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

Abstract

The application provides a negative electrode material, a negative electrode sheet containing the same and a battery. The negative electrode material comprises a silicon-based material and a first coating layer and a second coating layer arranged on the surface of the silicon-based material in sequence; the first coating layer contains fluorine elements; the second coating layer contains nitrogen elements; the mass percentage of the fluorine elements in the first coating layer is 1-3%; and the mass percentage of the nitrogen elements in the second coating layer is 1-4%. The negative electrode material provided by the application can improve the rate performance and fast-charging cycle stability of the battery by sequentially coating the first coating layer containing fluorine elements and the second coating layer containing nitrogen elements on the surface of the silicon-based material.

Description

TECHNICAL FIELD

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

[0002] In order to solve the problem of long charging time of new energy vehicles, developing fast charging technology has become one of the main directions of the industry. Under this background, using high rate to charge the vehicle power battery has become a trend in market application. Through high rate charging, the required charging time can be reduced, so the research on high rate power battery has become a key factor to promote the development of the industry.

[0003] At present, commercial fast charging is mainly concentrated in the graphite system, but the graphite negative electrode material directly applied in the fast charging battery has the disadvantage of low energy density of the battery.

[0004] Silicon-based materials are a new type of battery negative electrode material, which has a higher theoretical specific capacity than traditional graphite negative electrodes (up to 4200 mAh / g, more than 10 times that of graphite). In addition, silicon-based materials have low discharge platform, high charging and discharging efficiency, good safety, etc., which can improve the energy density and endurance of the battery, and are an ideal choice for the next generation of high-energy-density batteries.

[0005] However, silicon-based materials are prone to significant volume expansion during fast charging, with a volume change of up to 300%, which can cause a series of failure problems such as breaking of active material particles, peeling of active coating, and destruction of conductive network, which is not conducive to the fast charging performance of the battery and seriously affects the fast charging cycle life of the battery. SUMMARY

[0006] In order to solve the problem of significant volume expansion of existing silicon-based materials during fast charging, which affects the fast charging performance of the battery in which it is applied, the present application provides a negative electrode material, a negative electrode sheet containing the same and a battery.

[0007] According to a first aspect of the present application, a negative electrode material is provided, which includes a silicon-based material and a first coating layer, a second coating layer sequentially arranged on the surface of the silicon-based material; the first coating layer contains fluorine elements; the second coating layer contains nitrogen elements; the mass fraction of fluorine elements in the first coating layer is 1-3%; the mass fraction of nitrogen elements in the second coating layer is 1-4%.

[0008] In the negative electrode material provided by the application, by sequentially coating a first coating layer containing fluorine elements and a second coating layer containing nitrogen elements on the surface of the silicon-based material, and by controlling the content of fluorine elements in the first coating layer and the content of nitrogen elements in the second coating layer, the negative electrode material is applied to a negative electrode sheet and a battery, first, since the electronic conductivity of the silicon-based material is low, the first coating layer and the second coating layer are arranged on the surface of the silicon-based material, so as to improve the conductivity of the negative electrode material taking the silicon-based material as the main body, thereby improving the electrochemical performance and rate performance of the battery, second, the fluorine elements are doped in the first coating layer, the fluorine elements can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based material, the existence of the hydrogen bonds is conducive to enhancing the structural stability and flexibility of the negative electrode material, so that the negative electrode material can more stably withstand the volume change of the silicon-based material in the lithium ion deintercalation process, thereby realizing better fast charging cycle stability and improving the fast charging cycle life of the battery, third, the nitrogen elements are doped in the second coating layer, which can relieve the expansion of the silicon-based material in the fast charging process and improve the fast charging cycle stability. In summary, the negative electrode material provided by the application can improve the rate performance and fast charging cycle stability of the battery by sequentially coating a first coating layer containing fluorine elements and a second coating layer containing nitrogen elements on the surface of the silicon-based material.

[0009] If the mass content of the fluorine elements in the first coating layer is too small, the hydrogen bond force between the fluorine elements and the silicon-based material is too weak, the coating effect is poor, and if the mass content of the fluorine elements in the first coating layer is too large, the ion transmission distance is increased, resulting in a decrease in the rate performance of the battery; if the mass content of the nitrogen elements in the second coating layer is too small, the rate performance and the negative electrode surface structure stability of the battery cannot be well improved, and if the mass content of the nitrogen elements in the second coating layer is too large, the ion transmission distance is also increased, thereby reducing the rate performance of the battery.

[0010] Preferably, the first coating layer contains a first polymer, and the first polymer comprises a single monomer as shown in Formula I.

[0011]

[0012] R1, R2, R3, R4 are independently selected from one of —H, alkyl, —F, and at least one of R1, R2, R3, R4 is —F.

[0013] Preferably, the second coating layer contains a second polymer, and the second polymer comprises a second monomer as shown in Formula II.

[0014]

[0015] X contains a urea group.

[0016] The second coating layer of the negative electrode material contains a polar group urea group. On the one hand, the urea group improves the transmission speed of lithium ions through physical interaction with lithium ions, thereby improving the rate performance of the battery using the negative electrode material. On the other hand, the urea group can form a strong hydrogen bond with Li2O, Li2CO3, LiF and other components in the SEI film formed on the surface of the negative electrode, which is conducive to improving the structural stability of the negative electrode surface and further improving the fast charging cycle stability of the battery.

[0017] Preferably, the first polymer comprises a first monomer as shown in Formula III.

[0018]

[0019] Preferably, the second polymer comprises a second monomer as shown in Formula IV.

[0020]

[0021] Preferably, the number average molecular weight of the first polymer is 30,000-100,000.

[0022] Preferably, the number average molecular weight of the second polymer is 10,000-80,000.

[0023] By controlling the number average molecular weight of the first polymer for forming the first coating layer and the second polymer for forming the second coating layer within the above range, the adhesion strength of the first coating layer and the second coating layer can be maintained within an appropriate range, which is conducive to reducing the risk of the first coating layer and the second coating layer falling off or peeling off during repeated charging and discharging cycles of the negative electrode material. In addition, the first coating layer and the second coating layer have good mechanical strength, which can improve the buffering effect of the first coating layer and the second coating layer on the volume expansion of the silicon-based material during charging and discharging of the negative electrode material. Furthermore, the first coating layer and the second coating layer have good uniformity and compactness, which can improve the cycle stability of the negative electrode material during charging and discharging.

[0024] If the number average molecular weight of the first polymer for forming the first coating layer and the second polymer for forming the second coating layer is too small, the adhesion strength of the first coating layer and the second coating layer will not be high, and the first coating layer and the second coating layer are prone to fall off or peel off during charging and discharging cycles. In addition, the mechanical strength is low. If the number average molecular weight of the second polymer for forming the second coating layer is too large, it will increase the difficulty of coating, and also affect the uniformity and compactness of the first coating layer and the second coating layer.

[0025] Preferably, the mass fraction of the first coating layer in the negative electrode material is 2-4%.

[0026] Preferably, the thickness of the first coating layer is 3-80 nm.

[0027] Preferably, the mass ratio of the second coating layer in the negative electrode material is 1-5%.

[0028] Preferably, the thickness of the second coating layer is 2-100 nm.

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

[0030] If the mass ratio and thickness of the first coating layer and the second coating layer are too small, the first coating layer and the second coating layer are prone to rupture during the repeated charging and discharging process, thereby causing loss of active lithium; if the mass ratio and thickness of the first coating layer and the second coating layer are too large, the ion transmission performance of the negative electrode material will be poor, which will cause the rate performance and cycle performance of the negative electrode material to decrease.

[0031] Preferably, the particle size D50 of the negative electrode material is 6-10 μm.

[0032] Controlling the particle size D50 of the negative electrode material within the above range can not only keep the lithium ion transmission path within an appropriate range, but also ensure the compaction density of the negative electrode sheet prepared using the negative electrode material and the energy density of the battery.

[0033] Preferably, the specific surface area of the negative electrode material is 1-7 m 2 / g.

[0034] Preferably, the silicon content in the silicon-based material is 20-80 wt%.

[0035] Controlling the silicon content of the silicon-based material in the negative electrode material within the above range and applying it to the negative electrode sheet and the battery can not only ensure the energy density and rate performance of the battery, but also avoid the risk of rupture of the negative electrode material caused by excessive expansion of the silicon-based material during the charging and discharging process due to too high silicon content.

[0036] If the silicon content of the silicon-based material in the negative electrode material is too high, the silicon-based material expands too much during the charging and discharging process, which is more prone to rupture; if the silicon content of the silicon-based material in the negative electrode material is too low, the energy density and rate performance of the battery using it will decrease.

[0037] Preferably, the silicon-based material includes at least one of silicon monoxide (SiO) and silicon-carbon material (SiC).

[0038] Preferably, the proportion of mesopores in the inner pores and the outer pores of the silicon-carbon is 42-87%, and the proportion of micropores is less than 59%, wherein the diameter of the mesopores is 3-42 nm, and the diameter of the micropores is 0.6-2 nm.

[0039] Preferably, the negative electrode material is prepared by the following steps:

[0040] S1. mixing a fluorine source and a silicon-based material, and ball-milling at 230-400℃ for 5-20 h to obtain a silicon-based material containing a first coating layer;

[0041] S2. preparing a nitrogen source solution by using a nitrogen source and an organic solvent;

[0042] S3. applying the nitrogen source solution to the surface of the silicon-based material containing the first coating layer, and drying the nitrogen source solution to form a second coating layer on the surface of the silicon-based material containing the first coating layer, thereby obtaining the negative electrode material.

[0043] Preferably, S1 comprises the following operation: mixing a fluorine source and a silicon-based material, and ball-milling at 230-400℃ for 5-20 h to obtain a silicon-based material containing a first coating layer.

[0044] Preferably, in S2, the organic solvent comprises at least one of benzene, N-methylpyrrolidone (NMP), and N,N-dimethylformamide (DMF).

[0045] Preferably, in S2, the mass fraction of the nitrogen source solution is 3-25%.

[0046] Preferably, S3 comprises the following operation: mixing the silicon-based material containing the first coating layer and the nitrogen source solution, and stirring at 30-80℃ for 4-10 h to obtain a mixed solution, and spray-drying the mixed solution to obtain the negative electrode material.

[0047] Preferably, the inlet temperature of the spray-drying is 100-200℃, and the outlet temperature is 60-90℃.

[0048] According to a second aspect of the present application, a negative electrode sheet is provided, which comprises the negative electrode material described above.

[0049] According to a third aspect of the present application, a battery is provided, which comprises the negative electrode sheet described above.

[0050] The negative electrode material provided by the present application can be applied to the negative electrode sheet and the battery, and can endow the battery with excellent rate performance and fast-charging cycle stability. DETAILED DESCRIPTION

[0051] The technical features in the technical solutions provided by the present application are further described clearly and completely in combination with specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, rather than 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.

[0052] Embodiment 1

[0053] A battery is prepared by the following steps:

[0054] 1. Preparation of the negative electrode sheet

[0055] The negative electrode material, conductive agent conductive carbon black (SP), single-walled carbon nanotube (SWCNT), and binder polyacrylic acid (PAA) are mixed in a mass ratio of 80:9:1:10, and then added to a 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 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 prepared.

[0056] The above-mentioned negative electrode material includes a silicon material and a first coating layer and a second coating layer sequentially arranged on the surface of the silicon-based material. The first coating layer contains a first polymer, and the first polymer includes a first monomer as shown in Formula III. The second coating layer contains a second polymer, and the second polymer includes a second monomer as shown in Formula IV.

[0057]

[0058]

[0059] The above-mentioned negative electrode material is prepared by the following steps:

[0060] S1. The fluorine source (first polymer) and the silicon-based material are mixed, and then ball-milled at 300℃ for 12h to prepare a silicon-based material containing a first coating layer.

[0061] The fluorine source is prepared by the following steps: the first monomer is added to an organic solvent NMP, an initiator azobisisobutyronitrile is added, heated to 65℃ under nitrogen protection, and reacted for 7h to obtain a first polymer with a number average molecular weight of 50000-55000.

[0062] The silicon-based material involved in this embodiment is a silicon-carbon material.

[0063] S2. A nitrogen source (second polymer) is prepared into a nitrogen source solution with a mass fraction of 15% by using an organic solvent NMP.

[0064] The nitrogen source is prepared by the following steps: adding a second monomer to an organic solvent NMP, adding an initiator azobisisobutyronitrile, heating to 70°C under nitrogen protection for 10h to obtain a second polymer with a number average molecular weight of 50000-55000;

[0065] S3. The silicon-based material containing the first coating layer is mixed with the nitrogen source solution and stirred at 55°C for 7h to obtain a mixed solution. The mixed solution is spray dried to form a second coating layer on the surface of the silicon-based material containing the first coating layer, thereby obtaining a negative electrode material with a particle size D50 of 8μm. The inlet temperature of the spray drying is 150°C, and the outlet temperature is 75°C.

[0066] The thickness of the first coating layer is 40nm, the thickness of the second coating layer is 55nm, the mass fraction of fluorine in the first coating layer is 2.5%, and the mass fraction of nitrogen in the second coating layer is 2.5%.

[0067] 2. Preparation of the positive electrode sheet

[0068] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2, the conductive agent conductive carbon black (SP), and the binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:2:2, and then added to the 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.

[0069] 3. Preparation of the separator

[0070] A polyethylene (PE) film containing a ceramic layer is used as the separator.

[0071] 4. Preparation of the electrolyte

[0072] 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.

[0073] 5. Assembly of the battery

[0074] 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, the bare cell is obtained by winding. 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, the battery is obtained.

[0075] Embodiment 2

[0076] The embodiment provides a battery, and a difference between the battery and the battery of Embodiment 1 is that a preparation method of a negative electrode material used in a negative electrode sheet is different.

[0077] The negative electrode material used in the embodiment is prepared through the following steps:

[0078] The negative electrode material is prepared through the following steps:

[0079] S1. mixing a fluorine source and a silicon-based material, and ball milling at 230 DEG C for 20 h to obtain a silicon-based material containing a first coating layer;

[0080] The fluorine source is a first polymer with a number average molecular weight of 30,000-35,000.

[0081] S2. preparing a nitrogen source solution with a mass fraction of 3% by using a nitrogen source and an organic solvent NMP;

[0082] The nitrogen source is a second polymer with a number average molecular weight of 10,000-15,000.

[0083] S3. mixing the silicon-based material containing the first coating layer and the nitrogen source solution, and stirring at 30 DEG C for 10 h to obtain a mixed solution, and performing spray drying on the mixed solution to form a second coating layer on the surface of the silicon-based material containing the first coating layer, so as to obtain a negative electrode material with a particle size D50 of 6 μm, wherein an inlet temperature of the spray drying is 100 DEG C, and an outlet temperature is 60 DEG C.

[0084] The thickness of the first coating layer is 3 nm, the thickness of the second coating layer is 100 nm, the mass fraction of fluorine in the first coating layer is 1.3%, and the mass fraction of nitrogen in the second coating layer is 1.4%.

[0085] Except for the above difference, the materials, the formula, the ratio and the preparation operation used in the embodiment are strictly consistent with those of Embodiment 1.

[0086] Embodiment 3

[0087] The embodiment provides a battery, and a difference between the battery and the battery of Embodiment 1 is that a preparation method of a negative electrode material used in a negative electrode sheet is different.

[0088] The negative electrode material used in the embodiment is prepared through the following steps:

[0089] The negative electrode material is prepared through the following steps:

[0090] S1. mixing a fluorine source and a silicon-based material, and ball milling at 230 DEG C for 20 h to obtain a silicon-based material containing a first coating layer;

[0091] The first polymer has a number average molecular weight of 95000-100000.

[0092] S2. A nitrogen source solution with a mass fraction of 25% is prepared by using a nitrogen source and an organic solvent NMP.

[0093] The second polymer has a number average molecular weight of 75000-80000.

[0094] S3. The silicon-based material containing the first coating layer is mixed with the nitrogen source solution and stirred at 80°C for 4h to obtain a mixed solution. The mixed solution is spray dried to form a second coating layer on the surface of the silicon-based material containing the first coating layer, thereby obtaining a negative electrode material with a particle size D50 of 10μm. The inlet temperature of the spray drying is 200°C, and the outlet temperature is 90°C.

[0095] The thickness of the first coating layer is 80nm, the thickness of the second coating layer is 2nm, the mass fraction of fluorine in the first coating layer is 2.8%, and the mass fraction of nitrogen in the second coating layer is 3.5%.

[0096] Except for the above differences, the materials, formula, preparation operation and the like used in this embodiment are strictly consistent with those in Example 1.

[0097] Example 4

[0098] This embodiment provides a battery. Compared with Example 1, the difference is that the first polymer contained in the first coating layer of the negative electrode material is different, and the first monomer used is as shown in Formula V.

[0099]

[0100] Except for the above differences, the materials, formula, preparation operation and the like used in this embodiment are strictly consistent with those in Example 1.

[0101] Example 5

[0102] This embodiment provides a battery. Compared with Example 1, the difference is that the second polymer contained in the second coating layer of the negative electrode material is different, and the second monomer used is as shown in Formula VI.

[0103]

[0104] Except for the above differences, the materials, formula, preparation operation and the like used in this embodiment are strictly consistent with those in Example 1.

[0105] Example 6

[0106] The embodiment provides a battery, and compared with the embodiment 1, the difference in the constitution is that the second polymer contained in the second coating layer in the negative electrode material is different, and the second monomer adopted is as shown in the formula VII.

[0107]

[0108] In addition to the above difference, the materials, the formula ratio and the preparation operation adopted in the embodiment are strictly kept consistent with the embodiment 1.

[0109] Embodiment 7

[0110] The embodiment provides a battery, and compared with the embodiment 1, the difference in the constitution is that (1) in the preparation step S1 of the negative electrode material, the first monomer is added into the organic solvent NMP, an initiator azobisisobutyronitrile is added, and the first polymer with a number average molecular weight of 20000-25000 is prepared as a fluorine source by heating to 40 DEG C under nitrogen protection for 3h; (2) in the preparation step S2 of the negative electrode material, the second monomer is added into the organic solvent NMP, an initiator azobisisobutyronitrile is added, and the second polymer with a number average molecular weight of 8000-8500 is prepared as a nitrogen source by heating to 40 DEG C under nitrogen protection for 5h.

[0111] In addition to the above difference, the materials, the formula ratio and the preparation operation adopted in the embodiment are strictly kept consistent with the embodiment 1.

[0112] Embodiment 8

[0113] The embodiment provides a battery, and compared with the embodiment 1, the difference in the constitution is that (1) in the preparation step S1 of the negative electrode material, the first monomer is added into the organic solvent NMP, an initiator azobisisobutyronitrile is added, and the first polymer with a number average molecular weight of 20000-25000 is prepared as a fluorine source by heating to 40 DEG C under nitrogen protection for 3h; (2) in the preparation step S2 of the negative electrode material, the second monomer is added into the organic solvent NMP, an initiator azobisisobutyronitrile is added, and the second polymer with a number average molecular weight of 8000-8500 is prepared as a nitrogen source by heating to 40 DEG C under nitrogen protection for 5h.

[0114] In addition to the above difference, the materials, the formula ratio and the preparation operation adopted in the embodiment are strictly kept consistent with the embodiment 1.

[0115] Embodiment 9

[0116] The embodiment provides a battery, and compared with the embodiment 1, the difference in the constitution is that the thickness of the first coating layer of the negative electrode material is 2nm, and the thickness of the second coating layer is 120nm.

[0117] 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.

[0118] Example 10

[0119] This embodiment provides a battery, compared with Example 1, the difference is that the thickness of the first coating layer of the negative electrode material is 90 nm, and the thickness of the second coating layer is 1 nm.

[0120] 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.

[0121] Example 11

[0122] This embodiment provides a battery, compared with Example 1, the difference is that in the preparation step S3 of the negative electrode material, the particle size of the prepared negative electrode material is 4 μm.

[0123] 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.

[0124] Example 12

[0125] This embodiment provides a battery, compared with Example 1, the difference is that in the preparation step S3 of the negative electrode material, the particle size of the prepared negative electrode material is 13 μm.

[0126] 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.

[0127] Comparative Example 1

[0128] This comparative example provides a battery, compared with Example 1, the difference is that in the preparation process of the negative electrode sheet, the negative electrode material adopted is a silicon-based material without the first coating layer and the second coating layer.

[0129] Except for the above-mentioned differences, the materials, formula, ratio and preparation operation adopted in this comparative example are strictly consistent with those of Example 1.

[0130] Comparative Example 2

[0131] This comparative example provides a battery, compared with Example 1, the difference is that in the preparation process of the negative electrode sheet, the negative electrode material adopted is a silicon-based material without the first coating layer and only with the second coating layer, and the thickness of the second coating layer is 95 nm.

[0132] Except for the above-mentioned differences, the materials, formula, ratio and preparation operation adopted in this comparative example are strictly consistent with those of Example 1.

[0133] Comparative Example 3

[0134] This comparative example provides a battery, which is compared with Example 1, and the difference is that the negative electrode material used in the preparation process of the negative electrode sheet is a silicon-based material containing only the first coating layer without the second coating layer, and the thickness of the first coating layer is 95 nm.

[0135] Except for the above-mentioned difference, the materials, formula ratio and preparation operation used in this comparative example are strictly consistent with Example 1.

[0136] Comparative Example 4

[0137] This comparative example provides a battery, which is compared with Example 1, and the difference is that (1) in the preparation step S1 of the negative electrode material, the fluorine source (the first polymer) and the silicon-based material are mixed and then ball-milled at 300°C for 29h to obtain the silicon-based material containing the first coating layer; (2) in the preparation step S2 of the negative electrode material, the nitrogen source (the second polymer) is dissolved in the organic solvent NMP to obtain a nitrogen source solution with a mass fraction of 1.5%; (3) in the negative electrode material prepared in the preparation step S3 of the negative electrode material, the mass fraction of fluorine in the first coating layer is 4.5%, and the mass fraction of nitrogen in the second coating layer is 0.2%.

[0138] Except for the above-mentioned difference, the materials, formula ratio and preparation operation used in this comparative example are strictly consistent with Example 1.

[0139] Comparative Example 5

[0140] This comparative example provides a battery, which is compared with Example 1, and the difference is that (1) in the preparation step S1 of the negative electrode material, the fluorine source (the first polymer) and the silicon-based material are mixed and then ball-milled at 300°C for 3h to obtain the silicon-based material containing the first coating layer; (2) in the preparation step S2 of the negative electrode material, the nitrogen source (the second polymer) is dissolved in the organic solvent NMP to obtain a nitrogen source solution with a mass fraction of 29%; (3) in the negative electrode material prepared in the preparation step S3 of the negative electrode material, the mass fraction of fluorine in the first coating layer is 0.8%, and the mass fraction of nitrogen in the second coating layer is 4.8%.

[0141] Except for the above-mentioned difference, the materials, formula ratio and preparation operation used in this comparative example are strictly consistent with Example 1.

[0142] Comparative Example 6

[0143] This comparative example provides a battery, which is compared with Example 1, and the difference is that the monomers contained in the first coating layer and the second coating layer of the negative electrode material are different.

[0144] The negative electrode material involved in this comparative example is prepared by the following steps:

[0145] S1. A nitrogen source solution with a mass fraction of 15% is prepared by using a nitrogen source and an organic solvent NMP;

[0146] The nitrogen source is prepared by the following steps: a monomer as shown in Formula IV is added to an organic solvent NMP, an initiator azobisisobutyronitrile is added, and the mixture is heated to 70℃ under nitrogen protection for 10h to obtain a polymer with a number average molecular weight of 50000-55000 as the nitrogen source.

[0147] S2. The silicon-based material with a particle size D50 of 8μm is mixed with the nitrogen source solution and stirred at 55℃ for 7h to obtain a mixed solution. The mixed solution is spray dried to form a first coating layer on the surface of the silicon-based material, thereby obtaining a silicon-based material containing a first coating layer.

[0148] S3. The fluorine source and the silicon-based material containing a first coating layer are mixed, and then ball-milled at 300℃ for 12h to form a second coating layer on the surface of the silicon-based material containing a first coating layer, thereby obtaining a negative electrode material.

[0149] The fluorine source is prepared by the following steps: a monomer as shown in Formula III is added to an organic solvent NMP, an initiator azobisisobutyronitrile is added, and the mixture is heated to 65℃ under nitrogen protection for 7h to obtain a polymer with a number average molecular weight of 50000-55000 as the fluorine source.

[0150] The thickness of the first coating layer is 40nm, the thickness of the second coating layer is 55nm, the mass fraction of nitrogen in the first coating layer is 2.5%, and the mass fraction of fluorine in the second coating layer is 2.5%.

[0151] In addition to the above differences, the materials, formulation ratios and preparation operations used in the comparative examples are strictly consistent with those of Example 1.

[0152] Test Example

[0153] 1. Test subjects

[0154] In this test example, the batteries prepared in Examples 1-12 and Comparative Examples 1-6 are used as test subjects, and relevant performance tests are carried out.

[0155] 2. Test content

[0156] (1) First coulombic efficiency

[0157] At 25℃, the battery is charged at a rate of 0.33C to 4.2V, and then discharged at a rate of 0.33C to 2.5V. The first coulombic efficiency of the battery is calculated.

[0158] First coulombic efficiency (%) = battery 0.33C first discharge total capacity / battery 0.33C first charge total capacity x 100%.

[0159] (2) Capacity retention rate after 1C / 1C cycling at room temperature for 1200 cycles

[0160] The battery was charged at 1C rate to 4.2V under 25℃ condition with constant current and constant voltage, the cutoff current was 0.05C, and then rested for 10 min, followed by discharging the battery at 1C rate to 2.5V with constant current, and then rested for 10 min, which was one cycle of charge and discharge. The battery was cycled for 1200 cycles according to the above method, and the capacity retention rate of the battery after 1200 cycles of 1C / 1C charge and discharge was calculated.

[0161] The capacity retention rate of the battery after N cycles (%) = (discharge capacity of the Nth cycle / first discharge capacity) x 100%, and N was the cycle number of the battery.

[0162] (3) 6C rate performance at room temperature - constant current charge ratio

[0163] The battery was discharged at 1C rate to 2.5V under 25℃ condition with constant current, and then rested for 10 min, followed by charging the battery at 6C rate to 4.2V with constant current and constant voltage, the cutoff current was 0.05C, and then rested for 10 min, and 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. 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.

[0164] (4) 1C / 10C discharge capacity retention rate at room temperature

[0165] The battery was charged at 1C rate to 4.2V under 25℃ condition with constant current and constant voltage, the cutoff current was 0.05C, and then rested for 10 min, followed by discharging the battery at 1C rate to 2.5V, and the discharge capacity Q 1C was recorded as the initial discharge capacity; then the battery was charged at 1C rate to 4.2V under 25℃ condition with constant current and constant voltage, the cutoff current was 0.05C, and then rested for 10 min, followed by discharging the fully charged battery at 10C rate to 2.5V with constant current, and the discharge capacity Q 10C was recorded; and the discharge capacity retention rate of the 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%.

[0166] 3. Experimental results

[0167] Table 1. Results of relevant performance tests of the batteries

[0168]

[0169]

[0170] The results of relevant performance tests of the batteries prepared in Examples 1-12 and Comparative Examples 1-6 are shown in Table 1.

[0171] The negative electrode material used in the lithium battery provided by Comparative Example 1 is a silicon-based material without a coating layer, while the negative electrode material used in the batteries provided by Examples 1-12 comprises a silicon-based material and a fluorine-containing first coating layer and a nitrogen-containing second coating layer sequentially arranged on the surface of the silicon-based material. 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-in ratio at 6C rate, and the capacity retention rate at room temperature at 1C / 10C of the batteries provided by Examples 1-12 are all significantly higher than those of Comparative Example 1. The above results can indicate that by sequentially coating the surface of the silicon-based material with a first coating layer containing fluorine elements and a second coating layer containing nitrogen elements, the rate performance and fast-charging cycle stability of the battery applying the same can be improved.

[0172] By comparing the test results of Comparative Examples 1, 4, 5, and 6, it can be shown that compared with the first monomer and the second monomer of other structures, the first polymer formed by polymerization of the first monomer represented by Formula III and the second polymer formed by polymerization of the second monomer represented by Formula IV in Example 1 have better first coulombic efficiency, cycle performance, rate performance, and fast-charging performance when used to coat the silicon-based material to prepare the negative electrode material.

[0173] By comparing the test results of Comparative Examples 1, 7, and 8, it can be shown that the use of a first polymer with a number average molecular weight in the range of 30000-100000 and a second polymer with a number average molecular weight in the range of 10000-80000 to coat the silicon-based material results in a negative electrode material and a battery applying the same with better cycle performance and rate performance.

[0174] By comparing the test results of Example 1 and Comparative Examples 4 and 5, it can be shown that by controlling the content of fluorine elements in the first coating layer and the content of nitrogen elements in the second coating layer of the negative electrode material to be in the range of 1-3% and 1-4%, respectively, the rate performance and cycle performance of the negative electrode material and the battery applying the same can be better.

[0175] 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 is 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 within the protection scope of the present application.

Claims

1. A negative electrode material, characterized by: The negative electrode material comprises a silicon-based material and a first coating layer and a second coating layer arranged in sequence on the surface of the silicon-based material; The first coating layer contains fluorine elements, and the mass percentage of the fluorine elements in the first coating layer is 1-3%; the first coating layer contains a first polymer, and the first polymer comprises a single monomer as shown in Formula I; , formula I; The R1, the R2, the R3, and the R4 are independently selected from one of --H, an alkyl group, and --F, and at least one of the R1, the R2, the R3, and the R4 is --F; The second coating layer contains nitrogen elements, and the mass percentage of the nitrogen elements in the second coating layer is 1-4%; the second coating layer contains a second polymer, and the second polymer comprises a second monomer as shown in Formula II; X contains a urea group.

2. The negative electrode material of claim 1, wherein: The first polymer comprises a first monomer as shown in Formula III, and / or the second polymer comprises a second monomer as shown in Formula IV; , formula III; , formula IV.

3. The negative electrode material of claim 1, wherein: The number average molecular weight of the first polymer is 30,000-100,000, and / or the number average molecular weight of the second polymer is 10,000-80,000.

4. The negative electrode material of claim 1, wherein: The mass percentage of the first coating layer in the negative electrode material is 2-4%, and / or the thickness of the first coating layer is 3-80 nm.

5. The negative electrode material of claim 1, wherein: The mass percentage of the second coating layer in the negative electrode material is 1-5%, and / or the thickness of the second coating layer is 2-100 nm.

6. The negative electrode material of claim 1, wherein: The particle size D50 of the negative electrode material is 6-10 µm.

7. The negative electrode material of claim 1, wherein the carbon-based material is selected from the group consisting of graphite, carbon black, and carbon nanotubes. The negative electrode material is prepared by the following steps: S1. mixing a fluorine source and the silicon-based material, ball milling at 230-400°C for 5-20 h to obtain a silicon-based material containing the first coating layer; S2. preparing a nitrogen source solution by using a nitrogen source and an organic solvent; S3. applying the nitrogen source solution to the surface of the silicon-based material containing the first coating layer, and drying the nitrogen source solution to form the second coating layer on the surface of the silicon-based material containing the first coating layer, thereby obtaining the negative electrode material.

8. A negative electrode sheet characterized by comprising: The negative electrode sheet comprises the negative electrode material according to any one of claims 1-7.

9. A battery, characterized by: The battery comprises the negative electrode sheet according to claim 8.

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