Silicon-based composite material as well as preparation method and application thereof

Abstract

The invention provides a silicon-based composite material and a preparation method and application thereof. The silicon-based composite material comprises an inner core, a first coating layer and a second coating layer, wherein the inner core is sequentially coated with the first coating layer and the second coating layer; the inner core comprises a porous framework matrix and nano silicon particles, and the porous framework matrix comprises at least one of a porous carbon material, a porous metal material, a porous organic / inorganic metal framework material and a large-specific-surface graphite material; the first coating layer comprises at least one of a silicon-oxygen compound and carbon; and the second coating layer comprises at least one of a metal phosphorus-containing compound, a metal boron-containing compound, a metal nitrogen-containing compound, a metal oxide and a conductive polymer. According to the silicon-based composite material, the activity of nano silicon in the silicon-based composite material can be properly reduced, the stability of the silicon-based composite material can be improved, the mechanical stability of the silicon-based composite material can be improved, the huge expansive force of the nano silicon after lithium intercalation can be effectively relieved, meanwhile, the interface impedance can be reduced, and the performance of the silicon-based composite material is improved. And the silicon-based composite material has relatively good conductivity.

Description

Technical Field

[0001] The present application belongs to the technical field of lithium-ion batteries, and specifically relates to a silicon-based composite material and a preparation method and application thereof. Background Art

[0002] Since silicon-based negative electrodes have a high theoretical specific capacity (4200mAh / g), which is more than 10 times that of traditional graphite negative electrodes, they are considered to be one of the most promising negative electrode materials in the field of lithium-ion batteries. However, silicon-based materials have a huge volume expansion (~270%) during the charging and discharging process. The expansion stress generated after assembling into a battery cell may cause the battery cell to face battery cell failure and safety risks such as bulging, leakage, and opening. At the same time, the expansion of the silicon negative electrode causes the negative electrode interface SEI film to continuously decompose and form during the cycle of the battery cell, resulting in rapid capacity decay and poor cycle performance.

[0003] Vapor-deposited silicon carbon is a low-expansion silicon-based composite material currently in mass production on the market. The specific method is to deposit nano-silicon on a porous hard carbon matrix to synthesize a silicon-based composite material. Part of the pore structure is reserved inside the material to alleviate the expansion, which can reduce the expansion of silicon materials by at least 20%. The synthesized nano-silicon-based composite material is coated with a carbon layer by the CVD method to reduce the activity of nano-silicon and improve the stability of the nano-silicon-based composite material. At the same time, it can avoid direct contact between nano-silicon and electrolyte during the charging and discharging process of the battery cell, and reduce the side reactions generated at the negative electrode interface; however, the outer carbon layer of the vapor-deposited silicon-carbon material mainly adopts low-temperature coating, and the density of the coating layer may not be enough. There may be a small amount of pores on the surface, resulting in nano-silicon exposure, affecting the long-term service life of the battery cell. At the same time, the conductivity of the low-temperature coated carbon layer is poor, the interface conductivity and ion affinity of the coating layer are poor, and the charge transfer internal resistance at the interface is relatively high, showing poor power performance. Moreover, only coating with a carbon layer will also result in insufficient coating effect on nano-silicon, so it cannot effectively improve the stability of the nano-silicon-based composite material. Summary of the invention

[0004] In order to solve the problems and shortcomings existing in the prior art, the present application provides a silicon-based composite material and a preparation method and application thereof. The silicon-based composite material has two coating layers, which can not only appropriately reduce the activity of nano-silicon in the silicon-based composite material and improve the stability of the silicon-based composite material, but also improve the mechanical stability of the silicon-based composite material, effectively alleviate the huge expansion force of nano-silicon after lithium insertion, and at the same time reduce the interface impedance, so that the silicon-based composite material has better conductive properties.

[0005] According to the first aspect of the present application, a silicon-based composite material is provided, comprising an inner core, and a first coating layer and a second coating layer which sequentially coat the inner core; the inner core comprises a porous skeleton matrix and nano-silicon particles, the porous skeleton matrix comprises at least one of a porous carbon material, a porous metal material, a porous organic / inorganic metal framework material, and a large specific surface area graphite material; the first coating layer comprises at least one of a silicon oxide compound and carbon; the second coating layer comprises at least one of a metal phosphorus-containing compound, a metal boron-containing compound, a metal nitrogen-containing compound, a metal oxide, and a conductive polymer.

[0006] In the silicon-based composite material provided in the present application, the kernel includes a porous skeleton matrix and nano silicon particles, and then the outside of the kernel is coated with a first coating layer and a second coating layer in sequence. First, the first coating layer includes a silicon oxide or carbon, and the silicon oxide layer or the carbon layer can effectively passivate the nano silicon particles, so the activity of the silicon-based composite material can be effectively reduced, thereby improving the stability of the nano silicon-based composite material. However, for the silicon-containing particle kernel, only coating the first coating layer, the coating layer formed by it is not complete enough, and a more obvious protective effect cannot be played on the silicon-containing particle kernel. Even if the stability of the silicon-containing particle kernel can be improved to a certain extent, the improvement effect is limited, and the volume expansion of silicon during the charge and discharge process cannot be well alleviated, so the electrochemical properties of the obtained silicon-based composite material such as cycle performance, rate performance, etc. are still relatively poor.

[0007] Therefore, the present application continues to form a second coating layer on the basis of the first coating layer. First, the second coating layer in the present application is a material with good electrical conductivity and ion conductivity, and has good lithium ion adsorption performance, which is conducive to the transmission of lithium ions at the coating layer interface and effectively reduces the interface impedance. Second, the second coating layer improves the coating integrity of the surface of the silicon-based material, further optimizes the dense uniformity of the coating layer on the surface of the core, can effectively avoid direct contact between the electrolyte and the active material, and reduce the surface floating silicon content, thereby improving the interface stability, thereby optimizing the cycle stability of the silicon-based composite material. Third, there is a strong covalent effect between the anions and cations in the second coating layer, so that it has good mechanical stability as a coating layer, which is conducive to alleviating the huge expansion force of nano-silicon after lithium insertion. Thus, the introduction of the second coating layer effectively optimizes the electrical conductivity and lithium ion transmission capacity of the silicon-based composite material, while greatly improving the interface stability of the silicon-based composite material, and can improve the volume expansion mitigation ability of the silicon material, thereby optimizing the cycle performance, rate performance, etc. of the silicon-based composite material.

[0008] It should also be noted that in the silicon-based composite material provided in the present application, the electrochemical properties such as cycle performance and rate performance of the silicon-based composite material can be further optimized under the joint action of the first coating layer and the second coating layer. When only the second coating layer is coated, the activity of the nano-silicon in the synthesized silicon-carbon is relatively high, and there are safety hazards in the synthesis process. Moreover, too high activity of the nano-silicon will lead to an increase in side reactions with the electrolyte after penetration, which is not conducive to the stability and performance of the silicon-based material.

[0009] Preferably, the first coating layer includes silicon oxides; the second coating layer includes aluminum phosphorus compounds, aluminum boron compounds, aluminum nitrogen compounds, and aluminum oxides. The combination of the first coating layer and the second coating layer can provide a more significant protective effect on the silicon-containing core, while appropriately reducing the reactivity of silicon particles without excessively affecting the performance of silicon particles, and is therefore more conducive to taking into account the stability of the silicon-based composite material as well as the performance of capacity, charge and discharge performance, and other aspects.

[0010] Preferably, the pore volume of the porous skeleton matrix is ​​0.3-2.0 cm 3 / g, specific surface area is 10-2000m 2 / g.

[0011] Preferably, the conductive polymer includes at least one of polyaniline, polyacrylic acid, polypyrrole, phenolic resin and epoxy resin.

[0012] Preferably, the aluminum-phosphorus compound includes at least one of aluminum phosphate and aluminum metaphosphate; the aluminum-boron compound includes aluminum borate; the aluminum-nitrogen compound includes aluminum nitrate; and the aluminum oxide includes at least one of aluminum oxide and aluminum isopropoxide.

[0013] Preferably, the first coating layer includes silicon oxides; the second coating layer includes aluminum phosphate. The first coating layer and the second coating layer are respectively made of the above materials. Such a combination is more conducive to improving the interface stability, electrical conductivity and ion conductivity of the silicon-based composite material, and is also more conducive to reducing the interface impedance and alleviating the volume expansion effect of silicon, thereby being more conducive to the electrochemical performance of the silicon-based composite material in the battery and optimizing the cycle performance and rate performance of the battery.

[0014] Preferably, the thickness of the first coating layer is 0.5 to 10 nm; the thickness of the second coating layer is 0.1 to 20 nm. Controlling the thickness of the first coating layer and the second coating layer respectively can not only effectively protect the inner core of the silicon-containing particles, but also will not significantly reduce the conductivity of the silicon-based composite material and the lithium ion transmission efficiency. Moreover, ensuring that the thickness of the first coating layer and the second coating layer is within the above range is more conducive to the synergistic effect of the first coating layer and the second coating layer, so that the two coating layers will not affect each other's performance, but can further make up for each other's shortcomings, thereby further optimizing the electrochemical performance of the silicon-based composite material.

[0015] Preferably, the D50 of the silicon-based composite material is 5 to 15 μm. Controlling the D50 of the silicon-based composite material within the above range is more conducive to taking into account the surface activity, capacity performance, cycle stability, rate performance, processing performance and other aspects of the silicon-based composite material.

[0016] Preferably, the porous skeleton matrix includes porous carbon materials and porous organic / inorganic metal framework materials. Further, the types of the above-mentioned porous skeleton matrix are selected, and the three-dimensional network structure formed by the porous skeleton matrix and the nano-silicon particles are interwoven with each other is more stable, so that lithium ions can diffuse and migrate more quickly inside the electrode material during the charging and discharging process, thereby improving the charging and discharging efficiency of the battery and shortening the charging time. Moreover, under the above-mentioned combination, the volume expansion effect of the nano-silicon particles can be better alleviated, the volume change of the silicon particles can be limited, and under the joint action of the first coating layer and the second coating layer, the pulverization and structural damage of the electrode material caused by the volume change can be reduced, thereby improving the cycle stability and charging and discharging performance of the battery.

[0017] Preferably, the mass ratio of the porous carbon material to the porous organic / inorganic metal framework material is 80-95:5-20.

[0018] Preferably, the D50 of the porous carbon material is 3 to 10 μm, and the D50 of the porous organic metal framework material is 8 to 16 μm.

[0019] Preferably, in the core, the particle size of the nano silicon particles is 1 to 10 nm. Controlling the particle size of the nano silicon particles within the above range is more conducive to taking into account the performance of the nano silicon particles in terms of capacity, lithium ion transmission efficiency, expansion effect, distribution uniformity, etc., so that the silicon-based composite material has better comprehensive performance and is more conducive to its electrochemical performance in the battery.

[0020] According to the second aspect of the present application, a method for preparing the above-mentioned silicon-based composite material is provided, comprising the following steps: S1. placing a porous skeleton matrix precursor in a first inert gas atmosphere, keeping it warm at 500-1200°C for 0.5-6h, and obtaining a porous skeleton matrix after crushing and grading; S2. placing the porous skeleton matrix in a silicon-containing atmosphere, depositing it at 300-1000°C for 0.5-50h to obtain an inner core; S3. placing the inner core in an oxygen-containing atmosphere or a carbon-containing atmosphere, coating it at 100-800°C for 0.1-20h to complete coating of the first coating layer; S4. then placing the product obtained in S3 in a second coating layer precursor solution, stirring the reaction for 0.1-2h, and obtaining a silicon-based composite material after drying.

[0021] In the preparation method of the silicon-based composite material provided in the present application, through the above-mentioned steps, not only the nano-silicon particles can be uniformly deposited in the porous skeleton matrix, but also the first coating layer and the second coating layer can be uniformly and densely deposited on the surface of the silicon-containing particle core. In addition, in such steps, the second coating layer can be tightly connected to the surface of the first coating layer to form a more uniform and dense coating layer. Therefore, it can effectively protect the silicon-containing particle core and effectively alleviate the volume expansion effect of the silicon particles, thereby jointly optimizing the structural stability of the silicon-based composite material during the charge and discharge cycle, and effectively improving the cycle performance, rate performance and other properties of the silicon-based composite material.

[0022] Preferably, in S1, the carbon precursor of the porous carbon material includes at least one of phenolic resin, carbon black, MOF-5, furfuryl alcohol polymer, glucose, chitosan, polydopamine, and polystyrene.

[0023] Preferably, in S1, the temperature is increased to 500-1200°C at a heating rate of 1-4°C / min.

[0024] Preferably, in S1 , the first inert gas atmosphere includes nitrogen.

[0025] Preferably, in S2, the silicon-containing atmosphere includes at least one of monosilane, disilane, polysilane silole, and polysilane silole derivatives.

[0026] Preferably, in S2, the silicon-containing atmosphere further comprises a second inert gas atmosphere. Preferably, the second inert gas atmosphere comprises nitrogen.

[0027] Preferably, in S2, before the silicon-containing atmosphere is introduced, a second inert gas atmosphere is introduced in advance to raise the temperature to 300-1000°C.

[0028] Preferably, in S3, the oxygen-containing atmosphere comprises at least one of oxygen and air. Preferably, in S3, the oxygen-containing atmosphere comprises air.

[0029] Preferably, in S3, the carbon-containing atmosphere includes at least one of ethanol, methane, benzene, and benzene derivatives.

[0030] Preferably, in S3, the introduced atmosphere includes an oxygen-containing atmosphere. Preferably, in S3, the introduced oxygen-containing atmosphere is air.

[0031] Preferably, in S3, the introduced atmosphere includes an oxygen-containing atmosphere and a third inert gas atmosphere. Preferably, the volume ratio of the oxygen-containing atmosphere to the introduced atmosphere is 0.1-3%. Preferably, in S3, the introduced oxygen-containing atmosphere is air, and the introduced third inert gas atmosphere includes nitrogen.

[0032] Preferably, in S4, the second coating layer precursor solution includes Al(NO)3·9H2O and (NH4)2HPO4. Preferably, the mass ratio of Al(NO)3·9H2O to (NH4)2HPO4 is 2 to 5:1.

[0033] According to the third aspect of the present application, a negative electrode sheet is provided, comprising the above silicon-based composite material, or the silicon-based composite material prepared by the above silicon-based composite material preparation method. The negative electrode sheet obtained in the present application has a lower expansion rate, and the surface active material is less likely to fall off, because the silicon-based composite material has good mechanical properties and structural stability, so the negative electrode sheet prepared by the silicon-based composite material has higher stability and electrochemical performance.

[0034] According to the fourth aspect of the present application, a battery is provided, comprising the above silicon-based composite material, or the silicon-based composite material prepared by the above silicon-based composite material preparation method, or the above negative electrode sheet. The battery obtained in the present application has good cycle performance and rate performance, and is a silicon-based system battery with excellent performance. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only embodiments of a part of the present application, rather than all of the embodiments.

[0036] Example 1

[0037] 1. Preparation of silicon-based composite materials

[0038] The silicon-based composite material of this embodiment is prepared according to the following steps:

[0039] S1. The porous skeleton matrix precursor phenolic resin was placed in a nitrogen atmosphere, heated to 800°C at 2°C / min and kept at that temperature for 2h, and then crushed and classified to obtain a porous skeleton matrix; the D50 of the porous skeleton matrix (porous carbon material) was 8μm;

[0040] S2. The porous skeleton substrate is first placed in a nitrogen atmosphere and heated to 550°C, and then the nitrogen atmosphere is replaced with a first mixed atmosphere of a silicon-containing atmosphere and a nitrogen atmosphere, and deposited at 550°C for 5h to obtain a core; the silicon-containing atmosphere is a silane atmosphere, and the volume ratio of the silane atmosphere in the first mixed atmosphere is 15%;

[0041] S3. The core is placed in a second mixed atmosphere of an oxygen atmosphere and a nitrogen atmosphere, and coated at 300 ° C for 10 hours to complete the coating of the first coating layer (silicon oxide compound); the oxygen-containing atmosphere is an air atmosphere, and the volume ratio of the air atmosphere in the first mixed atmosphere is 1.5%;

[0042] S4. Then, the product obtained in S3 is placed in a second coating layer precursor solution, stirred to react for 1 hour, and after drying, the second coating layer (aluminum phosphate) is coated to obtain a silicon-based composite material; the second coating layer precursor solution is an ethanol solution of Al(NO)3·9H2O and (NH4)2HPO4, and the mass ratio of Al(NO)3·9H2O and (NH4)2HPO4 is 3.5:1.

[0043] And after testing, the thickness of the first coating layer is 5nm, the thickness of the second coating layer is 10nm, and the D50 of the silicon-based composite material is 8.01μm.

[0044] 2. Preparation of batteries

[0045] The above silicon-based composite material, graphite, conductive agent SP and binder PAA are mixed evenly in a mass ratio of 15:80:2.5:2.5, respectively, and water is used as a solvent to mix into a slurry with a solid content of 38-42%. Copper foil is used as a current collector, and it is coated and dried as a negative electrode. A ternary material is used as a positive electrode. The mass ratio of ternary material (NCM811), conductive agent SP, and binder PVDF is 97:1.2:1.8. NMP is used as a solvent. The above materials are mixed evenly and coated on aluminum foil. After coating and drying, it is used as a positive electrode. The electrolyte is 1 mol / L LiPF6 / EC+DMC+EMC, the volume ratio is 1:1:1, and the diaphragm is Celgard2400 diaphragm. The positive electrode, negative electrode and diaphragm are assembled into a soft-pack 396389 battery.

[0046] Example 2

[0047] 1. Preparation of silicon-based composite materials

[0048] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that the oxygen-containing atmosphere is adjusted to methane in S3. The rest of the operations are the same as those in Embodiment 1.

[0049] And after testing, the thickness of the first coating layer is 6nm, the thickness of the second coating layer is 11nm, and the D50 of the silicon-based composite material is 8.02μm.

[0050] 2. Preparation of batteries

[0051] The preparation of the battery in this example is consistent with that in Example 1.

[0052] Example 3

[0053] 1. Preparation of silicon-based composite materials

[0054] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that the mass ratio of Al(NO)3·9H2O to (NH4)2HPO4 in S4 is 1:1. The rest is the same as the operation in Embodiment 1.

[0055] And after testing, the thickness of the first coating layer is 6nm, the thickness of the second coating layer is 14nm, and the D50 of the silicon-based composite material is 8.01μm.

[0056] 2. Preparation of batteries

[0057] The preparation of the battery in this example is consistent with that in Example 1.

[0058] Example 4

[0059] 1. Preparation of silicon-based composite materials

[0060] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that, in S1, the porous skeleton is a porous organic / inorganic metal framework material, which is a Fe-MOF material, and the D50 of the final porous organic / inorganic metal framework material is also 8 μm. The rest is the same as the operation in Embodiment 1.

[0061] And after testing, the thickness of the first coating layer is 6.3nm, the thickness of the second coating layer is 11.4nm, and the D50 of the silicon-based composite material is 8.02μm.

[0062] 2. Preparation of batteries

[0063] The preparation of the battery in this example is consistent with that in Example 1.

[0064] Example 5

[0065] 1. Preparation of silicon-based composite materials

[0066] The preparation of the silicon-based composite material in this embodiment is different from that in embodiment 1 in that in S1, the porous skeleton matrix is ​​adjusted to a large-scale graphite material, and the final D50 of the large-scale graphite material is also 8 μm. The rest is the same as the operation in embodiment 1.

[0067] And after testing, the thickness of the first coating layer is 5.7nm, the thickness of the second coating layer is 10.7nm, and the D50 of the silicon-based composite material is 8.02μm.

[0068] 2. Preparation of batteries

[0069] The preparation of the battery in this example is consistent with that in Example 1.

[0070] Example 6

[0071] 1. Preparation of silicon-based composite materials

[0072] The preparation of the silicon-based composite material in this embodiment is different from that in Example 1 in that, in S1, the porous skeleton matrix is ​​adjusted to a porous carbon material and a porous organic / inorganic metal framework material, the type of the porous carbon material is consistent with that in Example 1, and the porous organic / inorganic metal framework material is a Fe-MOF material; here, the D50 of the porous carbon material and the porous organic metal framework material are 8 μm and 10 μm, respectively, and the mass ratio of the porous carbon material to the porous organic metal framework material is 90:10. The rest is consistent with the operation of Example 1.

[0073] And after testing, the thickness of the first coating layer is 6.7nm, the thickness of the second coating layer is 12.3nm, and the D50 of the silicon-based composite material is 8.23μm.

[0074] 2. Preparation of batteries

[0075] The preparation of the battery in this example is consistent with that in Example 1.

[0076] Example 7

[0077] 1. Preparation of silicon-based composite materials

[0078] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 6 in that the mass ratio of the porous carbon material to the porous organic metal framework material is adjusted to 70:30. The rest of the operations are the same as those in Embodiment 6.

[0079] And after testing, the thickness of the first coating layer is 7.6nm, the thickness of the second coating layer is 11.9nm, and the D50 of the silicon-based composite material is 8.62μm.

[0080] 2. Preparation of batteries

[0081] The preparation of the battery in this example is consistent with that in Example 6.

[0082] Example 8

[0083] 1. Preparation of silicon-based composite materials

[0084] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 6 in that, in S1, the D50 of the porous carbon material is adjusted to 12 μm; the rest of the operations are consistent with those in Embodiment 6.

[0085] And after testing, the thickness of the first coating layer is 6.8nm, the thickness of the second coating layer is 10.7nm, and the D50 of the silicon-based composite material is 11.82μm.

[0086] 2. Preparation of batteries

[0087] The preparation of the battery in this example is consistent with that in Example 6.

[0088] Example 9

[0089] 1. Preparation of silicon-based composite materials

[0090] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that the coating time at 450° C. is 10 h in S3. The rest of the operations are the same as those in Embodiment 1.

[0091] And after testing, the thickness of the first coating layer is 18.7nm, the thickness of the second coating layer is 11.3nm, and the D50 of the silicon-based composite material is 8.04μm.

[0092] 2. Preparation of batteries

[0093] The preparation of the battery in this example is consistent with that in Example 1.

[0094] Example 10

[0095] 1. Preparation of silicon-based composite materials

[0096] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that, in S4, the stirring reaction time is adjusted to 4 hours. The rest of the operations are the same as those in Embodiment 1.

[0097] And after testing, the thickness of the first coating layer is 5.4nm, the thickness of the second coating layer is 32nm, and the D50 of the silicon-based composite material is 8.05μm.

[0098] 2. Preparation of batteries

[0099] The preparation of the battery in this example is consistent with that in Example 1.

[0100] Embodiment 11

[0101] 1. Preparation of silicon-based composite materials

[0102] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that step S2 is omitted. In the process of S1, the porous skeleton matrix precursor phenolic resin is placed in a mixed gas for calcination. The calcination conditions are the same as those in Embodiment 1. The mixed gas includes 84.99% nitrogen, 15% silane gas, and 0.01% air (volume ratio). The rest is the same as the operation in Embodiment 1.

[0103] And after testing, the thickness of the first coating layer is 5.2nm, the thickness of the second coating layer is 10.2nm, and the D50 of the silicon-based composite material is 8.02μm.

[0104] 2. Preparation of batteries

[0105] The preparation of the battery in this example is consistent with that in Example 1.

[0106] Comparative Example 1

[0107] 1. Preparation of silicon-based composite materials

[0108] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that the operation in S4 is not performed, that is, the coating of the second coating layer is not performed. The rest of the operations are the same as those in Embodiment 1.

[0109] And after testing, the thickness of the first coating layer is 5.6nm, and the D50 of the silicon-based composite material is 8.01μm.

[0110] 2. Preparation of batteries

[0111] The preparation of the battery in this example is consistent with that in Example 1.

[0112] Comparative Example 2

[0113] 1. Preparation of silicon-based composite materials

[0114] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that the operation in S3 is not performed, that is, the coating of the first coating layer is not performed, and the coating of the second coating layer (S4) is performed directly. The rest of the operations are the same as those in Embodiment 1.

[0115] And after testing, the thickness of the second coating layer is 10.9nm, and the D50 of the silicon-based composite material is 8.01μm.

[0116] 2. Preparation of batteries

[0117] The preparation of the battery in this example is consistent with that in Example 1.

[0118] Comparative Example 3

[0119] 1. Preparation of silicon-based composite materials

[0120] The preparation of the silicon-based composite material in this embodiment is different from that in Embodiment 1 in that in S4, the second coating layer precursor solution is adjusted to a glucose solution with a mass fraction of 5%, so that the second coating layer is a carbon layer. The rest is the same as the operation in Embodiment 1.

[0121] And after testing, the thickness of the first coating layer is 5.8nm, the thickness of the second coating layer is 15nm, and the D50 of the silicon-based composite material is 8.02μm.

[0122] 2. Preparation of batteries

[0123] The preparation of the battery in this example is consistent with that in Example 1.

[0124] Test Case

[0125] 1. Experimental Construction Method

[0126] (1) Negative electrode expansion rate test

[0127] The expansion rate of the negative electrode sheets prepared in all the above embodiments and comparative examples was tested, and the expansion rate of the negative electrode sheet is the expansion rate of the thickness of the fully charged negative electrode sheet after rolling. The specific testing method is as follows: the thickness of the electrode sheet before battery injection is recorded as L1, and after the formation and capacity division processes, the thickness of the electrode sheet after full charge (1C constant current charging to 4.2V, constant voltage charging of 0.05C) is L2, and the electrode sheet expansion rate = (L2-L1) / L1.

[0128] (2) DCR test at 50% SOC before and after battery cycling

[0129] The batteries prepared in all the above embodiments and comparative examples were subjected to DCR test at 50% SOC before and after cycling. The specific test method is as follows: a charge and discharge test cabinet is used for testing, the battery after capacity division is discharged to 50% SOC at 1C, and left for 1h, with the end voltage of the standby being V1, and then discharged at 1C for 10s, with the end voltage of the discharge being V2, and DCR = (V1-V2) / I.

[0130] (3) Capacity retention test at room temperature (25±2℃) 1C / 1C cycle for 1000 cycles

[0131] The batteries prepared in all the above embodiments and comparative examples were tested for capacity retention rate of 1C / 1C cycle for 1000 weeks at room temperature. The specific testing method is as follows: a charge and discharge test cabinet was used for battery testing, 1C constant current and constant voltage charging, a cut-off current of 0.33C, standing for 10 minutes, and then discharged at 1C to 2.5V, which was considered a charge and discharge cycle. The ratio of the capacity of the battery at 1C discharge (4.2-2.5V) to the capacity of the first week to the 1000th week was the capacity retention rate of the battery for 1000 cycles.

[0132] (4) 8C charging constant current charging ratio test

[0133] The batteries prepared in all the above embodiments and comparative examples were subjected to an 8C charging constant current charging ratio test. The specific testing method is as follows: a charge and discharge test cabinet is used to test the battery, the battery is discharged to 2.5V at 1C, and charged to 4.2V at 8C constant current. The ratio of the constant current charging capacity to the total constant current and constant voltage charging capacity is the 8C charging constant current charging ratio.

[0134] 2. Experimental results

[0135] The test results of the negative electrode sheets and battery performances prepared in all the above embodiments and comparative examples are shown in Table 1.

[0136] Table 1 Test results of negative electrode sheets and battery performances prepared in the examples and comparative examples

[0137]

[0138] As can be seen from Table 1, the silicon-based negative electrode sheet prepared by using the silicon-based composite material provided in this application has a lower expansion rate, and the DCR growth rate of the battery prepared by it before and after the cycle is low, and it has good room temperature cycle performance and high rate performance. This is because the silicon-based composite material in this application has two specific coating layers, which can not only appropriately reduce the activity of nano-silicon in the silicon-based composite material, improve the stability of the silicon-based composite material, but also improve the mechanical stability of the silicon-based composite material, effectively alleviate the huge expansion force of nano-silicon after lithium insertion, and at the same time reduce the interface impedance, so that the silicon-based composite material has good conductivity, thereby effectively optimizing the various performances of the battery, with specific reference to Examples 1 to 11.

[0139] There is no second coating layer in Comparative Example 1, no first coating layer in Comparative Example 2, and a carbon layer in Comparative Example 3; the above factors cause the negative electrode sheet expansion rate to increase, the DCR growth rate to increase, and the room temperature cycle and high rate performance to decrease in Comparative Examples 1 to 3. This shows that there is a specific interaction between the first and second coating layers of the present application. Only when these two specific coating layers are coated together on the surface of the silicon-carbon particles can the stability of the silicon-based composite material be further improved and the volume expansion effect of silicon be alleviated, thereby further optimizing the relevant performance of the silicon-based negative electrode sheet and the battery.

[0140] Further comparing Example 1 with Examples 2 to 3, the first coating layer in Example 2 is a carbon layer, and the mass ratio of Al(NO)3·9H2O and (NH4)2HPO4 in the raw materials of the second coating layer in Example 3 is not in the range of 2 to 5:1, while the battery DCR growth rate in Examples 2 to 3 is higher, and the room temperature cycle and high rate performance of the battery are worse. This shows that under the specific raw material combination and specific raw material mass ratio of the first and second coating layers, it is more conducive to obtaining a silicon-based negative electrode sheet with a lower expansion rate and a battery with better conductivity, room temperature cycle and high rate performance.

[0141] Comparing Example 1 with Examples 4 to 8, the porous skeleton in Example 4 is a porous organic / inorganic metal framework material (Fe-MOF material), the porous skeleton matrix in Example 5 is a large specific surface graphite material, and the porous skeleton matrix in Example 6 is a porous carbon material and a porous organic / inorganic metal framework material, while the expansion rate of the silicon-based negative electrode sheet in Examples 4 to 5 is higher than that in Example 1, and the various performances of the battery are also worse, but the expansion rate of the silicon-based negative electrode sheet in Example 6 is lower than that in Example 1, and the various performances of the battery are also better than those in Example 1. This shows that the type selection and matching of the porous skeleton matrix will also affect the performance of the final silicon-based composite material, and then affect the performance of the silicon-based negative electrode sheet and the battery. Further comparing Example 6 with Examples 7 to 8, in Example 7, the mass ratio of the porous carbon material to the porous organic / inorganic metal framework material in the porous skeleton matrix is ​​not in the range of 80 to 95:5 to 20, and in Example 8, the D50 of the porous carbon material in the porous skeleton matrix is ​​larger, while the expansion rate of the silicon-based negative electrode sheet in Examples 7 to 8 is higher than that in Example 6, and the DCR growth rate is higher, and the room temperature cycle performance and high-rate performance are not as good as those in Example 6. This shows that the mass ratio of the porous carbon material and the porous organic / inorganic metal framework material and their D50 will affect the performance of the finally prepared silicon-based negative electrode sheet and the battery. Controlling the above parameters within a specific range is more conducive to obtaining a silicon-based negative electrode sheet with a lower expansion rate and a battery with better performance in all aspects.

[0142] Comparing Example 1 with Examples 9-10, the first coating layer in Example 9 is thicker, and the second coating layer in Example 10 is thicker, which results in a decrease in battery performance in Examples 9-10. This indicates that controlling the thickness of the first and second coating layers within a specific range is more conducive to taking into account various battery properties and improving the overall performance of the battery.

[0143] Comparing Example 1 and Example 11, step S2 in Example 1 is omitted in Example 11, and in the S1 process, the formation of the first coating layer and the deposition of silicon particles are completed in one step. However, this will result in an uneven first coating layer and uneven deposition of silicon particles. The first coating layer also does not have a significant protective effect, which ultimately causes a decrease in the room temperature cycle and high rate performance of the battery.

[0144] The above embodiments are only used to illustrate the technical solution of the present application rather than to limit the protection scope of the present application. Although the present application is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solution of the present application can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present application.

Claims

1. A silicon-based composite material, characterized in that: It comprises an inner core, and a first coating layer and a second coating layer which sequentially coat the inner core; The inner core comprises a porous skeleton matrix and nano silicon particles, wherein the porous skeleton matrix comprises at least one of a porous carbon material, a porous metal material, a porous organic / inorganic metal framework material, and a large specific surface graphite material; The first coating layer includes at least one of silicon oxide and carbon; The second coating layer includes at least one of a metal phosphorus-containing compound, a metal boron-containing compound, a metal nitrogen-containing compound, a metal oxide, and a conductive polymer.

2. The silicon-based composite material according to claim 1, characterized in that: The first coating layer includes silicon oxide; and the second coating layer includes aluminum phosphate.

3. The silicon-based composite material according to claim 1, characterized in that: The thickness of the first coating layer is 0.5 to 10 nm; The thickness of the second coating layer is 0.1-20 nm.

4. The silicon-based composite material according to claim 3, characterized in that: The D50 of the silicon-based composite material is 5 to 15 μm.

5. The silicon-based composite material according to claim 1, characterized in that: The porous skeleton matrix includes the porous carbon material and the porous organic / inorganic metal framework material; The mass ratio of the porous carbon material to the porous / inorganic organic metal framework material is 80-95:5-20.

6. The silicon-based composite material according to claim 1, characterized in that: The D50 of the porous carbon material is 3 to 10 μm, and the D50 of the porous organic metal framework material is 8 to 16 μm.

7. A method for preparing the silicon-based composite material according to claims 1 to 6, characterized in that: The steps include: S1. The porous skeleton matrix precursor is placed in a first inert gas atmosphere, kept at 500 to 1200 ° C for 0.5 to 6 hours, and crushed and classified to obtain the porous skeleton matrix; S2. placing the porous skeleton substrate in a silicon-containing atmosphere and depositing it at 300 to 1000° C. for 0.5 to 50 h to obtain the core; S3. placing the core in an oxygen-containing atmosphere or a carbon-containing atmosphere, coating at 100-800 ° C for 0.1-20h, completing the coating of the first coating layer; S4. Then, the product obtained in S3 is placed in the second coating layer precursor solution, stirred for reaction for 0.1 to 2 hours, and dried to obtain the silicon-based composite material.

8. The method for preparing the silicon-based composite material according to claim 6, characterized in that: In S2, the silicon-containing atmosphere includes at least one of monosilane, disilane, polysilane silole, and polysilane silole derivatives.

9. A negative electrode sheet, characterized in that: The invention comprises the silicon-based composite material as claimed in any one of claims 1 to 6, or the silicon-based composite material prepared by the method for preparing the silicon-based composite material as claimed in any one of claims 7 to 8.

10. A battery, characterized in that: It comprises the silicon-based composite material as claimed in any one of claims 1 to 6, or the silicon-based composite material prepared by the method for preparing the silicon-based composite material as claimed in any one of claims 7 to 8, or the negative electrode sheet as claimed in claim 9.

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