Composite negative electrode material, preparation method thereof, negative electrode sheet and lithium battery using the composite negative electrode material

Abstract

The application provides a composite negative electrode material and a preparation method and application thereof, a negative electrode sheet and a lithium battery. The composite negative electrode material comprises a silicon-based material and a coating layer arranged on the surface of the silicon-based material, the coating layer contains a copolymer, and the copolymer comprises a first monomer as shown in formula I and a second monomer as shown in formula II; X is a heterocycle containing an O atom; R1 is selected from one of -H, an alkyl group and -NH2, R2 is selected from one of -H, an alkyl group and -NH2, and at least one of R1 and R2 is -NH2. The composite negative electrode material provided by the application can be applied to the negative electrode sheet and the lithium battery, so that the first coulomb efficiency, the cycle performance, the rate performance and the fast charging performance of the lithium battery are improved, and the cycle life of the lithium battery is prolonged.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a composite negative electrode material, a preparation method thereof, a negative electrode sheet using the composite negative electrode material and a lithium battery. BACKGROUND

[0002] Graphite-based negative electrode materials are currently the most important negative electrode materials applied in lithium batteries, including natural graphite, artificial graphite, mesocarbon microbeads and other forms. Graphite-based negative electrode materials have high safety, good rate performance, small volume expansion (<10%) after lithium intercalation, good cycle performance, low price and no pollution. In the process of lithium ion intercalation into the negative electrode, lithium ions form graphite intercalation compounds LiC6 with graphite, which provides a theoretical specific capacity of 372 mAh / g. The low theoretical specific capacity of graphite-based negative electrode materials cannot meet the increasing capacity demand of lithium batteries, thus limiting the development prospects of graphite-based negative electrode materials.

[0003] Among the many lithium battery negative electrode materials, silicon is considered the most promising to replace graphite negative electrode because it can form Li x Si with lithium ions to obtain extremely high specific capacity. The theoretical specific capacity of silicon is as high as 4200 mAh / g, and the advantage of its relatively high reaction potential (~0.4 V vs. Li + / Li) makes it an ideal material for preparing lithium batteries with high energy density. However, the large volume expansion (>300%) and poor electronic conductivity (1.56 x 10 -3 S / cm) of silicon during charging and discharging will cause problems such as increased polarization, active material shedding, rapid capacity decay, etc., which limit its application in fast-charging lithium batteries. SUMMARY

[0004] In order to solve the problems of large volume expansion and poor conductivity of silicon negative electrode materials during charging and discharging, the present application provides a composite negative electrode material, a preparation method thereof, a negative electrode sheet using the composite negative electrode material and a lithium battery.

[0005] According to a first aspect of the present application, a composite negative electrode material is provided, which includes a silicon-based material and a coating layer arranged on the surface of the silicon-based material, the coating layer containing a copolymer, the copolymer including a first monomer as shown in Formula I and a second monomer as shown in Formula II;

[0006]

[0007]

[0008] X is a heterocycle containing an O atom;

[0009] R1 is selected from one of -H, alkyl, -NH2, R2 is selected from one of -H, alkyl, -NH2, and at least one of R1, R2 is -NH2.

[0010] In the composite negative electrode material provided by the application, the copolymer formed by the copolymerization of the first monomer as shown in Formula I and the second monomer as shown in Formula II is used to coat the silicon-based material. When the composite negative electrode material is applied to the negative electrode sheet and the lithium battery, firstly, the coating layer can protect the silicon-based material from directly contacting with the electrolyte, thereby reducing the loss of electrolyte components, protecting the stability of the electrode structure, improving the cycle life and the initial coulombic efficiency of the lithium battery; secondly, during the charging and discharging process of the lithium battery, the coating layer can buffer the volume expansion of the silicon-based material, reduce the risk of the rupture of the SEI film formed on the surface of the negative electrode sheet due to the volume expansion of the silicon-based material during the charging and discharging process, thereby improving the cycle performance of the lithium battery; thirdly, the copolymer used to form the coating layer has strong adhesion, and due to the presence of the first monomer, the coating layer formed by the copolymer has the characteristics of high electrical conductivity, which can improve the low electrical conductivity of the silicon-based material, thereby improving the rate performance of the lithium battery, and the electrical conductivity of the coating layer will not decrease significantly with the increase of temperature, which makes the lithium battery still have excellent rate performance in the case that the temperature of the lithium battery increases due to the chemical reaction inside the lithium battery during the cycle charging and discharging process, especially in the fast charging process, the lithium battery is prone to heat, and due to the presence of the coating layer, the lithium battery using the silicon-based material has good fast charging performance; fourthly, the -NH2 in the second monomer contained in the copolymer used to form the coating layer can form a hydrogen bond with the hydroxyl group on the surface of the silicon-based material, thereby improving the stability of the coating layer, and further improving the cycle performance of the lithium battery using the composite negative electrode material, so that the lithium battery can still have good cycle stability and high capacity retention rate after undergoing multiple charging and discharging cycles. Therefore, the composite negative electrode material provided by the application improves the initial coulombic efficiency, cycle performance, rate performance and fast charging performance of the lithium battery using the composite negative electrode material through the above four aspects, thereby prolonging the cycle life of the lithium battery.

[0011] Preferably, the copolymer includes the first monomer as shown in Formula III and the second monomer as shown in Formula IV.

[0012]

[0013] The composite negative electrode material involved in the present scheme uses the copolymer formed by the copolymerization of the first monomer as shown in Formula III and the second monomer as shown in Formula IV to coat the silicon-based material, so that the initial coulombic efficiency, cycle performance and rate performance of the lithium battery using the composite negative electrode material are further improved.

[0014] Preferably, the molar ratio of the first monomer to the second monomer is 1:1-10.

[0015] By controlling the molar ratio of the first monomer and the second monomer in the copolymer used for forming the coating layer within the above range, the conductivity of the coating layer is ensured, the conductivity of the silicon-based material is improved, and the hydrogen bonding force between the coating layer and the silicon-based material is ensured.

[0016] If the molar ratio of the first monomer and the second monomer is too small, i.e., the second monomer is too much, the improvement of the conductivity of the silicon-based material by the coating layer is too small; if the molar ratio of the first monomer and the second monomer is too large, i.e., the first monomer is too much, the hydrogen bonding force between the coating layer and the silicon-based material is too small

[0017] Preferably, the molar ratio of the first monomer and the second monomer is 1:5.

[0018] Preferably, the number average molecular weight of the copolymer is 5000-30000.

[0019] Controlling the number average molecular weight of the copolymer used for forming the coating layer within the above range has the following advantages: first, the adhesion strength of the coating layer is maintained within an appropriate range, which is conducive to reducing the risk of the coating layer falling off or peeling off during the charging and discharging process of the composite negative electrode material; second, the coating layer has good mechanical strength, which can improve the buffering effect of the coating layer on the volume expansion of silicon during the charging and discharging process of the composite negative electrode material; third, the coating layer has good uniformity and compactness, which can improve the cycle stability of the composite negative electrode material during the charging and discharging process.

[0020] If the number average molecular weight of the copolymer used for forming the coating layer is too small, the adhesion strength of the coating layer is not high, the coating layer is prone to fall off or peel off during the charging and discharging cycle process, and the mechanical strength of the copolymer is low; if the number average molecular weight of the copolymer used for forming the coating layer is too large, the solubility of the copolymer in organic solvents is poor, which increases the difficulty of coating and affects the uniformity and compactness of the coating layer.

[0021] Preferably, the particle size D50 of the composite negative electrode material is 4-9 μm.

[0022] Controlling the particle size D50 of the composite negative electrode material within the above range can maintain the lithium ion transmission path within an appropriate range and ensure the compaction density of the negative electrode sheet prepared using the composite negative electrode material and the energy density of the lithium battery.

[0023] Preferably, the specific surface area of the composite negative electrode material is 2-8 m 2 / g.

[0024] Preferably, the silicon content in the silicon-based material is 20-80 wt%.

[0025] The silicon content in the silicon-based material in the composite negative electrode material is controlled within the above range and applied to the negative electrode sheet and the lithium battery, which can ensure the energy density and rate performance of the lithium battery and avoid the risk of rupture of the composite negative electrode material caused by excessive volume expansion of the silicon-based material in the charging and discharging process due to too high silicon content.

[0026] If the silicon content in the silicon-based material of the composite negative electrode material is too high, the silicon-based material expands too much in the charging and discharging process, which is more likely to cause the risk of rupture; if the silicon content in the silicon-based material of the composite negative electrode material is too low, the energy density and rate performance of the lithium battery using the same will be reduced.

[0027] Preferably, the mass ratio of the coating layer in the composite negative electrode material is 1-8%.

[0028] Preferably, the thickness of the coating layer is 10-150 nm.

[0029] Controlling the mass ratio and thickness of the coating layer within the above range can further improve the lithium ion transmission performance, rate performance and cycle performance of the composite negative electrode material, and the coating layer has good flexibility.

[0030] If the mass ratio and thickness of the coating layer are too small, the coating layer is prone to rupture in the process of multiple cycles of charging and discharging, which further leads to the loss of active lithium; if the mass ratio and thickness of the coating layer are too large, the ion transmission performance of the composite negative electrode material will be poor, which leads to a decrease in its rate performance and cycle performance.

[0031] Preferably, the silicon-based material includes at least one of silicon monoxide (SiO) and silicon-carbon material (SiC).

[0032] Preferably, the silicon-carbon material includes a porous carbon framework, a silicon layer and a carbon layer, the porous carbon framework includes an inner layer region and an outer layer region wrapping the inner layer region, the inner layer region has a plurality of inner pores, the outer layer region has a plurality of outer pores, the silicon layer is arranged on the inner walls of the inner pores and the outer pores, and the carbon layer is arranged on the surface of the silicon layer in the outer pores, so that the silicon-carbon particles have inner pores in the inner layer region and outer pores in the outer layer region.

[0033] Preferably, in the inner pores and the outer pores of the silicon-carbon material, the proportion of mesopores is 42-86%, and the proportion of micropores is less than 58%, wherein the diameter of the mesopores is 3-40 nm, and the pore diameter of the micropores is 0.5-2 nm.

[0034] According to a second aspect of the present application, a preparation method of a composite negative electrode material is provided, including the following steps:

[0035] S1. Constructing a three-electrode system, wherein the three-electrode system contains a working electrode, a counter electrode and a reference electrode, the working electrode comprises one of a platinum electrode and a glassy carbon electrode, the counter electrode comprises a platinum electrode, and the reference electrode comprises a silver / silver chloride electrode;

[0036] S2. Preparing an electrolyte solution by using an electrolyte and an organic solvent, wherein the electrolyte comprises at least one of LiClO4 and tetrabutylammonium perchlorate;

[0037] S3. Dissolving a first monomer as shown in Formula I and a second monomer as shown in Formula II in the electrolyte solution to obtain a first mixed solution;

[0038] S4. Immersing the working electrode in the first mixed solution and applying a constant voltage of 0.8-1.8 V for 600-1500 s to prepare a copolymer by using a three-electrode system;

[0039] S5. Preparing a copolymer solution by using the copolymer and an organic solvent;

[0040] S6. Applying the copolymer solution to the surface of a silicon-based material, drying the copolymer solution on the surface of the silicon-based material to form a coating layer on the surface of the silicon-based material, and preparing a composite negative electrode material;

[0041]

[0042] X is a heterocycle containing an O atom;

[0043] R1 is selected from one of —H, an alkyl group and —NH2, R2 is selected from one of —H, an alkyl group and —NH2, and at least one of R1 and R2 is —NH2.

[0044] Compared with other copolymerization methods, the copolymer formed by dissolving the first monomer and the second monomer in the electrolyte solution and applying pressure to the first monomer and the second monomer to make them copolymerize by using the three-electrode system is used to coat the silicon-based material, the prepared composite negative electrode material is applied to the lithium battery, and the first coulomb efficiency, the cycle performance, the rate performance and the fast charging performance of the lithium battery can be improved, and the cycle life of the lithium battery is prolonged.

[0045] Preferably, in S3, the first monomer is as shown in Formula III, and the second monomer is as shown in Formula IV.

[0046]

[0047] Preferably, in S1, the concentration of the prepared electrolyte solution is 0.05-0.2 mol / L.

[0048] Preferably, in S3, the molar ratio of the first monomer to the second monomer is 1:1-10.

[0049] Preferably, in S3, the molar ratio of the first monomer to the second monomer is 1:1-5.

[0050] Preferably, in S4, the number average molecular weight of the prepared copolymer is 5000-30000.

[0051] Preferably, in S5, the organic solvent includes at least one of benzene, toluene, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF).

[0052] Preferably, in S5, the mass fraction of the copolymer solution is 5-18%.

[0053] Preferably, in S6, the particle size D50 of the silicon-based material is 4-9 μm.

[0054] Preferably, in S6, the specific surface area of the silicon-based material is 2-8 m 2 / g.

[0055] Preferably, in S6, the silicon content in the silicon-based material is 20-80 wt%.

[0056] Preferably, in S6, the mass fraction of the formed coating layer in the composite anode material is 1-8%.

[0057] Preferably, in S6, the thickness of the formed coating layer is 10-150 nm.

[0058] Preferably, in S6, the silicon-based material includes at least one of silicon monoxide (SiO) and silicon-carbon material (SiC).

[0059] Preferably, in the inner pores and outer pores of the silicon-carbon material, the proportion of mesopores is 42-86%, and the proportion of micropores is less than 58%, wherein the diameter of the pore mesopore is 3-40 nm, and the pore diameter of the micropore is 0.5-2 nm.

[0060] Preferably, S6 includes the following operations: mixing the silicon-based material with the copolymer solution and stirring at 40-80°C for 4-10 h to obtain a second mixed solution, and spray drying the mixed solution to prepare the composite anode material.

[0061] Preferably, in S6, the inlet temperature of the spray drying is 100-200°C, and the outlet temperature is 50-90°C.

[0062] According to a third aspect of the present application, a negative electrode sheet is provided, which includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector, and the negative electrode active material layer contains the composite anode material described above or the composite anode material prepared by the preparation method described above.

[0063] According to a fourth aspect of the present application, a lithium battery is provided, which comprises the negative electrode sheet described above.

[0064] The composite negative electrode material provided by the present application is applied to the negative electrode sheet and the lithium battery, which endows the battery with excellent first coulomb efficiency, cycle performance, rate capability and fast charging performance, and further prolongs the cycle life of the lithium battery. DETAILED DESCRIPTION

[0065] The technical features in the technical solutions provided by the present application will be further described clearly and completely in combination with specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0066] Embodiment 1

[0067] A lithium battery is prepared by the following steps:

[0068] 1. Preparation of the negative electrode sheet

[0069] The composite negative electrode material, the conductive agent conductive carbon black (SP), the single-walled carbon nanotube (SWCNT) and the binder polyacrylic acid (PAA) are mixed according to the mass ratio of 80:9:1:10, and then added to the solvent deionized water, mixed uniformly, and a negative electrode slurry with a solid content of 30% is prepared. The negative electrode slurry is coated on the two surfaces of the negative electrode current collector copper foil to form a negative electrode active material layer, and after vacuum drying and cold pressing, the negative electrode sheet is prepared.

[0070] The composite negative electrode material described above comprises a silicon-based material and a coating layer arranged on the surface of the silicon-based material, and the coating layer contains a copolymer, which comprises a first monomer as shown in Formula III and a second monomer as shown in Formula IV.

[0071]

[0072] The composite negative electrode material described above is prepared by the following steps:

[0073] S1. Constructing a three-electrode system comprising a working electrode, a counter electrode and a reference electrode;

[0074] Among them, the working electrode adopts a glassy carbon electrode, the counter electrode adopts a platinum sheet electrode, and the reference electrode adopts a silver / silver chloride electrode;

[0075] S2. An electrolyte solution with a concentration of 0.1 mol / L is prepared by using an electrolyte LiClO4 and an organic solvent THF;

[0076] S3. Dissolving the first monomer and the second monomer in the electrolyte solution to obtain a first mixture;

[0077] The molar ratio of the first monomer to the second monomer is 1:5.

[0078] S4. Injecting the mixture into a three-electrode system, ensuring that the working electrode is completely immersed in the first mixture, and applying a constant voltage of 1.3 V for 1000 s. After the reaction is completed, the working electrode is taken out and cleaned with dichloromethane to remove unreacted monomers and electrolyte residues, obtaining a copolymer with a number average molecular weight of 18000-20000;

[0079] S5. Preparing a copolymer solution with a mass fraction of 11% by using the copolymer and an organic solvent THF;

[0080] S6. Adding a silicon-based material to the copolymer solution and stirring at 60°C for 7h to obtain a second mixture. The second mixture is spray dried to form a coating layer with a thickness of 80 nm on the surface of the silicon-based material, and a composite negative electrode material with a particle size D50 of 6μm is prepared. The mass fraction of the coating layer in the composite negative electrode material is 5%, the inlet temperature of the spray drying is 150°C, and the outlet temperature is 70°C.

[0081] In this embodiment, the silicon-based material is a silicon-carbon material.

[0082] 2. Preparation of the positive electrode sheet

[0083] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2, a conductive agent conductive carbon black (SP), and a binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:2:2 and then added to a solvent N-methyl pyrrolidone (NMP) to obtain a positive electrode slurry with a solid content of 45%. The positive electrode slurry is coated on both surfaces of the positive electrode current collector aluminum foil to form a positive electrode active coating. After vacuum drying and cold pressing, a positive electrode sheet is obtained.

[0084] 3. Preparation of the separator

[0085] A polyethylene (PE) film containing a ceramic layer is used as the separator.

[0086] 4. Preparation of the electrolyte

[0087] Vinyl carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and fluoroethylene carbonate (FEC) are mixed in a mass ratio of 20:40:30:10 to obtain an organic solvent. Then, the fully dried lithium salt LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1M.

[0088] 5. Assembly of lithium battery

[0089] The above positive electrode sheet, the separator, and the negative electrode sheet are stacked in order with the separator between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and then are wound to obtain a bare battery cell. The bare battery cell is placed in an outer packaging shell, and after drying, the above electrolyte is injected. After vacuum packaging, standing, formation, shaping, and other processes, a lithium battery is obtained.

[0090] Example 2

[0091] The embodiment provides a lithium battery. Compared with example 1, the difference is that the preparation method of the composite negative electrode material used in the negative electrode sheet is different.

[0092] The composite negative electrode material used in the embodiment is prepared through the following steps:

[0093] S1. Constructing a three-electrode system containing a working electrode, a counter electrode, and a reference electrode;

[0094] Among them, the working electrode uses a glassy carbon electrode, the counter electrode uses a platinum sheet electrode, and the reference electrode uses a silver / silver chloride electrode;

[0095] S2. An electrolyte solution with a concentration of 0.05 mol / L is prepared by using an electrolyte LiClO4 and an organic solvent THF;

[0096] S3. The first monomer and the second monomer are dissolved in the electrolyte solution to obtain a first mixed solution;

[0097] Among them, the molar ratio of the first monomer to the second monomer is 1:1;

[0098] S4. The mixed solution is injected into the three-electrode system, the working electrode is completely immersed in the first mixed solution, and a constant voltage of 0.8V is used for pressure reaction for 1500s. After the reaction is completed, the working electrode is taken out and cleaned with dichloromethane to remove unreacted monomers and electrolyte residues, and a copolymer with a number average molecular weight of 3000-4000 is obtained;

[0099] S5. A copolymer solution with a mass fraction of 5% is prepared by using the copolymer and the organic solvent THF;

[0100] S6. The silicon-based material is added to the copolymer solution and stirred at 40°C for 10h to obtain a second mixed solution. The second mixed solution is spray dried to form a coating layer with a thickness of 10nm on the surface of the silicon-based material, and a composite negative electrode material with a particle size D50 of 4μm is prepared. The mass ratio of the coating layer in the composite negative electrode material is 1%, the inlet temperature of the spray drying is 100°C, and the outlet temperature is 50°C.

[0101] In addition to the above differences, the materials, formula, and preparation operations used in this embodiment are strictly consistent with those of Example 1.

[0102] Example 3

[0103] This embodiment provides a lithium battery, and the difference from Example 1 is that the preparation method of the composite negative electrode material used in the negative electrode sheet is different.

[0104] The composite negative electrode material used in this embodiment is prepared by the following steps:

[0105] S1. Construct a three-electrode system containing a working electrode, a counter electrode, and a reference electrode;

[0106] Among them, the working electrode uses a glassy carbon electrode, the counter electrode uses a platinum sheet electrode, and the reference electrode uses a silver / silver chloride electrode;

[0107] S2. Use electrolyte LiClO4 and organic solvent THF to prepare an electrolyte solution with a concentration of 0.2 mol / L;

[0108] S3. Dissolve the first monomer and the second monomer in the electrolyte solution to obtain a first mixture;

[0109] Among them, the molar ratio of the first monomer to the second monomer is 1:10;

[0110] S4. Inject the mixture into the three-electrode system, ensure that the working electrode is completely immersed in the first mixture, and use a constant voltage of 1.8V to pressurize the reaction for 600s. After the reaction is completed, the working electrode is taken out and cleaned with dichloromethane to remove unreacted monomers and electrolyte residues, and a copolymer with a number average molecular weight of 28000-30000 is obtained;

[0111] S5. Use the copolymer and organic solvent THF to prepare a copolymer solution with a mass fraction of 18%;

[0112] S6. Add the silicon-based material to the copolymer solution and stir at 80°C for 4h to obtain a second mixture. The second mixture is spray dried to form a coating layer with a thickness of 150nm on the surface of the silicon-based material, and a composite negative electrode material with a particle size D50 of 9μm is prepared. The mass fraction of the coating layer in the composite negative electrode material is 8%, the inlet temperature of the spray drying is 200°C, and the outlet temperature is 90°C.

[0113] In addition to the above differences, the materials, formula, and preparation operations used in this embodiment are strictly consistent with those of Example 1.

[0114] Example 4

[0115] The embodiment provides a lithium battery, and compared with the embodiment 1, the difference is that the coating layer in the composite negative electrode material contains different copolymers, and the second monomer used is shown as formula VI.

[0116]

[0117] In addition to the above difference, the materials, formula ratio and preparation operation used in the embodiment are strictly consistent with the embodiment 1.

[0118] Embodiment 5

[0119] The embodiment provides a lithium battery, and compared with the embodiment 1, the difference is that the coating layer in the composite negative electrode material contains different copolymers, and the second monomer used is shown as formula VI.

[0120]

[0121] In addition to the above difference, the materials, formula ratio and preparation operation used in the embodiment are strictly consistent with the embodiment 1.

[0122] Embodiment 6

[0123] The embodiment provides a lithium battery, and compared with the embodiment 1, the difference is that the coating layer in the composite negative electrode material contains different copolymers, and the second monomer used is shown as formula VI.

[0124]

[0125] In addition to the above difference, the materials, formula ratio and preparation operation used in the embodiment are strictly consistent with the embodiment 1.

[0126] Embodiment 7

[0127] The embodiment provides a lithium battery, and compared with the embodiment 1, the difference is that in the preparation step S4 of the composite negative electrode material, the mixed solution is injected into a three-electrode system, the working electrode is completely immersed in the first mixed solution, a constant voltage of 1.3V is used to pressurize the reaction for 500s, and the number average molecular weight of the prepared copolymer is 3000-4000.

[0128] In addition to the above difference, the materials, formula ratio and preparation operation used in the embodiment are strictly consistent with the embodiment 1.

[0129] Embodiment 8

[0130] The embodiment provides a lithium battery, and the difference compared with the embodiment 1 is that: in the preparation step S3 of the composite negative electrode material, the mixed solution is injected into a three-electrode system, the working electrode is completely immersed in the first mixed solution, a constant voltage of 1.3 V is adopted to pressurize the reaction for 2500 s, and the number average molecular weight of the prepared copolymer is 33000-35000.

[0131] In addition to the above difference, the materials, formula ratio and preparation operation adopted in the embodiment are strictly consistent with those in the embodiment 1.

[0132] Embodiment 9

[0133] The embodiment provides a lithium battery, and the difference compared with the embodiment 1 is that: in the preparation step S3 of the composite negative electrode material, the molar ratio of the first monomer to the second monomer is 1:7.

[0134] In addition to the above difference, the materials, formula ratio and preparation operation adopted in the embodiment are strictly consistent with those in the embodiment 1.

[0135] Embodiment 10

[0136] The embodiment provides a lithium battery, and the difference compared with the embodiment 1 is that: (1) in the preparation step S5 of the composite negative electrode material, the mass fraction of the copolymer solution is 3%; (2) in the preparation step S6 of the composite negative electrode material, the particle size D50 of the finally prepared composite negative electrode material is 3 mu m, the thickness of the formed coating layer is 7 nm, and the mass ratio of the coating layer in the composite negative electrode material is 0.5%.

[0137] In addition to the above difference, the materials, formula ratio and preparation operation adopted in the embodiment are strictly consistent with those in the embodiment 1.

[0138] Embodiment 11

[0139] The embodiment provides a lithium battery, and the difference compared with the embodiment 1 is that: (1) in the preparation step S5 of the composite negative electrode material, the mass fraction of the copolymer solution is 20%; (2) in the preparation step S6 of the composite negative electrode material, the particle size D50 of the finally prepared composite negative electrode material is 10 mu m, the thickness of the formed coating layer is 180 nm, and the mass ratio of the coating layer in the composite negative electrode material is 10%.

[0140] In addition to the above difference, the materials, formula ratio and preparation operation adopted in the embodiment are strictly consistent with those in the embodiment 1.

[0141] Comparative example 1

[0142] This comparative example provides a lithium battery, compared with Example 1, the difference is that the composite negative electrode material used in the preparation process of the negative electrode sheet is a silicon-based material without a coating layer.

[0143] In addition to the above-mentioned differences, the materials, formula ratio and preparation operation used in this comparative example are strictly consistent with Example 1.

[0144] Comparative Example 2

[0145] This comparative example provides a lithium battery, compared with Example 1, the difference is that the copolymer contained in the coating layer in the composite negative electrode material is different, and the first monomer used is as shown in Formula VIII;

[0146]

[0147] In addition to the above-mentioned differences, the materials, formula ratio and preparation operation used in this comparative example are strictly consistent with Example 1.

[0148] Comparative Example 3

[0149] This comparative example provides a lithium battery, compared with Example 1, the difference is that the copolymer contained in the coating layer in the composite negative electrode material is different, and the first monomer used is as shown in Formula IX;

[0150]

[0151] In addition to the above-mentioned differences, the materials, formula ratio and preparation operation used in this comparative example are strictly consistent with Example 1.

[0152] Comparative Example 4

[0153] This comparative example provides a lithium battery, compared with Example 1, the difference is that the copolymer contained in the coating layer in the composite negative electrode material is different, and the first monomer used is as shown in Formula X;

[0154]

[0155] In addition to the above-mentioned differences, the materials, formula ratio and preparation operation used in this comparative example are strictly consistent with Example 1.

[0156] Comparative Example 5

[0157] This comparative example provides a lithium battery, compared with Example 1, the difference is that the copolymer contained in the coating layer in the composite negative electrode material is different, and the second monomer used is as shown in Formula XI;

[0158]

[0159] In addition to the above differences, the materials, formulation ratios, and preparation operations used in the present comparative example were strictly consistent with those of Example 1.

[0160] Test Example

[0161] 1. Test Subject

[0162] The lithium batteries prepared in Examples 1 to 11 and Comparative Examples 1 to 5 were used as test subjects, and the relevant performance tests were performed.

[0163] 2. Test Contents

[0164] (1) Initial Coulombic Efficiency

[0165] The lithium battery was charged at 0.33C rate to 4.2V under the condition of 25°C, and then left for 10 min. Subsequently, the lithium battery was discharged at 0.33C rate to 2.5V, and then left for 10 min. The initial coulombic efficiency of the lithium battery was calculated.

[0166] Initial coulombic efficiency (%) = lithium battery 0.33C initial discharge total capacity / lithium battery 0.33C initial charge total capacity x 100%.

[0167] (2) Capacity Retention Rate after 1C / 1C Cycling for 1200 Cycles at Normal Temperature

[0168] The lithium battery was charged at 1C rate to 4.2V under the condition of 25°C, and then left for 10 min. Subsequently, the lithium battery was discharged at 1C rate to 2.5V, and then left for 10 min. This was one charge-discharge cycle. The lithium battery was subjected to charge-discharge cycling for 1200 cycles according to the above method, and the capacity retention rate after 1C / 1C charge-discharge cycling for 1200 cycles was calculated.

[0169] Capacity retention rate (%) of lithium battery after N cycles = (discharge capacity of the Nth cycle / initial discharge capacity) x 100%, and N is the number of cycles of the lithium battery.

[0170] (3) Normal Temperature 6C Rate Performance - Constant Current Charge Ratio

[0171] The lithium battery was discharged at 1C rate to 2.5V at 25℃, and then rested for 10 min. The lithium battery was charged at 6C rate to 4.2V at constant current and constant voltage, and the cut-off current was 0.05C, and then rested for 10 min. The 6C constant current charge capacity Q1, the 6C constant current and constant voltage total charge capacity Q2 and the highest temperature in the fast charging process (i.e. the highest temperature at 6C rate charging) were recorded. The 6C rate charging constant current charge ratio was calculated according to the following formula: 6C rate charging constant current charge ratio = 6C constant current charge capacity Q1 / 6C constant current and constant voltage total charge capacity Q2 x 100%, wherein the constant current and constant voltage total charge capacity Q2 = constant current charge capacity + constant voltage charge capacity.

[0172] (4) Normal temperature 1C / 10C discharge capacity retention rate

[0173] The lithium battery after the capacity test was charged at 1C rate to 4.2V at constant current and constant voltage at 25℃, and the cut-off current was 0.05C, and then rested for 10 min. The lithium battery was discharged at 1C rate to 2.5V, and the discharge capacity Q was recorded. 1C as the initial discharge capacity; then the lithium battery was charged at 1C rate to 4.2V at constant current and constant voltage at 25℃, and the cut-off current was 0.05C, and then rested for 10 min. The fully charged lithium battery was discharged at 10C rate to 2.5V at constant current, and the discharge capacity Q 10C was recorded. The discharge capacity retention rate of the lithium battery at 1C / 10C rate was calculated according to the following formula: discharge capacity retention rate (%) = discharge capacity at 10C rate Q 10C / discharge capacity at 1C rate Q 1C x 100%.

[0174] 3. Experimental results

[0175] Table 1. Test results of related properties of lithium batteries

[0176]

[0177] The test results of related properties of the lithium batteries prepared in Examples 1-11 and Comparative Examples 1-5 are shown in Table 1.

[0178] The composite negative electrode material used in the lithium battery provided by Comparative Example 1 is a silicon-based material without a coating layer, while the composite negative electrode material used in the lithium battery provided by Examples 1-11 is prepared by coating a silicon-based material with a copolymer formed by copolymerization of a first monomer represented by Formula I and a second monomer represented by Formula II. The test results show that the first coulombic efficiency, the capacity retention rate after 1200 cycles at room temperature at a 1C / 1C rate, the constant current charging ratio at a 6C rate, and the capacity retention rate at room temperature at a 1C / 10C rate of the lithium ion battery provided by Examples 1-11 are all significantly higher than those of the lithium ion battery provided by Comparative Example 1. The above results can indicate that coating a silicon-based material with a copolymer formed by copolymerization of a first monomer represented by Formula I and a second monomer represented by Formula II and applying the composite negative electrode material to a negative electrode sheet and a lithium battery can improve the first coulombic efficiency, the cycle performance, the rate performance, and the fast charging performance of the lithium battery, thereby prolonging the cycle life of the lithium battery.

[0179] The test results of Comparative Examples 1, 4, 5, 6 and Comparative Examples 2, 3, 4, 5 can indicate that, compared with the first monomers and the second monomers of other structures, the composite negative electrode material prepared by coating a silicon-based material with a copolymer formed by copolymerization of a first monomer represented by Formula III and a second monomer represented by Formula IV has better first coulombic efficiency, cycle performance, rate performance, and fast charging performance.

[0180] The test results of Comparative Examples 1, 7, 8 can indicate that the cycle performance and the rate performance of the negative electrode active material prepared by coating a silicon-based material with a copolymer having a number average molecular weight in the range of 5000-30000 are better.

[0181] The above examples are only used to illustrate the technical solutions of the present application and not to limit the protection scope of the present application. Although the present application has been described in detail with reference to the above examples, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently, and these modifications or replacements are all within the protection scope of the present application.

Claims

1. A composite negative electrode material, characterized by: The composite negative electrode material comprises a silicon-based material and a coating layer arranged on the surface of the silicon-based material, wherein the coating layer contains a copolymer, and the copolymer comprises a first monomer as shown in Formula I and a second monomer as shown in Formula II. X is a heterocycle containing an O atom. R1 is selected from one of --H, alkyl, and --NH2, R2 is selected from one of --H, alkyl, and --NH2, and at least one of R1 and R2 is --NH2.

2. The composite negative material of claim 1, wherein: The copolymer comprises a first monomer as shown in Formula III and a second monomer as shown in Formula IV.

3. The composite negative material of claim 1, wherein: The molar ratio of the first monomer to the second monomer is 1:1-10.

4. The composite negative material of claim 1, wherein: The number average molecular weight of the copolymer is 5000-30000.

5. The composite negative material of claim 1, wherein: The particle size D50 of the composite negative electrode material is 4-9 μm, and / or the specific surface area of the composite negative electrode material is 2-8 m 2 / g, and / or the silicon content in the silicon-based material is 20-80 wt%.

6. The composite negative material of claim 1, wherein: The mass ratio of the coating layer in the composite negative electrode material is 1-8%, and / or the thickness of the coating layer is 10-150 nm.

7. A method for preparing a composite negative electrode material, characterized by, The method comprises the following steps: S1. Constructing a three-electrode system, wherein the three-electrode system contains a working electrode, a counter electrode, and a reference electrode, the working electrode comprises one of a platinum electrode and a glassy carbon electrode, the counter electrode comprises a platinum electrode, and the reference electrode comprises a silver / silver chloride electrode; S2. Preparing an electrolyte solution by using an electrolyte and an organic solvent, wherein the electrolyte comprises at least one of LiClO4 and tetrabutylammonium perchlorate; S3. Dissolving a first monomer as shown in Formula I and a second monomer as shown in Formula II in the electrolyte solution to obtain a first mixture; S4. Immersing the working electrode in the first mixture and applying a constant voltage of 0.8-1.8 V for 600-1500 s to prepare a copolymer; S5. Preparing a copolymer solution by using the copolymer and an organic solvent; S6. Applying the copolymer solution to the surface of a silicon-based material and drying the copolymer solution on the surface of the silicon-based material to form a coating layer on the surface of the silicon-based material, thereby preparing the composite negative electrode material; X is a heterocycle containing an O atom. R1 is selected from one of --H, alkyl, and --NH2, R2 is selected from one of --H, alkyl, and --NH2, and at least one of R1 and R2 is --NH2.

8. The method of claim 7, wherein the composite negative material is prepared by the steps of: In S3, the first monomer is as shown in Formula III, and the second monomer is as shown in Formula IV. ​ 9. A negative electrode sheet characterized by comprising: The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector, and the negative electrode active material layer contains the composite negative electrode material as claimed in any one of claims 1-7 or the composite negative electrode material prepared by the preparation method as claimed in claim 8.

10. A lithium battery, characterized by: The lithium battery comprises the negative electrode sheet as claimed in claim 9.

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