Negative electrode sheet and lithium ion battery

Abstract

The application discloses a negative pole piece and a lithium ion battery, and relates to the technical field of batteries, and specifically discloses a negative pole piece and a lithium ion battery. The negative pole piece comprises a current collector, a first coating layer and a second coating layer which are stacked on at least one side of the current collector; the first coating layer and the second coating layer each comprise a negative active material, a first binder, a second binder, a first conductive agent and a second conductive agent; and the preparation of the negative pole piece satisfies the following conditions: 2≤C2 / C1≤4 and 1.05≤X / Y≤1.25, wherein C1 and C2 are the mass proportions of the first conductive agent in the first coating layer and the second coating layer respectively, and X and Y are the solid contents of the first coating layer slurry and the second coating layer slurry respectively. According to the application, the solid content of the slurry, the proportions of different active materials and conductive agents are designed and controlled in the double-coating pole piece, and the designed proportions satisfy a certain functional relationship, so that the ion conductivity of the battery can be effectively improved while the high conductivity of the pole piece is maintained, the direct-current internal resistance of the pole piece in the charging and discharging process is reduced, the rate charging and discharging performance is improved, and the lithium precipitation window is widened.

Description

Technical Field

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

[0002] With the increasing prominence of energy crises and environmental pollution, developing sustainable new energy sources and building a low-carbon society have become urgent priorities. Lithium-ion batteries, as a new type of high-energy green battery, have attracted significant attention. They retain the main advantages of lithium batteries—high voltage and high capacity—while also possessing significant characteristics such as long cycle life and good safety performance. They have shown broad application prospects and potential for huge economic benefits in portable electronic devices, electric vehicles, space technology, and the defense industry, rapidly becoming a widely studied research hotspot in recent years. Among these, the market share of power batteries and energy storage batteries is also increasing, but the performance indicators emphasized by power batteries and energy storage batteries differ. Power batteries used in electric vehicles place greater emphasis on energy density, cycle life, low-temperature discharge, and fast-charging performance.

[0003] To improve energy density, it is necessary to increase the specific capacity of battery active materials and the discharge voltage of the battery. For anode materials, silicon anodes blended with graphite are undoubtedly a better choice. Silicon has a much higher theoretical specific capacity than graphite, but its cycle stability is worse and its volume expansion is greater, so it needs to be used in combination with graphite. Silicon anodes are mainly divided into two categories: silicon suboxide and silicon-carbon materials. Silicon suboxide is further divided into conventional silicon suboxide and pre-lithiated / magnesium-plated silicon suboxide, while silicon-carbon is divided into milled nano-silicon-carbon and fumed silicon-carbon. Conventional silicon suboxide has a lower initial efficiency and can only be used in low-dosage silicon / graphite low-internal-resistance materials; pre-lithiated / magnesium-plated silicon suboxide and milled nano-silicon-carbon have much lower cycle stability at the same energy density than fumed silicon-carbon.

[0004] Vaporized silicon-carbon (VPC) uses hard carbon as a framework, within which silane is deposited in the gas phase. This silane decomposes into silicon at high temperatures, releasing hydrogen gas, which is then coated with a carbon layer. When this novel VPC material is combined with graphite in lithium-ion battery anode materials, it can significantly improve energy density, and its cycle stability can reach the level of batteries using graphite as the anode material alone.

[0005] However, there are still some problems to be solved when using fumed silicon-carbon materials as negative electrode active materials for lithium-ion batteries, such as high DC internal resistance during charging and discharging, poor rate performance, and narrow lithium plating window. Summary of the Invention

[0006] This invention provides a negative electrode sheet and a lithium-ion battery. By designing an electrode sheet with a double-layer coating structure and controlling the solid content of different coatings and the type and content ratio of conductive agents in the electrode sheet, the porosity and conductivity of the electrode sheet coating are adjusted. This allows the electrode sheet to maintain high conductivity while improving ionic conductivity, thereby effectively improving the problems of high DC internal resistance, poor rate performance, and narrow lithium plating window of lithium-ion batteries.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] The first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a current collector, a first coating disposed on at least one side of the current collector, and a second coating disposed on the first coating; the first coating and the second coating each comprise a negative electrode active material, a first binder, a second binder, a first conductive agent, and a second conductive agent;

[0009] The first conductive agent is selected from one or more of single-walled carbon nanotubes, graphene, and graphyne, and the second conductive agent is conductive carbon black and / or multi-walled carbon nanotubes.

[0010] The method for preparing the negative electrode sheet includes the following steps:

[0011] (1) The negative electrode active material, the first binder, the second binder, the first conductive agent, the second conductive agent and water are stirred evenly to obtain the first coating slurry, and the solid content of the first coating slurry is X;

[0012] (2) The first coating slurry prepared in step (1) is coated onto the current collector, and after drying, a first coating is formed. The mass ratio of the first conductive agent in the first coating is C1.

[0013] (3) The negative electrode active material, the first binder, the second binder, the first conductive agent, the second conductive agent and water are stirred evenly to obtain the second coating slurry, and the solid content of the second coating slurry is Y;

[0014] (4) The second coating slurry prepared in step (1) is coated on the first coating, and after drying, the second coating is formed to obtain the negative electrode sheet. The mass ratio of the first conductive agent in the second coating is C2.

[0015] The preparation of the negative electrode sheet satisfies the following relationship: 2≤C2 / C1≤4, 1.05≤X / Y≤1.25.

[0016] Furthermore, in the preparation process of the above negative electrode sheet, the order of steps (2) and (3) can be interchanged.

[0017] Furthermore, the current collector is one of copper foil, pure copper foil, porous copper foil, nickel / copper foam foil, and carbon-coated copper foil.

[0018] Furthermore, the negative electrode active material comprises one or more of silicon-carbon materials, silicon suboxide, pre-lithiated silicon suboxide, pre-magnesiated silicon suboxide, and carbon materials.

[0019] Furthermore, the silicon-carbon material comprises one or more of fumed silicon-carbon and milled silicon-carbon.

[0020] Furthermore, the carbon material includes one or more of artificial graphite, natural graphite, modified graphite, fast-charging graphite, soft carbon, and hard carbon.

[0021] Furthermore, the negative electrode active material preferably accounts for more than 92% of the mass in the first coating and the second coating.

[0022] Furthermore, the first adhesive is preferably one of polyacrylic acid, polyacrylonitrile, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose, and the second adhesive is preferably one of styrene-butadiene rubber and modified styrene-butadiene polymer.

[0023] Furthermore, the mass ratio of the first adhesive to the second adhesive in the first coating is preferably 2-4:1.

[0024] Furthermore, the mass ratio of the first adhesive to the second adhesive in the second coating is preferably 1-3:1.

[0025] Furthermore, the mass ratio of the first conductive agent and the second conductive agent in the first coating and the second coating is preferably less than 2%.

[0026] Furthermore, the mass ratio of the first conductive agent to the second conductive agent in the first coating is preferably 0.025-0.075:1.

[0027] Furthermore, the mass ratio of the first conductive agent to the second conductive agent in the second coating is preferably 0.075-0.3:1.

[0028] Furthermore, the solid content of the first coating slurry is preferably 49%-56%;

[0029] Furthermore, the solid content of the second coating slurry is preferably 45%-52%.

[0030] Furthermore, the mass ratio of the first coating to the second coating is preferably 1:0.5-2.

[0031] Furthermore, the first coating is applied in equal amounts to both sides of the current collector, and the second coating is applied in equal amounts to the first coating on both sides of the current collector.

[0032] Furthermore, the preferred single-sided coating weight of the negative electrode sheet is 80-120 mg, and the compaction density is 1.5-1.7 g / cm³. 3 .

[0033] A second aspect of the present invention provides a lithium-ion battery comprising the negative electrode sheet described in the first aspect.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] 1. This invention designs a negative electrode sheet with a double-layer coating structure. On the one hand, it controls the solid content ratio of the first coating to the second coating in the electrode sheet, so that the porosity of the electrode sheet is distributed in a gradient, with the porosity gradually increasing from the inside to the outside. This makes it easier for the electrolyte to penetrate the electrode sheet, increases the contact area between the electrolyte and the negative electrode active material, and improves the ionic conductivity. On the other hand, it controls the type and content of conductive agents in the first and second coatings, so that the electrode sheet can build a good electronic conductive network in different coatings and as a whole, improves the conductivity between the negative electrode active materials in the high-porosity coating, and thus maintains the high conductivity of the electrode sheet. Under the above synergistic effect, the ion conduction and electron conduction performance of the negative electrode sheet are improved, thereby effectively reducing the DC impedance of the lithium-ion battery constructed with this negative electrode sheet and improving its rate performance.

[0036] 2. This invention rationally designs the type and content of conductive agent, the solid content of slurry, and the type and amount of binder in the coating of the negative electrode sheet, and makes the negative electrode sheet satisfy the specific relationship 2≤C2 / C1≤4, 1.05≤X / Y≤1.25, so that the lithium-ion battery containing the negative electrode sheet can have low DC internal resistance, excellent rate performance, and a widened lithium plating window. Attached Figure Description

[0037] Figure 1 The bar chart shows the DC internal resistance test results for the examples and comparative examples. Each set of data in the chart corresponds to the DC internal resistance at 90% SOC, 50% SOC, and 20% SOC, respectively, from left to right.

[0038] Figure 2 The bar chart shows the rate performance test results for the examples and comparative examples. Each set of data in the chart corresponds to the discharge capacity percentage of 0.5C, 0.33C, 1C, 2C, 4C, and 5C relative to 0.5C, from left to right. Detailed Implementation

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items. "Comprising" or "containing" as used herein means that it may include or contain other components in addition to the stated components. "Comprising" or "containing" as used herein may also be replaced with the closed form "is" or "consisting of".

[0040] As described in the background section, when fumed silicon-carbon materials are combined with graphite in lithium-ion battery anode materials, the energy density can be significantly improved, and the cycle stability can reach the level of batteries that use graphite as the anode material alone. However, there are still some problems to be solved when using fumed silicon-carbon materials as anode active materials in lithium-ion batteries, such as high DC internal resistance during charging and discharging, poor rate performance, and narrow lithium plating window.

[0041] To solve the above-mentioned technical problems, embodiments of the present invention provide a negative electrode sheet, comprising a first coating disposed on at least one side of the current collector, and a second coating disposed on the first coating; both the first coating and the second coating comprise a negative electrode active material, a first binder, a second binder, a first conductive agent, and a second conductive agent;

[0042] The first conductive agent is selected from one or more of single-walled carbon nanotubes, graphene, and graphyne, and the second conductive agent is one or more of conductive carbon black and / or multi-walled carbon nanotubes.

[0043] The method for preparing the negative electrode sheet includes the following steps:

[0044] (1) The negative electrode active material, the first binder, the second binder, the first conductive agent, the second conductive agent and water are stirred evenly to obtain the first coating slurry, and the solid content of the first coating slurry is X;

[0045] (2) The first coating slurry prepared in step (1) is coated onto the current collector, and after drying, a first coating is formed. The mass ratio of the first conductive agent in the first coating is C1.

[0046] (3) The negative electrode active material, the first binder, the second binder, the first conductive agent, the second conductive agent and water are stirred evenly to obtain the second coating slurry, and the solid content of the second coating slurry is Y;

[0047] (4) The second coating slurry prepared in step (1) is coated on the first coating, and after drying, the second coating is formed to obtain the negative electrode sheet. The mass ratio of the first conductive agent in the second coating is C2.

[0048] The preparation of the negative electrode sheet satisfies the following relationship: 2≤C2 / C1≤4, 1.05≤X / Y≤1.25.

[0049] Currently, lithium-ion batteries using fumed silicon-carbon materials as the negative electrode active material suffer from problems such as high DC internal resistance, poor rate performance, and narrow lithium plating window. This invention considers the electrode layer. Assuming each part of the electrode coating possesses a good electronic conductivity network, the lithium-ion transport speed within the electrode becomes a crucial factor affecting the charge / discharge rate. In existing technologies, reducing the solid content of the coating slurry can increase the porosity of the coating, thereby allowing the electrolyte to penetrate the electrode more easily and improve ionic conductivity. However, this also increases the electrode resistance with increasing porosity, affecting the battery's rate performance. Furthermore, increasing the content of conductive agents in the coating will affect the battery's capacity. Therefore, effectively solving the aforementioned problems of high DC internal resistance, poor rate performance, and narrow lithium plating window without affecting battery capacity remains a pressing and challenging problem in this field.

[0050] To address the aforementioned technical problems, this invention designs an electrode with a double-layer coating structure. On one hand, by controlling the solid content of the first and second coatings, the porosity of the electrode is gradient-distributed, gradually increasing from the inside to the outside. This allows the electrolyte to penetrate the electrode more easily, increasing the contact area between the electrolyte and the negative electrode active material, and improving ionic conductivity. Simultaneously, by controlling the type and content of conductive agents in the first and second coatings, a good electronic conductivity network can be constructed in each coating, improving the conductivity between the negative electrode active materials at areas with high porosity, thereby maintaining the electrode's high conductivity. Furthermore, this invention unexpectedly discovered that when the negative electrode satisfies the specific relationships 2≤C2 / C1≤4 and 1.05≤X / Y≤1.25, lithium-ion batteries prepared using fumed silicon-carbon materials as the negative electrode active material can exhibit low DC internal resistance, significantly improved rate performance, and a broadened lithium plating window.

[0051] In some preferred embodiments, the current collector may be selected from one of copper foil, pure copper foil, porous copper foil, nickel / copper foam foil, and carbon-coated copper foil.

[0052] In some preferred embodiments, the aforementioned negative electrode active material may further include one or more of silicon carbide, silicon suboxide, pre-lithiated silicon suboxide, pre-magnesiated silicon suboxide, milled silicon carbide, artificial graphite, natural graphite, modified graphite, fast-charging graphite, soft carbon, and hard carbon; specifically, the negative electrode active material may be composed of fumed silicon carbide and artificial graphite, and the proportion of fumed silicon carbide and artificial graphite in the negative electrode active material is calculated according to the required specific capacity of the negative electrode active material.

[0053] In some preferred embodiments, the mass percentage of the negative electrode active material in the first coating and the second coating is preferably greater than 92%, more preferably greater than 94%, for example, a value within any range of 94%-95%, 95%-96%, or 96%-97%.

[0054] In some preferred embodiments, the first binder is preferably one of polyacrylic acid, polyacrylonitrile, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose, and the second binder is preferably one of styrene-butadiene rubber and modified styrene-butadiene polymer; for example, in the above-mentioned first coating and second coating, the first binder is polyacrylic acid, and the second binder is styrene-butadiene rubber.

[0055] More preferably, in the first coating, the mass ratio of the first binder to the second binder is preferably 2-4:1, such as 2:1, 2.5:1, 3:1, 3.5:1, 4:1, etc., including but not limited to the mass ratios listed above; in the second coating, the mass ratio of the first binder to the second binder is preferably 1-3:1, such as 2:1, 2.5:1, 3:1, etc., including but not limited to the mass ratios listed above. The type and amount of binder can synergistically work with the solid content of the slurry to adjust the porosity of the electrode sheet.

[0056] In some preferred embodiments, to avoid the battery capacity being affected by excessively high conductive agent content, the mass ratio of conductive agent in the coating needs to be controlled. For example, the sum of the mass ratios of the first conductive agent and the second conductive agent in the first coating and the second coating should be controlled to be less than 2%, for example, 1.5%.

[0057] In some preferred embodiments, by adding conductive materials with different conductivity and dimensions and controlling the content of different conductive materials, a good electronic conductive network can be constructed in the coating while controlling costs and not affecting battery capacity, thereby reducing electrode resistance. Specifically, in the above-mentioned first coating, the mass ratio of the first conductive agent to the second conductive agent is preferably 0.025-0.075:1, for example, 0.025:1, 0.03:1, 0.035:1, 0.04:1, 0.045:1, 0.05:1, 0.055:1, 0.06:1, 0.06... The mass ratios of the first conductive agent to the second conductive agent in the second coating are preferably 0.075-0.3:1, such as 0.075:1, 0.075:1, 0.08:1, 0.1:1, 0.12:1, 0.15:1, 0.2:1, 0.25:1, 0.3:1, etc., including but not limited to the mass ratios listed above. For example, the first conductive agent is a single-walled carbon nanotube, and the second conductive agent is conductive carbon black, which includes, but is not limited to, acetylene black and Ketjen black.

[0058] In some preferred embodiments, the porosity of the coating is controlled by adjusting the solid content in the coating slurry. Specifically, the solid content in the first coating slurry is preferably 49%-56%, such as 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, etc., including but not limited to the solid contents listed above. The solid content in the second coating slurry is preferably 45%-52%, such as 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, etc., including but not limited to the solid contents listed above. More preferably, the mass ratio of the first coating to the second coating is 1:0.5-2, for example, 1:1.

[0059] In some preferred embodiments, the first coating is applied equally to both sides of the current collector, and the second coating is applied equally to the first coating on both sides of the current collector.

[0060] In some preferred embodiments, the coating weight on one side of the negative electrode sheet is preferably 80-120 mg, for example 80 mg, 90 mg, 100 mg, 110 mg, or 120 mg; after cold pressing, the compaction density is 1.5-1.7 g / cm³. 3 For example, 1.6g / cm 3 The weight of a single-sided coating refers to the mass of the first coating plus the second coating on a single side with a unit area of ​​1540.25 square millimeters.

[0061] An embodiment of the present invention provides a lithium-ion battery comprising the aforementioned negative electrode sheet.

[0062] In some preferred embodiments of the present invention, the lithium-ion battery is a secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte.

[0063] The present invention will be further described below with reference to specific embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0064] Example 1

[0065] This embodiment relates to the preparation of a negative electrode sheet and a lithium-ion battery. The specific preparation process is as follows:

[0066] (1) Preparation of the first coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.06:1.44. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 54%.

[0067] (2) Preparation of the second coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 48%.

[0068] (3) Preparation of the negative electrode sheet: First, the first coating slurry is coated on the negative electrode copper foil, with a single-sided coating weight of 50 mg. After double-sided coating and baking, the second coating slurry is coated, with a single-sided coating weight of 50 mg. After double-sided coating, it is dried and cold-pressed to obtain a compaction density of 1.6 g / cm³. 3 The negative electrode plate;

[0069] (4) Preparation of the positive electrode sheet: Lithium octane nickel cobalt manganese oxide, PVDF binder, and conductive agent were weighed at a mass ratio of 97.8:0.9:1.3 and mixed evenly with NMP to obtain a positive electrode slurry. The positive electrode slurry was coated onto aluminum foil with a coating mass of 0.3 g. After double-sided coating, it was dried and cold-pressed to obtain a compaction density of 3.4 g / cm³. 3 Positive electrode plate;

[0070] (5) Preparation of electrolyte: Lithium hexafluorophosphate was dissolved in a mixed solution of EC, DEC and EMC (1:1:1), and vinylene carbonate was added to obtain an electrolyte with a concentration of 1.2M.

[0071] (6) Preparation of lithium-ion battery: The negative electrode sheet, the separator (polypropylene separator) and the positive electrode sheet are stacked and wound in sequence to obtain a bare cell. The tabs are ultrasonically welded, the bare cell is placed in the battery case, dried to remove moisture, electrolyte is injected into the battery case, and packaged. After formation and capacity testing, a lithium-ion battery is obtained.

[0072] Example 2

[0073] This embodiment relates to the preparation of a negative electrode sheet and a lithium-ion battery. The specific preparation process is as follows:

[0074] (1) Preparation of the first coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.05:3.15:0.06:1.44. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 54%.

[0075] (2) Preparation of the second coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:2.1:2.1:0.24:1.26. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 48%.

[0076] (3) Preparation of the negative electrode sheet: First, the first coating slurry is coated on the negative electrode copper foil, with a single-sided coating weight of 50 mg. After double-sided coating and baking, the second coating slurry is coated, with a single-sided coating weight of 50 mg. After double-sided coating, it is dried and cold-pressed to obtain a compaction density of 1.6 g / cm³. 3 The negative electrode plate;

[0077] (4) Preparation of the positive electrode sheet: Lithium octane nickel cobalt manganese oxide, PVDF binder, and conductive agent were weighed at a mass ratio of 97.8:0.9:1.3 and mixed evenly with NMP to obtain a positive electrode slurry. The positive electrode slurry was coated onto aluminum foil with a coating mass of 0.3 g. After double-sided coating, it was dried and cold-pressed to obtain a compaction density of 3.4 g / cm³. 3 Positive electrode plate;

[0078] (5) Preparation of electrolyte: Lithium hexafluorophosphate was dissolved in a mixed solution of EC, DEC and EMC (1:1:1), and vinylene carbonate was added to obtain an electrolyte with a concentration of 1.2M.

[0079] (6) Preparation of lithium-ion battery: The negative electrode sheet, the separator (polypropylene separator) and the positive electrode sheet are stacked and wound in sequence to obtain a bare cell. The tabs are ultrasonically welded, the bare cell is placed in the battery case, dried to remove moisture, electrolyte is injected into the battery case, and packaged. After formation and capacity testing, a lithium-ion battery is obtained.

[0080] Example 3

[0081] This embodiment relates to the preparation of a negative electrode sheet and a lithium-ion battery. The specific preparation process is as follows:

[0082] (1) Preparation of the first coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.06:1.44. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 54%.

[0083] (2) Preparation of the second coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.12:1.38. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 50%.

[0084] (3) Preparation of the negative electrode sheet: First, the first coating slurry is coated on the negative electrode copper foil, with a single-sided coating weight of 50 mg. After double-sided coating and baking, the second coating slurry is coated, with a single-sided coating weight of 50 mg. After double-sided coating, it is dried and cold-pressed to obtain a compaction density of 1.6 g / cm³. 3 The negative electrode plate;

[0085] (4) Preparation of the positive electrode sheet: Lithium octane nickel cobalt manganese oxide, PVDF binder, and conductive agent were weighed at a mass ratio of 97.8:0.9:1.3 and mixed evenly with NMP to obtain a positive electrode slurry. The positive electrode slurry was coated onto aluminum foil with a coating mass of 0.3 g. After double-sided coating, it was dried and cold-pressed to obtain a compaction density of 3.4 g / cm³. 3 Positive electrode plate;

[0086] (5) Preparation of electrolyte: Lithium hexafluorophosphate was dissolved in a mixed solution of EC, DEC and EMC (1:1:1), and vinylene carbonate was added to obtain an electrolyte with a concentration of 1.2M.

[0087] (6) Preparation of lithium-ion battery: The negative electrode sheet, the separator (polypropylene separator) and the positive electrode sheet are stacked and wound in sequence to obtain a bare cell. The tabs are ultrasonically welded, the bare cell is placed in the battery case, dried to remove moisture, electrolyte is injected into the battery case, and packaged. After formation and capacity testing, a lithium-ion battery is obtained.

[0088] Example 4

[0089] This embodiment relates to the preparation of a negative electrode sheet and a lithium-ion battery. The specific preparation process is as follows:

[0090] (1) Preparation of the first coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.06:1.44. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 54%.

[0091] (2) Preparation of the second coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.24:1.26. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 46%.

[0092] (3) Preparation of the negative electrode sheet: First, the first coating slurry is coated on the negative electrode copper foil, with a single-sided coating weight of 50 mg. After double-sided coating and baking, the second coating slurry is coated, with a single-sided coating weight of 50 mg. After double-sided coating, it is dried and cold-pressed to obtain a compaction density of 1.6 g / cm³. 3 The negative electrode plate;

[0093] (4) Preparation of the positive electrode sheet: Lithium octane nickel cobalt manganese oxide, PVDF binder, and conductive agent were weighed at a mass ratio of 97.8:0.9:1.3 and mixed evenly with NMP to obtain a positive electrode slurry. The positive electrode slurry was coated onto aluminum foil with a coating mass of 0.3 g. After double-sided coating, it was dried and cold-pressed to obtain a compaction density of 3.4 g / cm³. 3 Positive electrode plate;

[0094] (5) Preparation of electrolyte: Lithium hexafluorophosphate was dissolved in a mixed solution of EC, DEC and EMC (1:1:1), and vinylene carbonate was added to obtain an electrolyte with a concentration of 1.2M.

[0095] (6) Preparation of lithium-ion battery: The negative electrode sheet, the separator (polypropylene separator) and the positive electrode sheet are stacked and wound in sequence to obtain a bare cell. The tabs are ultrasonically welded, the bare cell is placed in the battery case, dried to remove moisture, electrolyte is injected into the battery case, and packaged. After formation and capacity testing, a lithium-ion battery is obtained.

[0096] Comparative Example 1

[0097] This comparative example relates to the preparation of a negative electrode sheet and a lithium-ion battery. The only difference from Example 1 is that the mass ratio of each component in the first coating slurry and the second coating slurry is different, as detailed below:

[0098] First coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.12:1.38, and the output solid content is 54%.

[0099] Second coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.12:1.38, and the output solid content is 48%.

[0100] Apart from the differences mentioned above, all other operations are the same.

[0101] Comparative Example 2

[0102] This comparative example relates to the preparation of a negative electrode sheet and a lithium-ion battery. The only difference from Example 1 is the preparation of the negative electrode sheet, as detailed below:

[0103] Preparation of the negative electrode sheet: First, a first coating slurry is coated onto the negative electrode copper foil, with a single-sided coating weight of 66 mg. After double-sided coating and baking, a second coating slurry is applied, with a single-sided coating weight of 34 mg. After double-sided coating, the sheet is dried and cold-pressed to obtain a compacted density of 1.6 g / cm³. 3 The negative electrode plate;

[0104] Apart from the differences mentioned above, all other operations are the same.

[0105] Comparative Example 3

[0106] This comparative example relates to the preparation of a negative electrode sheet and a lithium-ion battery. The only difference from Example 1 is the preparation of the negative electrode sheet, as detailed below:

[0107] Preparation of the negative electrode sheet: First, a first coating slurry is coated onto the negative electrode copper foil, with a single-sided coating weight of 34 mg. After double-sided coating and baking, a second coating slurry is applied, with a single-sided coating weight of 66 mg. After double-sided coating, the sheet is dried and cold-pressed to obtain a compacted density of 1.6 g / cm³. 3 The negative electrode plate;

[0108] Apart from the differences mentioned above, all other operations are the same.

[0109] Comparative Example 4

[0110] This comparative example relates to the preparation of a negative electrode sheet and a lithium-ion battery. The specific preparation process is as follows:

[0111] (1) Preparation of coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650 mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.06:1.44. Pre-stir the active material and conductive carbon black, then add 1 / 2 part of polyacrylic acid and deionized water and stir. Then add single-walled carbon nanotubes, 1 / 2 part of polyacrylic acid, styrene-butadiene rubber, and deionized water and stir. Adjust the content of deionized water to make the final output solid content 50%.

[0112] (2) Preparation of the negative electrode sheet: The coating slurry was coated onto the negative electrode copper foil, with a single-sided coating weight of 100 mg. After double-sided coating, the foil was dried and cold-pressed to obtain a compaction density of 1.6 g / cm³. 3 The negative electrode plate;

[0113] (3) Preparation of the positive electrode sheet: Lithium octane nickel cobalt manganese oxide, PVDF binder, and conductive agent were weighed at a mass ratio of 97.8:0.9:1.3 and mixed evenly with NMP to obtain a positive electrode slurry. The positive electrode slurry was coated onto aluminum foil with a coating mass of 0.3 g. After double-sided coating, it was dried and cold-pressed to obtain a compaction density of 3.4 g / cm³. 3 Positive electrode plate;

[0114] (4) Preparation of electrolyte: Lithium hexafluorophosphate was dissolved in a mixed solution of EC, DEC and EMC (1:1:1), and vinylene carbonate was added to obtain an electrolyte with a concentration of 1.2M.

[0115] (5) Preparation of lithium-ion battery: The negative electrode sheet, separator (polypropylene separator) and positive electrode sheet are stacked and wound in sequence to obtain a bare cell. The tabs are ultrasonically welded, the bare cell is placed in the battery case, dried to remove moisture, electrolyte is injected into the battery case, and packaged. After formation and capacity testing, a lithium-ion battery is obtained.

[0116] Comparative Example 5

[0117] This comparative example relates to the preparation of a negative electrode sheet and a lithium-ion battery. The only difference from Example 1 is the solid content of the second coating slurry, as detailed below:

[0118] First coating slurry: solid content of output is 54%;

[0119] Second coating slurry: The solid content of the output is 41%.

[0120] Apart from the differences mentioned above, all other operations are the same.

[0121] Comparative Example 6

[0122] This comparative example relates to the preparation of a negative electrode sheet and a lithium-ion battery. The only difference from Example 1 is that the mass ratio of each component in the first coating slurry and the second coating slurry is different, as detailed below:

[0123] First coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2.8:0.04:1.44, and the output solid content is 54%.

[0124] Second coating slurry: Weigh the active material (composed of fumed silicon carbide and artificial graphite, with the specific proportions calculated according to their respective capacities to form a composite material with a specific capacity of 650mAh / g), styrene-butadiene rubber, polyacrylic acid, single-walled carbon nanotubes, and conductive carbon black according to the mass ratio of 94.3:1.4:2. The output solid content is 48%.

[0125] Apart from the differences mentioned above, all other operations are the same.

[0126] Performance testing

[0127] The lithium-ion batteries prepared in the above embodiments and comparative examples were tested for DC internal resistance and rate performance. The specific procedures are as follows:

[0128] DC internal resistance test: At room temperature of 25℃, charge at 1C constant current to 4.25V, then charge at 4.25V constant voltage until the current decreases to 0.05C. Discharge at 1C for tmin (t=6, 24, 18min, adjusted to 90%, 50%, 20% SOC), let stand for 60min, then discharge at 2C constant current for 30s, charge at 2C constant current for 30s and adjust back to the original SOC. Repeat the discharge and charge to complete the HPPC discharge and charge at all SOCs.

[0129] Rate performance test: At room temperature (25℃), the battery was discharged at 1C constant current to 2.5V, charged at 1C constant current to 4.25V, and then charged at 4.25V constant voltage until the current decreased to 0.05C. The battery was then discharged at XC constant current to 2.5V (X is 0.33, 0.5, 1, 2, 4, 5). The discharge capacity ratio of the battery relative to 0.5C at different rates was recorded.

[0130] The parameters of the negative electrode sheets prepared in the above embodiments and comparative examples are as follows:

[0131] Table 1

[0132]

[0133] In Table 1, m1 is the weight of the first coating slurry applied to one side, and m2 is the weight of the second coating slurry applied to one side.

[0134] The test results for the DC internal resistance and rate performance are shown in Table 2 below:

[0135] Table 2

[0136]

[0137]

[0138] From Table 2 and Figure 1 It can be seen that the lithium-ion batteries prepared in Examples 1-4 all have low discharge DCR under different SOC conditions, proving that they have strong ion and electron conduction capabilities. In contrast, the discharge DCR of the comparative examples is relatively high, especially under low SOC conditions.

[0139] Compared to Example 1, the negative electrode sheet without a double-layer structure prepared in Comparative Example 4 exhibits the highest DCR in lithium-ion batteries assembled from it under 20% SOC conditions. Comparative Example 5, with an excessively high solid content ratio between the first and second coatings (X / Y value of 1.317, exceeding the maximum value of 1.25 within the preferred range), also shows a high DCR in its prepared lithium-ion battery, presumably due to a mismatch between conductivity and porosity in the second coating. Similarly, Comparative Example 6, with an excessively high ratio of the mass percentage of the first conductive agent in the second coating to that in the first coating (C2 / C1 value outside the preferred range of 2-4), also suffers from a high DCR in its prepared lithium-ion battery, presumably due to poor conductivity in the first coating leading to a high overall resistance of the negative electrode sheet.

[0140] Furthermore, Comparative Example 1 exhibited a higher DCR under various SOC conditions compared to Example 1. Although the design of the slurry solid content resulted in denser active particles near the current collector and looser active particles away from the current collector, leading to a higher porosity in the second coating after electrode drying compared to the first coating, thus increasing the contact area between the electrolyte and active particles and improving the ion transport rate, the high porosity of the second coating resulted in poorer conductivity compared to the first coating, leading to a higher battery DCR. The calculated C2 / C1 ratio was 1, which is outside the preferred range of 2-4. The relative mass ratio of the first and second coatings also affects the battery DCR. For example, negative electrode sheets with a relatively large (Comparative Example 2) or small (Comparative Example 3) mass resulted in batteries with higher DCRs under various SOC conditions compared to Example 1. This further demonstrates the importance of the mass ratio of the first and second coatings in balancing electron and ion conduction capabilities.

[0141] From Table 2 and Figure 2 It can be seen that the lithium-ion batteries prepared in Examples 1-4 have better rate performance, especially at high rates. They can still have more than 80% of the discharge capacity at a high rate of 5C. In contrast, the lithium-ion batteries prepared in the comparative example have more capacity loss when discharged at high rates.

[0142] Similar to the DC internal resistance test results above, the lithium-ion battery prepared in Comparative Example 4 had the worst rate discharge capability, especially under high rate conditions, where the capacity reduction was the greatest. While Comparative Example 5, with an excessively high ratio of solid content in the first coating to the second coating, and Comparative Example 6, with an excessively high ratio of the mass percentage of the first conductive agent in the second coating to the mass percentage of the first conductive agent in the first coating, or Comparative Example 1, with an excessively low ratio, had better rate performance than Comparative Example 4, they were still far lower than Example 1. This is because the mismatch between the conductivity and porosity of the negative electrode sheet led to a higher DCR in the battery, thus affecting the rate performance of the battery.

[0143] Furthermore, the lithium-ion batteries prepared in Comparative Examples 2 and 3 have a higher capacity at 5C than the other comparative examples, but significantly lower than that in Example 1. This also indicates that an excessively high or low mass ratio of the first coating to the second coating will affect the rate performance.

[0144] In summary, this invention designs a double-layer coated electrode structure and controls the solid content ratio of the first coating to the second coating, as well as the types of conductive agents in the coatings and the mass ratio of specific conductive agents in different coatings, to balance the conductivity and ion conduction performance of the battery. When the negative electrode meets the specific relationship 2≤C2 / C1≤4 and 1.05≤X / Y≤1.25, the prepared lithium-ion battery can have low DC internal resistance and significantly improved rate performance.

[0145] The embodiments described above are merely preferred examples to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode includes a current collector, a first coating disposed on at least one side of the current collector, and a second coating disposed on the first coating; both the first coating and the second coating include a negative electrode active material, a first binder, a second binder, a first conductive agent, and a second conductive agent; the negative electrode active material includes fumed silicon-carbon material; The first conductive agent is selected from one or more of single-walled carbon nanotubes, graphene, and graphyne, and the second conductive agent is conductive carbon black and / or multi-walled carbon nanotubes. The method for preparing the negative electrode sheet includes the following steps: (1) The negative electrode active material, the first binder, the second binder, the first conductive agent, the second conductive agent and water are stirred evenly to obtain the first coating slurry, and the solid content of the first coating slurry is X; (2) The first coating slurry prepared in step (1) is coated onto the current collector, and after drying, a first coating is formed. The mass ratio of the first conductive agent in the first coating is C1. (3) The negative electrode active material, the first binder, the second binder, the first conductive agent, the second conductive agent and water are stirred evenly to obtain the second coating slurry, and the solid content of the second coating slurry is Y; (4) The second coating slurry prepared in step (3) is coated on the first coating, and after drying, the second coating is formed to obtain the negative electrode sheet. The mass ratio of the first conductive agent in the second coating is C2. The negative electrode sheet is prepared in accordance with the following relationship: 2≤C2 / C1≤4, 1.05≤X / Y≤1.

25.

2. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material is composed of fumed silicon-carbon material and artificial graphite.

3. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material accounts for more than 92% of the mass in both the first and second coatings.

4. The negative electrode sheet according to claim 1, characterized in that, The first adhesive is one or more of polyacrylic acid, polyacrylonitrile, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose; The second adhesive is one or more of styrene-butadiene rubber and modified styrene-butadiene polymer.

5. The negative electrode sheet according to claim 4, characterized in that, The mass ratio of the first adhesive to the second adhesive in the first coating is 2-4:1; The mass ratio of the first adhesive to the second adhesive in the second coating is 1-3:

1.

6. The negative electrode sheet according to claim 1, characterized in that, In both the first and second coatings, the mass percentage of the first conductive agent and the second conductive agent is less than 2%. The mass ratio of the first conductive agent to the second conductive agent in the first coating is 0.025-0.075:1; The mass ratio of the first conductive agent to the second conductive agent in the second coating is 0.075-0.3:

1.

7. The negative electrode sheet according to claim 1, characterized in that, The solid content of the first coating slurry is 49%-56%; The solid content of the second coating slurry is 45%-52%.

8. The negative electrode sheet according to claim 1, characterized in that, The mass ratio of the first coating to the second coating is 1:0.5-2.

9. The negative electrode sheet according to claim 1, characterized in that, The negative electrode sheet has a single-sided coating weight of 80-120 mg and a compaction density of 1.5-1.7 g / cm³. 3 .

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the negative electrode sheet as described in any one of claims 1 to 9.

Images