Negative electrode active material, negative electrode sheet containing same, and lithium-ion battery

Abstract

A negative electrode active material, a negative electrode sheet containing same, and a lithium-ion battery. The negative electrode active material comprises a silicon-based material and a coating layer provided on a surface of the silicon-based material. The coating layer contains a copolymer. The copolymer comprises a first monomer as shown in formula I and a second monomer as shown in formula II, where X and Y each independently contain at least one of an aryl group or an alkenyl group, and Z is selected from one of -Cl, -Br, and -I.

Description

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

[0001] This application claims priority to Chinese Patent Application No. 2024119606500, filed with the Chinese Patent Office on December 27, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of battery technology, specifically to a negative electrode active material, a negative electrode sheet containing the same, and a lithium-ion battery. Background Technology

[0003] With the rapid development of mobile electronic devices, electric vehicles, and grid energy storage, the development of lithium-ion batteries with high energy density, high power density, long cycle life, and high safety has become a research hotspot and focus in the energy storage field. Developing anode materials with high capacity, high rate capability, and high cycle stability is an important way to achieve this goal. Silicon materials have attracted widespread attention due to their abundant reserves and extremely high theoretical charge specific capacity. Technical issues

[0004] Silicon materials undergo tremendous volume expansion (greater than 300%) during charging and discharging. They also have inherently low conductivity, and their lithium storage capacity decays rapidly during cycling, making them unsuitable for practical applications. Technical solutions

[0005] According to a first aspect of this application, a negative electrode active material is provided, the negative electrode active material comprising a silicon-based material and a coating layer disposed 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 groups;

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

[0009] According to a second aspect of this application, a method for preparing the above-mentioned negative electrode active material is provided, comprising the following steps:

[0010] S1. The first monomer and the second monomer undergo a copolymerization reaction to obtain a copolymer;

[0011] S2. Prepare a copolymer solution using the copolymer;

[0012] 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 a coating layer on the surface of the silicon-based material.

[0013] According to a third aspect of this application, a negative electrode sheet is provided, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active coating disposed on the surface of the negative electrode current collector, the negative electrode active coating containing the aforementioned negative electrode active material or a negative electrode active material prepared using the aforementioned method for preparing the negative electrode active material.

[0014] According to a fourth aspect of this application, a lithium-ion battery is provided, the lithium-ion battery including the aforementioned negative electrode. Beneficial effects

[0015] In the negative electrode active material provided in this application, a copolymer formed by copolymerization of a first monomer as shown in Formula I and a second monomer as shown in Formula II is used to coat the silicon-based material. This has the following effects:

[0016] First, since silicon-based materials are prone to expansion during charging and discharging, the copolymer formed by the first monomer shown in Formula I and the second monomer shown in Formula II has good flexibility. Using this copolymer to coat the silicon-based material can effectively buffer the volume expansion of the silicon-based material during charging and discharging, making the negative electrode active material less prone to cracking during expansion, thereby improving the cycle life and cycle capacity retention rate of the negative electrode active material in actual use.

[0017] Secondly, by using the aforementioned copolymer to coat silicon-based materials, the structural and chemical stability of the negative electrode active material can be improved, thereby reducing the continuous formation of an unstable SEI film on the surface of the negative electrode due to silicon volume expansion.

[0018] Third, since silicon-based materials have low electronic conductivity, the coating layer formed by coating silicon-based materials with the above-mentioned copolymer can improve the conductivity of the negative electrode active material to a certain extent, thereby improving the rate performance of the negative electrode active material and enabling lithium-ion batteries using the negative electrode active material provided in this application to have excellent fast charging performance. Embodiments of the present invention

[0019] In some embodiments, the structure of the first monomer is shown in Formula III, and the structure of the second monomer is shown in Formula IV.

[0020] The first monomer shown in Formula III contains a long dodecane chain, which has good flexibility. The second monomer shown in Formula IV contains a benzene ring, which has rigidity. The copolymer formed by the copolymerization reaction of the first monomer shown in Formula III and the second monomer shown in Formula IV contains both a long dodecane chain and a benzene ring. Therefore, the copolymer has both good flexibility and rigidity. After coating silicon-based materials with this copolymer, a negative electrode active material is obtained. This coating layer can not only improve the buffering effect of silicon volume expansion during charging and discharging, but also improve the flexibility of the negative electrode sheet using this negative electrode active material. It is beneficial to reduce the risk of peeling off the negative electrode active coating containing this negative electrode active material. Through the above two aspects, the cycle life of lithium-ion batteries using this negative electrode active material is extended, so that the capacity retention rate of lithium-ion batteries can still be maintained at a high level after multiple charge and discharge cycles.

[0021] In some embodiments, the number average molecular weight of the copolymer is 4,000 to 50,000.

[0022] By controlling the number-average molecular weight of the copolymer used to form the coating layer within the range of 4,000 to 50,000, firstly, the bonding strength of the coating layer can be kept within a suitable range, which helps to reduce the risk of the coating layer falling off or peeling off during multiple charge-discharge cycles of the negative electrode active material. Secondly, it gives the coating layer good mechanical strength, which can improve the buffering effect of the coating layer on the volume expansion of silicon during the charge-discharge process of the negative electrode active material. Thirdly, it gives the coating layer good uniformity and density, which can improve the cycle stability of the negative electrode active material during the charge-discharge process.

[0023] If the number-average molecular weight of the copolymer used to form the coating layer is too small, the bonding strength of the coating layer will be low, and the coating layer will easily fall off or peel off during charge-discharge cycles, and the mechanical strength of the copolymer will be low. If the number-average molecular weight of the copolymer used to form the coating layer is too large, the copolymer will have poor solubility in organic solvents, which will increase the difficulty of coating and also affect the uniformity and density of the coating layer.

[0024] In some implementations, the molar ratio of the first monomer to the second monomer is 1 to 1.2:1.

[0025] By controlling the molar ratio of the first monomer and the second monomer in the copolymer used to form the coating layer within the above-mentioned range, the coating layer made from the copolymer formed by the copolymerization reaction of the first monomer and the second monomer can have good mechanical strength and flexibility, reducing 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.

[0026] If the molar ratio of the first monomer to the second monomer is too high, that is, there is too much first monomer, although the coating layer has good flexibility, its rigidity is poor and the coating layer is prone to cracking; if the molar ratio of the first monomer to the second monomer is too low, that is, there is too little first monomer, although the coating layer has strong rigidity, the coating layer is prone to cracking due to silicon expansion.

[0027] In some implementations, the coating layer accounts for 2-8% of the mass of the negative electrode active material.

[0028] In some embodiments, the thickness of the coating layer is 10~130 nm.

[0029] By controlling the mass ratio and thickness of the coating layer within the above range, the lithium-ion transport performance, rate performance and cycle performance of the negative electrode active material can be improved, and the coating layer has good flexibility.

[0030] If the mass percentage of the coating layer is too small or the thickness is too thin, the coating layer is prone to cracking during multiple charge-discharge cycles, which leads to the loss of active lithium. If the mass percentage of the coating layer is too large or the thickness is too thick, the ion transport performance of the negative electrode active material will be poor, resulting in a decrease in its rate performance and cycle performance.

[0031] In some embodiments, the particle size D50 of the negative electrode active material is 4~9 μm.

[0032] In some embodiments, the specific surface area of ​​the negative electrode active material is 1~8 m². 2 / g.

[0033] Controlling the specific surface area of ​​the negative electrode active material within the above range can improve the stability of the negative electrode active material, allowing its capacity to be better utilized during cyclic charging and discharging.

[0034] If the specific surface area of ​​the negative electrode active material is too small, its contact area with the electrolyte will be too small, which may lead to a high internal resistance in the lithium-ion battery using the negative electrode active material, thus limiting the effectiveness of the negative electrode active material. If the specific surface area of ​​the negative electrode active material is too large, the negative electrode active material is more likely to agglomerate during the preparation of the negative electrode sheet, making the material more difficult to disperse, and thus affecting the performance of the negative electrode sheet.

[0035] This application uses a copolymer formed by the copolymerization of a first monomer and a second monomer to coat a silicon-based material, thereby giving the resulting negative electrode active material good structural and chemical stability. This improves the cycle performance and fast-charging performance of lithium-ion batteries using this negative electrode active material, allowing the lithium-ion battery to maintain a high capacity retention rate after multiple charge-discharge cycles, thus improving the battery's cycle life.

[0036] In some embodiments, in S1, the copolymer is prepared by the following steps:

[0037] S1-1. Mix the first monomer, the second monomer, and the alkali and catalyst, and then heat and reflux the mixture under an inert gas protective atmosphere for 5-12 h to obtain the prepolymer.

[0038] S1-2. 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.

[0039] This scheme first uses the Suzuki coupling reaction to couple the first and second monomers in the presence of alkali and catalyst, and then adds an initiator to open the double bonds and carry out a copolymerization reaction to obtain a copolymer. The copolymer obtained by the above method has better performance. When this copolymer is applied to the coating layer, it can improve the structural stability, chemical stability, rate performance and cycle performance of the negative electrode active material.

[0040] In some embodiments, S1-2 includes the following operations: mixing the prepolymer with an organic solvent and an initiator, reacting the mixture under an inert gas protective atmosphere at 40-100°C for 3-12 h to obtain a reaction solution, adding the reaction solution to a precipitation solvent to obtain a copolymer precipitate, and washing and drying the precipitate to obtain the copolymer.

[0041] In some embodiments, in S1-2, the precipitation solvent includes at least one of propanol, isopropanol, and acetone.

[0042] In some embodiments, in S1, the amount of initiator is 0.1 to 0.9% of the total mass of the first monomer and the second monomer.

[0043] In some embodiments, in S1-1, the base includes at least one of sodium carbonate, potassium carbonate, potassium phosphate, and sodium bicarbonate.

[0044] In some embodiments, in S1-1, the catalyst includes at least one of Pd(dppf)Cl2 and Pd(PPh3)4.

[0045] In some embodiments, in S1-2, the organic solvent includes at least one of benzene, toluene, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF).

[0046] In some embodiments, in S1-2, the initiator includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, and benzoyl peroxide (BPO).

[0047] In some embodiments, the copolymer solution has a mass fraction of 3 to 20% in S2.

[0048] In some embodiments, S3 includes the following operation: mixing the silicon-based material with the copolymer solution and stirring at 30~80°C for 4~10 h to obtain a mixture, and spray drying the mixture to obtain the negative electrode active material.

[0049] In some embodiments, in S3, the inlet temperature of the spray dryer is 100~200°C and the outlet temperature is 60~90°C.

[0050] In some embodiments, the silicon-based material includes at least one of silicon suboxide (SiO) and silicon carbide (SiC).

[0051] In some embodiments, the silicon-carbon material includes a porous carbon framework, a silicon layer, and a carbon layer. The porous carbon framework includes an inner region and an outer region that surrounds the inner region. The inner region has multiple internal pores, and the outer region has multiple external pores. The silicon layer is disposed on the inner walls of the internal and external pores, and the carbon layer is disposed on the surface of the silicon layer in the external pores, so that the silicon-carbon particles have internal pores in the inner region and external pores in the outer region.

[0052] In some embodiments, in the internal and external pores of the silicon-carbon material, the proportion of mesopores is 42-86%, and the proportion of micropores is less than 58%. Among them, the diameter of mesopores is 3-40 nm, and the pore diameter of micropores is 0.5-2 nm.

[0053] Applying the negative electrode active material provided in this application to the negative electrode sheet gives the negative electrode sheet good flexibility, which plays a certain role in buffering the volume expansion of the negative electrode active material during the charging and discharging process, reducing the expansion rate of the negative electrode sheet, and improving the cycle performance and rate performance of the negative electrode sheet.

[0054] Applying the negative electrode active material provided in this application to a negative electrode sheet, and then applying the negative electrode sheet to a lithium-ion battery, is beneficial to improving the cycle performance and rate performance of the lithium-ion battery.

[0055] Example 1

[0056] A lithium-ion battery is prepared by the following steps:

[0057] 1. Preparation of negative electrode sheet

[0058] The negative electrode active material, conductive agent conductive carbon black (SP), single-walled carbon nanotubes (SWCNT), and binder polyacrylic acid (PAA) are mixed in a mass ratio of 80:9:1:10 and then added to deionized water as a solvent. After mixing evenly, a negative electrode slurry with a solid content of 30% is obtained. The negative electrode slurry is coated on both surfaces of the negative electrode current collector copper foil to form a negative electrode active coating. After vacuum drying and cold pressing, a negative electrode sheet is obtained.

[0059] The aforementioned 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 first monomer as shown in Formula III and a second monomer as shown in Formula IV.

[0060]

[0061] The above-mentioned negative electrode active material is prepared by the following steps:

[0062] S1. The first monomer and the second monomer are mixed at a molar ratio of 1:1 and added to the organic solvent THF. Sodium bicarbonate and Pd(PPh3)4 are added, and the mixture is heated under reflux for 8 h under argon protection 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 added to ethyl acetate / hexane for recrystallization to obtain a prepolymer. The prepolymer is mixed with THF and azobisisobutyronitrile and reacted under argon protection at 70°C for 8 h to obtain a reaction solution. The reaction solution is added to propanol to obtain a copolymer precipitate. After washing and drying, a copolymer with a number average molecular weight between 20,000 and 22,000 is obtained.

[0063] S2. Dissolve the copolymer in THF to prepare a copolymer solution with a mass fraction of 12%;

[0064] S3. Add the silicon-based material to the copolymer solution and stir at 50°C for 7 h to obtain a mixture. Spray dry the mixture to form a coating layer with a thickness of 70 nm on the surface of the silicon-based material to obtain a negative electrode active material with a particle size D50 of 6 μm. The mass ratio of the coating layer in the negative electrode active material is 5%. The inlet temperature of the spray dryer is 150°C and the outlet temperature is 75°C.

[0065] In this embodiment, the silicon-based material used is silicon-carbon material.

[0066] 2. Preparation of the positive electrode sheet

[0067] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2, conductive carbon black (SP) and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:2:2 and then added to the solvent N-methylpyrrolidone (NMP). After being mixed evenly, a positive electrode slurry with a solid content of 45% is obtained. 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, the positive electrode sheet is obtained.

[0068] 3. Preparation of the diaphragm

[0069] A polyethylene (PE) film with a ceramic layer is used as the separator.

[0070] 4. Preparation of electrolyte

[0071] Ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and fluoroethylene carbonate (FEC) were mixed in a mass ratio of 20:40:30:10 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 M.

[0072] 5. Assembly of lithium-ion batteries

[0073] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with the electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0074] Example 2

[0075] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in structure is that the preparation method of the negative electrode active material used in the negative electrode sheet is different.

[0076] The negative electrode active material used in this embodiment is prepared through the following steps:

[0077] S1. The first monomer and the second monomer are mixed at a molar ratio of 1.1:1 and added to the organic solvent THF. Sodium bicarbonate and Pd(PPh3)4 are added, and the mixture is heated under reflux for 5 h under argon protection 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 added to ethyl acetate / hexane for recrystallization to obtain a prepolymer. The prepolymer is mixed with THF and azobisisobutyronitrile and reacted under argon protection at 40°C for 12 h to obtain a reaction solution. The reaction solution is added to propanol to obtain a copolymer precipitate. After washing and drying, a copolymer with a number average molecular weight between 4000 and 6000 is obtained.

[0078] S2. Dissolve the copolymer in THF to prepare a copolymer solution with a mass fraction of 3%;

[0079] S3. The silicon-based material is added to the copolymer solution and stirred at 30°C for 10 h to obtain a mixture. The mixture is then spray-dried to form a coating layer with a thickness of 10 nm on the surface of the silicon-based material, thereby obtaining a negative electrode active material with a particle size D50 of 4 μm. The mass percentage 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.

[0080] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0081] Example 3

[0082] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in structure is that the preparation method of the negative electrode active material used in the negative electrode sheet is different.

[0083] The negative electrode active material used in this embodiment is prepared through the following steps:

[0084] S1. The first monomer and the second monomer are mixed at a molar ratio of 1.2:1 and added to the organic solvent THF. Sodium bicarbonate and Pd(PPh3)4 are added, and the mixture is heated under reflux for 12 h under argon protection 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 added to ethyl acetate / hexane for recrystallization to obtain a prepolymer. The prepolymer is mixed with THF and azobisisobutyronitrile and reacted under argon protection at 100°C for 3 h to obtain a reaction solution. The reaction solution is added to propanol to obtain a copolymer precipitate. After washing and drying, a copolymer with a number average molecular weight between 48,000 and 50,000 is obtained.

[0085] S2. Dissolve the copolymer in THF to prepare a copolymer solution with a mass fraction of 20%;

[0086] S3. The silicon-based material is added to the copolymer solution and stirred at 80°C for 4 h to obtain a mixture. The mixture is then spray-dried to form a coating layer with a thickness of 130 nm on the surface of the silicon-based material, thus obtaining a negative electrode active material with a particle size D50 of 9 μm. The mass percentage of the coating layer in the negative electrode active material is 8%. The inlet temperature of the spray dryer is 100°C and the outlet temperature is 60°C.

[0087] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0088] Example 4

[0089] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in structure is that the coating layer in the negative electrode active material contains a different copolymer, and the first monomer used is as shown in Formula V.

[0090]

[0091] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0092] Example 5

[0093] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in structure is that the coating layer in the negative electrode active material contains a different copolymer, and the first monomer used is as shown in Formula VI.

[0094]

[0095] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0096] Example 6

[0097] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in structure is that the coating layer in the negative electrode active material contains a different copolymer, and the first monomer used is as shown in Formula VII.

[0098]

[0099] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0100] Example 7

[0101] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in structure is that the coating layer in the negative electrode active material contains a different copolymer, and the second monomer used is as shown in Formula VIII.

[0102]

[0103] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0104] Example 8

[0105] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in composition is that the coating layer in the negative electrode active material contains a different copolymer, and the second monomer used is as shown in Formula IX.

[0106]

[0107] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0108] Example 9

[0109] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in composition is that in the preparation step S1 of the negative electrode active material, the prepolymer is mixed with THF and azobisisobutyronitrile and reacted at 70°C for 2 h under argon protection, and the number average molecular weight of the obtained copolymer is 2000~3000.

[0110] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0111] Example 10

[0112] This embodiment provides a lithium-ion battery. Compared with Embodiment 1, the difference in composition is that in the preparation step S1 of the negative electrode active material, the prepolymer is mixed with THF and azobisisobutyronitrile and reacted at 70°C for 14 h under argon protection, and the number average molecular weight of the obtained copolymer is 52,000~54,000.

[0113] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0114] Example 11

[0115] This embodiment provides a lithium-ion battery. Compared with embodiment 1, the difference in composition is: (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 final negative electrode active material is 2 μm, the thickness of the coating layer is 7 nm, and the mass ratio of the coating layer in the negative electrode active material is 1%.

[0116] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0117] Example 12

[0118] This embodiment provides a lithium-ion battery. Compared with embodiment 1, the difference in composition is: (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 final negative electrode active material is 12 μm, the thickness of the coating layer is 150 nm, and the mass ratio of the coating layer in the negative electrode active material is 12%.

[0119] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0120] Comparative Example 1

[0121] This comparative example provides a lithium-ion battery. Compared with Example 1, the difference in structure is that the negative electrode active material used in the preparation of the negative electrode sheet is a silicon-based material without a coating layer.

[0122] Apart from the differences mentioned above, the materials, formulation ratios, and preparation procedures used in this comparative example are strictly consistent with those in Example 1.

[0123] Comparative Example 2

[0124] This comparative example provides a lithium-ion battery. Compared with Example 1, the difference in composition is that, in the preparation process of the negative electrode sheet, the coating layer of the negative electrode active material used only contains the first monomer as shown in Formula III and does not contain the second monomer as shown in Formula IV. That is, the silicon-based material is coated with the first monomer as shown in Formula III. Specifically, a monomer solution with a mass fraction of 12% is prepared by using the first monomer as shown in Formula III and THF. The silicon-based material is mixed with the monomer solution and then spray-dried to obtain the negative electrode active material.

[0125] Apart from the differences mentioned above, the materials, formulation ratios, and preparation procedures used in this comparative example are strictly consistent with those in Example 1.

[0126] Comparative Example 3

[0127] This comparative example provides a lithium-ion battery. Compared with Example 1, the difference in composition is that, in the preparation process of the negative electrode sheet, the coating layer of the negative electrode active material used only contains the second monomer as shown in Formula IV and does not contain the first monomer as shown in Formula III. That is, the silicon-based material is coated with the second monomer as shown in Formula IV. Specifically, a monomer solution with a mass fraction of 12% is prepared by using the second monomer as shown in Formula IV and THF. The silicon-based material is mixed with the monomer solution and then spray-dried to obtain the negative electrode active material.

[0128] Apart from the differences mentioned above, the materials, formulation ratios, and preparation procedures used in this comparative example are strictly consistent with those in Example 1.

[0129] Comparative Example 4

[0130] This comparative example provides a lithium-ion battery. Compared with Example 1, the difference in structure 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 as shown in Formula X, and the second monomer is as shown in Formula XI.

[0131]

[0132] The negative electrode active material used in this comparative example was prepared through the following steps:

[0133] S1. Mix the first monomer and the second monomer in a molar ratio of 1:1 and dissolve them in THF to prepare a monomer solution with a mass fraction of 12%;

[0134] S2. Add the silicon-based material to the monomer solution and stir at 50°C for 7 h to obtain a mixture. Spray dry the mixture to form a coating layer with a thickness of 70 nm on the surface of the silicon-based material, and obtain a negative electrode active material with a particle size D50 of 6 μm.

[0135] Apart from the differences mentioned above, the materials, formulation ratios, and preparation procedures used in this comparative example are strictly consistent with those in Example 1.

[0136] Test case

[0137] 1. Participants

[0138] This test example uses the lithium-ion batteries prepared in Examples 1-12 and Comparative Examples 1-4 as test objects to conduct relevant performance tests.

[0139] 2. Test Content

[0140] (1) First Coulomb efficiency

[0141] At 25°C, the lithium-ion battery was charged at a constant current and constant voltage of 0.33C to 4.2 V, allowed to stand for 10 min, and then discharged at a constant current of 0.33C to 2.5 V, allowed to stand for 10 min. The initial coulombic efficiency of the lithium-ion battery was calculated.

[0142] Initial coulombic efficiency (%) = Total capacity of lithium-ion battery during initial discharge at 0.33C / Total capacity of lithium-ion battery during initial charge at 0.33C × 100%.

[0143] (2) Capacity retention rate after 1200 cycles at room temperature (1C / 1C)

[0144] At 25°C, the lithium-ion battery was charged at a constant current and constant voltage of 1C to 4.2V, with a cutoff current of 0.05C, and allowed to stand for 10 minutes. Then, the lithium-ion battery was discharged at a constant current of 1C to 2.5V and allowed to stand for 10 minutes. This constitutes one charge-discharge cycle. The lithium-ion battery was subjected to 1200 charge-discharge cycles using the above method. The capacity retention rate of the lithium-ion battery after 1200 charge-discharge cycles at 1C / 1C was calculated.

[0145] The capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity of the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0146] (3) Room temperature 6C rate performance - constant current charge ratio

[0147] At 25℃, the lithium battery was discharged at a constant current rate of 1C to 2.5V, left to stand for 10 minutes, and then charged at a constant current and constant voltage rate of 6C to 4.2V with a cutoff current of 0.05C. After standing for 10 minutes, the lithium battery's 6C constant current charging capacity Q1, total 6C constant current and constant voltage charging capacity Q2, and the highest temperature during fast charging (i.e., the highest temperature during 6C charging) were recorded. The 6C charging constant current charging ratio was calculated using the following formula: 6C charging constant current charging ratio = 6C constant current charging capacity Q1 / 6C constant current and constant voltage charging capacity Q2 × 100%, where the total constant current and constant voltage charging capacity Q2 = constant current charging capacity + constant voltage charging capacity.

[0148] (4) Capacity retention rate at room temperature 1C / 10C discharge

[0149] The lithium-ion battery, after capacity testing, was charged at 25℃ using a 1C rate under constant current and constant voltage conditions to 4.2 V, with a cutoff current of 0.05C. After resting for 10 min, the lithium-ion battery was then discharged at a 1C rate under constant current conditions to 2.5 V, and its discharge capacity Q was recorded. 1C The initial discharge capacity was used as the initial discharge capacity. Then, the lithium-ion battery was charged at 25°C with a 1C rate to 4.2V under constant current and constant voltage, with a cutoff current of 0.05C. After resting for 10 minutes, the fully charged lithium-ion battery was discharged at a 10C rate with a constant current to 2.5V, and its discharge capacity Q was recorded. 10C The discharge capacity retention rate of a lithium-ion battery at 1C / 10C rates is calculated using the following formula: Discharge capacity retention rate (%) = Discharge capacity Q at 10C rate 10C Discharge capacity Q at 1C rate 1C ×100%.

[0150] 3. Experimental Results

[0151] Table 1. Performance test results of lithium-ion batteries

[0152] Group Initial Coulombic Efficiency (%) Capacity Retention Rate after 1200 Cycles at Room Temperature 1C / 1C (%) Constant Current Charge Ratio at 6C Rate (%) Capacity Retention Rate after Discharge at Room Temperature 1C / 10C (%) Example 1 82.1 85.6 80.5 85.8 Example 2 81.9 84.8 80.1 84.9 Example 3 81.7 85.1 79.8 84.8 Example 4 80.7 82.8 77.6 82.1 Example 5 80.1 82.1 76.8 81.7 Example 6 79.6 80.2 75.8 81.0 Example 7 79.3 78.1 75.9 80.2 Example 8 78.8 80.1 78.1 83.5 Example 9 79.1 81.9 75.9 81.8 Example 10 80.2 80.2 78.8 79.9 Example 11 80.5 78.9 78.5 81.8 Example 12 78.9 81.7 76.6 78.3 Comparative Example 1 68.3 60.4 62.1 65.6 Comparative Example 2 75.8 65.9 68.9 72.5 Comparative Example 3 74.1 66.2 69.2 73.9 Comparative Example 4 77.8 69.8 72.6 74.5

[0153] 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.

[0154] The negative electrode active material used in the lithium-ion battery provided in 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 in Examples 1-12 is prepared by coating a silicon-based material with a copolymer formed by copolymerization of a first monomer as shown in Formula I and a second monomer as shown in Formula II. Test results show that the initial coulombic efficiency, capacity retention after 1200 cycles at room temperature 1C / 1C, constant current charge ratio at 6C rate, and capacity retention at room temperature 1C / 10C discharge are all significantly higher than those of Comparative Example 1. These results can... This invention describes the use of a copolymer formed by the copolymerization of a first monomer as shown in Formula I and a second monomer as shown in Formula II to coat silicon-based materials. The coating layer effectively buffers the volume expansion of the silicon-based materials during charging and discharging, making the negative electrode active material less prone to cracking during expansion. This improves the cycle life and cycle capacity retention of lithium-ion batteries using the negative electrode active material in actual use. Furthermore, the coating layer can improve the conductivity of the negative electrode active material to a certain extent, thereby enhancing its rate performance and giving lithium-ion batteries using it excellent fast-charging performance.

[0155] Comparing the test results of Examples 1, 4, 5, 6, 7, and 8, it can be shown that, compared with the first and second monomers of other structures, the negative electrode active material prepared by coating the silicon-based material with the copolymer formed by the copolymerization reaction of the first monomer as shown in Formula III and the second monomer as shown in Formula IV in Example 1 has better cycle performance and rate performance.

[0156] Comparing the test results of Examples 1, 9, and 10, it can be shown that the negative electrode active material prepared by coating silicon-based materials with copolymers with a number average molecular weight in the range of 4,000 to 50,000 has better cycle performance and rate performance.

Claims

1. A negative electrode active material, the negative electrode active material comprising a silicon-based material and a coating layer disposed 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; X and Y independently contain at least one of aryl and alkenyl groups; The Z is selected from one of —Cl, —Br, and —I.

2. The negative electrode active material as described in claim 1, wherein: The structure of the first monomer is shown in Formula III, and the structure of the second monomer is shown in Formula IV.

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

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

1.

5. The negative electrode active material as described in claim 1, wherein: 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.

6. The negative electrode active material as described in claim 1, wherein: 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.

7. The method for preparing the negative electrode active material as described in claim 1, comprising the following steps: S1. The first monomer and the second monomer undergo a copolymerization reaction 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.

8. The method for preparing the negative electrode active material as described in claim 7, wherein, In S1, the copolymer is prepared by the following steps: S1-1. The first monomer, the second monomer, and the alkali and catalyst are mixed and heated under an inert gas protective atmosphere and refluxed for 5-12 h to obtain the prepolymer; S1-2. 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.

9. A negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active coating disposed on the surface of the negative electrode current collector, the negative electrode active coating containing the negative electrode active material as described in any one of claims 1 to 6 or a negative electrode active material prepared by the preparation method of the negative electrode active material as described in any one of claims 7 to 8.

10. A lithium-ion battery, the lithium-ion battery comprising the negative electrode as described in claim 9.

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