electron transfer speed is slow, thus also degrading the cycle performance of the silicon negative electrode material.

[0017] In some embodiments, the thickness of the shell in the silicon negative electrode material is 10-50 nm. The coating thickness of the shell should not be too small or too large. If the coating thickness is too small, the metal-doped silicon core cannot be protected, and the corrosion of the electrolyte on it will increase, especially the corrosion of the metal will be more obvious, which will increase the side reaction and reduce the cycle stability of the silicon negative electrode material. At the same time, if the coating thickness is too small, when the silicon in the metal-doped silicon core expands in volume during the cycle process, the shell will not be able to resist the expansion stress and will break, degrading the structural stability of the silicon negative electrode material and further degrading the cycle stability of the silicon negative electrode material. If the coating thickness is too large, the surface porosity of the silicon negative electrode material will decrease, which is not conducive to the infiltration of the electrolyte, reduces the electron transfer rate and lithium ion migration rate, and also degrades the cycle performance of the silicon negative electrode material.

[0018] In some embodiments, the metal elements include germanium and tin. With the combination of germanium and tin, the lithium ions have more excellent deintercalation performance in the silicon negative electrode material, and the framework formed by the metal-doped silicon core is more stable under the combination of these two metal elements, because it is more conducive to relieving the volume stress of the silicon-based negative electrode material, improving the structural stability of the silicon-based negative electrode material, and optimizing the fast charging performance.

[0019] In some embodiments, the mass ratio of germanium and tin is 1:1-3, and the D50 of the silicon negative electrode material is 2.0-3.0 μm. When the above two conditions are controlled at the same time, the cycle performance and high-rate performance of the silicon negative electrode material are better. This is because when the mass ratio of germanium and tin is specific and the particle size is specific, the lattice arrangement of the metal-doped silicon core formed by germanium and tin is more stable, which can better alleviate the volume expansion of silicon and also more obviously improve the conductivity, thereby further optimizing the cycle performance and high-rate fast-charging performance of the silicon negative electrode material.

[0020] In some embodiments, the molecular weight of the conductive polymer is 20,000-70,000. If the molecular weight is too high, the silicon particles are too dense on the surface, which is not conducive to the transmission of lithium ions. If the molecular weight is too small, the flexibility is poor, which is not conducive to relieving the volume expansion effect of the silicon particles.

[0021] In some embodiments, the conductive polymer includes at least one of polyacetylene, polythiophene, polyoxoborate-based polymer, poly(3,4-ethylenedioxythiophene), poly(4-phenyl sulfonic acid), polyaniline, and polypyrrole.

[0022] In some embodiments, the conductive polymer includes polyaniline and polyacetylene. Polyacetylene is a very good conductive polymer with high conductivity, which can play a good conductive role. Polyaniline has a strong rigidity due to the benzene ring, which can improve the stability of the SEI film. In addition, the N in polyaniline can form a hydrogen bond with the silicon surface hydroxyl group, improve the interface connection tightness of the metal-doped silicon core and the conductive polymer shell, make the silicon negative electrode material still maintain the original stable structure under high-rate charging and discharging cycles, and not cause the core-shell to separate, thereby further improving the fast-charging performance of the silicon negative electrode material.

[0023] In some embodiments, when the conductive polymer includes polyaniline and polyacetylene, the mass ratio of polyaniline and polyacetylene is 1:1-3.

[0024] In some embodiments, the preparation of the above-mentioned silicon negative electrode material comprises the following steps: S1. mixing and grinding a metal oxide containing metal elements, silicon particles and a reduced metal powder, then under a protective atmosphere, the obtained mixture is raised to 400-700°C at a temperature raising speed of 4-7 ℃ / min, and after holding for 3-8 h, an alloying product is obtained; S2. after cooling to room temperature, the alloying product is ground, and under a nitrogen atmosphere, the obtained mixture is raised to 600-900 ℃ at a temperature raising speed of 4-7 ℃ / min, and after holding for 5-8 h, a nitriding product is obtained; S3. the nitriding product is put into an acid solution, and reacted at 50-80 ℃ for 6-12 h, and after washing and drying the reaction product, a metal-doped silicon is obtained; S4. a conductive polymer solution is prepared, the metal-doped silicon is added into the conductive polymer solution, and stirred at 40-100 ℃ for 5-10 h, and the obtained mixture is spray-dried to obtain a silicon negative electrode material with a core-shell structure. The reaction principle of the above-mentioned steps is that S1 is an alloying process of calcination, the local high temperature generated thereby reduces the metal oxide to introduce the silicon / metal (doped metal) / magnesium compound into the silicon phase, then S2 continues to nitride under a nitrogen atmosphere to obtain nitrided magnesium and silicon / metal (doped metal), and finally S3 obtains the product silicon-metal (doped metal) through acid etching, i.e. the metal-doped silicon. After obtaining the metal-doped silicon, it is mixed with the conductive polymer solution, and spray-dried to obtain the silicon negative electrode material with a core-shell structure.

[0025] In some embodiments, in S1, the molar ratio of the metal oxide, the silicon particles and the reduced metal powder is 1:15-20:6-10.

[0026] In some embodiments, in S1, the D50 of the silicon particles is 0.3-3 μm.

[0027] In some embodiments, in S1, the protective atmosphere comprises argon.

[0028] In some embodiments, in S3, the acid solution is a hydrochloric acid solution with a concentration of 1 mol / L.

[0029] In some embodiments, in S4, in the conductive polymer solution, the mass fraction of the conductive polymer is 20-40%.

[0030] In some embodiments, in S4, the mass ratio of the metal-doped silicon to the conductive polymer solution is 15-45:1.

[0031] In some embodiments, in S4, the organic solvent used for preparing the conductive polymer solution comprises at least one of N,N-dimethylacetamide, dimethyl sulfoxide, toluene and dichloromethane. The conductive polymer can be dissolved in the organic solvent at 30-60 ℃.

[0032] Example 1

[0033] 1. Preparation of silicon negative electrode material

[0034] The silicon negative electrode material of the present example is prepared according to the following steps:

[0035] S1. GeO2 containing metal elements, silicon particles (metallurgical grade), and Mg powder are mixed and ground, and then the obtained mixture is heated to 550℃ at a heating rate of 5℃ / min under an argon atmosphere, and after holding for 5.5h, an alloying product is obtained; the molar ratio of GeO2, silicon particles, and Mg powder is 1:45:90, and the D50 of the silicon particles is 1.5μm;

[0036] S2. After cooling to room temperature, the alloying product is ground, and then heated to 750℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, and after holding for 6h, a nitriding product is obtained;

[0037] S3. The nitriding product is placed in a hydrochloric acid solution (1mol / L) and reacted at 65℃ for 9h, and after the reaction product is washed and dried, Ge-doped silicon is obtained;

[0038] S4. A conductive polymer solution is prepared, the Ge-doped silicon is added to the conductive polymer solution, and stirred at 70℃ for 7.5h, and then the obtained mixture system is spray dried to obtain a silicon negative electrode material with a core-shell structure; the conductive polymer is polyacetylene+polystyrene, and the mass ratio of polyacetylene and polystyrene is 1:2, the number average molecular weight of polyacetylene is 40000, and the number average molecular weight of polystyrene is 50000; in the conductive polymer solution, the mass fraction of the conductive polymer is 30wt%, and the mass ratio of the Ge-doped silicon and the conductive polymer solution is 30:1, so that the D50 of the final silicon negative electrode material is 2.1μm, the thickness of the conductive polymer shell is 20~40nm, and the mass ratio of Ge in the silicon negative electrode material is 0.6%.

[0039] The structure of the silicon 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 only has an inner core.

[0040] 2. Preparation of negative electrode sheet and battery

[0041] (1) Preparation of negative electrode sheet

[0042] The silicon negative electrode material, conductive agent SP, conductive agent SWCNT, and binder PAA prepared above are dissolved in a solvent in a mass percentage of 80:9:1:10, and the solid content is controlled at 30%, and then coated on a copper foil current collector, vacuum dried, and a negative electrode sheet is prepared.

[0043] (2) Preparation of battery

[0044] A soft package battery was assembled by using the above negative electrode sheet, a conventional mature process to prepare a ternary positive electrode sheet (the positive electrode active material was NCM811, the mass ratio of NCM811, conductive SP and binder PVDF was 93:3:4), 1 mol / L LiPF6 / EC+DMC+EMC (v / v=1:1:1) electrolyte, PE+aluminum oxide separator, and a conventional production process for the shell.

[0045] Example 2

[0046] 1. Preparation of silicon negative electrode material

[0047] In the preparation of the silicon negative electrode material of the present embodiment, the difference from Example 1 is that in S1, the molar ratio of GeO2, silicon particles and Mg powder is 1:65:130, so that the mass ratio of Ge in the silicon negative electrode material is 0.3%. The remaining operations are consistent with Example 1.

[0048] 2. Preparation of negative electrode sheet and battery

[0049] The preparation of the negative electrode sheet and battery in the present embodiment is consistent with Example 1.

[0050] Example 3

[0051] 1. Preparation of silicon negative electrode material

[0052] In the preparation of the silicon negative electrode material of the present embodiment, the difference from Example 1 is that in S1, the molar ratio of GeO2, silicon particles and Mg powder is 1:30:60, so that the mass ratio of Ge in the silicon negative electrode material is 1%. The remaining operations are consistent with Example 1.

[0053] 2. Preparation of negative electrode sheet and battery

[0054] The preparation of the negative electrode sheet and battery in the present embodiment is consistent with Example 1.

[0055] Example 4

[0056] 1. Preparation of silicon negative electrode material

[0057] In the preparation of the silicon negative electrode material of the present embodiment, the difference from Example 1 is that in S1, GeO2 is replaced by SnO2. The remaining operations are consistent with Example 1.

[0058] 2. Preparation of negative electrode sheet and battery

[0059] The preparation of the negative electrode sheet and battery in the present embodiment is consistent with Example 1.

[0060] Example 5

[0061] 1. Preparation of silicon negative electrode material

[0062] In the preparation of the silicon negative electrode material of the present example, different from Example 1 is that in S1, GeO2 is replaced by GeO2 and SnO2, and the molar ratio of GeO2 and SnO2 is 1:2, and the D50 of the obtained silicon negative electrode material is 2.8 μm. The rest of the operations are consistent with Example 1.

[0063] 2. Preparation of negative electrode sheet and battery

[0064] The preparation of the negative electrode sheet and battery in the present example is consistent with Example 1.

[0065] Example 6

[0066] 1. Preparation of silicon negative electrode material

[0067] In the preparation of the silicon negative electrode material of the present example, different from Example 1 is that in S1, GeO2 is replaced by GeO2 and SnO2, and the molar ratio of GeO2 and SnO2 is 1:0.8, and the D50 of the obtained silicon negative electrode material is 2.4 μm. The rest of the operations are consistent with Example 1.

[0068] 2. Preparation of negative electrode sheet and battery

[0069] The preparation of the negative electrode sheet and battery in the present example is consistent with Example 1.

[0070] Example 7

[0071] 1. Preparation of silicon negative electrode material

[0072] In the preparation of the silicon negative electrode material of the present example, different from Example 1 is that in S1, GeO2 is replaced by GeO2 and SnO2, and the molar ratio of GeO2 and SnO2 is 1:0.8, and the D50 of the obtained silicon negative electrode material is 3.5 μm. The rest of the operations are consistent with Example 1.

[0073] 2. Preparation of negative electrode sheet and battery

[0074] The preparation of the negative electrode sheet and battery in the present example is consistent with Example 1.

[0075] Example 8

[0076] 1. Preparation of silicon negative electrode material

[0077] In the preparation of the silicon negative electrode material of the present example, different from Example 1 is that in S4, the mass fraction of the conductive polymer in the conductive polymer solution is 20wt%, so that the thickness of the final conductive polymer shell is 10-15 nm. The rest of the operations are consistent with Example 1.

[0078] 2. Preparation of negative electrode sheet and battery

[0079] The negative electrode sheet and the battery in this example were prepared in the same manner as in Example 1.

[0080] Example 9

[0081] 1. Preparation of silicon negative electrode material

[0082] In the preparation of the silicon negative electrode material in this example, the difference from Example 1 is that in S4, the mass fraction of the conductive polymer in the conductive polymer solution is 40 wt%, so that the thickness of the final conductive polymer shell is 60-70 nm. The remaining operations are the same as in Example 1.

[0083] 2. Preparation of negative electrode sheet and battery

[0084] The negative electrode sheet and the battery in this example were prepared in the same manner as in Example 1.

[0085] Example 10

[0086] 1. Preparation of silicon negative electrode material

[0087] In the preparation of the silicon negative electrode material in this example, the difference from Example 1 is that in S4, the conductive polymer is polyaniline. The remaining operations are the same as in Example 1.

[0088] 2. Preparation of negative electrode sheet and battery

[0089] The negative electrode sheet and the battery in this example were prepared in the same manner as in Example 1.

[0090] Example 11

[0091] 1. Preparation of silicon negative electrode material

[0092] In the preparation of the silicon negative electrode material in this example, the difference from Example 1 is that in S4, the conductive polymer is polyoxaborate-based polymer with a number average molecular weight of 50000. The remaining operations are the same as in Example 1.

[0093] 2. Preparation of negative electrode sheet and battery

[0094] The negative electrode sheet and the battery in this example were prepared in the same manner as in Example 1.

[0095] Example 12

[0096] 1. Preparation of silicon negative electrode material

[0097] In the preparation of the silicon negative electrode material in this example, the difference from Example 1 is that in S4, the conductive polymer is poly(4-phenyl sulfonic acid) with a number average molecular weight of 50000. The remaining operations are the same as in Example 1.

[0098] 2. Preparation of negative electrode sheet and battery

[0099] The negative electrode sheet and the battery in this example were prepared in the same manner as in Example 1.

[0100] Example 13

[0101] 1. Preparation of silicon negative electrode material

[0102] In the preparation of the silicon negative electrode material in this example, the difference from Example 1 is that in S4, the conductive polymer is polythiophene and polypyrrole, and the number average molecular weight is 40000 and 50000, respectively, and the mass ratio of polythiophene to polypyrrole is 1:2. The remaining operations are the same as in Example 1.

[0103] 2. Preparation of negative electrode sheet and battery

[0104] The negative electrode sheet and the battery in this example were prepared in the same manner as in Example 1.

[0105] Example 14

[0106] 1. Preparation of silicon negative electrode material

[0107] In the preparation of the silicon negative electrode material in this example, the difference from Example 1 is that in S4, the number average molecular weight of the conductive polymers polyaniline and polyacetylene is 15000 and 17000, respectively. The remaining operations are the same as in Example 1.

[0108] 2. Preparation of negative electrode sheet and battery

[0109] The negative electrode sheet and the battery in this example were prepared in the same manner as in Example 1.

[0110] Example 15

[0111] 1. Preparation of silicon negative electrode material

[0112] In the preparation of the silicon negative electrode material in this example, the difference from Example 1 is that in S4, the number average molecular weight of the conductive polymers polyaniline and polyacetylene is 75000 and 80000, respectively. The remaining operations are the same as in Example 1.

[0113] 2. Preparation of negative electrode sheet and battery

[0114] The negative electrode sheet and the battery in this example were prepared in the same manner as in Example 1.

[0115] Comparative Example 1

[0116] 1. Preparation of silicon negative electrode material

[0117] In the preparation of the silicon negative electrode material in this example, the difference from Example 1 is that the steps of S1-S3 are not performed, and in S4, the silicon particles are directly mixed with the conductive polymer solution and spray dried. The remaining operations are the same as in Example 1.

[0118] 2. Preparation of negative electrode sheet and battery

[0119] The negative electrode sheet and battery in the present comparative example were prepared in the same manner as in Example 1.

[0120] Comparative Example 2

[0121] 1. Preparation of silicon negative electrode material

[0122] In the preparation of the silicon negative electrode material in the present comparative example, the difference from Example 1 is that the step of S4 is not performed, and the Ge-doped silicon prepared in S3 is directly used as the silicon negative electrode material. The remaining operations are the same as in Example 1.

[0123] 2. Preparation of negative electrode sheet and battery

[0124] The negative electrode sheet and battery in the present comparative example were prepared in the same manner as in Example 1.

[0125] Comparative Example 3

[0126] 1. Preparation of silicon negative electrode material

[0127] In the preparation of the silicon negative electrode material in the present comparative example, the difference from Example 1 is that in S1, the molar ratio of GeO2, silicon particles, and Mg powder is 1:80:160, so that the mass ratio of Ge in the silicon negative electrode material is 0.1%. The remaining operations are the same as in Example 1.

[0128] 2. Preparation of negative electrode sheet and battery

[0129] The negative electrode sheet and battery in the present comparative example were prepared in the same manner as in Example 1.

[0130] Comparative Example 4

[0131] 1. Preparation of silicon negative electrode material

[0132] In the preparation of the silicon negative electrode material in the present comparative example, the difference from Example 1 is that in S1, the molar ratio of GeO2, silicon particles, and Mg powder is 1:20:40, so that the mass ratio of Ge in the silicon negative electrode material is 1.3%. The remaining operations are the same as in Example 1.

[0133] 2. Preparation of negative electrode sheet and battery

[0134] The negative electrode sheet and battery in the present comparative example were prepared in the same manner as in Example 1.

[0135] Comparative Example 5

[0136] 1. Preparation of silicon negative electrode material

[0137] In the preparation of the silicon negative electrode material of the present comparative example, the difference from Example 1 is that the Ge-doped silicon obtained in S3 is coated with a carbon layer shell, i.e., the operation of S4 is changed as follows: the Ge-doped silicon is introduced into a fluidized bed type atmosphere furnace, heated to 800°C under an inert atmosphere, and ethylene, a carbon source gas, is introduced, and heat preservation is performed for 5h, then the carbon source gas is turned off, and the temperature is lowered to room temperature to obtain the silicon negative electrode material. In the above process, the flow rate of the carbon source gas ethylene is controlled to be 50 L / min, the amount of Ge-doped silicon used is consistent with Example 1, and the D50, shell thickness, and mass ratio of Ge in the silicon negative electrode material obtained are consistent with Example 1. The remaining operations are consistent with Example 1.

[0138] 2. Preparation of negative electrode sheet and battery

[0139] The preparation of the negative electrode sheet and battery in the present comparative example is consistent with Example 1.

[0140] Comparative Example 6

[0141] 1. Preparation of silicon negative electrode material

[0142] In the preparation of the silicon negative electrode material of the present comparative example, the difference from Example 1 is that the GeO2 obtained in S1 is replaced with CuO. The remaining operations are consistent with Example 1.

[0143] 2. Preparation of negative electrode sheet and battery

[0144] The preparation of the negative electrode sheet and battery in the present comparative example is consistent with Example 1.

[0145] Test Example

[0146] 1. Experimental construction method

[0147] The batteries prepared in all the above examples and comparative examples are subjected to relevant electrochemical performance tests, which are specifically tested on a LAND battery test system of Wuhan Jinuo Electronics Co., Ltd. at room temperature (25°C), and the charge and discharge voltage is limited to 2.5V~4.2V. The calculation method or test conditions of the initial efficiency (first charge and discharge efficiency), cycle performance and rate performance are as follows:

[0148] (1) Initial efficiency: total capacity of 0.33C first discharge of battery / total capacity of 0.33C first charge of battery

[0149] (2) Capacity retention rate at room temperature 2C / 2C cycle for 500 cycles

[0150] 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, and recording the capacity retention rate of the battery at 2C / 2C cycle for 500 cycles.

[0151] (3) 6C rate performance at room temperature

[0152] 6C rate charge: 1C constant current discharge to 2.5V at 25℃, rest for 10min, 6C constant current constant voltage charge to 4.2V, 0.05C cut-off, record the constant current charge capacity, constant current constant voltage total capacity and the highest temperature in the fast charging process, constant current charge ratio = constant current charge capacity / constant current constant voltage total capacity.

[0153] 2. Experimental results

[0154] The first efficiency (first charge-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.

[0155] Table 1. Performance test results of batteries prepared in examples and comparative examples

[0156] Group First efficiency (%) Room temperature 2C / 2C cycle 500 cycles capacity retention rate (%) 6C rate charge-constant current charge ratio (%) 6C rate charge-highest temperature (℃) Example 1 85.29 0.38 0.03 6 Example 2 82.38 8.87 8.73 8 Example 3 83.18 9.27 9.03 7 Example 4 81.98 7.27 7.23 9 Example 5 86.49 1.38 1.23 6 Example 6 85.39 0.48 0.63 8 Example 7 85.79 0.58 0.33 8 Example 8 84.08 6.87 8.93 7 Example 9 81.98 7.57 7.73 8 Example 10 82.18 5.47 5.24 1 Example 11 81.58 7.07 6.64 0 Example 12 82.48 5.77 6.94 0 Example 13 81.88 6.67 5.93 9 Example 14 82.18 7.97 7.43 8 Comparative example 1 72.07 5.25 8.56 0 Comparative example 2 73.27 6.85 9.65 8 Comparative example 3 74.67 7.96 1.15 5 Comparative example 4 75.57 8.26 4.85 0 Comparative example 5 76.08 1.77 0.24 8 Comparative example 6 74.77 7.96 2.55 4

[0157] As can be seen from Table 1, the battery prepared by using the silicon negative electrode material provided in the present application has excellent conductivity and ion conductivity, and the silicon negative electrode material itself has stable structure, which can effectively relieve the volume expansion stress of silicon in the charge-discharge cycle process, thereby optimizing the first efficiency, room temperature cycle performance and high rate fast charging performance of the battery. It is a kind of fast charging negative electrode material with excellent performance, and specific reference is made to Examples 1-14.

[0158] In the silicon negative electrode material of Comparative Example 1, the core is not doped with metal, and the silicon negative electrode material has poor conductivity. In addition, there is no metal to serve as a buffer for the silicon, and the volume expansion effect of the silicon negative electrode material is more obvious, which leads to poor initial efficiency, room temperature cycle performance and high-rate charging performance of the battery prepared therefrom, and the battery has high temperature during high-rate charging and poor heat resistance.

[0159] In the silicon negative electrode material of Comparative Example 2, only the core is doped with metal, and there is no conductive polymer layer. Therefore, the core cannot be effectively protected, the corrosion of the electrolyte to the core is strengthened, and the conductivity of the material is also reduced without the coating of the conductive polymer, which leads to poor initial efficiency, room temperature cycle performance and high-rate charging performance of the battery prepared therefrom. In addition, without the protection of the conductive polymer layer, the side reaction is more obvious, and the temperature of the battery during high-rate charging is also obviously increased.

[0160] In the silicon negative electrode materials of Comparative Examples 3 and 4, the mass ratio of the metal element in the silicon negative electrode material is too low or too high. The mass ratio of the metal element that is too low cannot effectively improve the conductivity of the silicon material and relieve the volume effect, and the mass ratio of the metal element that is too high may affect the performance of the silicon itself, degrade the capacity, etc., and ultimately lead to a decrease in the performance of the silicon negative electrode material. Therefore, the initial efficiency, room temperature cycle performance and high-rate charging performance of the battery prepared therefrom are poor.

[0161] In the silicon negative electrode material of Comparative Example 5, the conductive polymer layer is replaced by a carbon layer. Compared with the carbon layer, the conductive polymer has good flexibility and mechanical properties, and can effectively relieve the volume expansion of the silicon material during the charging and discharging process of the battery. Therefore, the initial efficiency, room temperature cycle performance and high-rate charging performance of the battery using the carbon layer as the shell are relatively poor.

[0162] In the silicon negative electrode material of Comparative Example 6, the metal of the metal-doped silicon core is Cu. The specific capacity of Cu for storing lithium is low, and the volume change during charging and discharging is not obvious, which cannot effectively relieve the strain-stress during the cycle process. Therefore, the initial efficiency, room temperature cycle performance and high-rate charging performance of the battery prepared therefrom are relatively poor.

[0163] Further comparison of Examples 1, 2 and 3 shows that, compared with Example 1, the mass ratio of the metal Ge in the silicon negative electrode material of Examples 2 and 3 is relatively low and relatively high, respectively. Both the low and high mass ratios of the metal Ge affect the performance of the silicon negative electrode material, and ultimately affect the performance of the battery. This shows that further limiting the amount of metal doped in the silicon negative electrode material is more conducive to balancing the performance of the silicon negative electrode material in multiple aspects, and further optimizing the performance of the battery.

[0164] Comparing Example 1 with Examples 4-5, it can be found that when the type of the doped metal is changed, the silicon negative electrode material can also have good conductivity, ion conductivity and structural stability, which can be seen from the high initial efficiency and good room temperature cycle performance and high rate charging performance of the battery. It can be further seen that when the doped metal is Ge and Sn, the battery exhibits better performance, which indicates that when these two metals are used together to dope silicon, the conductivity or ion conductivity of the silicon negative electrode material can be further improved, and the volume expansion of silicon can be further relieved, thereby further improving the performance of the silicon negative electrode material and optimizing the performance of the battery.

[0165] Comparing Examples 5-7, it can be found that when the mass ratio of Ge and Sn is changed, the D50 of the silicon negative electrode material will also change, and with this change, the performance of the silicon negative electrode material also changes, and ultimately the performance of the battery also changes. As can be seen from these examples, when the mass ratio of Ge and Sn is 1:2, the battery prepared from the silicon negative electrode material has better performance.

[0166] Comparing Example 1 with Examples 8-9, compared with Example 1, the thickness of the conductive polymer shell in Examples 8 and 9 is relatively low and relatively high, respectively. When the thickness of the conductive polymer shell is relatively low, the protection effect of the conductive polymer on the core is relatively poor, and the relief of the volume expansion of the silicon material is also relatively limited, so the performance of the battery prepared therefrom also decreases. When the thickness of the conductive polymer shell is relatively high, the lithium ion transport path is increased, and the internal resistance is also increased, thereby causing the performance of the battery prepared therefrom to also decrease.

[0167] Comparing Example 1 with Examples 10-13, it can be found that when other conductive polymers are used as the shell, the silicon negative electrode material still has good conductivity, ion 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 these silicon negative electrode materials. And through further comparison, when the conductive polymer is a combination of polyaniline and polyacetylene, the battery prepared from the silicon negative electrode material exhibits better electrochemical performance.

[0168] Comparing Example 1 with Examples 14-15, compared with Example 1, the number average molecular weight of the conductive polymer in Examples 14 and 15 is relatively small and relatively large, respectively. When the molecular weight is small, the polymer has low flexibility and low mechanical strength, the protection effect on the core and the volume effect relief performance decrease, and therefore the performance of the battery decreases. When the molecular weight is large, the accumulation on the surface of the core is too dense, which is not conducive to the transmission of lithium ions, and also increases the internal resistance, thereby also causing the performance of the battery to decrease.

Claims

1. A silicon negative electrode material, comprising a core and a shell covering the core, wherein the core comprises metal-doped silicon, and the shell comprises a conductive polymer; a mass percentage of metal elements in the silicon negative electrode material is 0.3-1%; the metal elements comprise at least one of germanium and tin.

2. The silicon negative electrode material of claim 1, wherein: In the silicon negative electrode material, a thickness of the shell is 10-50 nm.

3. The silicon negative electrode material of claim 1, wherein: the metal elements comprise germanium and tin.

4. The silicon negative electrode material of claim 3, wherein: a mass ratio of the germanium to the tin is 1:1-3, and a D50 of the silicon negative electrode material is 2.0-3.0 μm.

5. The silicon negative material of claim 1, wherein: a molecular weight of the conductive polymer is 20,000-70,000.

6. The silicon anode material of claim 1, wherein: the conductive polymer comprises at least one of polyacetylene, polythiophene, polyoxaboride-based polymer, poly(3,4-ethylenedioxythiophene), poly(4-phenyl sulfonic acid), polyaniline, and polypyrrole.

7. The silicon negative material of claim 6, wherein: the conductive polymer comprises polyaniline and polyacetylene. 8.The silicon negative electrode material of claim 1, wherein a preparation method thereof comprises the following steps: S1. mixing and grinding a metal oxide containing the metal elements, silicon particles, and a reduced metal powder, then heating the obtained mixture to 400-700℃ at a heating rate of 4-7 ℃ / min under a protective atmosphere, and keeping the temperature for 3-8 h to obtain an alloying product; S2. grinding the alloying product after cooling to room temperature, and heating the alloying product to 600-900 ℃ at a heating rate of 4-7 ℃ / min under a nitrogen atmosphere, and keeping the temperature for 4-8 h to obtain a nitriding product; S3. putting the nitriding product into an acid solution, and reacting at 50-80 ℃ for 6-12 h, and then washing and drying the reaction product to obtain the metal-doped silicon; S4. preparing a conductive polymer solution, adding the metal-doped silicon into the conductive polymer solution, and stirring at 40-100 ℃ for 5-10 h, and then spray-drying the obtained mixture to obtain the silicon negative electrode material with a core-shell structure. 9.A negative electrode sheet, comprising the silicon negative electrode material of any one of claims 1-8. 10.A battery, comprising the negative electrode sheet of claim 9.

Images