Negative electrode sheet, lithium-ion battery and electric device

Abstract

Disclosed in the present application are a negative electrode sheet, a lithium-ion battery and an electric device. The negative electrode plate 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 comprises a negative electrode active material. The negative electrode active material comprises 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. In the present solution, by matching the particle sizes of the graphite, hard carbon and silicon-based materials, the shortcoming of low compaction density of hard carbon is effectively overcome, and the overall compaction density of the negative electrode sheet is improved, which are beneficial to an improvement in the energy density of a battery, conducive to the infiltration of an electrolyte and an improvement in the electrolyte retention of the electrode sheet, and also contribute to the rapid intercalation of lithium ions and an improvement in the rate performance.

Description

Negative plate, lithium ion battery and electric device TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a negative plate, a lithium ion battery and an electric device. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy density, no memory effect, long cycle life, fast charging and discharging, and low self-discharge, and are widely used in consumer electronics, electric vehicles and energy storage fields. Among them, the selection of negative active material is directly related to the energy density of lithium ion batteries, and also affects the rate performance. Due to the advantages of low lithium intercalation potential, high theoretical specific capacity, and price and environmental friendliness, graphite material is the main choice of current lithium ion battery negative active material. However, as the market demand for energy density and charging rate of lithium ion batteries is getting higher and higher, the single graphite material has been unable to meet the current demand.

[0003] Therefore, the common method is to mix silicon-based materials in the negative active material. The theoretical lithium storage capacity of silicon is as high as 4200mAh / g, and the lithium intercalation platform is slightly higher than that of graphite, and the safety hidden danger is small, which is an excellent substitute for graphite-based negative materials. However, silicon shows a volume change of up to 300% during charging and discharging, so it is easy to cause silicon particles to powder, the internal conductive network of the electrode to be damaged, and high-rate charging to cause lithium precipitation and silicon expansion problems more obvious, which cannot meet the market demand for fast charging. Therefore, how to more effectively alleviate the volume expansion of silicon particles, ensure the cycle stability of the battery, and obtain a silicon-based negative material that takes into account energy density, cycle performance and rate performance, is still a technical hotspot that needs to be solved in the current lithium ion battery field.

[0004] SUMMARY

[0005] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a negative plate, a lithium ion battery and an electric device.

[0006] The first aspect of the present application provides a negative plate, the negative plate comprising a negative current collector and a negative active material layer located on at least one side of the negative current collector, the negative active material layer comprising a negative active material, the negative active material comprising a graphite material, a hard carbon material and a silicon-based material.

[0007] Among them, 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, 1.1≤D'50 / D"50≤3.

[0008] The negative plate according to the first aspect of the present application has the following beneficial effects:

[0009] Compared with graphite, hard carbon has higher reversible capacity, which is beneficial to improve the energy density of lithium ion battery, and also has good isotropy, which can facilitate the deintercalation of lithium ions; but the compaction density of hard carbon cannot be very high, which seriously affects the energy density of the battery. The silicon-based material has higher specific capacity, which greatly improves the energy density of the battery, but there is a large volume expansion during the intercalation of lithium. The present scheme effectively improves the short board of low compaction density of hard carbon by matching the particle sizes of graphite, hard carbon and silicon-based material, improves the overall compaction density of the negative electrode sheet, and is beneficial to the improvement of the energy density of the battery. Due to the good isotropy and wide interlayer spacing of hard carbon, it is beneficial to the infiltration of electrolyte, improves the liquid retention of the sheet, and also helps the rapid intercalation of lithium ions, and improves the rate performance. The small particle size silicon-based material is distributed between the gaps of hard carbon and graphite, which can further improve the compaction density of the sheet, and the volume expansion during the intercalation of lithium is also further limited, thereby effectively improving the cycle performance of the battery.

[0010] In addition, since the particle hardness of the silicon-based material is greater than that of hard carbon and graphite, it plays a role of pulley during the rolling process of the negative electrode sheet, which can alleviate the locking caused by the angular shape of hard carbon and graphite, is beneficial to the sliding between particles, avoids the local overpressure caused by particle cracking, and maintains the cycle performance, rate performance and energy density.

[0011] In some embodiments of the present application, D50 is 10-17 μm, for example, it can be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm.

[0012] In some embodiments of the present application, D'50 is 6-10 μm, for example, it can be 6 μm, 7 μm, 8 μm, 9 μm, 10 μm.

[0013] In some embodiments of the present application, D"50 is 3-7 μm, for example, it can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm.

[0014] In some embodiments of the present application, 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 the D peak area A D when the hard carbon material is subjected to Raman spectrum test. Wherein the D peak is a characteristic peak appearing near 1350 cm -1 , which belongs to the defect (disorder) carbon structure; the G peak is a characteristic peak appearing near 1580 cm -1 , which belongs to the ordered carbon structure.

[0015] The defect degree I of the hard carbon material represents the graphitization degree of the hard carbon material. When the graphitization degree of the hard carbon material is high, the lithium intercalation of the material is more dependent on the interlayer lithium storage, the specific surface area is lower, the pore structure is less, the volume expansion of the material during the cycle process cannot be relieved, and the carbon layer is easily peeled off. When the graphitization degree of the hard carbon material is low, the number of defect active sites increases, and the contribution to the capacity increases. The appropriate defects can improve the reversible capacity. At the same time, the isotropy is better, which is beneficial to the adsorption and storage of lithium and weakens the volume expansion in the thickness direction, and is beneficial to maintaining the stability of the cycle. When the defect degree I of the hard carbon is in the above range, the ordered structure and the defect structure can be coordinated and balanced, the reversible specific capacity and the first cycle coulombic efficiency are optimal, and the cycle performance and the expansion rate of the battery are effectively improved.

[0016] In some embodiments of the present application, 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

[0017] In some embodiments of the present application, the compaction density P, the porosity ε and the thickness H of the negative electrode active material layer of the negative electrode sheet satisfy formula 1: 15≤(kε+H) / P≤39.05; in formula 1: 1.26≤k≤1.40, P is calculated as a dimensionless value with g / cm 2 as a unit, and H is calculated as a dimensionless value with μm as a unit. By adjusting the value of k within a limited range and making the compaction density P, the porosity ε and the thickness H of the negative electrode sheet satisfy the relationship of formula 1, the negative electrode sheet can still have good porosity under high compaction density, which is beneficial to the infiltration of the electrolyte into the sheet and improves the diffusion efficiency of the active ions.

[0018] In some embodiments of the present application, the compaction density P of the negative electrode sheet is 1.55-1.7 g / cm 2 .

[0019] In some embodiments of the present application, the compaction density P1 of the graphite material is 1.6-1.75 g / cm 2 .

[0020] In some embodiments of the present application, the compaction density P2 of the hard carbon material is 1.3-1.5 g / cm 2 .

[0021] In some embodiments of the present application, the compaction density P3 of the silicon-based material is 1-1.3 g / cm 2 .

[0022] In some embodiments of the present application, the porosity ε of the negative electrode sheet is 20-38%.

[0023] In some embodiments of the present application, the thickness H of the negative electrode active material layer is 25-60 μm.

[0024] In some embodiments of the present application, the mass fraction W1 of the graphite material in the negative electrode active material is 75-92%.

[0025] In some embodiments of the present application, the mass fraction W2 of the hard carbon material in the negative electrode active material is 3-10%.

[0026] In some embodiments of the present application, the mass fraction W3 of the silicon-based material in the negative electrode active material is 5-15%.

[0027] In some embodiments of the present application, the silicon-based material includes at least one of elemental silicon, silicon oxide compound, and silicon alloy.

[0028] In some embodiments of the present application, the silicon-based material further includes a carbon layer on the surface of at least one of elemental silicon, silicon oxide compound, and silicon alloy.

[0029] In some embodiments of the present application, the carbon layer in the silicon-based material has a coating amount of 1-10 wt%, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 wt%.

[0030] In some embodiments of the present application, the hard carbon material can be prepared by any method known to those skilled in the art, including but not limited to discharge plasma furnace method of multi-field action of joule heat and high pressure, starch / rice preparation method, petroleum coke preparation method, biomass preparation method, etc.

[0031] In some embodiments of the present application, the raw material of the negative electrode active material layer further includes 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, etc. The binder includes but is not limited to at least one of polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose, butadiene rubber, polyurethane, polyvinyl alcohol, polyvinyl butyral, etc.

[0032] In some embodiments of the present application, the negative electrode active material layer includes 70-99 wt% of the negative electrode active material, 0.5-6 wt% of the conductive agent, and 0.5-20 wt% of the binder.

[0033] In some embodiments of the present application, the negative electrode active material, the conductive agent, and the binder are prepared into the negative electrode active material layer by dispersing the negative electrode active material, the conductive agent, and the binder in a solvent and then coating on the negative electrode current collector to dry to obtain the negative electrode active material layer.

[0034] The present application also relates to a method for preparing a negative electrode sheet, the method comprising the following steps:

[0035] The slurry of the negative electrode active material is coated on the negative electrode current collector to form the negative electrode sheet.

[0036] In some embodiments of the present application, the slurry of the negative electrode active material comprises the negative electrode active material, and at least one of a conductive agent, a binder. In some specific embodiments, the slurry of the negative electrode active material further comprises a solvent.

[0037] In some embodiments of the present application, the method for coating the slurry of the negative electrode active material comprises at least one of spraying, dip coating, blade coating, transfer coating, extrusion coating, flow coating, micro gravure coating, and the like.

[0038] In a third aspect of the present application, a lithium ion battery is provided, the lithium ion battery comprising the negative electrode sheet as described above.

[0039] In some embodiments of the present application, the lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The positive electrode sheet, the negative electrode sheet, and the separator are wound or stacked to form an electrode core, and the lithium ion battery is formed.

[0040] In some embodiments of the present application, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer on at least one side of the positive electrode current collector.

[0041] In some embodiments of the present application, the positive electrode current collector and the negative electrode current collector are independently selected from at least one of metal foils (such as aluminum foils, silver foils, tin foils, iron foils, titanium foils, nickel foils, copper foils, or alloy foils of the above-mentioned metals), metal meshes (such as aluminum meshes, silver meshes, tin meshes, iron meshes, titanium meshes, nickel meshes, copper meshes, or alloy meshes of the above-mentioned metals), and the like.

[0042] In some embodiments of the present application, the positive electrode current collector is an aluminum foil, and the negative electrode current collector is a copper foil.

[0043] In some embodiments of the present application, the positive electrode active material layer comprises a positive electrode active material, which is at least one of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium nickel cobalt manganate, lithium nickel cobalt aluminum, lithium nickel manganese aluminum, and the like.

[0044] In some embodiments of the present application, the raw material of the positive electrode active material layer further includes at least one of a conductive agent, 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, etc. The binder includes, but is not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose, butadiene rubber, polyurethane, polyvinyl alcohol, polyvinyl butyral, etc.

[0045] In some embodiments of the present application, the positive electrode active material layer includes 70-99 wt% of the positive electrode active material, 0.5-6 wt% of the conductive agent and 0.5-20 wt% of the binder.

[0046] In some embodiments of the present application, the electrolyte can be at least one of a solid-state electrolyte or an electrolyte solution.

[0047] In some embodiments of the present application, the electrolyte solution includes an electrolyte salt and an organic solvent, and the specific types and compositions of the electrolyte salt and the organic solvent are not specifically limited.

[0048] In some embodiments of the present application, the electrolyte solution further includes at least one of an additive, such as a positive electrode film-forming additive, a negative electrode film-forming additive and a cycle and low-temperature improving additive, etc.

[0049] In some embodiments of the present application, the separator includes a polymer separator, and the polymer separator includes at least one of polyethylene, polypropylene, polyvinylidene fluoride and a multi-layer composite film thereof.

[0050] In a fourth aspect of the present application, a power utilization device is provided, and the power utilization device includes the lithium ion battery as described above.

[0051] In the present application, the power utilization device refers to any device that can utilize electric energy and convert it into mechanical energy, thermal energy, light energy or other one or more forms of energy, such as an electric motor, an electric heating machine, an electric light source, etc. The power utilization device includes a mobile device, an electric vehicle, an electric train, a ship and a satellite, an energy storage system, etc. The mobile device can be a mobile phone, a notebook computer, a drone, a sweeping robot, an electronic cigarette, etc. The electric vehicle can be a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.

[0052] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter. DETAILED DESCRIPTION

[0053] The idea and the technical effects of the present application will be described clearly and completely in combination with the embodiments, so that the purpose, features and effects of the present application can be fully understood. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments of the present application, other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0054] The embodiments of the present application will be described in detail below. The described embodiments are exemplary and are used to explain the present application, but cannot be understood as limiting the present application.

[0055] In the description of the present application, the meaning of several is more than one, the meaning of multiple is more than two, greater than, less than, more than, etc. are understood as not including the number, above, below, within, etc. are understood as including the number, the meaning of about is within the number ±20%, 10%, 8%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.2%, 0.1%, etc. If the first, second is described, it is only used for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features or implicitly indicating the sequence of indicated technical features.

[0056] In the description of the present application, the description of the terms "one embodiment", "some embodiments", "exemplary embodiment", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in combination with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the exemplary description of the above terms does not necessarily mean the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics can be combined in any one or more embodiments or examples in a suitable manner.

[0057] The present application will be described below in combination with specific embodiments.

[0058] Embodiments 1-13

[0059] Embodiments 1-13 provide a negative electrode sheet and a lithium ion battery comprising the negative electrode sheet, which are distinguished with reference to Table 1 in terms of the ratio between the 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, the compaction rebound density, etc.

[0060] The preparation process of the negative electrode sheet is as follows:

[0061] The negative active material (graphite material, hard carbon material, silicon-based material), conductive agent (conductive carbon black SP and carbon nanotube CNT mixed, mass ratio 9:1), binder (styrene-butadiene rubber SBR and lithium polyacrylate PAALi mixed, mass ratio 0.5:1.8) were mixed according to a mass ratio of 97.7:1.1:1.2 to prepare a negative active material slurry, which was uniformly coated on both sides of the negative current collector, then cold-pressed and slitted to obtain a negative electrode sheet.

[0062] The preparation process of the positive electrode sheet is as follows:

[0063] The positive active material LiCoO2, conductive agent acetylene black, conductive carbon nanotube, and binder polyvinylidene fluoride PVDF were dispersed in an N-methyl pyrrolidone NMP solvent system according to a mass ratio of 97.6:0.5:0.6:1.3, uniformly coated on both sides of the positive current collector aluminum foil, and then slitted after drying to prepare a positive electrode sheet.

[0064] Separator: PE surface coated with ceramic mixture as a separator.

[0065] Electrolyte: Ethylene carbonate EC, propylene carbonate PC, diethyl carbonate DEC, and propyl propionate PP were mixed according to a volume ratio of 1.2:1:4:4 to obtain a mixed organic solvent, and then the fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent according to a proportion of 1 mol / L to prepare an electrolyte.

[0066] Full cell preparation: The above positive electrode sheet, separator, and negative electrode sheet were wound to prepare a bare cell, and then packaged and injected with electrolyte to prepare a finished lithium ion battery.

[0067] Table 1. Comparison of parameters of examples and comparative examples

[0068] Comparative examples 1-15

[0069] Comparative examples 1-15 each provide a negative electrode sheet and a lithium ion battery, which are different from examples 1-13, as shown in Table 1.

[0070] In the above examples and comparative examples:

[0071] The graphite material was purchased from Sunsheng New Material Co., Ltd. The graphite material with model number DM-APG-0017-0002 was used in examples 1-3 and 6-13 and comparative examples 1-15, the graphite material with model number DM-APG-0020-0002 was used in example 4, and the graphite material with model number DM-APG-0027-0141 was used in example 5.

[0072] The silicon-based material was purchased from Jin Silicon Technology Co., Ltd., the silicon-based material with model number YP-APS-0090-0000 was used in Examples 1-3, 6, 8-13 and Comparative Examples 1-15, the silicon-based material with model number YP-APS-0098-0000 was used in Example 4, the silicon-based material with model number YP-APS-0168-0000 was used in Example 5, and the silicon-based material with model number YP-APS-0054-0000 was used in Example 7, which were different porous carbon-coated silicon materials.

[0073] The hard carbon material was prepared as follows: 9.3 g of sucrose was dissolved in 100 mL of distilled water, poured into a Teflon hot tank, heated to 200℃, and kept at a constant temperature for 6 h. The product was placed in a tube furnace, heated to 1100℃ at a rate of 5℃ / min under an argon atmosphere, and calcined and carbonized for 2 h. The hard carbon was obtained. By controlling the carbonization temperature at 1100℃ and 1300℃, respectively, hard carbon materials with different hardness can be obtained.

[0074] The performance of the negative electrode sheets and lithium batteries of the examples and comparative examples was detected by the following method:

[0075] (1) Energy density

[0076] Capacity calibration: 0.2C discharge to 3.0V; 0.5C constant current constant voltage charging to 4.5V, cutoff 0.02C; then 0.2C discharge to 3.0V, record capacity C0and energy W0; platform voltage V0= W0 / C0.

[0077] The half-state thickness of the battery was tested by 600PPG, and the length and width of the battery were measured by two-dimensional software.

[0078] The energy density was calculated according to the following formula: energy density = [C0×V0 / (length×width×thickness)]×1000; wherein the units of each are as follows: platform voltage V0(V), capacity C0(mAh), energy W0(mWh), length, width, and thickness (mm).

[0079] (2) Cycle test

[0080] 25℃ environment, the following method is used for cycle test, 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. 500th week capacity retention rate = the discharge capacity of the 500th week / the discharge capacity of the 1st week x 100%; 500th week expansion rate = the full charge thickness of the 500th week / the initial half charge thickness x 100%;

[0081] All the battery thicknesses are tested using 600 PPG.

[0082] (3) Liquid retention amount test Liquid retention amount = liquid injection amount - liquid loss amount.

[0083] The results are shown in Table 2:

[0084] Table 2. Performance test results of examples and comparative examples

[0085] Comparing Comparative Examples 1-3, as the proportion of the three negative electrode active materials changes, the porosity of the negative electrode sheet formed gradually decreases, the liquid retention amount also gradually decreases, and the energy density and cycle performance also change. Comparing Comparative Examples 1, 4 and 5, different particle sizes of graphite, hard carbon and silicon-based materials are used, the porosity changes continuously, and the liquid retention amount, energy density and cycle performance also change. Comparing Comparative Examples 2-4, the porosity of Example 4 is between Examples 2-3, and the three are relatively close, but because the silicon-based material has poorer wettability for electrolyte, and the content of the silicon-based material in Example 4 is higher, the liquid retention amount is lower than that of Example 3. Comparing Comparative Examples 7-9, as the coating thickness increases, the liquid retention amount gradually increases, but the cycle performance gradually decreases. Comparing Comparative Examples 12-13, as the defect degree of the hard carbon material increases, the energy density, cycle performance and liquid retention amount all gradually decrease.

[0086] Comparing Comparative Example 1 and Comparative Example 1, after the hard carbon material is omitted and replaced by graphite material, the porosity of Comparative Example 1 decreases significantly, although the value of (kε+H) / P still meets the requirements, the liquid retention amount decays obviously, the energy density also decreases to a certain extent, and the expansion rate of 500th week cycle is close to doubled, and the capacity retention rate also decreases by about 10%.

[0087] Comparing Comparative Example 1 and Comparative Example 2, after the silicon-based material is omitted and replaced by graphite material, the porosity increases, the liquid retention amount increases, but the energy density decreases significantly, the expansion rate of 500th week cycle still maintains at a high level, and the capacity retention rate also decreases significantly.

[0088] Comparing Example 1 with Comparative Example 3, the pure graphite system in Comparative Example 3 has the lowest expansion rate and the highest capacity retention rate in 500 cycles, but the liquid retention amount and the energy density are also at the lowest level.

[0089] Comparing Example 1 with Comparative Examples 4-5, the defect degree I of hard carbon represents the graphitization degree of the hard carbon material. When the graphitization degree of hard carbon is high, the lithium storage of the material is more dependent on the interlayer lithium storage, the specific surface area is lower, and the pore structure is less, which cannot alleviate the volume expansion of the material during the cycle process, and is easy to cause the peeling of the carbon layer. On the contrary, when the graphitization degree of hard carbon is low, the number of defect active sites increases, and the contribution to the capacity increases. The appropriate defects can improve the reversible capacity; at the same time, the isotropy is better, which is conducive to the adsorption and storage of lithium, and weakens the volume expansion in the thickness direction, which is conducive to maintaining the stability of the cycle. Therefore, the defect degrees of the hard carbon materials in Comparative Examples 4 and 5 are too low or too high, which do not meet the defect degree requirement, the proportion of ordered structure and disordered structure in the hard carbon material is unbalanced, the gain of the cycle performance of the battery is greatly attenuated, the expansion rate in 500 cycles and the capacity retention rate are at the level of Comparative Example 1 or 2, which has obvious disadvantages compared with Example 1; and the influence on the liquid retention is small, and the liquid retention amount only decreases slightly.

[0090] Comparing Example 3 with Comparative Examples 6-7, in Comparative Examples 6 and 7, the (kε+H) / P is too low or too high through the compaction density P, the porosity ε and the thickness H of the negative electrode active material layer, which makes the negative electrode sheet not conducive to the infiltration of the electrolyte into the sheet under high pressure, and further limits the improvement of the diffusion efficiency of the active ions.

[0091] Comparing Example 1 with Comparative Examples 8-9, the hardness of the silicon-based material in Comparative Example 8 is too low, and the hardness of the hard carbon material in Comparative Example 9 is too high. The particle hardness of the silicon-based material is greater than that of hard carbon and graphite, which plays the role of a pulley during the rolling process of the negative electrode sheet, can alleviate the locking caused by the angular shape of hard carbon and graphite, is conducive to the sliding between particles, and avoids the local overpressure leading to particle cracking. The hardness of silicon itself is relatively large, and reducing the hardness requires a thicker carbon layer, which affects the volume ratio of the carbon layer and the development of the energy density of the battery, and also prolongs the path of lithium ion insertion into silicon. The hardness of hard carbon is too large, which will cause internal physical short circuit of the battery, and the most direct performance is the accelerated cycle decay. Therefore, the energy density and the capacity retention rate in Comparative Examples 8 and 9 both decrease compared with Example 1, and the thickness expansion rate increases significantly.

[0092] Comparative Example 1 and Comparative Examples 10-13, the ratio of D50 / D'50 in Comparative Examples 10-11 is too large or too small, and the ratio of D'50 / D"50 in Comparative Examples 12-13 is too low or too high, which seriously affects the structural stability of the material itself under the same compaction density, and the porosity of the electrode plate is low, resulting in unsatisfactory liquid retention effect. For example, in Comparative Examples 10-11, the graphite particle size is too large, and the silicon-based material and the hard carbon material are densely distributed in the gap between the graphite; the hard carbon material particle size is too large, the gap formed by the hard carbon and the graphite is increased, and a large amount of silicon-based material is distributed in the gap between the two, and the volume expansion of the silicon-based material caused during the operation process cannot be relieved. Therefore, the 500-week thickness expansion rate of the two comparative examples is even higher than that of Comparative Example 1, and the capacity retention rate is lower than that of Comparative Example 1. In Comparative Examples 12-13, when the particle size of the silicon-based material is too large, it is difficult to embed in the gap between the hard carbon and the graphite; if the hard carbon is too large, the silicon-based material cannot be well retained in the gap between the hard carbon and the graphite. In Example 1, the silicon-based material with a suitable particle size is distributed in the gap between the hard carbon and the graphite, which not only improves the compaction density of the electrode plate, but also further limits the volume expansion during the lithium intercalation process, thereby improving the cycle performance of the battery.

[0093] Comparative Example 4 and Comparative Examples 14 and 15, the porosity in Comparative Examples 14-15 is low, and the compaction density of the negative electrode plate is too high or too low, although it still satisfies Formula 1, but one or more of the energy density, cycle capacity retention rate, expansion rate and liquid retention amount has decreased to a certain extent, and some performances are lower than those of Comparative Examples 1-3.

[0094] Therefore, by combining the above examples and Comparative Examples 1-15, the lithium ion battery using the negative electrode plate provided by the present application still has good capacity retention rate and low battery expansion rate after long-term cycle charging and discharging, which is significantly better than Comparative Examples 1-2; compared with the pure graphite system of Comparative Example 3, the energy density of the battery is also greatly improved, which can better meet the market demand, and the good porosity enhances the infiltration of the electrolyte, which provides protection for the long cycle stability of the battery. Therefore, the negative electrode plate provided by the present application can significantly improve the cycle performance of the lithium ion battery, solve the battery failure problem caused by large material expansion, and the battery also has good liquid retention and high energy density.

[0095] The above describes the present application in detail in combination with the examples, but the present application is not limited to the above examples, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the purpose of the present application. In addition, the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

Claims

1. A negative electrode sheet, characterized by, The negative electrode sheet 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 including a negative electrode active material, the negative electrode active material including 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.

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 Mohs hardness S1 of the graphite material is 0.8-2, the Mohs hardness S2 of the hard carbon material is 3-5, and the Mohs hardness S3 of the silicon-based material is 5-7, and S2 5. The negative electrode sheet according to claim 1, wherein The compaction density P, the porosity ε of the negative electrode sheet, and the thickness H of the negative electrode active material layer satisfy formula 1: 15≤(kε+H) / P≤39.

05. In formula 1 : 1.26 < k < 1.40, P is calculated as a dimensionless value in g / cm 2 calculated as a dimensionless value in g / cm, H is calculated as a dimensionless value in μm.

6. The negative electrode sheet according to claim 5, characterized by The compact density P of the negative electrode sheet is 1.55-1.7 g / cm 2 ; the compact density P1 of the graphite material is 1.6-1.75 g / cm 2 ; the compact density P2 of the hard carbon material is 1.3-1.5 g / cm 2 ; and the compact density P3 of the silicon-based material is 1-1.3 g / cm 2 .

7. The negative electrode sheet according to claim 5, wherein The porosity ε of the negative electrode sheet is 20-38%, and the thickness H of the negative electrode active material layer is 25-60 μm.

8. The negative electrode sheet according to claim 1, characterized by 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%.

9. A lithium-ion battery, characterized by The negative electrode sheet of any one of claims 1-8.

10. An electrical device, characterized by The lithium ion battery of claim 9.

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