A negative electrode material and a battery including the same

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a negative electrode material and a battery comprising the same. In the application, lithium carbonate is distributed on the surface of active particles in the form of nanoparticles, is coated with amorphous carbon and carbon nanotubes, and the carbon nanotubes are distributed inside and / or on the surface of the amorphous carbon, thereby constructing a composite inorganic layer (i.e. coating layer). The coating layer has high structural stability and chemical stability in high-temperature charge-discharge cycles, the battery assembled from the negative electrode material has the advantages of high high-temperature cycle capacity retention rate and small thickness expansion rate, and also has relatively high specific capacity and initial coulombic efficiency.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically to a negative electrode material and a battery including the negative electrode material. Background Technology

[0002] During the charging and discharging process of a lithium-ion battery, the electrolyte reacts on the surface of the negative electrode material, forming an interfacial material separating the solid and liquid phases. This interfacial material is called the solid electrolyte interface (SEI) film. The SEI film can suppress the co-intercalation of solvent molecules and plays a crucial role in the stable cycling of lithium-ion batteries. However, under high-temperature charging and discharging conditions, the SEI film is prone to dissolution, decomposition, and regrowth. The newly formed SEI film has a loose structure and cannot effectively suppress the further occurrence of side reactions, resulting in continuous consumption of active materials and continuous thickening of the SEI film. Ultimately, this leads to excessively rapid capacity decay and excessive thickness expansion of the lithium-ion battery. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention aims to provide a negative electrode material and a battery comprising the negative electrode material. The negative electrode material can solve the problems of excessive capacity decay and excessive thickness expansion during high-temperature charge-discharge cycles. Furthermore, the negative electrode material also has high specific capacity and initial coulombic efficiency.

[0004] The objective of this invention is achieved through the following technical solution:

[0005] A negative electrode material comprising active particles, lithium carbonate particles, amorphous carbon, and carbon nanotubes; wherein the lithium carbonate particles are distributed on the surface of the active particles, and the amorphous carbon and carbon nanotubes are coated on the surfaces of the active particles and the lithium carbonate particles.

[0006] According to an embodiment of the present invention, the amorphous carbon and carbon nanotubes are mixed to form a coating layer (i.e., a composite inorganic layer) that coats the surfaces of the active particles and lithium carbonate particles. Preferably, the coating layer completely coats the surfaces of the active particles and lithium carbonate particles.

[0007] According to an embodiment of the present invention, the content of each component in the negative electrode material, by weight, is as follows:

[0008]

[0009] Preferably, the content of each component in the negative electrode material, by weight, is as follows:

[0010] 95 parts, 95.5 parts, 96 parts, 96.5 parts, 97 parts, 97.5 parts, 98 parts, 98.5 parts, 99 parts, and 99.5 parts of active granules;

[0011] Lithium carbonate granules in the following quantities: 0.03 parts, 0.04 parts, 0.05 parts, 0.06 parts, 0.07 parts, 0.08 parts, 0.09 parts, 0.1 parts, 0.15 parts, 0.2 parts, 0.25 parts, 0.3 parts, 0.35 parts, 0.4 parts, 0.45 parts, 0.5 parts, 0.55 parts, 0.6 parts;

[0012] Amorphous carbon in the following quantities: 0.45 parts, 0.5 parts, 0.55 parts, 0.6 parts, 0.65 parts, 0.7 parts, 0.75 parts, 0.8 parts, 0.85 parts, 0.9 parts, 0.95 parts, 1 part, 1.5 parts, 2 parts, 2.5 parts, 3 parts, 3.5 parts, and 4 parts.

[0013] Carbon nanotubes: 0.02 parts, 0.03 parts, 0.04 parts, 0.05 parts, 0.06 parts, 0.07 parts, 0.08 parts, 0.09 parts, 0.1 parts, 0.15 parts, 0.2 parts, 0.25 parts, 0.3 parts, 0.35 parts, 0.4 parts.

[0014] According to embodiments of the present invention, when the content of each component is within the above-mentioned range, the negative electrode material has a high specific capacity and initial coulombic efficiency, good coating integrity, and a small specific surface area. When the content of active particles is less than 95 parts, or the content of amorphous carbon is greater than 4 parts, the specific capacity and initial coulombic efficiency of the negative electrode material are low; when the content of active particles is greater than 99.5 parts, or the content of lithium carbonate particles is greater than 0.6 parts, or the content of amorphous carbon is less than 0.45 parts, the coating integrity of the negative electrode material is poor, and it cannot solve the problem of excessive capacity decay and excessive thickness expansion during high-temperature charge-discharge cycles; when the content of lithium carbonate particles is less than 0.03 parts, the performance improvement effect on the negative electrode material is small; when the content of carbon nanotubes is greater than 0.4 parts, the specific surface area of ​​the negative electrode material is large, and there are more surface side reactions, which will deteriorate the performance of the battery.

[0015] According to embodiments of the present invention, the active particles are substances with reversible lithium insertion and delithiation capabilities, including but not limited to at least one of artificial graphite, natural graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-oxygen complexes, silicon-carbon complexes, and lithium titanate.

[0016] According to an embodiment of the present invention, the median particle size Dv50 of the active particles based on the bulk density distribution is 4μm to 20μm, for example, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, or 20μm. When the Dv50 of the active particles is within the above range, the specific surface area of ​​the negative electrode material is small, and the particle distribution uniformity in the negative electrode sheet is good. When the Dv50 of the active particles is less than 4μm, the specific surface area of ​​the negative electrode material is too large, and there are more surface side reactions; when the Dv50 of the active particles is greater than 20μm, scratches and other phenomena are prone to occur during coating, affecting the performance of the battery.

[0017] According to embodiments of the present invention, the lithium carbonate particles have a particle size of less than or equal to 100 nm, for example, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, or 95 nm. When this condition is met, amorphous carbon can be well coated on the surface of the lithium carbonate particles, and the lithium carbonate particles can adhere to the surface of the active particles. When this condition is not met (particle size greater than 100 nm), the lithium carbonate particles are not easily adhered to the surface of the active particles, and the amorphous carbon cannot effectively encapsulate the lithium carbonate particles.

[0018] According to embodiments of the present invention, the carbon nanotubes are dispersed inside and / or on the surface of amorphous carbon and a coating layer formed by the carbon nanotubes; or, the carbon nanotubes are dispersed inside and / or on the surface of amorphous carbon.

[0019] According to embodiments of the present invention, the average diameter of the carbon nanotubes is less than or equal to 100 nm, for example, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, or 95 nm, and the average length is 0.2 μm to 50 μm, for example, 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 8 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, or 50 μm. When this condition is met, the utilization rate of carbon nanotubes per unit mass is high, which can effectively improve the conductivity of the surface of active particles. When the average diameter of carbon nanotubes is greater than 100 nm, the utilization rate of carbon nanotubes per unit mass is low; when the average length of carbon nanotubes is less than 0.2 μm, it is not easy to form an effective conductive path; when the average length of carbon nanotubes is greater than 50 μm, it is not easy to disperse and adhere to the surface of active particles and lithium carbonate particles.

[0020] The present invention also provides a method for preparing the above-mentioned negative electrode material, comprising the following steps:

[0021] (1) Disperse lithium source material, carbon nanotubes and carbon source material in deionized water and stir evenly to obtain the first mixture;

[0022] (2) Add the active particles to the first mixture and stir evenly to obtain the second mixture;

[0023] (3) Evaporate the deionized water in the second mixture to obtain the first powder;

[0024] (4) Under the protection of inert gas, the first powder is calcined to obtain the second powder, namely the negative electrode material.

[0025] According to an embodiment of the present invention, the method further includes the following steps:

[0026] (5) The second powder is cleaned with deionized water and then dried to obtain the negative electrode material.

[0027] The lithium source material is selected from lithium carboxymethyl cellulose and / or lithium polyacrylate, and the weight content of lithium in the lithium source material is 1.0% to 4.0%.

[0028] The average diameter of the carbon nanotubes is less than or equal to 100 nm, and the average length is 0.2 μm to 50 μm.

[0029] The carbon source material is selected from water-soluble carbon-containing polymers and / or polymers, such as at least one of cellulose, carboxymethyl cellulose, starch, chitosan, polyvinylpyrrolidone, polyacrylic acid, and polyacrylamide.

[0030] The active particles are selected from one or more of the following: artificial graphite, natural graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-oxygen composites, silicon-carbon composites, and lithium titanate.

[0031] The mass ratio of the active particles to carbon nanotubes is 95–99.5:0.025–0.45;

[0032] The mass ratio of the active particles to the lithium source material is 95–99.5:0.15–4.5;

[0033] The mass ratio of the active particles to the carbon source is 95–99.5:0.95–9.5;

[0034] The inert gas is one or more of nitrogen, helium, argon, xenon, and radon;

[0035] The calcination process is as follows: The temperature is increased to 200–350℃ (e.g., 200℃, 220℃, 240℃, 250℃, 280℃, 290℃, 300℃, 320℃, 340℃, or 350℃) at a heating rate of 2–5℃ / min, and held for 1–4 hours. Then, the temperature is increased to 600–1200℃ (e.g., 600℃, 700℃, 800℃, 900℃, 1000℃, 1100℃, or 1200℃) at a heating rate of 1–4℃ / min, and held for 2–6 hours.

[0036] After calcination, the lithium source material can form lithium carbonate particles that adhere to the surface of the active particles.

[0037] After calcination, the carbon source material can form amorphous carbon that coats the surface of the active particles and lithium carbonate particles.

[0038] The cleaning process is as follows: Mix the second powder and deionized water at a mass ratio of 1:2 to 1:19, stir mechanically for 1 to 3 hours, then filter out the liquid to obtain the cleaned powder. Repeat this process 2 to 4 times.

[0039] The present invention also provides a negative electrode sheet, wherein the negative electrode sheet comprises the above-mentioned negative electrode material.

[0040] The present invention also provides a battery comprising the above-described negative electrode material; or, the battery comprising the above-described negative electrode sheet.

[0041] According to an embodiment of the present invention, the battery includes a positive electrode, a separator, an electrolyte, and a casing.

[0042] According to an embodiment of the present invention, the charge / discharge cutoff voltage of the battery is greater than or equal to 4.45V.

[0043] According to an embodiment of the present invention, the battery retains a capacity of more than 86% and a thickness expansion rate of less than 10% after 800 charge-discharge cycles at 45°C and 1.5C / 0.5C.

[0044] Beneficial effects:

[0045] This invention distributes lithium carbonate in the form of nanoparticles on the surface of active particles, coats it with amorphous carbon and carbon nanotubes, and distributes the carbon nanotubes inside and / or on the surface of the amorphous carbon, constructing a composite inorganic layer (i.e., a coating layer). Compared with the prior art, the lithium carbonate in this invention is in the form of nanoparticles, which can avoid the coating layer cracking caused by lithium intercalation expansion of the negative electrode; the use of amorphous carbon as a surface coating material can significantly improve the structural toughness of the coating layer; the coating of lithium carbonate with amorphous carbon can prevent the dissolution or decomposition of lithium carbonate particles under high temperature conditions, and at the same time prevent the dissolution of lithium carbonate in aqueous slurries, thus improving the stability of the negative electrode material structure and processing technology; the carbon nanotubes of this invention can significantly improve the surface conductivity of the negative electrode material and reduce the capacity decay caused by ohmic polarization. The coating layer of this invention has high structural and chemical stability in high-temperature charge-discharge cycles, and the battery assembled from the negative electrode material has the advantages of high high-temperature cycle capacity retention and small thickness expansion rate, while also having high specific capacity and initial coulombic efficiency. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the structure of the negative electrode material prepared in Example 1 of the present invention; 11 is an active particle, 12 is a lithium carbonate particle, 13 is a carbon nanotube, and 14 is amorphous carbon.

[0047] Figure 2 A scanning electron microscope (SEM) image of the negative electrode material prepared in Example 1 of this invention.

[0048] Figure 3 The image shows the X-ray diffraction (XRD) pattern of the negative electrode material prepared in Example 1 of this invention.

[0049] Figure 4 The image shows the X-ray diffraction (XRD) pattern of the negative electrode material prepared in Comparative Example 3 of this invention.

[0050] Figure 5 Capacity retention curves of lithium-ion batteries assembled with negative electrode materials prepared for each embodiment and comparative example.

[0051] Figure 6 Thickness expansion curves of lithium-ion batteries assembled from the negative electrode materials prepared for each embodiment and comparative example. Detailed Implementation

[0052] The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0053] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0054] In the description of this invention, it should be noted that the terms "first," "second," etc., are used for descriptive purposes only and are not intended to indicate or imply relative importance.

[0055] Example 1

[0056] The negative electrode material in this embodiment was obtained using the following preparation method:

[0057] (1) Disperse 1.63g of lithium carboxymethyl cellulose, 0.075g of carbon nanotubes and 4.35g of cellulose in 400mL of deionized water and stir evenly to obtain the first mixture;

[0058] Among them, the lithium content of carboxymethyl cellulose lithium is 3.1% by weight, and the carbon nanotubes have an average diameter of 10 nm and an average length of 2 μm;

[0059] (2) Add 100g of artificial graphite to the first mixture and stir evenly to obtain the second mixture;

[0060] Among them, the median particle size Dv50 of artificial graphite is 13 μm;

[0061] (3) Evaporate the deionized water in the second mixture to obtain the first powder;

[0062] (4) Under the protection of inert gas, the first powder is calcined to obtain the second negative powder;

[0063] (5) The second powder is cleaned with deionized water and then dried to obtain the negative electrode material.

[0064] The calcination process is as follows: the temperature is increased to 250℃ at a heating rate of 3℃ / min and held for 2 hours, then increased to 800℃ at a heating rate of 2℃ / min and held for 4 hours.

[0065] The cleaning process is as follows: Mix the powder and deionized water at a mass ratio of 1:9, stir mechanically for 2 hours, then filter out the liquid to obtain the cleaned powder. Repeat this process twice.

[0066] Using the above calcination process, artificial graphite and carbon nanotubes were calcined separately. The weight retention rate was calculated by dividing the weight after calcination by the weight before calcination, which allowed for the estimation of the content of artificial graphite and carbon nanotubes in the negative electrode material. In this embodiment, the weight retention rates of both artificial graphite and carbon nanotubes were approximately 100%.

[0067] Figure 1 A schematic diagram of the structure of the negative electrode material prepared in Example 1 is given.

[0068] The negative electrode material in this embodiment has the following characteristics:

[0069] (1) The negative electrode material includes artificial graphite, lithium carbonate particles distributed on the surface of artificial graphite, amorphous carbon coated on the surface of artificial graphite and lithium carbonate particles, and carbon nanotubes.

[0070] (2) In this negative electrode material, the weight content of artificial graphite is 97.65%, the weight content of carbon nanotubes is 0.07%, the weight content of lithium carbonate particles is 0.26%, and the weight content of amorphous carbon is 2.02%.

[0071] (3) The particle size of lithium carbonate particles is less than 100 nm, and carbon nanotubes are dispersed on the surface of active particles and lithium carbonate particles.

[0072] Figure 2 SEM images of the negative electrode material prepared in Example 1 are provided. From the images, it can be seen that there are protrusions with a size of less than 100 nm on the surface of the negative electrode material, corresponding to amorphous carbon-coated lithium carbonate particles. At the same time, carbon nanotubes with a length of several micrometers can be observed.

[0073] (4) Lithium carbonate particles have a crystalline structure.

[0074] Figure 3 The XRD pattern of the negative electrode material prepared in Example 1 is shown. The diffraction peak at 26°-27° is the diffraction peak of the {002} crystal plane of artificial graphite. The diffraction peak of lithium carbonate (Li2CO3) is also marked in the figure.

[0075] Example 2

[0076] The preparation method of the negative electrode material in this embodiment is similar to that in Example 1, except that the amount of lithium carboxymethyl cellulose in step (1) is reduced from 1.63g to 0.2g, and the amount of cellulose in step (1) is reduced from 4.35g to 1.04g.

[0077] Compared with Example 1, the negative electrode material of this embodiment has a similar carbon nanotube content, but the weight contents of artificial graphite, lithium carbonate particles, and amorphous carbon are 99.45%, 0.03%, and 0.45%, respectively.

[0078] Example 3

[0079] The preparation method of the negative electrode material in this embodiment is similar to that in Example 1, except that the amount of lithium carboxymethyl cellulose in step (1) is increased from 1.63g to 3.85g, and the amount of cellulose in step (1) is increased from 4.35g to 8.56g.

[0080] Compared with Example 1, the negative electrode material of this embodiment has a similar carbon nanotube content, but the weight contents of artificial graphite, lithium carbonate particles, and amorphous carbon are 95.33%, 0.60%, and 4.00%, respectively.

[0081] Example 4

[0082] The preparation method of the negative electrode material in this embodiment is similar to that in Example 1, except that the amount of carbon nanotubes in step (1) is reduced from 0.075g to 0.02g.

[0083] Compared with Example 1, the negative electrode material of this embodiment has similar contents of lithium carbonate particles and amorphous carbon, but the weight contents of artificial graphite and carbon nanotubes are 97.70% and 0.02%, respectively.

[0084] Example 5

[0085] The preparation method of the negative electrode material in this embodiment is similar to that in Example 1, except that the amount of carbon nanotubes in step (1) is increased from 0.075g to 0.41g.

[0086] Compared with Example 1, the negative electrode material of this embodiment has similar contents of lithium carbonate particles and amorphous carbon, but the weight contents of artificial graphite and carbon nanotubes are 97.32% and 0.40%, respectively.

[0087] Comparative Example 1

[0088] The preparation method of the negative electrode material in this comparative example is similar to that in Example 1, except that 1.63g of lithium carboxymethyl cellulose in step (1) is replaced with 2.80g of carboxymethyl cellulose.

[0089] Compared with Example 1, the negative electrode material of this comparative example has similar contents of artificial graphite and carbon nanotubes, but the weight contents of lithium carbonate particles and amorphous carbon are 0% and 2.28%, respectively.

[0090] Comparative Example 2

[0091] The preparation method of the negative electrode material in this comparative example is similar to that in Example 1, except that 0.075g of carbon nanotubes and 4.35g of cellulose in step (1) are replaced with 4.54g of cellulose.

[0092] Compared with Example 1, the negative electrode material of this comparative example has similar contents of artificial graphite and lithium carbonate particles, but the weight contents of carbon nanotubes and amorphous carbon are 0% and 2.09%, respectively.

[0093] Comparative Example 3

[0094] The preparation method of the negative electrode material in this comparative example is similar to that in Example 1, except that cellulose is not added in step (1).

[0095] Compared with Example 1, the negative electrode material of this comparative example has similar contents of lithium carbonate and carbon nanotubes, but the weight contents of artificial graphite and amorphous carbon are 99.34% and 0.33%, respectively.

[0096] Figure 4 The XRD pattern of the negative electrode material prepared in Comparative Example 3 is shown. The diffraction peak at 26°-27° is the diffraction peak of the {002} crystal plane of artificial graphite. There is no diffraction peak of lithium carbonate (Li2CO3) in the figure, which indicates that the content of amorphous carbon is too low and cannot effectively coat the lithium carbonate particles.

[0097] Comparative Example 4

[0098] The preparation method of the negative electrode material in this comparative example is similar to that in Example 1, except that in step (1), lithium carboxymethyl cellulose and carbon nanotubes are not added, but only 6.02g of cellulose is added.

[0099] Compared with Example 1, the negative electrode material of this comparative example has a similar content of artificial graphite, but the weight contents of lithium carbonate particles, carbon nanotubes and amorphous carbon are 0%, 0% and 2.35%, respectively.

[0100] Comparative Example 5

[0101] The negative electrode material in this comparative example is the artificial graphite in step (1) of Example 1.

[0102] Coin Cell Preparation and Testing

[0103] (1) Preparation of button cells

[0104] The negative electrode material, sodium carboxymethyl cellulose, styrene-butadiene rubber, and carbon black were mixed in a mass ratio of 97.1:1.2:1.4:0.3. Deionized water was added, and the mixture was stirred under vacuum to obtain a negative electrode slurry with a concentration of 5.5 mg / cm³. 2The areal density was determined by coating the negative electrode slurry onto an 8μm thick copper foil, drying it at 80℃, slicing it, and then transferring it to a 100℃ vacuum oven for 12 hours of drying. After rolling in the drying environment, the compaction density was approximately 1.4 g / cm³. 3 Then, a punching machine was used to make round discs with a diameter of about 1.2 cm.

[0105] Under an inert atmosphere, 13 wt% of fully dried lithium hexafluorophosphate and 10 wt% of fluoroethylene carbonate were rapidly added to ethylene carbonate and stirred until homogeneous to obtain the desired electrolyte.

[0106] In a glove box, the aforementioned disc is used as the working electrode, a lithium metal sheet as the counter electrode, and a polyethylene membrane as the separator. Electrolyte is added to assemble a coin cell.

[0107] (2) Performance testing of button cells

[0108] Using the LAND test system, the specific capacity and initial coulombic efficiency of the negative electrode material were calculated by discharging to 0.005V at 50μA, allowing it to stand for 10 minutes, and then charging to 1.5V at 50μA.

[0109] Table 1. Specific capacity and initial coulombic efficiency test results of the anode materials in each embodiment and comparative example.

[0110] Group Capacity (mAh / g) First Coulomb efficiency Example 1 352 92.2％ Example 2 354 92.9％ Example 3 349 91.2％ Example 4 354 93.2％ Example 5 352 91.8％ Comparative Example 1 353 92.3％ Comparative Example 2 351 92.4％ Comparative Example 3 347 90.9％ Comparative Example 4 351 92.3％ Comparative Example 5 349 91.1％

[0111] As can be seen from Table 1, the amorphous carbon and carbon nanotube contents of the negative electrode materials in Examples 1-5 are moderate, the coating effect is good, and the specific capacity and first coulombic efficiency are high; the amorphous carbon content of the negative electrode material in Comparative Example 3 is too low, the coating integrity is poor, and the specific capacity and first coulombic efficiency are low; the negative electrode material in Comparative Example 5 has no coating, and the specific capacity and first coulombic efficiency are low.

[0112] Preparation and testing of lithium-ion batteries

[0113] (1) Preparation of negative electrode

[0114] A negative electrode material, sodium carboxymethyl cellulose, styrene-butadiene rubber, and carbon black were mixed in a mass ratio of 97.1:1.2:1.4:0.3. Deionized water was added, and the mixture was stirred under vacuum to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated onto a copper foil with a thickness of 8 μm and dried to obtain a copper foil with a negative electrode coating. The areal density of the negative electrode coating was 9.2 mg / cm³. 2 The copper foil containing the negative electrode coating was transferred to an oven at 100°C and dried for 10 hours, with a concentration of 1.75 g / cm³. 3 The material is compacted and rolled, then cut to obtain the negative electrode sheet.

[0115] (2) Preparation of lithium-ion batteries

[0116] Lithium cobalt oxide, polyvinylidene fluoride, and carbon black were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone was added, and the mixture was stirred under vacuum until a homogeneous positive electrode slurry was formed. The positive electrode slurry was then uniformly coated onto an aluminum foil with a thickness of 10 μm and dried to obtain an aluminum foil with a positive electrode coating. The areal density of the positive electrode coating was 17.0 mg / cm³. 2 The aluminum foil containing the positive electrode coating was transferred to an oven at 120°C and dried for 8 hours, with a yield of 4.1 g / cm³. 3 The compaction density is achieved by rolling, and then slitting to obtain the desired positive electrode sheet.

[0117] Under an inert atmosphere, a mixed solution was prepared according to the mass ratio of EC:PC:PP:LiPF6:FEC:PS = 13:13:50:15:5:4, and stirred evenly to obtain the desired electrolyte.

[0118] The positive electrode, an 8μm thick polyethylene separator, and the negative electrode are stacked in sequence, ensuring that the separator acts as a separator between the positive and negative electrodes. Then, the cells are wound to obtain bare cells. The bare cells are placed in an aluminum-plastic film casing and dried. Electrolyte is injected into the bare cells. After processes such as encapsulation, settling, formation, secondary sealing, and sorting, the desired lithium-ion battery is obtained.

[0119] (3) Performance testing of lithium-ion batteries

[0120] Using the LAND testing system at a test temperature of 45℃, the initial discharge capacity was obtained by constant-current charging at 1.5C to 4.45V, constant-voltage charging at 0.05C, resting for 10 minutes, and discharging at 0.5C to 3V. The thickness of the lithium-ion battery was measured by constant-current charging at 1.5C to 3.88V, constant-voltage charging at 0.02C, and this was taken as the initial thickness. Cyclic testing was then performed using this charge-discharge cycle: constant-current charging at 1.5C to 4.45V, constant-voltage charging at 0.05C, resting for 10 minutes, discharging at 0.5C to 3V, and resting for 10 minutes. The capacity retention rate per week could be obtained based on the initial discharge capacity. The full-charge thickness of the lithium-ion battery was measured every 100 weeks, and the thickness expansion rate per 100 weeks could be obtained based on the initial thickness.

[0121] Figure 5 Cycle capacity retention curves of lithium-ion batteries assembled with anode materials prepared in various embodiments and comparative examples are shown.

[0122] Figure 6 Cyclic thickness expansion curves of lithium-ion batteries assembled with anode materials prepared in various embodiments and comparative examples are shown.

[0123] from Figure 5 and Figure 6As can be seen, the negative electrode materials in Examples 1-5 contain lithium carbonate particles and carbon nanotubes on their surface, and the lithium carbonate particles are coated with amorphous carbon, resulting in a stable surface structure. The assembled lithium-ion batteries exhibit a capacity retention rate higher than 86% and a thickness expansion rate less than 10% after 800 cycles. In contrast, the negative electrode material in Comparative Example 1 does not contain lithium carbonate particles, the negative electrode material in Comparative Example 2 does not contain carbon nanotubes, the negative electrode material in Comparative Example 3 has too little amorphous carbon to effectively encapsulate the lithium carbonate particles, the negative electrode material in Comparative Example 4 contains neither lithium carbonate particles nor carbon nanotubes, and Comparative Example 5 is uncoated artificial graphite. The lithium-ion batteries assembled from the negative electrode materials of each comparative example exhibit a capacity retention rate lower than 86% and a thickness expansion rate greater than 11% after 800 cycles.

[0124] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode material, characterized in that, The negative electrode material is composed of active particles, lithium carbonate particles, amorphous carbon, and carbon nanotubes; the lithium carbonate particles are distributed on the surface of the active particles, and the amorphous carbon and carbon nanotubes are coated on the surfaces of the active particles and the lithium carbonate particles. The lithium carbonate particles have a particle size of less than or equal to 100 nm; The content of each component in the negative electrode material, by weight, is as follows: 95 to 99.5 parts of active granules; Lithium carbonate granules: 0.03 parts to 0.6 parts; 0.45 to 4 parts of amorphous carbon; 0.02 to 0.4 parts of carbon nanotubes.

2. The negative electrode material according to claim 1, characterized in that, The amorphous carbon and carbon nanotubes are mixed to form a coating layer that coats the surfaces of the active particles and lithium carbonate particles.

3. The negative electrode material according to claim 1, characterized in that, The active particles are selected from at least one of artificial graphite, natural graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-oxygen complex, silicon-carbon complex, and lithium titanate.

4. The negative electrode material according to claim 1, characterized in that, The median particle size Dv50 of the active particles based on the volume density distribution is 4μm~20μm.

5. The negative electrode material according to claim 1, characterized in that, The carbon nanotubes are dispersed within and / or on the surface of amorphous carbon and the coating layer formed by the carbon nanotubes.

6. The negative electrode material according to claim 1, characterized in that, The carbon nanotubes have an average diameter of less than or equal to 100 nm and an average length of 0.2 μm to 50 μm.

7. The negative electrode material according to any one of claims 1-6, characterized in that, The preparation method of the negative electrode material includes the following steps: (1) Disperse lithium source material, carbon nanotubes and carbon source material in deionized water and stir evenly to obtain the first mixture; (2) Add the active particles to the first mixture and stir evenly to obtain the second mixture; (3) Evaporate the deionized water in the second mixture to obtain the first powder; (4) Under the protection of inert gas, the first powder is calcined to obtain the second powder, i.e., the negative electrode material; wherein the lithium source material is selected from lithium carboxymethyl cellulose and / or lithium polyacrylate; The carbon source is selected from at least one of cellulose, carboxymethyl cellulose, starch, chitosan, polyvinylpyrrolidone, polyacrylic acid, and polyacrylamide; The calcination process is as follows: the temperature is increased to 200-350℃ at a heating rate of 2-5℃ / min, held for 1-4 hours, and then increased to 600-1200℃ at a heating rate of 1-4℃ / min.

8. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the negative electrode material according to any one of claims 1-7.

9. A battery, characterized in that, The battery comprises the negative electrode material according to any one of claims 1-7; or, the battery comprises the negative electrode sheet according to claim 8.

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