A lithium ion battery negative electrode sheet and a lithium ion battery using the same

Abstract

The application discloses a lithium ion battery negative pole piece, which comprises a negative pole current collector and an active material layer coated on the negative pole current collector, and the raw material of the active material layer comprises a granular graphite mixture with different average particle diameters; the granular graphite with a smaller average particle diameter has a larger specific surface area and more pores, can effectively store more electrolyte, and guarantees long cycle life of the negative pole piece; and the addition of the granular graphite with a smaller average particle diameter can effectively reduce the volume of the whole graphite material mixture, thereby effectively improving the compaction density of the negative pole piece. The application provides a lithium ion battery applying the above negative pole piece, and the lithium ion battery can effectively improve the problem of active material falling of the negative pole piece caused by large volume expansion of a silicon-based negative pole during charging and discharging, guarantees high energy density and long cycle life of the lithium ion battery, and meets the application requirement of long cruising range of a power battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a negative electrode sheet of a lithium-ion battery and a lithium-ion battery using the electrode sheet. Background Art

[0002] In March 2017, four ministries and commissions including the Ministry of Industry and Information Technology of China jointly issued the "Action Plan for Promoting the Development of the Automotive Power Battery Industry", requiring that by 2020, the monomer energy density of lithium-ion power batteries reach 300 Wh / kg; the existing material system can no longer meet the needs of future development, so it is necessary to develop positive and negative electrode materials with high energy density, and at the same time increase the compaction density of the electrode sheet. The theoretical specific lithium storage capacity of silicon (Si) is 4200 mAh / g, and among all elements that can alloy and store lithium, the specific capacity of silicon is the highest. At room temperature, each silicon atom can bind up to 3.75 lithium atoms at most, obtaining the Li 15 Si4 alloy phase, and the corresponding actual specific lithium storage capacity is at most 3579 mAh / g, which is about 10 times the theoretical specific capacity of graphite (372 mAh / g), and it is a new negative electrode material with the most potential to replace graphite. However, the charge and discharge process of silicon (Si) is accompanied by a huge volume effect, and the volume expansion reaches more than 300%. Such a huge volume effect will cause strong mechanical stress, resulting in the loss of contact between the electrode active material and the current collector, that is, the loss of the electrode active substance, causing a rapid attenuation of the reversible capacity of the electrode and a shortening of the cycle life.

[0003] In recent years, a new type of silicon-oxygen negative electrode material SiOx (0 < x < 2) has begun to show its advantages in use as a negative electrode material for lithium-ion batteries: the introduction of oxygen can generate an inert component during the first lithium intercalation, which is beneficial to reducing the absolute volume change of silicon during the lithium deintercalation and intercalation process; at the same time, nanosizing the silicon-based material can reduce the volume expansion effect of the silicon-based material to a certain extent and improve the cycle stability of the material. The above methods can effectively improve the initial volume effect problem of the silicon-based material, but during the charge and discharge cycle process, the silicon crystal repeatedly deintercalates and intercalates lithium highly, and the volume effect becomes more and more serious, and the electrode powdering also becomes more and more serious, resulting in the deterioration of the long-term cycle performance of the silicon-based negative electrode. At the same time, in recent years, in response to the above-mentioned problems, the majority of scientific researchers and battery practitioners have basically focused on improving and optimizing the silicon-based material itself, and there are few discussions on how to reduce the problem of poor cycle caused by the shedding of electrode active substances caused by the expansion of the silicon-based negative electrode from the negative electrode sheet, and few people study how to increase the compaction density of the silicon-based negative electrode sheet to increase the energy density of the battery.

[0004] Therefore, there is an urgent need to develop a technology that can effectively improve the problem of active material shedding from silicon-based anodes caused by large volume expansion during charging and discharging, ensure excellent long-term cycle life of lithium-ion batteries using such silicon-based anodes, and at the same time, silicon-based anodes have high compaction density and good overall performance. Summary of the Invention

[0005] To address the shortcomings of the prior art, this invention provides a lithium-ion battery negative electrode sheet. This negative electrode sheet includes a negative current collector and an active material layer coated on the current collector. The active material layer contains a graphite mixture composed of two types of granular graphite with an average particle size difference of 5-14 μm. The smaller average particle size of the granular graphite in this mixture often has a larger specific surface area and more porosity compared to the larger average particle size, effectively storing more electrolyte and ensuring a long cycle life for the negative electrode sheet. Furthermore, the addition of the smaller average particle size graphite effectively reduces the overall volume of the graphite mixture, thereby effectively increasing the compaction density of the negative electrode sheet. This invention provides a lithium-ion battery using the above-mentioned negative electrode sheet. This lithium-ion battery effectively improves the problem of active material shedding from the negative electrode sheet caused by the large volume expansion during charging and discharging of silicon-based negative electrodes, ensuring high energy density and long cycle life of the lithium-ion battery, meeting the application requirements for long driving range in power batteries.

[0006] The technical effects to be achieved by this invention are accomplished through the following solutions:

[0007] The lithium-ion battery negative electrode sheet of the present invention includes a negative electrode current collector and an active material layer coated on the negative electrode current collector. The raw materials of the active material layer include silicon-based active materials, a graphite material mixture, a conductive agent, and a binder. The graphite material mixture is composed of two types of particulate graphite with an average particle size difference of 5-14 μm.

[0008] Furthermore, the average particle sizes of the two types of granular graphite are in the ranges of 9-18 μm and 4-7 μm, respectively.

[0009] Furthermore, the granular graphite is one or more of natural graphite, artificial graphite, mesophase carbon microspheres, soft carbon, or hard carbon, or a mixture thereof.

[0010] The graphite material mixture is added to the raw materials of the active material layer by mixing two types of granular graphite with an average particle size difference of 5-14 μm. Compared with granular graphite with a uniform average particle size, the mixture of two granular graphite with a difference in average particle size not only ensures the compaction density of the negative electrode sheet, but also effectively stores more electrolyte, ensuring a long cycle life of the negative electrode sheet. When larger and smaller average particle size granular graphite are used in combination, the larger average particle size granular graphite generally has a good normal particle distribution, which can effectively ensure the compaction density of the negative electrode sheet, while the smaller average particle size granular graphite, due to its large specific surface area and numerous pores, can effectively store more electrolyte, ensuring a long cycle life of the negative electrode sheet. Moreover, the addition of smaller average particle size granular graphite can further improve the compaction density of the negative electrode sheet to a certain extent. While granular graphite materials with uniform average particle size can improve the compaction density of the negative electrode by using a larger average particle size or the cycle life by using a smaller average particle size, they cannot achieve the superior performance of a blend of granular graphite materials that balances both high compaction density and long cycle life. The larger average particle size of the granular graphite is 9-18 μm, a common particle size for negative electrode active materials. The smaller average particle size is 4-7 μm. In a blend of granular graphite materials with this difference in average particle size, the smaller average particle size often has a larger specific surface area and more pores, effectively storing more electrolyte and ensuring a longer cycle life for the negative electrode. Furthermore, the addition of larger average particle size granular graphite effectively ensures a good normal particle distribution in the graphite material, thereby effectively improving the compaction density of the negative electrode.

[0011] Furthermore, the negative electrode current collector is a copper foil coated with a graphene material coating with a thickness of 1-4 μm, and the surface of the graphene material coating contains carboxyl groups and hydroxyl groups.

[0012] The surface of the graphene material coating applied on the negative current collector contains carboxyl groups and hydroxyl groups. During coating, dehydration condensation reaction occurs between the carboxyl groups and the hydroxyl groups on the surface of the silicon-based active material, and hydrogen bond binding occurs between the hydroxyl groups on the graphene material coating and the hydroxyl groups on the surface of the silicon-based active material, enabling the silicon-based active material and the negative current collector to be well bonded together, enhancing the adhesion of the negative electrode sheet, and thus effectively solving the problem of loss of electrode active material caused by sheet dropout during the manufacturing process and charge-discharge cycling of the silicon-based negative electrode sheet, and effectively improving the cycle life of the silicon-based negative electrode sheet. Among them, the graphite is oxidized by the graphite oxide reduction method to change the free electron pairs of the graphite lamellae and modify its surface with oxygen-containing functional groups (such as carboxyl, hydroxyl, carbonyl, and epoxy groups), and then the graphene material coating with carboxyl groups and hydroxyl groups on its surface can be prepared.

[0013] Furthermore, the mass ratio of the silicon-based active material, granular graphite with a larger average particle size, and granular graphite with a smaller average particle size is (1 - 7):(12 - 16):(1 - 3); this can not only effectively improve the tap density of the negative electrode sheet, ensure the long cycle life of the negative electrode sheet, but also effectively ensure good adhesion between the silicon-based active material and the surface of the negative current collector.

[0014] Furthermore, the coating areal density of the active material layer is 10mg / cm 2 -25mg / cm 2 . This can not only ensure good conductivity of the active material layer, but also effectively improve the tap density of the negative electrode sheet.

[0015] Furthermore, the silicon-based active material is one or a mixture of more than one of silicon monoxide (SiO x , 0 < x < 2), nanosilicon, or silicon / carbon composite materials. Among all the elements capable of alloying with lithium for lithium storage, silicon has the highest specific capacity. The specific capacity of each silicon atom to combine with lithium atoms is about 10 times that of the theoretical specific capacity of graphite, making it the most promising new negative electrode material to replace graphite.

[0016] Furthermore, the conductive agent includes one or a mixture of several of the following: conductive carbon black (SP), Ketjen black (ECP), conductive graphite (KS), carbon nanotubes, and carbon nanofibers. Conductive carbon black is characterized by small particle size, large and rough specific surface area, high structure, and clean surface. Ketjen black requires only a very low addition amount to achieve high conductivity. Conductive graphite, in addition to high conductivity, also possesses corrosion resistance, wear resistance, high temperature resistance, high strength, and light weight. Carbon nanotubes and carbon nanofibers not only have good conductivity but also good heat transfer properties. Using one or a mixture of the above materials as the conductive agent for the active material layer can effectively ensure good conductivity of the negative electrode sheet. The binder includes CMC adhesive or SBR binder, which are commonly used binder combinations in negative electrode materials. The CMC adhesive is a hydroquinone cellulose sol with a solid content of 1%-5%.

[0017] The negative electrode of the lithium-ion battery of the present invention is as described above.

[0018] Furthermore, it includes positive electrode, negative electrode, separator, electrolyte, and battery casing;

[0019] The battery casing is made of steel, aluminum, or aluminum-plastic film, preferably 21700 steel. The separator is made of PE, PP, or PP / PE / PE film. The electrolyte contains a polyester solvent and a lithium salt solute, or other non-aqueous electrolyte. Preferably, the polyester solvent is dimethyl carbonate (DMC), and the lithium salt solute is lithium hexafluorophosphate (LiPF6). The positive electrode includes a positive current collector and a positive active material coated on the current collector. The positive active material is one or a mixture of several of the following: nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, lithium cobalt oxide, lithium iron phosphate, and lithium manganese oxide. The positive current collector is preferably an aluminum foil with a thickness of 10-20 μm. The lithium-ion battery prepared using the above-mentioned negative electrode with long cycle life and high compaction density can effectively improve the problem of active material shedding from the negative electrode caused by the large volume expansion of silicon-based negative electrodes during charging and discharging, ensuring high energy density and long cycle life of the lithium-ion battery, and meeting the application requirements of long driving range for power batteries.

[0020] The present invention has the following advantages:

[0021] 1. The present invention uses a method of mixing granular graphite with an average particle size difference into the raw materials of the active material layer. This not only effectively improves the compaction density of the negative electrode sheet, but also the smaller average particle size granular graphite has a larger specific surface area and more pores, which is conducive to storing more electrolyte and ensuring the long cycle life and high energy density of the negative electrode sheet and the lithium-ion battery using the negative electrode sheet.

[0022] 2. The graphene material coating surface of the present invention contains carboxyl groups and hydroxyl groups, which can react with the groups of the active material layer to form a tight bond, effectively solving the problem of electrode active material loss caused by electrode shedding during the silicon-based negative electrode sheet manufacturing process and charge-discharge cycle process, and effectively improving the cycle life of the negative electrode sheet and the lithium-ion battery using the negative electrode sheet. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the negative electrode sheet of the lithium-ion battery in this invention;

[0024] Figure 2 This is a comparison chart of the cycle performance of lithium-ion batteries in Example 1 and Comparative Examples 1 and 2 of this invention.

[0025] The annotations in the attached figures are explained as follows:

[0026] 100, Negative current collector; 110, Copper foil; 120, Graphene material coating (surface contains carboxyl and hydroxyl groups); 200, Active material layer. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0028] The lithium-ion battery negative electrode sheet in the embodiments of the present invention is shown in the attached figure. Figure 1 As shown, it includes a negative electrode current collector 100 and an active material layer 200 coated on the negative electrode current collector. The negative electrode current collector is a copper foil 110 with a graphene material coating 120 with a thickness of 1-4 μm on its surface.

[0029] Example 1:

[0030] The preparation of the negative electrode sheet for lithium-ion batteries is as follows:

[0031] (1) Preparation of CMC adhesive solution: Add 1.5% by weight of CMC powder to deionized water (CMC + water adhesive solution, CMC solid content is 1.8%), stir and disperse evenly to obtain CMC adhesive solution;

[0032] (2) Preparation of active material slurry: 15% by weight of silicon-based active material, 70% by weight of granular graphite with an average particle size of 9 μm, 15% by weight of granular graphite with an average particle size of 4 μm, and 0.2% by weight of carbon nanotube conductive agent are physically dry-mixed and stirred evenly. Then, the above-mentioned CMC adhesive is added and stirred to disperse. 1.5% by weight of SBR binder and deionized water are added to adjust the solid content of the slurry to 45% and stirred evenly. The obtained slurry is filtered and demagnetized to obtain the active material slurry.

[0033] (3) The above-mentioned active material slurry was coated onto the surface of an 8μm copper foil coated with a 2μm graphene material coating (the graphene surface contains carboxyl and hydroxyl groups). After drying and dehydration, and rolling, the negative electrode sheet of the lithium-ion battery in Example 1 was obtained. The double-sided surface density of the coating was 22 mg / cm². 2 The coating, baking, and drying temperature was 80℃, and the electrode sheet compaction density was 1.78 g / cm³. 3 .

[0034] The preparation of lithium-ion batteries is as follows:

[0035] (1) The lithium-ion battery negative electrode sheet prepared above is cut and the tabs are welded to cut it into strips with a width of 64.5 mm.

[0036] (2) The nickel-cobalt-aluminum ternary cathode LiNi 0.85 Co 0.10 Mn 0.05 O2 material, conductive carbon nanotubes, and binder PVDF are mixed and added to N-methylpyrrolidone, and stirred to disperse evenly to prepare a positive electrode slurry. The obtained positive electrode slurry is coated onto the surface of a 12μm aluminum foil positive electrode current collector, and then dried, rolled, slit, and had tabs welded to obtain a lithium-ion positive electrode sheet. The areal density of the coating on both sides is 54 mg / cm². 2 The coating, baking, and drying temperature was 110℃, and the electrode sheet compaction density was 3.55 g / cm³. 3 The electrode cutting width is 63mm;

[0037] (3) Use a polyethylene diaphragm (PE) film to separate the negative electrode sheet from the positive electrode sheet from step (1) and step (2), and wind it with a circular winding needle. Finally, place the wound electrode assembly into a 21700 steel shell and assemble it (weld the bottom of the negative electrode ear, roll the groove, and weld the cap). Inject the electrolyte into the drying oven and then seal it to obtain a 21700 lithium-ion battery.

[0038] (4) Finally, the battery is formed, aged and tested in a conventional manner to obtain the lithium-ion battery of Example 1.

[0039] Example 2:

[0040] Compared with Example 1, Example 2 differs in that: in step (2) of preparing the negative electrode sheet, 60% of granular graphite with an average particle size of 9 μm and 25% of granular graphite with an average particle size of 4 μm are used, and the solid content of the slurry is 40%; in step (3) of preparing the negative electrode sheet, the compaction density of the electrode sheet by roller pressing is 1.82 g / cm³. 3 .

[0041] Example 3:

[0042] Compared with Example 1, Example 3 differs in that: in step (2) of preparing the negative electrode sheet, 70% of granular graphite with an average particle size of 11 μm and 15% of granular graphite with an average particle size of 6 μm are used, and the conductive agent is 0.2% conductive carbon black; in step (3) of preparing the negative electrode sheet, the thickness of the graphene material coating is 1 μm, and the compaction density of the electrode sheet by roller pressing is 1.80 g / cm³. 3 .

[0043] Example 4:

[0044] Compared with Example 1, Example 4 differs in that: in step (2) of preparing the negative electrode sheet, 60% of granular graphite with an average particle size of 11 μm and 15% of granular graphite with an average particle size of 6 μm are used, the conductive agent is 0.1% conductive carbon black and 0.1% Ketjen black, and the solid content of the slurry is 40%; in step (3) of preparing the negative electrode sheet, the thickness of the graphene material coating is 1 μm, and the areal density of the active material coating is 18 mg / cm³. 2 The compaction density of the electrode rolls is 1.75 g / cm³. 3 .

[0045] Example 5:

[0046] Compared with Example 1, Example 5 differs in that: in step (2) of preparing the negative electrode sheet, 75% of granular graphite with an average particle size of 15 μm and 15% of granular graphite with an average particle size of 5 μm are used, and the conductive agent is 0.2% conductive graphite; in step (3) of preparing the negative electrode sheet, the thickness of the graphene material coating is 3 μm, and the areal density of the active material coating is 25 mg / cm³. 2 The compaction density of the electrode sheet by roller pressing is 1.79 g / cm³. 3 .

[0047] Example 6:

[0048] Compared with Example 1, Example 6 differs in that: in step (2) of preparing the negative electrode sheet, 65% of granular graphite with an average particle size of 15 μm and 15% of granular graphite with an average particle size of 7 μm are used, the conductive agent is 0.1% conductive carbon black and 0.1% conductive graphite, and the solid content of the slurry is 40%; in step (3) of preparing the negative electrode sheet, the thickness of the graphene material coating is 3 μm, and the areal density of the active material coating is 20 mg / cm³. 2 The compaction density of the electrode sheet by roller pressing is 1.79 g / cm³. 3 .

[0049] Example 7:

[0050] Compared with Example 1, Example 7 differs in that: in step (2) of preparing the negative electrode sheet, 65% of granular graphite with an average particle size of 18 μm and 20% of granular graphite with an average particle size of 7 μm are used, the conductive agent is 0.05% conductive carbon black, 0.05% Ketjen black and 0.1% carbon nanotubes, and the solid content of the slurry is 40%; in step (3) of preparing the negative electrode sheet, the thickness of the graphene material coating is 4 μm, and the compaction density of the electrode sheet is 1.85 g / cm³. 3 .

[0051] Example 8:

[0052] Compared with Example 1, Example 8 differs in that: in step (2) of preparing the negative electrode sheet, 70% of granular graphite with an average particle size of 18 μm and 10% of granular graphite with an average particle size of 6 μm are used, and the conductive agent is 0.1% conductive graphite and 0.1% carbon nanofibers; in step (3) of preparing the negative electrode sheet, the thickness of the graphene material coating is 4 μm, and the areal density of the active material coating is 20 mg / cm³. 2 The compaction density of the electrode sheet by roller pressing is 1.83 g / cm³. 3 .

[0053] Comparative Example 1:

[0054] Compared with Example 1, Comparative Example 1 differs in that: in step (2) of preparing the negative electrode sheet, 15% of granular graphite with an average particle size of 4 μm was not added, but 85% of granular graphite with an average particle size of 9 μm was added; in step (3) of preparing the negative electrode sheet, the compaction density of the electrode sheet by roller pressing is 1.65 g / cm³. 3 .

[0055] Comparative Example 2:

[0056] Compared with Example 1, Comparative Example 2 differs in that: in step (2) of preparing the negative electrode sheet, 15% of granular graphite with an average particle size of 4 μm was not added, but 85% of granular graphite with an average particle size of 9 μm was added; in step (3) of preparing the negative electrode sheet, the copper foil coated with the active material slurry is ordinary copper foil without a graphene coating on the surface, and the roller-pressed compaction density of the electrode sheet is 1.65 g / cm³. 3 .

[0057] The differences between Examples 1-8 and Comparative Examples 1 and 2 are shown in the table below:

[0058]

[0059] Comparison of the compaction and liquid absorption performance of negative electrode sheets in lithium-ion batteries

[0060] The negative electrode sheets coated in Examples 1-8 and Comparative Example 1 were cut into sheets 20 cm long × 4 cm wide. 50 μL of electrolyte PC solvent was taken using a micro-sampler and dropped onto each electrode sheet. The liquid absorption time for each negative electrode sheet was recorded (the shorter the absorption time, the higher the actual compaction in use; the control limit is 15 min). The test results are shown in the table below:

[0061]

[0062] As can be seen from the table, the liquid absorption performance of the negative electrode sheet of the present invention (Examples 1-8) is significantly improved compared with that of the conventional negative electrode sheet (Comparative Example 1), and the compaction density of the negative electrode sheet is 1.75~1.85 g / cm³. 3 Within the range, it still meets the requirements (absorption time is within the control limit of 15 minutes).

[0063] Comparison of energy densities of lithium-ion batteries

[0064] Using the energy density calculation formula: Energy density (Wh / kg) = (Battery capacity (Ah) × Nominal voltage (V)) ÷ (Battery weight (g) ÷ 1000), the gravimetric energy density of the lithium-ion batteries in Examples 1-8 and Comparative Example 1 was calculated. The calculation results are shown in the table below:

[0065]

[0066] By comparing the battery energy density, it can be seen that the lithium-ion batteries of Examples 1-8 of the present invention have a high energy density, reaching 292~301Wh / kg.

[0067] The comparison of the liquid absorption performance of the negative electrode sheet in lithium-ion batteries and the comparison of the energy density of lithium-ion batteries show that the negative electrode sheet prepared by the present invention using a mixture of granular graphite materials with varying average particle sizes has good liquid absorption performance, ensuring that the negative electrode sheet can quickly absorb liquid even under high compaction density. Moreover, it can effectively improve the filling density of the negative electrode sheet. The increase in the compaction density of the negative electrode sheet allows more active material to be filled in the same 21700 cylindrical battery, increasing the battery capacity and thus significantly improving the energy density of lithium-ion batteries.

[0068] Comparison of adhesion of negative electrode sheets

[0069] The negative electrode sheets of Examples 1-8 and Comparative Example 2 before rolling were cut into 15cm long × 2cm wide sheets, and the adhesion was tested for each sheet. The test results are shown in the table below:

[0070]

[0071] As can be seen from the table, the adhesion of the negative electrode sheet produced by the present invention (Examples 1-8) is significantly improved compared with the adhesion of the conventional negative electrode sheet (Comparative Example 2), by 1.25-2.2 times.

[0072] Comparison of the appearance of the negative electrode sheet of a fully charged lithium-ion battery

[0073] The lithium-ion batteries of Examples 1-8 and Comparative Example 2 were fully charged (0.5C constant current charging to 4.2V, 4.2V constant voltage charging to the cutoff current of 0.01C), and then dissected and photographed in the fully charged state to observe the appearance of the negative electrode. The results are shown in the table below:

[0074]

[0075] As can be seen from the table, the negative electrode active material of the present invention (Examples 1-8) is still well bonded to the foil when fully charged, but the negative electrode active material and foil of the conventional negative electrode (Comparative Example 2) have a large area of ​​detachment when fully charged.

[0076] The results of comparing the adhesion of the negative electrode sheet and the appearance of the negative electrode sheet of a fully charged lithium-ion battery show that the negative electrode sheet of the lithium-ion battery of the present invention is coated with a graphene material coating containing carboxyl and hydroxyl groups on the copper foil of the negative electrode current collector. During the coating process, the carboxyl groups of the graphene material coating and the hydroxyl groups on the surface of the silicon-based active material undergo a dehydration condensation reaction. At the same time, the hydroxyl groups on the graphene material coating and the hydroxyl groups on the surface of the silicon-based active material undergo hydrogen bonding, which makes the silicon-based active material and the negative electrode current collector bond well together, thereby effectively improving the adhesion of the negative electrode sheet and making it difficult for the negative electrode active material to fall off the foil after charging and discharging expansion.

[0077] ambient temperature cycling test

[0078] The batteries from Example 1, Comparative Examples 1 and 2 were charged at a constant current of 0.5C to 4.15V, then charged at a constant voltage of 4.15V to a cutoff current of 0.01C, and then discharged at a constant current of 1C to 2.75V. Cyclic testing was performed until the capacity retention rate was below 80%. The test was then completed, and the results are as follows: Figure 2 As shown.

[0079] Depend on Figure 2 It can be seen that the lithium-ion battery of Example 1 of the present invention exhibits better cycle performance. After 1000 cycles, the battery capacity retention rate is still 80%, while the cycle performance of Comparative Examples 1 and 2 is poor. Comparative Example 1 has a capacity retention rate of 80% after about 570 cycles, and Comparative Example 2 has a capacity retention rate of 80% after about 200 cycles.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and not to limit them. Although the embodiments of the present invention have been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the embodiments of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet for a lithium-ion battery, comprising a negative electrode current collector and an active material layer coated on the negative electrode current collector, characterized in that: The raw materials of the active material layer include a silicon-based active material, a mixture of graphite materials, a conductive agent, and a binder; the mixture of graphite materials is composed of two granular graphites with an average particle size difference of 5-14 μm. The negative current collector is a copper foil coated with a graphene material coating with a thickness of 1-4 μm on the surface, and the surface of the graphene material coating contains carboxyl groups and hydroxyl groups. The average particle sizes of the two granular graphites are respectively in the ranges of 9-18 μm and 4-7 μm. The mass ratio of the silicon-based active material, the granular graphite with a larger average particle size, and the granular graphite with a smaller average particle size is (1-7):(12-16):(1-3).

2. The lithium-ion battery negative electrode sheet as described in claim 1, characterized in that: The granular graphite is a mixture of one or more of natural graphite, artificial graphite, mesocarbon microbeads, soft carbon, or hard carbon.

3. The lithium-ion battery negative electrode sheet as described in claim 1, characterized in that: The surface density of the active material layer coating is 10 mg / cm³. 2 -25mg / cm 2 .

4. The lithium-ion battery negative electrode sheet as described in claim 1, characterized in that: The silicon-based active material is a mixture of one or more of silicon monoxide (SiOx, 0 < x < 2), nano-silicon, or silicon / carbon composite materials.

5. The lithium-ion battery negative electrode sheet as described in claim 1, characterized in that: The conductive agent includes a mixture of one or several of conductive carbon black SP, Ketjen black ECP, conductive graphite KS, carbon nanotubes, and carbon nanofibers; the binder includes CMC solution or SBR binder, and the CMC solution is a hydroxyethyl cellulose hydrogel with a solid content of 1%-5%.

6. A lithium-ion battery, the negative electrode sheet of which is as described in any one of claims 1-5.

7. The lithium-ion battery as described in claim 6, comprising a positive electrode, a negative electrode, a separator, an electrolyte, and a battery casing, characterized in that: The battery casing is a steel casing, an aluminum casing, or an aluminum-plastic film casing, and the separator is a PE film, a PP film, or a PP / PE / PE film; the positive electrode sheet includes a positive current collector and a positive active material coated on the positive current collector, and the positive active material is a mixture of one or several of nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, lithium cobaltate, lithium iron phosphate, and lithium manganate.

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