A method for preparing a negative electrode sheet for a lithium ion battery and a lithium ion battery

Abstract

The application relates to a negative electrode sheet preparation method for a lithium ion battery and the lithium ion battery. The preparation method comprises the following steps: mixing a CMC-Li binder and a PAA binder to obtain a negative electrode glue solution; uniformly mixing the negative electrode glue solution with a negative electrode active material, a conductive agent, NMP, NH4HCO3 and deionized water, and then adding an SBR binder to prepare a negative electrode slurry with an output viscosity of 6500-9500 Pa.s; coating the negative electrode slurry on the surface of a negative electrode current collector, drying and rolling to obtain a negative electrode sheet. The scheme provided by the application can form a porous structure in the negative electrode sheet during the preparation process of the negative electrode sheet, and a porous electrode negative electrode sheet with high thickness is prepared, the tortuosity of the thick electrode sheet is reduced, the transmission rate of lithium ions in the negative electrode sheet is improved, the concentration polarization of the electrode sheet is reduced, and the rate performance and cycle performance of the battery cell are improved.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery preparation technology, and in particular to a method for preparing a negative electrode sheet for lithium-ion batteries and a lithium-ion battery. Background Technology

[0002] As the mainstay of modern energy storage, lithium-ion batteries have energy density as one of the core indicators for measuring battery performance, which is directly related to the battery's application potential in many fields.

[0003] Silicon-carbon materials, as a novel type of lithium-ion battery anode material, possess significant advantages. Silicon has a very high theoretical specific capacity, approximately 4200 mAh / g, far exceeding graphite's 372 mAh / g. This allows silicon-carbon anodes to significantly improve battery energy density, providing strong support for extending device lifespan and increasing the driving range of electric vehicles. However, silicon materials suffer from severe volume expansion during charge and discharge, reaching 300%-400%. Such drastic volume changes easily lead to electrode material pulverization and detachment, thereby affecting battery cycle life and stability.

[0004] Applying silicon-carbon materials to prepare thick negative electrode sheets can accommodate more active materials, significantly improving the energy density of the battery. However, as the electrode thickness increases, the transport distance of lithium ions inside the electrode becomes longer, leading to a decrease in ion transport efficiency and making the battery more susceptible to polarization, which affects the charge-discharge performance and cycle life of the battery. Summary of the Invention

[0005] To address or partially address the problems existing in related technologies, this application provides a method for preparing a negative electrode sheet for lithium-ion batteries and a lithium-ion battery. This method can form a porous structure on the surface of the electrode sheet during the preparation process, thereby creating a porous thick electrode negative electrode sheet. This reduces the tortuosity of the thick electrode negative electrode sheet, increases the conduction rate of lithium ions in the negative electrode sheet, reduces the concentration polarization of the electrode sheet, and improves the rate performance and cycle performance of the battery cell.

[0006] The first aspect of this application provides a method for preparing a negative electrode sheet for a lithium-ion battery, comprising the following steps:

[0007] 1) The negative electrode adhesive is obtained by mixing CMC-Li binder and PAA binder;

[0008] 2) After uniformly mixing the negative electrode adhesive with the negative electrode active material, conductive agent, NMP, NH4HCO3 and deionized water, add SBR binder to prepare a negative electrode slurry with an output viscosity of 6500~9500 Pa·s;

[0009] 3) The negative electrode slurry is coated onto the surface of the negative electrode current collector, dried, and rolled to obtain a negative electrode sheet; the NH4HCO3 decomposes to generate gas during the drying step, which promotes the formation of a porous structure in the negative electrode sheet.

[0010] In some embodiments of this application, step 2) includes:

[0011] 2a) Take a portion of the negative electrode solution from step 1) as a primary mixed negative electrode solution, mix the primary mixed negative electrode solution with the negative electrode active material, conductive agent and deionized water, stir for 90~120min at a speed of 100~300rpm to obtain a primary conductive solution.

[0012] 2b) Take a portion of the negative electrode solution from step 1) as a secondary mixed negative electrode solution, add the secondary mixed negative electrode solution to the primary conductive solution, stir for 60~80 minutes at a speed of 1800~2000 rpm to obtain the secondary conductive solution;

[0013] 2c) Take a portion of the negative electrode adhesive from step 1) as the three-stage mixed negative electrode adhesive, add the three-stage mixed negative electrode adhesive, deionized water, NMP and NH4HCO3 to the secondary conductive adhesive, stir for 60~90min at a speed of 1800~2100rpm; after mixing evenly, add SBR binder to prepare a negative electrode slurry with an output viscosity of 6500~9500Pa·s.

[0014] In some embodiments of this application, the volume ratio of the primary mixed negative electrode solution, the secondary mixed negative electrode solution, and the tertiary mixed negative electrode solution is (1.5~2.5):1:(1.5~2.5).

[0015] In some embodiments of this application, the total amount of the binder is 2.5 to 3.5 wt% of the sum of the masses of the negative electrode adhesive, the negative electrode active material, and the conductive agent.

[0016] The amount of NH4HCO3 used is 0.6~1.2wt% of the sum of the mass of the negative electrode solution, the negative electrode active material, and the conductive agent.

[0017] In some embodiments of this application, the ratio of the total amount of the adhesive to the amount of NH4HCO3 is (2.5~4):1.

[0018] In some embodiments of this application, the negative electrode active material is selected from a composite material of graphite and silicon carbide, and the mass ratio of graphite to silicon carbide is (90~94):(3~7).

[0019] In some embodiments of this application, the conductive agent is selected from at least one of conductive carbon black, acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers, and superconducting carbon.

[0020] In some embodiments of this application, the negative current collector is selected from copper foil with a conductive carbon black base coating, and the thickness of the conductive carbon black base coating is 0.5~1μm.

[0021] In some embodiments of this application, in step 1), a negative electrode adhesive is obtained by mixing a CMC-Li binder with a solid content of 1 to 1.5 wt% and a PAA binder with a solid content of 3 to 8 wt%, and the mass ratio of the CMC-Li binder to the PAA binder is 1:(2 to 5).

[0022] In some embodiments of this application, the drying temperature in step 3) is 95~110℃.

[0023] In some embodiments of this application, the thickness of the negative electrode active material layer on the negative electrode sheet is 50~70μm.

[0024] The second aspect of this application provides a negative electrode sheet, which is prepared by the above-described method.

[0025] A third aspect of this application provides a lithium-ion battery, including a negative electrode sheet prepared by the above-described preparation method.

[0026] The fourth aspect of this application provides an electrical device or various energy storage systems that use batteries as energy storage elements, including the aforementioned lithium-ion batteries.

[0027] The technical solution provided in this application may include the following beneficial effects:

[0028] By introducing a pore-forming agent NH4HCO3, which is free of other magnetic impurities and pollution, during the preparation of the negative electrode sheet, it decomposes upon heating during the electrode drying process to form CO2, H2O, and NH3, thereby creating a porous structure in the electrode sheet. After rolling, a porous thick electrode negative electrode sheet is formed, which reduces the tortuosity of the negative electrode sheet, shortens the migration distance of lithium ions, improves the conduction rate of lithium ions in the negative electrode sheet, reduces the concentration polarization of the electrode sheet, and improves the rate performance and cycle performance of the thick electrode cell.

[0029] The technical solution of this application also includes: by strictly controlling the mixing order between the adhesive and the negative electrode active material and conductive agent during the preparation of the negative electrode sheet, it is beneficial for the adhesive to better wet the main material, increase the bonding force between the negative electrode active material, conductive agent and other particles and between the particles and the negative electrode current collector, which helps to slow down the pulverization of negative electrode particles and the peeling between negative electrode particles and negative electrode current collector caused by repeated volume shrinkage and expansion of silicon-carbon negative electrode, delay the aging of lithium-ion battery negative electrode sheets and extend the service life of the cell.

[0030] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Attached Figure Description

[0031] The above and other objects, features and advantages of this application will become more apparent from the following description of exemplary embodiments in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of this application.

[0032] Figure 1 This is a schematic diagram of the process of preparing the negative electrode sheet in Example 1 of this application. Detailed Implementation

[0033] To facilitate understanding of the present invention, it will be described in detail below. However, before describing the present invention in detail, it should be understood that the present invention is not limited to the specific embodiments described. It should also be understood that the terminology used herein is for describing specific embodiments only and is not intended to be restrictive.

[0034] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The singular forms “a,” “the,” and “the” used in this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0035] Where numerical ranges are provided, it should be understood that every intermediate value between the upper and lower limits of the range and any other specified or intermediate value within the specified range is covered within the present invention. The upper and lower limits of these smaller ranges may be independently included in the smaller range and are also covered within the present invention, subject to any explicitly excluded limits within the specified range. Where a specified range includes one or two limits, the range excluding any or both of those included limits is also included within the present invention. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0036] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. While any methods and materials, or equivalents thereof, may be used in the practice or testing of this invention, preferred methods and materials are now described.

[0037] Applying silicon-carbon materials to prepare thick negative electrode sheets can accommodate more active materials, significantly improving the energy density of the battery. However, as the electrode thickness increases, the transport distance of lithium ions inside the electrode becomes longer, leading to a decrease in ion transport efficiency and making the battery more susceptible to polarization, which affects the charge-discharge performance and cycle life of the battery.

[0038] To address the aforementioned problems, this application provides a method for preparing a negative electrode sheet for lithium-ion batteries, a negative electrode sheet, and a lithium-ion battery. This method enables the formation of a porous structure on the electrode surface during the negative electrode sheet preparation process, resulting in a porous thick-electrode negative electrode sheet. This reduces the tortuosity of the thick electrode, increases the lithium-ion transport rate within the negative electrode sheet, reduces concentration polarization, and improves the rate performance and cycle performance of the battery cell.

[0039] The method for preparing a negative electrode sheet for lithium-ion batteries according to the embodiments of this application includes the following steps:

[0040] 1) The negative electrode adhesive is obtained by mixing CMC-Li binder and PAA binder;

[0041] 2) After uniformly mixing the negative electrode adhesive with the negative electrode active material, conductive agent, NMP, NH4HCO3 and deionized water, add SBR binder to prepare a negative electrode slurry with an output viscosity of 6500~9500 Pa·s.

[0042] 3) The negative electrode slurry is coated onto the surface of the negative electrode current collector, dried, and rolled to obtain the negative electrode sheet; NH4HCO3 decomposes to produce gas during the drying step, which promotes the formation of a porous structure in the negative electrode sheet.

[0043] This application's embodiments introduce a non-magnetic, non-polluting pore-forming agent, NH4HCO3, during the preparation of the negative electrode slurry. During the electrode drying process, NH4HCO3 decomposes upon heating, forming CO2, H2O, and NH3, which are then discharged, creating a porous structure within the electrode. After rolling, this forms a porous thick-electrode negative electrode. This not only allows for the fabrication of a thicker negative electrode capable of accommodating more active material but also reduces electrode tortuosity, shortens the lithium-ion migration distance, increases the lithium-ion conduction rate within the electrode, reduces concentration polarization, and improves the rate performance and cycle life of the thick-electrode battery cell. Furthermore, the pore-forming agent, ammonium bicarbonate, decomposes at a low temperature, reaching CO2, H2O, and NH3 at 60°C. No additional heating step is required during the thick electrode preparation process; the decomposition is complete during the baking process after coating, without introducing other impurity ions and thus not affecting the battery cell's performance. The preparation method is simple, easy to operate, and low-cost, making it suitable for industrial applications.

[0044] Moreover, the above-mentioned negative electrode preparation process is simple and easy to operate, and the introduced pore-forming agent is low in cost, making it suitable for large-scale production and possessing extremely high industrial production application value.

[0045] In some embodiments, the discharge viscosity of the negative electrode slurry is preferably 8000~9500 Pa·s, more preferably 8500~9000 Pa·s. Optimizing the discharge viscosity of the negative electrode slurry enables it to spread evenly on the surface of the negative electrode current collector during subsequent coating processes, ensuring uniform distribution of the negative electrode active material and conductive agent on the current collector surface. This ensures smooth and uniform lithium-ion insertion and extraction, improving battery capacity and cycle life. Simultaneously, it allows for uniform distribution of the binder, increasing the adhesion between negative electrode particles (such as the negative electrode active material and conductive agent) and between the negative electrode particles and the surface of the negative electrode current collector. This ensures the negative electrode particles are firmly attached to the current collector surface, preventing powder shedding and increased battery internal resistance, thus improving battery cycle performance.

[0046] Furthermore, optimizing the discharge viscosity of the negative electrode slurry allows for uniform dispersion of the pore-forming agent within the slurry, ensuring its even presence in the negative electrode active material layer and preventing precipitation or aggregation. After drying, the pore-forming agent decomposes under heat, forming uniformly distributed and regularly structured pores on the electrode surface. These pores provide channels for electrolyte storage and lithium-ion transport, improving the battery's ionic conductivity and rate performance. If the viscosity is too low, the pore-forming agent may precipitate or aggregate in the slurry, resulting in uneven distribution and causing the negative electrode active material layer to easily collapse, failing to effectively form pores, or forming pores that are too small to meet the requirements of electrolyte and lithium-ion transport. If the viscosity is too high, it hinders the drying and compaction process of the electrode, and the pore-forming agent also cannot be uniformly dispersed, resulting in uneven distribution and irregular pore structures. This leads to uneven local electrolyte distribution, affecting the insertion and extraction of lithium ions in the electrode, reducing the battery's charge / discharge efficiency and cycle life.

[0047] In some implementations, step 2) above may specifically include:

[0048] 2a) Take a portion of the negative electrode solution from step 1) as a primary mixed negative electrode solution, mix the primary mixed negative electrode solution with the negative electrode active material, conductive agent and deionized water, stir for 90~120min at a speed of 100~300rpm to obtain a primary conductive solution.

[0049] 2b) Take a portion of the negative electrode solution from step 1) as a secondary mixed negative electrode solution, add the secondary mixed negative electrode solution to the primary conductive solution, stir for 60~80min at a speed of 1800~2000rpm to obtain the secondary conductive solution;

[0050] 2c) Take a portion of the negative electrode solution from step 1) as the three-stage mixed negative electrode solution, add the three-stage mixed negative electrode solution, deionized water, NMP and NH4HCO3 to the secondary conductive solution, stir for 60~90min at a speed of 1800~2100rpm; after mixing evenly, add SBR binder to prepare a negative electrode slurry with an output viscosity of 6500~9500Pa·s.

[0051] In this embodiment, PAA and CMC-Li are mixed and stirred in step 1) to prepare a negative electrode adhesive. CMC-Li acts as a dispersant for PAA, which helps PAA to be better dispersed in the dispersant. Then, the adhesive is mixed with the main material in batches (i.e., first mixing of negative electrode adhesive, second mixing of negative electrode adhesive, and third mixing of negative electrode adhesive). This helps the adhesive to better wet the main material, reduce the surface tension of the PAA binder, and increase the adhesion between negative electrode particles and between negative electrode particles and current collector. This helps to slow down the pulverization of negative electrode particles and the peeling from the current collector caused by repeated volume shrinkage and expansion of silicon-carbon negative electrode, thereby delaying the aging of the battery negative electrode sheet and extending the service life of the battery cell.

[0052] Further, in step 1), a negative electrode adhesive is obtained by mixing a CMC-Li binder with a solid content of 1~1.5 wt% and a PAA binder with a solid content of 3~8 wt%, and the mass ratio of the CMC-Li binder to the PAA binder is 1:(2~5); preferably 2:7. Specifically, the solid content of the CMC-Li binder can be 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, etc., or any value within the above range; preferably 1.2%. The solid content of the PAA binder can be 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, etc., or any value within the above range; preferably 6 wt%.

[0053] CMC-Li binders with a solid content of 1-1.5 wt% can be prepared using water as a solvent. PAA binders with a solid content of 3-8 wt% can be prepared using water as a solvent.

[0054] In some embodiments, the preferred mass ratio of CMC-Li adhesive, PAA adhesive, and SBR adhesive is 2:7:6.

[0055] In some embodiments, the volume ratio of the primary mixed negative electrode solution, the secondary mixed negative electrode solution, and the tertiary mixed negative electrode solution is (1.5~2.5):1:(1.5~2.5); preferably 2:1:2.

[0056] In this embodiment, by adjusting the mixing temperature, stirring speed, and order of adhesive addition, the materials can be mixed more evenly, allowing the pore-forming agent to be evenly dispersed in the slurry and forming a stable high bonding force between the negative electrode particles. When the negative electrode is made into an electrode sheet, it can effectively improve the service life of the negative electrode sheet.

[0057] In some embodiments, the total amount of binder is 2.5 to 3.5 wt% of the sum of the masses of the negative electrode adhesive, the negative electrode active material, and the conductive agent. Specifically, the total amount of binder can be 2.5%, 2.8%, 3%, 3.2%, 3.5%, or any value within the above range, of the sum of the masses of the negative electrode adhesive, the negative electrode active material, and the conductive agent.

[0058] In some embodiments, the amount of NH4HCO3 used is 0.6 to 1.2 wt% of the sum of the masses of the negative electrode adhesive, the negative electrode active material, and the conductive agent. Specifically, the amount of NH4HCO3 used is 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, etc., or any value within the above range, of the sum of the masses of the negative electrode adhesive, the negative electrode active material, and the conductive agent.

[0059] Furthermore, the ratio of the total amount of adhesive to the amount of NH4HCO3 is (2.5~4):1. Specifically, the ratio of the total amount of adhesive to the amount of NH4HCO3 is 2.5:1, 2.8:1, 3:1, 3.2:1, 3.5:1, 3.8:1, 4:1, etc., or any value within the above range.

[0060] In this embodiment, by optimizing the dosage of binder and pore-forming agent, the binder can better wet the main material, increasing the adhesion between negative electrode particles and between particles and the negative electrode current collector, thereby slowing down particle pulverization or peeling from the current collector, delaying the aging of the negative electrode sheet, and extending the service life of the battery cell. Furthermore, it allows the pore-forming agent to be uniformly distributed in the negative electrode slurry, enabling rapid decomposition of the pore-forming agent during the drying process, forming uniform and structurally consistent pores on the electrode surface, thus improving the battery's dynamic performance.

[0061] In some embodiments, the negative electrode active material is selected from a composite material of graphite and silicon carbide, wherein the mass ratio of graphite to silicon carbide is (90~94):(3~7). Further, the silicon carbide material has a silicon content of 20~80wt% and a carbon content of 80~20wt%.

[0062] In some embodiments, the conductive agent is selected from at least one of conductive carbon black (SP), acetylene black (ACET), Ketjen black, carbon dots (CDs), carbon nanotubes (CNT), graphene (GPE), carbon nanofibers (CNF), and superconducting carbon; preferably conductive carbon black.

[0063] In some embodiments, the negative current collector is selected from copper foil with a conductive carbon black primer, and the thickness of the conductive carbon black primer is 0.5~1μm.

[0064] In some embodiments, the areal density of the negative electrode slurry coated on the surface of the negative electrode current collector is 90~120 g / m². 2Specifically, the areal density of the negative electrode slurry coating on the surface of the negative electrode current collector is 90 g / m³. 2 95g / m 2 100g / m 2 105g / m 2 110g / m 2 115g / m 2 120g / m 2 etc., or any value within the above range.

[0065] In some embodiments, the thickness of the negative electrode sheet is 50~70μm. Specifically, the thickness of the thick electrode is 50μm, 55μm, 60μm, 65μm, 70μm, etc., or any value within the above range.

[0066] In some embodiments, the mass percentage of deionized water in the negative electrode slurry is 4-10 wt%; preferably 5 wt%.

[0067] In the negative electrode slurry applicable to the embodiments of this application, the components negative electrode graphite, negative electrode silicon carbide, adhesive, SBR, NMP, CNT, and NH4HCO3 are mixed in a mass ratio of 91:5:1.8:1.2:0.5:1:1 to prepare the negative electrode slurry.

[0068] In some embodiments, the temperature is controlled within the range of 20~35°C during the preparation of the negative electrode sheet.

[0069] The negative electrode sheet involved in the embodiments of this application is prepared by the above method.

[0070] The lithium-ion battery involved in the embodiments of this application includes the above-described negative electrode sheet or the negative electrode sheet prepared by the above method.

[0071] Lithium-ion batteries also include electrolyte, separator, and positive electrode. The negative electrode, separator, and positive electrode are sequentially stacked and then used to form a cell through winding or stacking processes. The cell is then placed into a pre-formed aluminum-plastic film, baked, and then the electrolyte is injected into the dried cell, immersing it in the electrolyte. Following vacuum sealing, settling, and formation processes, the lithium-ion battery is complete.

[0072] The electrolyte, separator, and positive electrode can all be made from any known materials, and this application does not limit them.

[0073] In some embodiments, the positive electrode includes a positive current collector and a positive electrode material layer coated on the surface of the positive current collector. The positive current collector is a conventional metal foil or a composite current collector, such as aluminum foil.

[0074] In some embodiments, the positive electrode material layer is formed by coating the surface of the positive electrode current collector with a positive electrode slurry.

[0075] In some embodiments, the positive electrode slurry includes a positive electrode active material, a conductive agent, and a binder. The positive electrode active material includes, but is not limited to, one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, and ternary LiNixCoyMnzO2 materials (where x+y+z=1, x≥y). The conductive agent includes, but is not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The binder includes, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins.

[0076] In some embodiments, the diaphragm described in this application can be arbitrarily selected from known porous diaphragms with good chemical and mechanical stability. The diaphragm material can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The diaphragm can be a single-layer film or a multi-layer composite film, without particular limitation. When the diaphragm is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0077] In some embodiments, the electrolyte described in this application may be selected from electrolytes comprising a solvent, additives, and a lithium salt. The lithium salt may be selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorosulfonyl imide, lithium difluoromethyl imide, lithium difluorooxalate phosphate, and lithium perchlorate. The concentration of the lithium salt in the electrolyte is 0.9 mol / L to 1.3 mol / L. The solvent may be selected from one or more of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, and dimethyl carbonate, wherein the carboxylic acid ester solvent is selected from one or more of ethyl propionate, ethyl acetate, and propyl propionate. The additive may be selected from one or more of ethylene carbonate, vinylene carbonate, ethylene ethylene carbonate, methylene vinyl carbonate, fluoroethylene carbonate, trifluoromethyl vinyl carbonate, and difluoroethylene carbonate. The components can be freely combined without particular limitation.

[0078] This application also provides embodiments of an electrical device or various energy storage systems that use batteries as energy storage elements. Electrical devices include, but are not limited to, mobile phones, tablets, computers, electric toys, power tools, electric vehicles, electric cars, ships, and spacecraft.

[0079] To make the present invention easier to understand, the present application will be further described in detail below with reference to embodiments. These embodiments are for illustrative purposes only and are not limited to the scope of application of the present application. Unless otherwise specified, the raw materials or components used in the present application can be obtained commercially or by conventional methods.

[0080] Example 1

[0081] (1) Preparation of electrolyte

[0082] Propylene carbonate (PC), propyl propionate (PP), ethyl propionate (EP), and ethylene carbonate (EC) were mixed in a mass ratio of 1:1:1:1. Based on the total mass of the electrolyte, 12.6% of fluoroethylene carbonate was added. After mixing thoroughly, LiPF6 was added to obtain an electrolyte with a LiPF6 concentration of 1.1 mol / L.

[0083] (2) Preparation of positive electrode sheet

[0084] Lithium nickel cobalt manganese oxide (LiNi) is used as the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2), conductive agent CNT (Carbon Nanotube), and binder PVDF (polyvinylidene fluoride) are mixed in a mass ratio of 95:1.5:1.5 and thoroughly stirred in N-methylpyrrolidone solvent to form a uniform positive electrode slurry. The positive electrode slurry is then uniformly coated onto the positive electrode current collector Al foil, and then dried, rolled, and compacted to obtain the positive electrode sheet.

[0085] (3) Preparation of negative electrode sheet

[0086] I. Preparation of negative electrode solution

[0087] a. Using water as a solvent, prepare a CMC-Li binder with a solid content of 1.2% and a PAA binder with a solid content of 6%.

[0088] b. Prepare a mixed adhesive solution by mixing CMC-Li adhesive with 1.2% solid content and PAA adhesive with 6% solid content at a mass ratio of 2:7, stirring for 30 minutes and vacuum degassing for 120 minutes.

[0089] II. Preparation of negative electrode slurry

[0090] a. Mix the negative electrode main material graphite and silicon carbon at a mass ratio of 91:5, stir for 5 minutes, with a rotation speed of 200 rpm and a revolution speed of 20 rpm, and control the temperature at 32℃.

[0091] b. Based on step (a), add 2 / 5 of the adhesive, CNT, and 1 / 5 of the water to the mixing tank. Mix for 100 minutes, with a rotation speed of 200 rpm and a revolution speed of 20 rpm. Control the temperature at 32°C and scrape the material from the mixing paddle.

[0092] c. Based on step (b), add 1 / 5 more adhesive solution, stir for 80 minutes, rotate at 1800 rpm, revolve at 20 rpm, and control the temperature at 32℃.

[0093] d. Based on step (c), add the remaining 2 / 5 of the adhesive, 4 / 5 of the water, NMP, and NH4HCO3. Stir for 80 minutes, with a rotation speed of 2000 rpm and a revolution speed of 20 rpm. Control the temperature at 32℃. Test the viscosity after stirring.

[0094] e. Based on step (d), add SBR binder and adjust the viscosity by adding water according to the measured viscosity to obtain a negative electrode slurry with an output viscosity in the range of 6500 Pa·s.

[0095] The negative electrode graphite, negative electrode silicon carbide, adhesive, SBR, NMP, CNT, and NH4HCO3 are added in a mass ratio of 91:5:1.8:1.2:0.5:1:1, and deionized water accounts for 5% of the mass ratio of the negative electrode slurry.

[0096] III. Preparation of negative electrode sheet

[0097] The negative electrode slurry obtained in step (2) is coated onto a copper foil with a 0.5 μm thick SP undercoat to form an areal density of 90 g / m². 2 The negative electrode sheet is dried in an oven at 100℃, then rolled and slit to obtain a thick negative electrode with a thickness of 65μm. The electrode preparation process is as follows: Figure 1 As shown.

[0098] (4) Preparation of lithium-ion batteries

[0099] PE porous polymer film is used as the separator.

[0100] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The stacked electrodes and separator are then wound together to form a core. The bare core is placed in a pre-formed aluminum-plastic film, and the electrolyte prepared above is injected into the baked and dried core. After vacuum sealing, settling, and formation processes, the lithium-ion battery is successfully manufactured.

[0101] Example 2

[0102] The difference from Example 1 is that the discharge viscosity of the negative electrode slurry is 8000 Pa·s.

[0103] Example 3

[0104] The difference from Example 1 is that the discharge viscosity of the negative electrode slurry is 9500 Pa·s.

[0105] Example 4

[0106] The difference from Example 1 is that negative electrode graphite, negative electrode silicon carbide, adhesive, SBR, NMP, CNT, and NH4HCO3 are added in a mass ratio of 91:5:1.8:1.2:0.5:1:10.

[0107] Example 5

[0108] The difference from Example 1 is that negative electrode graphite, negative electrode silicon carbide, adhesive, SBR, NMP, CNT, and NH4HCO3 are added in a mass ratio of 91:5:1.8:1.2:0.5:1:0.5.

[0109] Example 6

[0110] The difference from Example 1 is that negative electrode graphite, negative electrode silicon carbide, adhesive, SBR, NMP, CNT, and NH4HCO3 are added in a mass ratio of 92:5:0.8:0.2:0.5:2:1.

[0111] Comparative Example 1

[0112] The difference from Example 1 is that NH4HCO3 is not added in the preparation of the negative electrode slurry.

[0113] Comparative Example 2

[0114] The difference from Example 1 is that the discharge viscosity of the negative electrode slurry is 6000 Pa·s.

[0115] Comparative Example 3

[0116] The difference from Example 1 is that the discharge viscosity of the negative electrode slurry is 10000 Pa·s.

[0117] Comparative Example 4

[0118] The difference from Example 1 is that it includes the following steps:

[0119] CMC-Li binder, PAA binder, SBR binder, graphite, silicon carbide, CNT, NMP, NH4HCO3 and deionized water were mixed together in a certain proportion to prepare a negative electrode slurry with an output viscosity of 6500 Pa·s.

[0120] The negative electrode slurry is coated onto the surface of the negative electrode current collector, dried at 100°C, and rolled to obtain a thick electrode.

[0121] Lithium-ion battery performance testing:

[0122] The 25℃ cycle test involved charging the lithium-ion batteries to 4.45V at a constant current and voltage of 1C in a (25±2)℃ constant temperature chamber, with a cutoff current of 0.05C, followed by discharging to 3V at 1C. Multiple charge-discharge cycles were performed under these conditions. The capacity retention rate after 800 cycles was calculated, and the average capacity retention rate of 5 batteries in each group after the 25℃ cycle was recorded in Table 1.

[0123] Capacity retention (%) = Discharge capacity at 800th cycle (mAh) / Discharge capacity at 3rd cycle (mAh) × 100%

[0124] (2) Storage test at 25℃

[0125] The lithium-ion battery was charged to 4.5V at 1C constant current and constant voltage in a constant temperature chamber at (25±2)℃, and the cutoff current was 0.05C. The cell thickness H1 was measured. The cell was then stored in a constant temperature chamber at (25±2)℃ for 36 hours. The cell was then taken out in a high temperature oven and the thickness H2 was measured. Five cells were used in each group. The thickness expansion rate was calculated and the average value was recorded in Table 1.

[0126] High-temperature storage thickness expansion rate (%) = (H2-H1) / H1*100%

[0127] Table 1

[0128]

[0129] Data from Example 1 and Comparative Example 1 show that adding pore-forming agent NH4HCO3 during the preparation of the negative electrode slurry for the thick negative electrode sheet allows the gas generated by its thermal decomposition to form pores on the negative electrode sheet, thus creating a porous negative electrode sheet. This facilitates contact between the electrolyte and the electrode sheet, promoting lithium ion embedding within the pores. Compared to non-porous electrodes, this significantly reduces the tortuosity of the electrode sheet, shortens the lithium ion migration distance, and improves the battery's kinetic performance. Simultaneously, it alleviates the volume expansion and contraction of the silicon negative electrode during charge and discharge, delaying electrode aging. Furthermore, after the electrolyte enters the pores, a solid SEI film forms on the surface of the negative electrode particles at the pores during the formation stage. The formation of the SEI film at the pores prevents further reaction between the electrolyte and the negative electrode active material, thereby improving the cell's cycle performance and lifespan.

[0130] Data from Examples 1-3, Comparative Examples 2 and 3 show that the viscosity of the negative electrode slurry is closely related to the pore-forming effect of the pore-forming agent NH4HCO3 on the electrode. If the viscosity of the negative electrode slurry is too low, the pore-forming agent may precipitate or aggregate in the slurry, resulting in uneven distribution. This can lead to the collapse of the negative electrode active material layer, preventing effective pore formation or resulting in pores that are too small to meet the requirements of electrolyte and lithium-ion transport. Consequently, capacity retention cannot be maintained, and the ability to mitigate the volume expansion and contraction of the silicon negative electrode is poor, affecting the battery cell's lifespan. Conversely, if the viscosity of the negative electrode slurry is too high, it hinders the drying and compaction process of the electrode, and the pore-forming agent cannot be evenly dispersed, also resulting in uneven distribution. This leads to irregular pore structures, causing uneven local electrolyte distribution, affecting the insertion and extraction of lithium ions in the electrode, and reducing the battery's charge / discharge efficiency and cycle life.

[0131] Therefore, limiting the discharge viscosity of the negative electrode slurry to within 6500~9500 Pa·s is beneficial to the formation of porous negative electrode sheets, which can improve the lithium-ion transport rate, slow down the pulverization of negative electrode particles caused by the volume change of silicon negative electrode and the peeling between them and the current collector, thereby delaying the aging of the battery negative electrode sheets and improving the cycle performance and service life of the battery cell.

[0132] Data from Example 1 and Comparative Example 4 show that adding the negative electrode adhesive in batches helps the adhesive better wet the main material, increases the bonding force between particles such as negative electrode active material and conductive agent, and between particles and negative electrode current collector, helps to slow down the pulverization of negative electrode particles caused by repeated volume shrinkage and expansion of silicon-carbon negative electrode, as well as the peeling between negative electrode particles and negative electrode current collector, delays the aging of lithium-ion battery negative electrode sheets, and extends the service life of the cell.

[0133] Data from Examples 1, 4, and 5 show that if the amount of pore-forming agent NH4HCO3 is too high, it tends to settle in the negative electrode slurry and cannot be uniformly dispersed on the surface of the negative electrode sheet to form a uniform porous negative electrode sheet. This is detrimental to maintaining the cycle performance of the battery cell and also hinders the mitigation of electrode aging caused by the volume expansion and contraction of the silicon negative electrode. Conversely, if the amount of pore-forming agent NH4HCO3 is too low, it also fails to form sufficient pores on the surface of the negative electrode sheet, which is detrimental to maintaining the cycle performance of the battery cell and also hinders the mitigation of electrode aging caused by the volume expansion and contraction of the silicon negative electrode.

[0134] Data from Examples 1 and 6 show that if the amount of binder is too low, it will be difficult to strongly bond the negative electrode particles together and between the negative electrode particles and the negative electrode current collector. This is not conducive to slowing down the pulverization of the negative electrode particles and the peeling off from the current collector caused by the repeated volume contraction and expansion of the silicon-carbon negative electrode, thus accelerating the aging of the battery negative electrode sheet and deteriorating the battery cell lifespan.

[0135] It should be noted that the embodiments described above are only for explaining this application and do not constitute any limitation on this application. This application has been described with reference to typical embodiments, but it should be understood that the terms used therein are descriptive and explanatory terms, not limiting terms. Modifications and revisions can be made to this application within the scope of the claims as prescribed, and without departing from the scope and spirit of this application. Although the application described herein relates to specific methods, materials, and embodiments, it does not mean that this application is limited to the specific examples disclosed herein; on the contrary, this application can be extended to all other methods and applications with the same function.

Claims

1. A method for preparing a negative electrode sheet for lithium-ion batteries, characterized in that, Includes the following steps: 1) A negative electrode solution is obtained by mixing CMC-Li binder and PAA binder; including mixing CMC-Li binder with a solid content of 1~1.5wt% and PAA binder with a solid content of 3~8wt% to obtain a negative electrode solution, and the mass ratio of CMC-Li binder to PAA binder is 1:(2~5). 2) After uniformly mixing the negative electrode adhesive with the negative electrode active material, conductive agent, NMP, NH4HCO3 and deionized water, add SBR binder to prepare a negative electrode slurry with an output viscosity of 6500~9500 Pa·s; 3) The negative electrode slurry is coated onto the surface of the negative electrode current collector, dried, and rolled to obtain a negative electrode sheet; the NH4HCO3 decomposes to generate gas during the drying step, which promotes the formation of a porous structure in the negative electrode sheet; The negative electrode active material is selected from a composite material of graphite and silicon carbon, and the mass ratio of graphite to silicon carbon is (90~94):(3~7); the negative electrode current collector is selected from copper foil with conductive carbon black base coating, and the thickness of the conductive carbon black base coating is 0.5~1μm; the thickness of the negative electrode active material layer on the negative electrode sheet is 50~70μm.

2. The preparation method according to claim 1, characterized in that, Step 2) includes: 2a) Take a portion of the negative electrode solution from step 1) as a primary mixed negative electrode solution, mix the primary mixed negative electrode solution with the negative electrode active material, conductive agent and deionized water, stir for 90~120min at a speed of 100~300rpm to obtain a primary conductive solution. 2b) Take a portion of the negative electrode solution from step 1) as a secondary mixed negative electrode solution, add the secondary mixed negative electrode solution to the primary conductive solution, stir for 60~80 minutes at a speed of 1800~2000 rpm to obtain the secondary conductive solution; 2c) Take a portion of the negative electrode adhesive from step 1) as the three-stage mixed negative electrode adhesive, add the three-stage mixed negative electrode adhesive, deionized water, NMP and NH4HCO3 to the secondary conductive adhesive, stir for 60~90min at a speed of 1800~2100rpm; after mixing evenly, add SBR binder to prepare a negative electrode slurry with an output viscosity of 6500~9500Pa·s.

3. The preparation method according to claim 2, characterized in that, The volume ratio of the primary mixed negative electrode solution, the secondary mixed negative electrode solution, and the tertiary mixed negative electrode solution is (1.5~2.5):1:(1.5~2.5).

4. The preparation method according to claim 1, characterized in that, The total amount of the binder is 2.5~3.5 wt% of the sum of the mass of the negative electrode adhesive, the negative electrode active material, and the conductive agent. The amount of NH4HCO3 used is 0.6~1.2wt% of the sum of the mass of the negative electrode solution, the negative electrode active material, and the conductive agent.

5. The preparation method according to claim 4, characterized in that, The ratio of the total amount of the binder to the amount of NH4HCO3 is (2.5~4):

1.

6. The preparation method according to claim 1, characterized in that, The conductive agent is selected from at least one of conductive carbon black, acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers, and superconducting carbon.

7. The preparation method according to claim 1, characterized in that, The drying temperature in step 3) is 95~110℃.

8. A lithium-ion battery, comprising a negative electrode sheet prepared by any one of the preparation methods described in claims 1 to 7.

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