Negative electrode sheet, lithium ion battery, and electric device

Abstract

The application discloses a negative electrode sheet, a lithium ion battery and an electric device. The negative electrode sheet comprises a negative electrode sheet, a lithium ion battery and an electric device. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, the negative electrode active material comprising a graphite material, a hard carbon material and a silicon-based material; the particle size D50 of the graphite material, the particle size D'50 of the hard carbon material and the particle size D''50 of the silicon-based material satisfy 1.5 <= D50 / D'50 <= 2.5 and 1.1 <= D'50 / D''50 <= 3. Through the size particle diameter matching of the graphite, the hard carbon and the silicon-based material, the short board of the low hard carbon compaction density is effectively improved, the overall compaction density of the negative electrode sheet is improved, the energy density of the battery is improved, the electrolyte is infiltrated, the liquid retention of the electrode sheet is improved, the rapid embedding of lithium ions is promoted, and the rate performance is improved.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to negative electrode sheets, lithium-ion batteries and electrical equipment. Background Technology

[0002] Lithium-ion batteries possess advantages such as high energy density, no memory effect, long cycle life, rapid charge and discharge, and low self-discharge, making them widely used in consumer electronics, electric vehicles, and energy storage. The choice of negative electrode active material directly affects the energy density of lithium-ion batteries and also influences rate performance. Graphite materials are currently the primary choice for negative electrode active materials in lithium-ion batteries due to their low lithium intercalation potential, high theoretical specific capacity, and advantages in price and environmental friendliness. However, as market demands for higher energy density and charging rates in lithium-ion batteries continue to increase, graphite materials alone can no longer meet current requirements.

[0003] Therefore, a common approach is to dope silicon-based materials into the negative electrode active material. Silicon has a theoretical lithium storage capacity as high as 4200 mAh / g, a lithium intercalation platform slightly higher than graphite, and fewer safety hazards, making it an excellent alternative to graphite-based negative electrode materials. However, silicon exhibits a volume change of up to 300% during charge and discharge, which easily leads to silicon particle pulverization and damage to the internal conductive network of the electrode. Furthermore, high-rate charging causes more pronounced lithium plating and silicon expansion problems, failing to meet the market's fast-charging demands. Therefore, how to more effectively mitigate silicon particle volume expansion, ensure battery cycle stability, and obtain silicon-based negative electrode materials that balance energy density, cycle performance, and rate performance remains a pressing technological challenge in the current lithium-ion battery field. Summary of the Invention

[0004] This application aims to solve at least one of the technical problems existing in the prior art. To this end, this application proposes a negative electrode, a lithium-ion battery, and an electrical device.

[0005] In a first aspect, this application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, which includes graphite material, hard carbon material and silicon-based material.

[0006] Among them, the particle size D50 of graphite material, the particle size D′50 of hard carbon material, and the particle size D″50 of silicon-based material satisfy 1.5≤D50 / D′50≤2.5 and 1.1≤D′50 / D″50≤3.

[0007] The negative electrode sheet according to the first aspect of this application has the following beneficial effects:

[0008] Compared to graphite, hard carbon has a higher reversible capacity, which is beneficial for improving the energy density of lithium-ion batteries. It also possesses good isotropy, facilitating lithium-ion insertion and extraction. However, hard carbon's compaction density cannot be very high, severely impacting the cell's energy density. Silicon-based materials, on the other hand, have a higher specific capacity, significantly improving battery energy density, but they experience significant volume expansion during lithium insertion. This solution effectively addresses the low compaction density of hard carbon by combining graphite, hard carbon, and silicon-based materials of varying particle sizes, thereby increasing the overall compaction density of the negative electrode and ultimately improving battery energy density. The good isotropy and wide interlayer spacing of hard carbon facilitate electrolyte wetting, improving electrolyte retention and promoting rapid lithium-ion insertion, thus enhancing rate performance. The small-particle-size silicon-based material distributed between the hard carbon and graphite not only further increases the electrode's compaction density but also further limits the volume expansion during lithium insertion, effectively improving battery cycle performance.

[0009] In addition, since silicon-based materials have a higher particle hardness than hard carbon and graphite, they act as pulleys during the rolling process of the negative electrode sheet. This can alleviate the locking caused by the sharp edges of hard carbon and graphite, which is conducive to the slippage between particles, avoids particle breakage due to local overpressure, and maintains its cycle performance, rate performance and energy density.

[0010] In some embodiments of this application, D50 is 10 to 17 μm, for example, it can be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, or 17 μm.

[0011] In some embodiments of this application, D′50 is 6 to 10 μm, for example, it can be 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm.

[0012] In some embodiments of this application, D″50 is 3 to 7 μm, for example, it can be 3 μm, 4 μm, 5 μm, 6 μm, or 7 μm.

[0013] In some embodiments of this application, the defect degree I of the hard carbon material is 0.15 to 0.38; where I is the area A of the G peak when the hard carbon material is subjected to Raman spectroscopy. G With peak area A of D D The ratio. Among them, peak D is at 1350 cm⁻¹. -1 The characteristic peaks appearing nearby belong to the defective (disordered) carbon structure; the G peak is at 1580 cm⁻¹. -1 The characteristic peaks that appear nearby belong to an ordered carbon structure.

[0014] The defect degree I of hard carbon materials characterizes their graphitization degree. High graphitization in hard carbon materials means lithium intercalation relies more on interlayer lithium storage, resulting in a lower specific surface area and fewer pores, which fails to mitigate volume expansion during cycling and easily leads to carbon layer peeling. Conversely, low graphitization in hard carbon materials results in more defective active sites, increasing their contribution to capacity; a suitable number of defects can improve reversible capacity. Furthermore, better isotropy facilitates lithium adsorption and storage, reduces volume expansion in the thickness direction, and helps maintain cycle stability. When the defect degree I of hard carbon is within the aforementioned range, a coordination and balance between ordered and defective structures is achieved, resulting in optimal reversible specific capacity and first-cycle coulombic efficiency, effectively improving battery cycle performance and expansion rate.

[0015] In some embodiments of this application, the Mohs hardness S1 of the graphite material is 0.8 to 2, the Mohs hardness S2 of the hard carbon material is 3 to 5, and the Mohs hardness S3 of the silicon-based material is 5 to 7, and S2 <S3。

[0016] In some embodiments of this application, the compaction density P, porosity ε, and thickness H of the negative electrode active material layer satisfy Equation 1: 15≤(kε+H) / P≤39.05; where: 1.26≤k≤1.40, and P is expressed in g / cm³. 2 The unit is dimensionless, and H is also dimensionless, calculated in μm. By adjusting the value of k within a limited range, and ensuring that the compaction density P, porosity ε, and active layer thickness H of the negative electrode sheet satisfy the relationship of Equation 1, the negative electrode sheet can still have good porosity under high compaction density, which is beneficial for electrolyte wetting of the electrode sheet and improves the diffusion efficiency of active ions.

[0017] In some embodiments of this application, the compaction density P of the negative electrode sheet is 1.55–1.7 g / cm³. 2 .

[0018] In some embodiments of this application, the compaction density P1 of the graphite material is 1.6–1.75 g / cm³. 2 .

[0019] In some embodiments of this application, the compaction density P2 of the hard carbon material is 1.3–1.5 g / cm³. 2 .

[0020] In some embodiments of this application, the compaction density P3 of the silicon-based material is 1–1.3 g / cm³. 2 .

[0021] In some embodiments of this application, the porosity ε of the negative electrode is 20-38%.

[0022] In some embodiments of this application, the thickness H of the negative electrode active material layer is 25–60 μm.

[0023] In some embodiments of this application, the mass fraction W1 of graphite material in the negative electrode active material is 75-92%.

[0024] In some embodiments of this application, the hard carbon material accounts for 3 to 10% of the mass fraction W2 of the negative electrode active material.

[0025] In some embodiments of this application, the silicon-based material accounts for 5-15% of the mass fraction W3 of the negative electrode active material.

[0026] In some embodiments of this application, the silicon-based material includes at least one of elemental silicon, silicon oxide, and silicon alloy.

[0027] In some embodiments of this application, the silicon-based material is further coated with a carbon layer on the surface of at least one of elemental silicon, silicon oxide, and silicon alloy.

[0028] In some embodiments of this application, the amount of carbon layer coating in the silicon-based material is 1 to 10 wt%, for example, it can be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 wt%.

[0029] In some embodiments of this application, the hard carbon material can be prepared using any preparation method known to those skilled in the art, including but not limited to the preparation method using a discharge plasma furnace with Joule heating and high pressure, starch / rice preparation method, petroleum coke preparation method, biomass preparation method, etc.

[0030] In some embodiments of this application, the raw materials for the negative electrode active material layer further include at least one of a conductive agent and a binder. The conductive agent includes, but is not limited to, conductive graphite (such as KS-6, KS-15, SFG-6, SFG-15, SO, etc.), conductive carbon black (such as Super P, Super S, 350G, acetylene black, Ketjen black, etc.), conductive carbon fiber (such as VGCF, CNT), and graphene. The binder includes, but is not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose, styrene-butadiene rubber, polyurethane, polyvinyl alcohol, and polyvinyl butyral.

[0031] In some embodiments of this application, the negative electrode active material layer comprises 70-99 wt% negative electrode active material, 0.5-6 wt% conductive agent and 0.5-20 wt% binder.

[0032] In some embodiments of this application, when the negative electrode active material, conductive agent and binder are made into a negative electrode active material layer, the negative electrode active material layer is obtained by dispersing the negative electrode active material, conductive agent and binder in a solvent and then coating them onto the negative electrode current collector and drying them.

[0033] This application also relates to a method for preparing a negative electrode sheet, the method comprising the following steps:

[0034] A negative electrode sheet is made by coating a slurry of negative electrode active material onto a negative electrode current collector.

[0035] In some embodiments of this application, the slurry of the negative electrode active material includes the negative electrode active material and at least one of a conductive agent and a binder. In some specific embodiments, the slurry of the negative electrode active material also includes a solvent.

[0036] In some embodiments of this application, the method of coating the slurry of the negative electrode active material includes at least one of spraying, dip coating, blade coating, transfer coating, extrusion coating, cast coating, and microgravure coating.

[0037] A third aspect of this application provides a lithium-ion battery comprising the aforementioned negative electrode.

[0038] In some embodiments of this application, a lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode, negative electrode, and separator are formed into a cell by at least one of the following methods: winding, stacking, etc., and then manufactured into a lithium-ion battery.

[0039] In some embodiments of this application, the positive electrode sheet includes a positive current collector and a positive active material layer located on at least one side of the positive current collector.

[0040] In some embodiments of this application, the positive current collector and the negative current collector are each independently selected from at least one of the following metal materials: metal foil (such as aluminum foil, silver foil, tin foil, iron foil, titanium foil, nickel foil, copper foil or alloy foil of the above metals) and metal mesh (such as aluminum mesh, silver mesh, tin mesh, iron mesh, titanium mesh, nickel mesh, copper mesh or alloy mesh of the above metals).

[0041] In some embodiments of this application, the positive current collector is aluminum foil and the negative current collector is copper foil.

[0042] In some embodiments of this application, the positive electrode active material layer includes a positive electrode active material, which is at least one of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium nickel manganese aluminum oxide.

[0043] In some embodiments of this application, the raw materials for the positive electrode active material layer further include at least one of a conductive agent and a binder. The conductive agent includes, but is not limited to, conductive graphite (such as KS-6, KS-15, SFG-6, SFG-15, SO, etc.), conductive carbon black (such as Super P, Super S, 350G, acetylene black, Ketjen black, etc.), conductive carbon fiber (such as VGCF, CNT), and graphene. The binder includes, but is not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose, styrene-butadiene rubber, polyurethane, polyvinyl alcohol, and polyvinyl butyral.

[0044] In some embodiments of this application, the positive electrode active material layer comprises 70-99 wt% positive electrode active material, 0.5-6 wt% conductive agent, and 0.5-20 wt% binder.

[0045] In some embodiments of this application, the electrolyte may be at least one of a solid electrolyte or an electrolyte solution.

[0046] In some embodiments of this application, the electrolyte includes an electrolyte salt and an organic solvent, wherein the specific types and compositions of the electrolyte salt and the organic solvent are not specifically limited.

[0047] In some embodiments of this application, the electrolyte also includes additives, such as at least one of positive electrode film-forming additives, negative electrode film-forming additives, and additives for improving cycle life and low temperature.

[0048] In some embodiments of this application, the diaphragm comprises a polymer diaphragm, which includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, and their multilayer composite membranes.

[0049] A fourth aspect of this application provides an electrical device comprising the aforementioned lithium-ion battery.

[0050] Electrical equipment refers to any device that can utilize electrical energy and convert it into mechanical energy, thermal energy, light energy, or one or more other forms of energy, such as electric motors, electric heaters, and electric light sources. This includes mobile devices, electric vehicles, electric trains, ships and satellites, and energy storage systems. Mobile devices can be mobile phones, laptops, drones, robot vacuum cleaners, e-cigarettes, etc.; electric vehicles can be pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.

[0051] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Detailed Implementation

[0052] The following will clearly and completely describe the concept and technical effects of this application in conjunction with embodiments, so as to fully understand the purpose, features and effects of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are all within the scope of protection of this application.

[0053] The embodiments of this application are described in detail below. The described embodiments are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0054] In the description of this application, "several" means one or more, "multiple" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number, and "approximately" means within the range of ±20%, 10%, 8%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.2%, 0.1%, etc. of the stated number. The use of "first" and "second" is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0055] In the description of this application, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0056] The present application will be described below with reference to specific embodiments.

[0057] Examples 1-13

[0058] Examples 1-13 provide a negative electrode sheet and a lithium-ion battery containing the negative electrode sheet. The differences are shown in Table 1, which are the ratio of negative electrode active materials, the particle size and Mohs hardness of the negative electrode active materials, the porosity of the negative electrode sheet, the thickness of the negative electrode active material layer, the compaction density of the negative electrode active material layer, the defect degree of the hard carbon material, and the compaction rebound density.

[0059] The preparation process of the negative electrode is as follows:

[0060] A negative electrode active material (graphite material, hard carbon material, silicon-based material), conductive agent (conductive carbon black SP and carbon nanotubes CNT mixed in a mass ratio of 9:1), and binder (styrene-butadiene latex SBR and lithium polyacrylate PAALi mixed in a mass ratio of 0.5:1.8) are mixed in a mass ratio of 97.7:1.1:1.2 to prepare a negative electrode active material slurry. The slurry is then uniformly coated on both sides of the negative electrode current collector, and then cold-pressed and slit to obtain the negative electrode sheet.

[0061] The preparation process of the positive electrode is as follows:

[0062] The positive electrode active material LiCoO2, conductive agent acetylene black, conductive carbon nanotubes, and binder polyvinylidene fluoride (PVDF) were fully dispersed in an N-methylpyrrolidone (NMP) solvent system at a mass ratio of 97.6:0.5:0.6:1.3. The mixture was then uniformly coated onto both sides of the positive electrode current collector aluminum foil, dried, and slit to form a positive electrode sheet.

[0063] Separator: A ceramic mixture is coated on the PE surface to serve as a separator.

[0064] Electrolyte: Ethyl carbonate EC, propylene carbonate PC, diethyl carbonate DEC, and propyl propionate PP are mixed in a volume ratio of 1.2:1:4:4 to obtain a mixed organic solvent. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent at a ratio of 1 mol / L to prepare the electrolyte.

[0065] Full cell preparation: The above-mentioned positive electrode sheet, separator, and negative electrode sheet are wound to form a bare cell, which is then packaged and injected with electrolyte to produce a finished lithium-ion battery.

[0066] Table 1. Comparison of parameters between the examples and comparative examples

[0067]

[0068]

[0069] Comparative Examples 1-15

[0070] Comparative Examples 1 to 15 provide a negative electrode and a lithium-ion battery, respectively. The differences between them and Examples 1 to 13 are shown in Table 1.

[0071] In the above embodiments and comparative examples:

[0072] The graphite material was purchased from Shanshan New Materials Co., Ltd. Examples 1-3, Examples 6-13, and Comparative Examples 1-15 used graphite material with model number DM-APG-0017-0002. Example 4 used graphite material with model number DM-APG-0020-0002. Example 5 used graphite material with model number DM-APG-0027-0141.

[0073] The silicon-based materials were purchased from Jinsilicon Technology Co., Ltd. Examples 1-3, 6, 8-13, and Comparative Examples 1-15 used silicon-based materials with model number YP-APS-0090-0000, Example 4 used silicon-based materials with model number YP-APS-0098-0000, Example 5 used silicon-based materials with model number YP-APS-0168-0000, and Example 7 used silicon-based materials with model number YP-APS-0054-0000. These are different porous carbon-coated silicon materials.

[0074] The preparation method of hard carbon material is as follows: 9.3g of sucrose is dissolved in 100mL of distilled water, poured into a Teflon heating vessel, and heated to 200℃. The mixture is reacted at a constant temperature for 6 hours. The product is then placed in a tube furnace and calcined at 1100℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain hard carbon. Controlling the carbonization heating rate to 5, 2, and 1℃ / min yields hard carbon materials with different defect degrees; controlling the carbonization temperature to 1100℃ and 1300℃ yields hard carbon materials with different hardness.

[0075] The performance of the negative electrode sheets and lithium batteries of the embodiments and comparative examples was tested using the following methods:

[0076] (1) Energy density

[0077] Capacity calibration: Discharge at 0.2C to 3.0V; charge at 0.5C constant current and constant voltage to 4.5V, cut off at 0.02C; then discharge at 0.2C to 3.0V, record the capacity C0 and energy W0; plateau voltage V0 = W0 / C0.

[0078] The thickness of the battery in its half-charge state was tested using 600PPG, and the length and width of the battery were measured using 2D software.

[0079] The energy density is calculated using the following formula: Energy density = [C0 × V0 / (length × width × thickness)] × 1000; where the units are as follows: platform voltage V0 (V), capacity C0 (mAh), energy W0 (mWh), length, width, and thickness (mm).

[0080] (2) Loop Test

[0081] In a 25℃ environment, perform cyclic testing according to the following methods: charging mode: 2.8C CC to 4.25V, 2C CC to 4.35V, CV to 1.8C, 1.8C CC to 4.4V, CV to 1.5C, 1.5C CC to 4.5V, CV to 1.2C, 1.2C CC to 4.55V, CV to 0.26C; discharging mode: 0.7C DC to 3.0V.

[0082] 500-week capacity retention = discharge capacity in week 500 / discharge capacity in week 1 × 100%;

[0083] 500-week expansion rate = Fully charged thickness at week 500 / Initial half-charged thickness × 100%;

[0084] All battery thicknesses were tested using a 600PPG method.

[0085] (3) Liquid retention test

[0086] Liquid retention = Injection volume - Liquid loss.

[0087] The results are shown in Table 2:

[0088] Table 2. Performance test results of the examples and comparative examples

[0089] Serial Number Energy density (Wh / L) 500-week expansion rate (%) 500-week capacity retention rate (%) Liquid retention (g) Example 1 800 11.99 85.64 7.31 Example 2 815 13.14 84.39 6.98 Example 3 790 10.79 86.75 6.65 Example 4 800 11.41 86.16 6.42 Example 5 800 11.68 85.99 7.49 Example 6 800 12.54 85.07 6.93 Example 7 800 11.90 85.77 6.96 Example 8 815 12.68 84.79 7.12 Example 9 828 13.24 84.02 7.39 Example 10 793 9.76 87.81 7.56 Example 11 807 13.01 84.45 5.97 Example 12 804 11.57 86.05 7.01 Example 13 797 12.45 85.03 6.97 Comparative Example 1 792 20.11 75.56 6.03 Comparative Example 2 770 18.71 77.23 7.62 Comparative Example 3 760 9.03 89.88 6.34 Comparative Example 4 800 19.09 76.54 7.23 Comparative Example 5 800 18.88 77.26 7.19 Comparative Example 6 790 11.22 85.76 7.66 Comparative Example 7 810 17.97 78.34 5.78 Comparative Example 8 790 21.88 74.11 6.51 Comparative Example 9 800 20.65 75.31 6.37 Comparative Example 10 800 22.34 73.55 6.23 Comparative Example 11 800 21.22 74.33 6.87 Comparative Example 12 800 20.67 75.53 6.46 Comparative Example 13 800 20.89 75.19 6.76 Comparative Example 14 815 23.67 71.88 5.66 Comparative Example 15 780 17.23 79.03 7.13

[0090] Comparing Examples 1-3, as the proportions of the three negative electrode active materials changed, the porosity of the formed negative electrode sheet gradually decreased, and the electrolyte retention also gradually decreased. Simultaneously, the energy density and cycle performance also changed accordingly. Comparing Examples 1, 4, and 5, using graphite, hard carbon, and silicon-based materials with different particle sizes, the porosity continuously changed, and the electrolyte retention, energy density, and cycle performance also changed accordingly. Comparing Examples 2-4, the porosity of Example 4 was between that of Examples 2 and 3, and the three were relatively close. However, because silicon-based materials have poorer wettability to the electrolyte, and the content of silicon-based materials in Example 4 was higher, the electrolyte retention was actually lower than that of Example 3. Comparing Examples 7-9, as the coating thickness increased, the electrolyte retention gradually increased, but the cycle performance gradually decreased. Comparing Examples 12-13, as the defect degree of the hard carbon material increased, the energy density, cycle performance, and electrolyte retention all gradually decreased.

[0091] Comparing Example 1 and Comparative Example 1, after replacing the hard carbon material with graphite material, the porosity of Comparative Example 1 decreased significantly. Although the value of (kε+H) / P still met the requirements, the liquid retention decreased significantly, the energy density also decreased to a certain extent, and the expansion rate after 500 cycles nearly doubled, and the capacity retention rate also decreased by about 10%.

[0092] Comparing Example 1 and Comparative Example 2, Comparative Example 2, after replacing silicon-based materials with graphite materials, showed an increase in porosity and liquid retention, but a significant decrease in energy density. The expansion rate after 500 cycles remained high, and the capacity retention rate also decreased significantly.

[0093] Comparing Example 1 and Comparative Example 3, Comparative Example 3 uses a pure graphite system, which has the lowest expansion rate and the highest capacity retention rate after 500 cycles, but also the lowest liquid retention and energy density.

[0094] Comparing Example 1 and Comparative Examples 4-5, the hard carbon defect degree I characterizes the graphitization degree of the hard carbon material. High graphitization of hard carbon means that lithium intercalation relies more on interlayer lithium storage, resulting in a lower specific surface area and fewer pore structures. This makes it difficult to mitigate volume expansion during cycling and easily leads to carbon layer peeling. Conversely, low graphitization of hard carbon results in more defective active sites, increasing its contribution to capacity. A suitable amount of defects can improve reversible capacity; simultaneously, better isotropy facilitates lithium adsorption and storage, reduces volume expansion in the thickness direction, and helps maintain cycle stability. Therefore, the hard carbon materials in Comparative Examples 4 and 5 have either too low or too high defect degrees, failing to meet the defect degree requirements. The imbalance between ordered and disordered structures in the hard carbon materials significantly reduces the gain on battery cycle performance. The expansion rate and capacity retention rate after 500 cycles are at the level of Comparative Examples 1 or 2, showing a significant disadvantage compared to Example 1; while the impact on liquid retention is relatively small, with only a slight decrease in liquid retention.

[0095] Comparing Example 3 and Comparative Examples 6-7, in Comparative Examples 6 and 7, the compaction density P, porosity ε, and thickness H of the negative electrode active material layer, (kε+H) / P are either too low or too high. This makes it difficult for the electrolyte to wet the negative electrode under high compaction, thus limiting the improvement of the diffusion efficiency of active ions.

[0096] Comparing Example 1 and Comparative Examples 8-9, the silicon-based material in Comparative Example 8 had excessively low hardness, while the hard carbon material in Comparative Example 9 had excessively high hardness. The silicon-based material particles are harder than hard carbon and graphite, acting as a pulley during the negative electrode rolling process. This helps alleviate the locking caused by the angular shapes of hard carbon and graphite, facilitating particle slippage and preventing particle breakage due to localized overpressure. Silicon itself has high hardness; reducing hardness requires a thicker carbon layer. The volume ratio of the carbon layer affects the battery's energy density and prolongs the lithium-ion insertion path into silicon. Excessive hardness of hard carbon can lead to internal physical short circuits in the battery, most directly resulting in accelerated cycle degradation. Therefore, the energy density and capacity retention rate in Comparative Examples 8 and 9 decreased compared to Example 1, while the thickness expansion rate increased significantly.

[0097] Comparing Example 1 with Comparative Examples 10-13, the ratio of D50 / D′50 in Comparative Examples 10-11 is too large or too small, while the ratio of D′50 / D″50 in Comparative Examples 12-13 is too low or too high. Under the same compaction density, the particles are squeezed together, which seriously affects the structural stability of the material itself. Moreover, the porosity of the electrode is low, resulting in an unsatisfactory liquid retention effect. For example, in Comparative Examples 10-11, the graphite particle size is too large, and the silicon-based material and hard carbon material are densely distributed in the gaps between the graphite particles; the hard carbon material particle size is too large, and the gaps formed between the hard carbon and graphite are enlarged, with a large amount of silicon-based material aggregated in the gaps between them. The volume expansion of silicon-based materials during operation is essentially unmitigated. Therefore, the thickness expansion rate at 500 cycles in both comparative examples is even higher than in Comparative Example 1, and the capacity retention rate is lower. In Comparative Examples 12-13, when the particle size of the silicon-based material is too large, it is difficult to embed into the gaps between hard carbon and graphite; if the hard carbon is too large, the silicon-based material cannot be well retained in the gaps between hard carbon and graphite. However, in Example 1, the silicon-based material with a suitable particle size is distributed between the gaps between hard carbon and graphite, which not only increases the electrode compaction density but also further limits the volume expansion during lithium intercalation, thus improving the battery's cycle performance.

[0098] Comparing Example 4 with Comparative Examples 14 and 15, Comparative Examples 14 to 15 have lower porosity and excessively high or low compaction density of the negative electrode. Although they still satisfy Equation 1, they show a certain degree of reduction in one or more aspects such as energy density, cycle capacity retention rate, expansion rate, and liquid retention. Some performance characteristics are lower than those of Comparative Examples 1 to 3.

[0099] Therefore, combining the above embodiments and Comparative Examples 1-15: the lithium-ion battery using the negative electrode sheet provided in this application still exhibits good capacity retention and low battery expansion rate after long-term charge-discharge cycles, significantly superior to Comparative Examples 1-2; compared to the pure graphite system of Comparative Example 3, the battery's energy density is also significantly improved, better meeting market demands, while the good porosity enhances electrolyte wetting, ensuring the battery's long-cycle stability. Thus, the negative electrode sheet provided in this application can significantly improve the cycle performance of lithium-ion batteries, solving the battery failure problem caused by large material expansion, while the cell also possesses good electrolyte retention and high energy density.

[0100] The present application has been described in detail above with reference to the embodiments. However, the present application is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present application. Furthermore, unless otherwise specified, the embodiments and features in the embodiments of the present application can be combined with each other.

Claims

1. A negative electrode sheet, characterized by, The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer on at least one side of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, the negative electrode active material comprising a graphite material, a hard carbon material and a silicon-based material; wherein the particle size D50 of the graphite material, the particle size D'50 of the hard carbon material and the particle size D''50 of the silicon-based material satisfy 1.5≤D50 / D'50≤2.5 and 1.1≤D'50 / D''50≤3; the Mohs hardness S1 of the graphite material is 0.8-2, the Mohs hardness S2 of the hard carbon material is 3-5, the Mohs hardness S3 of the silicon-based material is 5-7, and S2<S3; the mass fraction W1 of the graphite material in the negative electrode active material is 75-92%, the mass fraction W2 of the hard carbon material in the negative electrode active material is 3-10%, and the mass fraction W3 of the silicon-based material in the negative electrode active material is 5-15%; the negative electrode sheet is obtained by coating a negative electrode active material slurry comprising the negative electrode active material to a negative electrode current collector and then cold pressing.

2. The negative electrode sheet according to claim 1, characterized by D50 is 10-17 μm, D'50 is 6-10 μm, and D''50 is 3-7 μm.

3. The negative electrode sheet according to claim 1, characterized by the defect degree I of the hard carbon material is 0.15-0.38; Wherein, I is the ratio of the G peak area A G and D peak area A D when the hard carbon material is subjected to Raman spectrum test.

4. The negative electrode sheet according to claim 1, characterized by the compaction density P, the porosity ɛ of the negative electrode sheet and the thickness H of the negative electrode active material layer satisfy formula 1: 15≤(kɛ+H) / P≤39.05; In formula 1 : 1.26 < k < 1.40, P is calculated as a dimensionless value in g / cm 3 calculated as a dimensionless value in g / cm, H is calculated as a dimensionless value in μm.

5. The negative electrode sheet according to claim 4, characterized by The compaction density P of the negative electrode sheet is 1.55 to 1.7 g / cm 3 .

6. The negative electrode sheet according to claim 4, characterized by the porosity ɛ of the negative electrode sheet is 20-38%, and the thickness H of the negative electrode active material layer is 25-60 μm.

7. A lithium-ion battery, characterized by The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer on at least one side of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, the negative electrode active material comprising a graphite material, a hard carbon material and a silicon-based material; 8. An electrical device, characterized by wherein the particle size D50 of the graphite material, the particle size D'50 of the hard carbon material and the particle size D''50 of the silicon-based material satisfy 1.5≤D50 / D'50≤2.5 and 1.1≤D'50 / D''50≤3; the Mohs hardness S1 of the graphite material is 0.8-2, the Mohs hardness S2 of the hard carbon material is 3-5, the Mohs hardness S3 of the silicon-based material is 5-7, and S2<S3; the mass fraction W1 of the graphite material in the negative electrode active material is 75-92%, the mass fraction W2 of the hard carbon material in the negative electrode active material is 3-10%, and the mass fraction W3 of the silicon-based material in the negative electrode active material is 5-15%; the negative electrode sheet is obtained by coating a negative electrode active material slurry comprising the negative electrode active material to a negative electrode current collector and then cold pressing. D50 is 10-17 μm, D'50 is 6-10 μm, and D''50 is 3-7 μm. the defect degree I of the hard carbon material is 0.15-0.38; the compaction density P, the porosity ɛ of the negative electrode sheet and the thickness H of the negative electrode active material layer satisfy formula 1: 15≤(kɛ+H) / P≤39.05; the porosity ɛ of the negative electrode sheet is 20-38%, and the thickness H of the negative electrode active material layer is 25-60 μm. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer on at least one side of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, the negative electrode active material comprising a graphite material, a hard carbon material and a silicon-based material; wherein the particle size D50 of the graphite material, the particle size D'50 of the hard carbon material and the particle size D''50 of the silicon-based material satisfy 1.5≤D50 / D'50≤2.5 and 1.1≤D'50 / D''50≤3; the Mohs hardness S1 of the graphite material is 0.8-2, the Mohs

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