A negative electrode active material, a negative electrode sheet containing the same, and a lithium ion battery

Abstract

The application provides a negative electrode active material, a negative electrode sheet containing the same and a lithium ion battery. The negative electrode active material 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, and the copolymer comprises a first monomer as shown in formula I and a second monomer as shown in formula II; X and Y independently contain at least one of an aryl group and an alkenyl group. The copolymer formed by 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, can well buffer the volume expansion of the silicon-based material in the charging and discharging process, makes the negative electrode active material not easy to break in the expansion process, improves the stability of the negative electrode active material, reduces the continuous generation of unstable SEI films due to the volume expansion of silicon, and also improves the conductivity of the negative electrode active material, so that the battery using the negative electrode active material has excellent rate performance and cycle performance, and the cycle life of the battery is prolonged.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative electrode active material, a negative electrode sheet containing the same and a lithium ion battery. BACKGROUND

[0002] With the rapid development of mobile electronic devices, electric vehicles and power grid energy storage, developing lithium ion batteries with high energy density, high power density, long cycle life and high safety is a research hotspot and focus in today's energy storage field. Developing negative electrode materials with high capacity, high rate and high cycle stability is an important way to achieve this goal.

[0003] Silicon materials have attracted widespread attention due to their abundant reserves, extremely high theoretical charge specific capacity and other advantages. However, silicon materials have problems such as huge volume expansion (more than 300%) and inherent low conductivity during charging and discharging, and the lithium storage capacity of silicon materials rapidly decays during the cycle process, which cannot meet the actual application requirements. SUMMARY

[0004] In order to solve the problems of existing silicon materials, such as huge volume expansion during charging and discharging, low conductivity and poor cycle performance, the present application provides a negative electrode active material, a negative electrode sheet containing the same and a lithium ion battery.

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

[0006]

[0007] X and Y independently contain at least one of aryl and alkenyl;

[0008] Z is selected from one of —Cl, —Br, —I.

[0009] In the negative electrode active material provided by the application, the silicon-based material is coated by a copolymer formed by copolymerization of a first monomer shown in Formula I and a second monomer shown in Formula II. First, the copolymer formed by the first monomer shown in Formula I and the second monomer shown in Formula II has good flexibility, and the coating of the silicon-based material by the copolymer can well buffer the volume expansion of the silicon-based material during charging and discharging, so that the negative electrode active material is not prone to rupture during expansion, thereby improving the cycle life and cycle capacity retention rate of the negative electrode active material in actual use. Second, the coating of the silicon-based material by the copolymer can improve the structural stability and chemical stability of the negative electrode active material, thereby reducing the continuous generation of unstable SEI film on the surface of the negative electrode sheet due to the volume expansion of silicon. Third, the coating layer formed by the coating of the silicon-based material by the copolymer can improve the conductivity of the negative electrode active material to some extent, thereby improving the rate performance of the negative electrode active material, so that the lithium ion battery using the negative electrode active material provided by the application has excellent fast charging performance.

[0010] Preferably, the structure of the first monomer is shown in Formula III, and the structure of the second monomer is shown in Formula IV.

[0011]

[0012] The first monomer shown in Formula III contains a dodecane long chain, and the dodecane long chain has good flexibility. The second monomer shown in Formula IV contains a benzene ring, and the benzene ring has rigidity. The copolymer formed by the copolymerization of the first monomer shown in Formula III and the second monomer shown in Formula IV contains both the dodecane long chain and the benzene ring, and thus the copolymer has both good flexibility and rigidity. The negative electrode active material obtained by coating the silicon-based material by the copolymer has a coating layer that can further improve the buffering effect on the volume expansion of silicon during charging and discharging and improve the flexibility of the negative electrode sheet using the negative electrode active material, which is conducive to reducing the risk of peeling of the negative electrode active coating containing the negative electrode active material, thereby prolonging the cycle life of the lithium ion battery using the negative electrode active material and maintaining the capacity retention rate of the lithium ion battery at a high level after multiple cycles of charging and discharging.

[0013] Preferably, the number average molecular weight of the copolymer is 4000-50000.

[0014] The number average molecular weight of the copolymer used to form the coating layer is controlled within the range of 4000-50000, which can keep the adhesion strength of the coating layer within an appropriate range, reduce the risk of the coating layer falling off or peeling off during the repeated charging and discharging process of the negative electrode active material, make the coating layer have good mechanical strength, improve the buffering effect of the coating layer on the volume expansion of silicon during the charging and discharging process of the negative electrode active material, and make the coating layer have good uniformity and compactness, thereby improving the cycle stability of the negative electrode active material during the charging and discharging process.

[0015] If the number average molecular weight of the copolymer used to form the coating layer is too small, the adhesion strength of the coating layer will not be high, the coating layer is prone to fall off or peel off during the charging and discharging cycle, and the mechanical strength of the copolymer is low; if the number average molecular weight of the copolymer used to form the coating layer is too large, the solubility of the copolymer in organic solvents will be poor, which will increase the difficulty of coating and affect the uniformity and compactness of the coating layer.

[0016] Preferably, the molar ratio of the first monomer to the second monomer is 1-1.2:1.

[0017] By controlling the molar ratio of the first monomer to the second monomer in the copolymer used to form the coating layer within the above range, the coating layer made of the copolymer formed by the copolymerization of the first monomer and the second monomer has good mechanical strength and flexibility, and the risk of cracking of the coating layer due to the volume expansion of the silicon-based material during the charging and discharging process of the negative electrode active material is reduced.

[0018] If the molar ratio of the first monomer to the second monomer is too high, that is, the first monomer is too much, the coating layer is flexible but has poor rigidity and is prone to cracking; if the molar ratio of the first monomer to the second monomer is too low, that is, the first monomer is too little, the coating layer has strong rigidity but is prone to cracking due to the expansion of silicon.

[0019] Preferably, the mass fraction of the coating layer in the negative electrode active material is 2-8%.

[0020] Preferably, the thickness of the coating layer is 10-130 nm.

[0021] Controlling the mass fraction 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 negative electrode active material, and the coating layer has good flexibility.

[0022] If the mass proportion of the coating layer is too small or the thickness is too thin, the coating layer is prone to breakage during multiple cycles of charging and discharging, thereby causing loss of active lithium; if the mass proportion of the coating layer is too large or the thickness is too thick, the ion transport performance of the negative active material is poor, which causes the rate capability and cycle performance of the negative active material to decrease.

[0023] Preferably, the particle size D50 of the negative active material is 4-9 μm.

[0024] Preferably, the specific surface area of the negative active material is 1-8 m 2 / g.

[0025] Controlling the specific surface area of the negative active material within the above range can improve the stability of the negative active material, so that the capacity of the negative active material can be better exerted during the cycle of charging and discharging.

[0026] If the specific surface area of the negative active material is too small, the contact area between the negative active material and the electrolyte is too small, which can cause the internal resistance of the lithium ion battery using the negative active material to be too high, so that the capacity of the negative active material is limited; if the specific surface area of the negative active material is too large, the negative active material is more prone to agglomeration during the preparation of the negative electrode sheet, so that the dispersion of the material is more difficult, thereby affecting the performance of the negative electrode sheet.

[0027] According to a second aspect of the present application, a preparation method of the above negative active material is provided, comprising the following steps:

[0028] S1. copolymerization of the first monomer and the second monomer to obtain a copolymer;

[0029] S2. preparing a copolymer solution by using the copolymer;

[0030] S3. applying the copolymer solution to the surface of the 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.

[0031] The present application uses the copolymer formed by the copolymerization of the first monomer and the second monomer to coat the silicon-based material, so that the prepared negative active material has good structural stability and chemical stability, the cycle performance and fast charging performance of the lithium ion battery using the negative active material are improved, the lithium ion battery still maintains a high capacity retention rate after multiple cycles of charging and discharging, thereby improving the cycle life of the battery.

[0032] Preferably, in S1, the copolymer is prepared by the following steps:

[0033] S1-1. mixing the first monomer, the second monomer, an alkali and a catalyst, and then heating and refluxing under an inert gas protective atmosphere for 5-12 h to obtain a prepolymer;

[0034] S1-2. mixing the prepolymer with an organic solvent and an initiator, and then reacting at 40-100°C for 3-12 hours under an inert gas atmosphere to obtain a copolymer.

[0035] The copolymer is prepared by coupling the first monomer and the second monomer in the presence of a base and a catalyst through Suzuki coupling reaction, and then adding an initiator to open the double bond and induce copolymerization. The copolymer prepared by the above method has better performance. When the copolymer is applied to the coating layer, the structural stability, chemical stability, rate performance and cycle performance of the negative active material can be further improved.

[0036] Preferably, S1-2 includes the following operations: mixing the prepolymer with an organic solvent and an initiator, and then reacting at 40-100°C for 3-12 hours under an inert gas atmosphere to obtain a reaction solution, adding the reaction solution to a precipitation solvent to obtain a copolymer precipitate, and then washing and drying to obtain the copolymer.

[0037] Preferably, in S1-2, the precipitation solvent includes at least one of propanol, isopropanol, and acetone.

[0038] Preferably, in S1, the amount of the initiator is 0.1-0.9% of the total mass of the first monomer and the second monomer.

[0039] Preferably, in S1-1, the base includes at least one of sodium carbonate, potassium carbonate, potassium phosphate, and sodium bicarbonate.

[0040] Preferably, in S1-1, the catalyst includes at least one of Pd(dppf)Cl2 and Pd(PPh3)4.

[0041] Preferably, in S1-2, the organic solvent includes at least one of benzene, toluene, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF).

[0042] Preferably, in S1-2, the initiator includes at least one of azobisisobutyronitrile, azobisisoheptyl nitrile, and benzoyl peroxide (BPO).

[0043] Preferably, in S2, the mass fraction of the copolymer solution is 3-20%.

[0044] Preferably, S3 includes the following operations: mixing the silicon-based material with the copolymer solution, and then stirring at 30-80°C for 4-10 hours to obtain a mixture, and then spray drying the mixture to obtain the negative active material.

[0045] Preferably, in S3, the inlet temperature of the spray dryer is 100-200°C, and the outlet temperature is 60-90°C.

[0046] Preferably, the silicon-based material comprises at least one of silicon monoxide (SiO) and silicon-carbon material (SiC).

[0047] Preferably, the silicon-carbon material comprises a porous carbon skeleton, a silicon layer and a carbon layer, the porous carbon skeleton comprises 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 porosities in the inner layer region and outer porosities in the outer layer region.

[0048] Preferably, in the inner porosities and the outer porosities 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.

[0049] According to a third aspect of the present application, a negative electrode sheet is provided, which comprises a negative electrode current collector and a negative electrode active coating arranged on the surface of the negative electrode current collector, and the negative electrode active coating contains the negative electrode active material or the negative electrode active material prepared by the preparation method of the negative electrode active material.

[0050] The application of the negative electrode active material to the negative electrode sheet makes the negative electrode sheet have good flexibility, plays a certain buffering role on the volume expansion of the negative electrode active material in the charging and discharging process, reduces the expansion rate of the negative electrode sheet, and improves the cycle performance and rate performance of the negative electrode sheet.

[0051] According to a fourth aspect of the present application, a lithium ion battery is provided, which comprises the negative electrode sheet.

[0052] The application of the negative electrode active material to the negative electrode sheet and the application of the negative electrode sheet to the lithium ion battery are beneficial to improving the cycle performance and rate performance of the lithium ion battery. DETAILED DESCRIPTION

[0053] The technical features of the technical solutions provided by the present application will be further described clearly and completely in combination with the specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0054] Embodiment 1

[0055] A lithium ion battery is prepared by the following steps:

[0056] 1. Preparation of a negative electrode sheet

[0057] The negative electrode active material, the conductive agent conductive carbon black (SP), the single-walled carbon nanotube (SWCNT), and the binder polyacrylic acid (PAA) are mixed in a mass ratio of 80:9:1:10 and then added to a solvent deionized water to obtain a negative electrode slurry with a solid content of 30%, the negative electrode slurry is coated on two surfaces of a negative electrode current collector copper foil to form a negative electrode active coating, and then a negative electrode sheet is prepared through vacuum drying and cold pressing.

[0058] The negative electrode active material includes 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 includes a first monomer as shown in Formula III and a second monomer as shown in Formula IV.

[0059]

[0060] The negative electrode active material is prepared through the following steps:

[0061] S1. The first monomer and the second monomer are mixed in a molar ratio of 1:1 and then added to an organic solvent THF, sodium bicarbonate and Pd(PPh3)4 are added, and the mixture is heated to reflux under argon protection for 8 h to obtain a prepolymer solution, the prepolymer solution is cooled, diluted with ethyl acetate, washed with brine, dried, filtered, and concentrated to obtain a concentrate, the concentrate is recrystallized in ethyl acetate / hexane to obtain a prepolymer, the prepolymer is mixed with THF and azobisisobutyronitrile and then reacted at 70°C under argon protection for 8 h to obtain a reaction liquid, the reaction liquid is added to propanol to obtain copolymer precipitation, and the copolymer precipitation is washed and dried to obtain a copolymer with a number average molecular weight of 20,000-22,000;

[0062] S2. The copolymer is dissolved in THF to obtain a copolymer solution with a mass fraction of 12%;

[0063] S3. The silicon-based material is added to the copolymer solution and stirred at 50°C for 7 h to obtain a mixture, the mixture is spray dried to form a coating layer with a thickness of 70 nm on the surface of the silicon-based material, and a negative electrode active material with a particle size D50 of 6 μm is prepared, wherein the mass ratio of the coating layer in the negative electrode active material is 5%, the inlet temperature of the spray drying is 150°C, and the outlet temperature is 75°C;

[0064] In this embodiment, the silicon-based material is a silicon-carbon material.

[0065] 2. Preparation of the positive electrode sheet

[0066] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1O2, conductive agent conductive carbon black (SP) and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:2:2, then added to solvent N-methyl pyrrolidone (NMP), mixed uniformly, to prepare 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 prepared.

[0067] 3. Preparation of the separator

[0068] A polyethylene (PE) film with a ceramic layer is used as the separator.

[0069] 4. Preparation of the electrolyte

[0070] 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.

[0071] 5. Assembly of the lithium ion battery

[0072] The above positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator between the positive and negative electrode sheets to serve as a separator, then wound to obtain a bare cell, the bare cell is placed in an outer packaging shell, dried, and then injected with the above electrolyte, after vacuum packaging, standing, formation, shaping, and other processes, a lithium ion battery is obtained.

[0073] Example 2

[0074] This example provides a lithium ion battery, which differs from Example 1 in that the preparation method of the negative electrode active material used in the negative electrode sheet is different.

[0075] The negative electrode active material used in this example is prepared by the following steps:

[0076] S1. The first monomer and the second monomer are mixed in a molar ratio of 1.1:1, then added to an organic solvent THF, sodium bicarbonate and Pd(PPh3)4 are added, heated to reflux under argon protection for 5h to obtain a prepolymer solution, cooled, diluted with ethyl acetate, washed with salt water, dried, filtered, concentrated to obtain a concentrate, the concentrate is added to ethyl acetate / hexane for recrystallization to obtain a prepolymer, the prepolymer is mixed with THF and azobisisobutyronitrile, then reacted under argon protection at 40°C for 12h to obtain a reaction solution, the reaction solution is added to propanol to obtain a copolymer precipitate, which is washed and dried to obtain a copolymer with a number average molecular weight of 4000-6000;

[0077] S2. The copolymer is dissolved in THF to prepare a copolymer solution with a mass fraction of 3%;

[0078] S3. The silicon-based material is added to the copolymer solution and stirred at 30°C for 10h to obtain a mixture, and the mixture is spray dried to form a coating layer with a thickness of 10nm on the surface of the silicon-based material, thereby preparing a negative electrode active material with a particle size D50 of 4μm, wherein the mass ratio of the coating layer in the negative electrode active material is 2%, the inlet temperature of the spray drying is 200°C, and the outlet temperature is 90°C.

[0079] Except for the above differences, the materials, formula, and preparation operations used in this embodiment are strictly consistent with those of Example 1.

[0080] Example 3

[0081] This embodiment provides a lithium ion battery, and the difference from Example 1 is that the preparation method of the negative electrode active material used in the negative electrode sheet is different.

[0082] The negative electrode active material used in this embodiment is prepared by the following steps:

[0083] S1. The first monomer and the second monomer are mixed according to a molar ratio of 1.2:1, and then added to an organic solvent THF, sodium bicarbonate and Pd(PPh3)4 are added, and the mixture is heated and refluxed under argon protection for 12h to obtain a prepolymer solution. After cooling, the solution is diluted with ethyl acetate, washed with brine, dried, filtered, and concentrated to obtain a concentrate. The concentrate is recrystallized from ethyl acetate / hexane to obtain a prepolymer. The prepolymer is mixed with THF and azobisisobutyronitrile, and the mixture is reacted under argon protection at 100°C for 3h to obtain a reaction solution. The reaction solution is added to propanol to obtain a copolymer precipitate, which is washed and dried to obtain a copolymer with a number average molecular weight of 48000-50000;

[0084] S2. The copolymer is dissolved in THF to prepare a copolymer solution with a mass fraction of 20%;

[0085] S3. The silicon-based material is added to the copolymer solution and stirred at 80°C for 4h to obtain a mixture, and the mixture is spray dried to form a coating layer with a thickness of 130nm on the surface of the silicon-based material, thereby preparing a negative electrode active material with a particle size D50 of 9μm, wherein the mass ratio of the coating layer in the negative electrode active material is 8%, the inlet temperature of the spray drying is 100°C, and the outlet temperature is 60°C.

[0086] Except for the above differences, the materials, formula, and preparation operations used in this embodiment are strictly consistent with those of Example 1.

[0087] Example 4

[0088] This example provides a lithium ion battery, compared with example 1, the difference is that the coating layer in the negative active material contains different copolymer, the first monomer used is shown as formula V;

[0089]

[0090] In addition to the above differences, the materials used, the formulation and the preparation operation of this example are strictly consistent with example 1.

[0091] Example 5

[0092] This example provides a lithium ion battery, compared with example 1, the difference is that the coating layer in the negative active material contains different copolymer, the first monomer used is shown as formula VI;

[0093]

[0094] In addition to the above differences, the materials used, the formulation and the preparation operation of this example are strictly consistent with example 1.

[0095] Example 6

[0096] This example provides a lithium ion battery, compared with example 1, the difference is that the coating layer in the negative active material contains different copolymer, the first monomer used is shown as formula VII;

[0097]

[0098] In addition to the above differences, the materials used, the formulation and the preparation operation of this example are strictly consistent with example 1.

[0099] Example 7

[0100] This example provides a lithium ion battery, compared with example 1, the difference is that the coating layer in the negative active material contains different copolymer, the second monomer used is shown as formula VIII;

[0101]

[0102] In addition to the above differences, the materials used, the formulation and the preparation operation of this example are strictly consistent with example 1.

[0103] Example 8

[0104] This example provides a lithium ion battery, compared with example 1, the difference is that the coating layer in the negative active material contains different copolymer, the second monomer used is shown as formula IX;

[0105]

[0106] Except for the above-mentioned differences, the materials, formula, ratio and preparation operation adopted in this embodiment are strictly consistent with those of Example 1.

[0107] Example 9

[0108] This embodiment provides a lithium ion battery, and the difference compared with Example 1 is that in the preparation step S1 of the negative electrode active material, the prepolymer is mixed with THF and azobisisobutyronitrile, and then reacted under argon protection at 70°C for 2h, and the number average molecular weight of the copolymer prepared is 2000-3000.

[0109] Except for the above-mentioned differences, the materials, formula, ratio and preparation operation adopted in this embodiment are strictly consistent with those of Example 1.

[0110] Example 10

[0111] This embodiment provides a lithium ion battery, and the difference compared with Example 1 is that in the preparation step S1 of the negative electrode active material, the prepolymer is mixed with THF and azobisisobutyronitrile, and then reacted under argon protection at 70°C for 14h, and the number average molecular weight of the copolymer prepared is 52000-54000.

[0112] Except for the above-mentioned differences, the materials, formula, ratio and preparation operation adopted in this embodiment are strictly consistent with those of Example 1.

[0113] Example 11

[0114] This embodiment provides a lithium ion battery, and the difference compared with Example 1 is that (1) in the preparation step S2 of the negative electrode active material, the mass fraction of the copolymer solution is 1%; (2) in the preparation step S3 of the negative electrode active material, the particle size D50 of the finally prepared negative electrode active material is 2μm, the thickness of the formed coating layer is 7nm, and the mass ratio of the coating layer in the negative electrode active material is 1%.

[0115] Except for the above-mentioned differences, the materials, formula, ratio and preparation operation adopted in this embodiment are strictly consistent with those of Example 1.

[0116] Example 12

[0117] The embodiment provides a lithium ion battery, and compared with the embodiment 1, the difference is that (1) in the preparation step S2 of the negative electrode active material, the mass fraction of the copolymer solution is 25%; (2) in the preparation step S3 of the negative electrode active material, the particle size D50 of the finally prepared negative electrode active material is 12 mu m, the thickness of the formed coating layer is 150 nm, and the mass ratio of the coating layer in the negative electrode active material is 12%.

[0118] In addition to the above-mentioned difference, the materials, formula ratio and preparation operation adopted in the embodiment are strictly consistent with those in the embodiment 1.

[0119] Comparative example 1

[0120] The comparative example provides a lithium ion battery, and compared with the embodiment 1, the difference is that in the preparation process of the negative electrode sheet, the negative electrode active material used is a silicon-based material without a coating layer.

[0121] In addition to the above-mentioned difference, the materials, formula ratio and preparation operation adopted in the comparative example are strictly consistent with those in the embodiment 1.

[0122] Comparative example 2

[0123] The comparative example provides a lithium ion battery, and compared with the embodiment 1, the difference is that in the preparation process of the negative electrode sheet, the coating layer in the negative electrode active material only contains the first monomer shown in formula III, and does not contain the second monomer shown in formula IV, that is, the silicon-based material is coated with the first monomer shown in formula III, and specifically, the first monomer shown in formula III and THF are used to prepare a monomer solution with a mass fraction of 12%, and then the silicon-based material is mixed with the monomer solution and then spray dried to prepare the negative electrode active material.

[0124] In addition to the above-mentioned difference, the materials, formula ratio and preparation operation adopted in the comparative example are strictly consistent with those in the embodiment 1.

[0125] Comparative example 3

[0126] The comparative example provides a lithium ion battery, and compared with the embodiment 1, the difference is that in the preparation process of the negative electrode sheet, the coating layer in the negative electrode active material only contains the second monomer shown in formula IV, and does not contain the first monomer shown in formula III, that is, the silicon-based material is coated with the second monomer shown in formula IV, and specifically, the second monomer shown in formula IV and THF are used to prepare a monomer solution with a mass fraction of 12%, and then the silicon-based material is mixed with the monomer solution and then spray dried to prepare the negative electrode active material.

[0127] In addition to the above-mentioned difference, the materials, formula ratio and preparation operation adopted in the comparative example are strictly consistent with those in the embodiment 1.

[0128] Comparative Example 4

[0129] The present comparative example provides a lithium ion battery, which is compared with Example 1, and the difference is that the material composition in the coating layer of the negative electrode active material is different, the first monomer contained in the coating layer is shown as Formula X, and the second monomer is shown as Formula XI.

[0130]

[0131] The negative electrode active material used in the present comparative example is prepared by the following steps:

[0132] S1. The first monomer and the second monomer are mixed in a molar ratio of 1:1 and then dissolved in THF to prepare a monomer solution with a mass fraction of 12%;

[0133] S2. The silicon-based material is added to the monomer solution and stirred at 50°C for 7h to obtain a mixture, and the mixture is spray dried to form a coating layer with a thickness of 70nm on the surface of the silicon-based material, thereby preparing a negative electrode active material with a particle size D50 of 6μm.

[0134] Except for the above-mentioned difference, the materials, formulation ratios and preparation operations used in the present comparative example are strictly consistent with those of Example 1.

[0135] Test Example

[0136] 1. Test Subjects

[0137] The lithium ion batteries prepared in Examples 1-12 and Comparative Examples 1-4 are used as test subjects in the present test example, and relevant performance tests are carried out.

[0138] 2. Test Contents

[0139] (1) First coulombic efficiency

[0140] At 25°C, the lithium ion battery is charged at a rate of 0.33C to 4.2V, and then discharged at a rate of 0.33C to 2.5V, and the first coulombic efficiency of the lithium ion battery is calculated.

[0141] First coulombic efficiency (%) = lithium ion battery 0.33C first discharge total capacity / lithium ion battery 0.33C first charge total capacity x 100%.

[0142] (2) Capacity retention rate at room temperature 1C / 1C cycle for 1200 cycles

[0143] The lithium ion battery was charged at 1C rate to 4.2V at 25℃, the cut-off current was 0.05C, and was rested for 10 min, then discharged at 1C rate to 2.5V, and was rested for 10 min, which was one cycle of charge and discharge. The lithium ion battery was cycled for 1200 times according to the above method, and the capacity retention rate of the lithium ion battery after 1200 cycles of charge and discharge at 1C / 1C was calculated.

[0144] The capacity retention rate (%) of the lithium ion battery after N cycles = (discharge capacity of the Nth cycle / first discharge capacity) x 100%, and N was the cycle number of the lithium ion battery.

[0145] (3) Normal temperature 6C rate performance - constant current charge ratio

[0146] The lithium battery was discharged at 1C rate to 2.5V at 25℃, and was rested for 10 min, then charged at 6C rate to 4.2V, and was rested for 10 min, the 6C constant current charge capacity Q1, 6C constant current and constant voltage charge total capacity Q2 and the highest temperature in the fast charging process (i.e. the highest temperature of 6C rate charging) were recorded, and 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 charge total capacity Q2 x 100%, wherein the constant current and constant voltage charge total capacity Q2 = constant current charge capacity + constant voltage charge capacity.

[0147] (4) Normal temperature 1C / 10C discharge capacity retention rate

[0148] The lithium ion battery was charged at 1C rate to 4.2V at 25℃, the cut-off current was 0.05C, and was rested for 10 min, then discharged at 1C rate to 2.5V, and the discharge capacity Q 1C was recorded as the initial discharge capacity; then the lithium ion battery was charged at 1C rate to 4.2V at 25℃, the cut-off current was 0.05C, and was rested for 10 min, and the fully charged lithium ion battery was discharged at 10C rate to 2.5V, and the discharge capacity Q 10C was recorded; and the discharge capacity retention rate of the lithium ion 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%.

[0149] 3. Experimental results

[0150] Table 1. Test results of related performances of lithium ion batteries

[0151]

[0152] The relevant performance test results of the lithium ion batteries prepared in Examples 1-12 and Comparative Examples 1-4 are shown in Table 1.

[0153] The negative electrode active material used in the lithium ion battery provided by Comparative Example 1 is a silicon-carbon material without a coating layer, while the negative electrode active material used in the lithium ion batteries provided by Examples 1-12 is coated with a copolymer formed by copolymerization of the first monomer shown in Formula I and the second monomer shown in Formula II. The test results show that the first coulombic efficiency, the capacity retention rate at room temperature after 1200 cycles at 1C / 1C, the constant current charge input ratio at 6C rate, and the capacity retention rate at room temperature at 1C / 10C of the lithium ion batteries provided by Examples 1-12 are all significantly higher than those of the lithium ion battery provided by Comparative Example 1. The above results can indicate that coating the silicon-based material with the copolymer formed by copolymerization of the first monomer shown in Formula I and the second monomer shown in Formula II can provide good buffering effect for the volume expansion of the silicon-based material during charging and discharging, so that the negative electrode active material is less likely to break during expansion, thereby improving the cycle life and cycle capacity retention rate of the lithium ion battery using the negative electrode active material in actual use. In addition, the coating layer can improve the conductivity of the negative electrode active material to some extent, thereby improving the rate performance of the negative electrode active material and enabling the lithium ion battery using the same to have excellent fast charging performance.

[0154] The test results of Comparative Examples 1, 4, 5, 6, 7, and 8 can indicate that, compared with other structures of the first monomer and the second monomer, the negative electrode active material prepared by coating the silicon-based material with the copolymer formed by copolymerization of the first monomer shown in Formula III and the second monomer shown in Formula IV has better cycle performance and rate performance.

[0155] The test results of Comparative Examples 1, 9, and 10 can indicate that the negative electrode active material prepared by coating the silicon-based material with the copolymer having a number average molecular weight in the range of 4000-50000 has better cycle performance and rate performance.

[0156] 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 negative electrode active material, characterized in that: The negative electrode active material includes a silicon-based material and a coating layer disposed on the surface of the silicon-based material. The coating layer contains a copolymer, which includes a copolymer obtained by reacting a first monomer as shown in Formula I and a second monomer as shown in Formula II with an alkali catalyst and polymerizing the copolymer. Formula I; Formula II.

2. The negative electrode active material as described in claim 1, characterized in that: The number-average molecular weight of the copolymer is 4000~50000.

3. The negative electrode active material as described in claim 1, characterized in that: The molar ratio of the first monomer to the second monomer is 1~1.2:

1.

4. The negative electrode active material as described in claim 1, characterized in that: The coating layer accounts for 2-8% of the mass of the negative electrode active material, and / or the thickness of the coating layer is 10-130 nm.

5. The negative electrode active material as described in claim 1, characterized in that: The particle size D50 of the negative electrode active material is 4~9 μm, and / or the specific surface area of ​​the negative electrode active material is 1~8 m². 2 / g.

6. The method for preparing the negative electrode active material as described in claim 1, characterized in that, Includes the following steps: S1. The first monomer, the second monomer, and an alkali and a catalyst are mixed and heated under an inert gas atmosphere and refluxed for 5-12 h to obtain a prepolymer; the prepolymer is mixed with an organic solvent and an initiator and reacted under an inert gas atmosphere at 40-100°C for 3-12 h to obtain the copolymer. S2. Prepare a copolymer solution using the copolymer; S3. Apply the copolymer solution to the surface of the silicon-based material and allow the copolymer solution on the surface of the silicon-based material to dry in order to form the coating layer on the surface of the silicon-based material.

7. A negative electrode sheet, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active coating disposed on the surface of the negative electrode current collector, wherein the negative electrode active coating contains the negative electrode active material as described in any one of claims 1 to 5 or the negative electrode active material prepared by the method for preparing the negative electrode active material as described in claim 6.

8. A lithium-ion battery, characterized in that: The lithium-ion battery includes the negative electrode as described in claim 7.

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