Battery cell, method of manufacturing the same, and battery

Abstract

The application relates to the technical field of battery preparation, and discloses a battery cell, a preparation method thereof and a battery. The disclosed battery cell comprises a negative electrode sheet, a diaphragm and a positive electrode sheet which are sequentially stacked; a transition layer is arranged between the negative electrode sheet and the diaphragm; the material of the transition layer is PVDF or PMMA; the thickness of the transition layer is a; the thickness area of the transition layer close to the negative electrode sheet (10%-30%) a is a compact area, the porosity of which is 10%-30%; the middle area of the transition layer is a transition area, the porosity of which is 30%-50%; the remaining thickness area of the transition layer is a high-porosity area, the porosity of which is 50%-75%; and there is no abrupt interface between the areas. The disclosed battery cell realizes the synergistic promotion of side reaction inhibition, low impedance and high cycle stability due to the arrangement of the transition layer, and the cycle life and safety of the battery are improved. The battery provided by the application is suitable for electric vehicles, energy storage systems or consumer electronic devices.

Description

Technical Field

[0001] This invention relates to the field of battery manufacturing technology, and more specifically, to battery cells, their manufacturing methods, and batteries. Background Technology

[0002] In the processes of coating, baking, die-cutting, handling, and winding / stacking, lithium-ion battery negative electrode sheets commonly suffer from problems such as surface scratches by dust, active material powder shedding, and particle residue leading to internal defects in the cell. These issues can easily cause lithium plating, increased internal resistance, and decreased cycle life, thereby reducing battery yield and safety.

[0003] Current industry practices include: negative pressure dust collection, ion air cleaning, roller surface cleaning; PVDF coating on the positive electrode edge / diaphragm; increasing the proportion of binder in the negative electrode slurry or adding PVDF as a binder, etc.

[0004] The above solution has obvious flaws: 1. Physical dust removal only removes surface dust and cannot form an effective physical protective layer on the negative electrode surface; 2. The PVDF coating on the diaphragm or positive electrode side is mostly used for insulation, wetting, or bonding, and is not designed for protection in the graphite negative electrode process; 3. Adding a binder to the slurry only achieves internal bonding and cannot form an independent surface protective structure.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a battery cell and its manufacturing method.

[0007] This invention is implemented as follows: In a first aspect, embodiments of the present invention provide a battery cell, comprising a negative electrode sheet, a separator and a positive electrode sheet stacked sequentially; A transition layer is disposed between the negative electrode and the separator. The transition layer is made of at least one of PVDF and PMMA. The thickness of the transition layer is 'a'. The thicker region of the transition layer near the negative electrode (10%~30%) is the dense region, with a porosity of 10%~30% and an average pore size of 10~30 nm. The middle region of the transition layer (40%~60%) is the transition region, with a porosity of 30%~50% and an average pore size of 30~50 nm. The remaining thickness of the transition layer is the high-porosity region, with a porosity of 50%~75% and an average pore size of 50~150 nm. The average pore size of the high-porosity region is greater than that of the transition region, and the average pore size of the transition region is greater than that of the dense region. There are no abrupt interfaces between the dense region, transition region, and high-porosity region; a ranges from 1 to 7 μm.

[0008] In an optional embodiment, the active material of the surface active layer of the negative electrode sheet is one or more of artificial graphite, natural graphite, silicon-based negative electrode, hard carbon, and soft carbon.

[0009] In an optional embodiment, α is 1.8~6μm, the thickness of the dense region is 0.4~1.5μm, the porosity is 10~25%, and the average pore size is 10~22nm; the thickness of the transition region is 0.8~3μm, the porosity is 31~46%, and the average pore size is 32~48nm; the thickness of the high-porosity region is 0.6~1.5μm, the porosity is 53~70%, and the average pore size is 52~100nm.

[0010] Secondly, embodiments of the present invention provide a method for preparing the aforementioned battery cell, comprising: A first pre-pressing material is provided, the first pre-pressing material includes a negative electrode sheet and a first protective layer bonded to the surface of the negative electrode sheet, the first protective layer has a thickness of 0.5~4μm, a porosity of 10%~35%, and an average pore size of 10~35nm; A second pre-compression material is provided, the second pre-compression material comprising a diaphragm and a second protective layer bonded to the surface of the diaphragm, the second protective layer having a thickness of 1~4μm, a porosity of 52%~77%, and an average pore size of 55~155nm; The materials forming the first and second protective layers are protective materials, and the protective materials are selected from at least one of PVDF and PMMA. Following the method of first protective layer to second protective layer, the first pre-pressed material, the second pre-pressed material and the positive electrode sheet are sequentially arranged for winding or stacking, and hot-pressed at 80~140℃ and 0.5~2MPa for 1~10min, so that the molecular chains of the first protective layer and the second protective layer diffuse and entangle with each other, and the two layers fuse to form a transition layer with a dense region, a transition region and a high porosity region.

[0011] In an optional embodiment, the thickness of the first protective layer is 1~3μm, the porosity is 12~26%, and the average pore size is 11~25nm; the thickness of the second protective layer is 1.5~4μm, the porosity is 55~73%, and the average pore size is 60~140nm.

[0012] In an optional embodiment, the method for preparing the first pre-compression material includes: A negative electrode sheet is prepared in advance, and a first protective liquid containing protective material is coated on the surface of the negative electrode sheet to form a wet film. Then, it is baked and cured at 60~140℃ to evaporate the solvent, and finally rolled at room temperature. Alternatively, an aqueous or oil-based negative electrode slurry can be coated onto the surface of the negative electrode current collector, followed by a first protective liquid containing a protective material, and then baked at 60~140℃ to cure and allow the solvent to evaporate.

[0013] In an optional embodiment, the concentration of the protective material in the first protective liquid is 5-20 wt%; optionally, the solvent in the first protective liquid is selected from at least one of DMF, NMP, THF and ethyl acetate.

[0014] In an optional implementation, the second protective layer is formed on the membrane surface by means of: A second protective liquid containing protective material and pore-forming agent is coated on the surface of the diaphragm to obtain an intermediate product. The intermediate product is then placed in a coagulation bath to solidify the protective material and form a porous structure. The coagulation bath is a mixture of water and alcohol with a volume ratio of 1:1~30 and a temperature of 5~40℃. Optionally, the alcohol is methanol.

[0015] In an optional embodiment, the concentration of the protective material in the second protective liquid is 5-20 wt%, the mass ratio of the pore-forming agent to the protective material is 2-10:100, and the pore-forming agent is selected from at least one of polyethylene glycol and polyvinylpyrrolidone. In an optional embodiment, the solvent in the second protective liquid is selected from at least one of DMF, NMP, THF, and ethyl acetate.

[0016] Thirdly, embodiments of the present invention provide a battery, including a battery cell provided in embodiments of the present invention, or a battery cell prepared by a preparation method provided in embodiments of the present invention.

[0017] The present invention has the following beneficial effects: The battery cell provided in this invention features a transition layer with a dense region near the negative electrode, characterized by low porosity and average pore size, used to protect and suppress negative electrode side reactions. On the separator side, it is a highly porous region with high porosity and large average pore size, used for ion transport and liquid retention. The middle transition region, with porosity and average pore size between the dense and highly porous regions, serves as a transition zone. There are no abrupt interfaces between the dense, transition, and highly porous regions; the transition from the dense to the highly porous region is natural. Compared to the abrupt interfaces in traditional multilayer structures, this transition layer in this invention provides a smoother ion transport path, resulting in increased ionic conductivity and significantly reduced interface impedance. Therefore, the battery cell provided by this invention, due to the transition layer, achieves a synergistic improvement in side reaction suppression, low impedance, and high cycle stability. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure formed by the negative electrode sheet, transition layer, and separator in the battery cell provided in an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of the negative electrode, separator, and positive electrode stacked in Example 1 before hot pressing. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0021] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0022] An embodiment of the present invention provides a battery cell comprising a negative electrode sheet, a separator and a positive electrode sheet stacked sequentially; like Figure 1 As shown, a transition layer is disposed between the negative electrode and the separator. The transition layer is made of at least one of PVDF and PMMA, and its thickness is 'a'. The thicker region of the transition layer near the negative electrode (10%~30%) is the dense region, with a porosity of 10%~30% and an average pore size of 10~30 nm. The middle region of the transition layer (40%~60%) is the transition region, with a porosity of 30%~50% and an average pore size of 30~50 nm. The remaining thick region of the transition layer is the high-porosity region, with a porosity of 50%~75% and an average pore size of 50~150 nm. The average pore size of the high-porosity region is greater than that of the transition region, and the average pore size of the transition region is greater than that of the dense region. There are no abrupt interfaces between the dense region, transition region, and high-porosity region; a ranges from 1 to 7 μm.

[0023] The battery cell provided in this invention features a transition layer with a dense region near the negative electrode, characterized by low porosity and average pore size, used to protect and suppress negative electrode side reactions. On the separator side, it is a highly porous region with high porosity and large average pore size, used for ion transport and liquid retention. The middle transition region, with porosity and average pore size between the dense and highly porous regions, serves as a transition zone. There are no abrupt interfaces between the dense, transition, and highly porous regions; the transition from the dense to the highly porous region is natural. Compared to the abrupt interfaces in traditional multilayer structures, this transition layer in this invention provides a smoother ion transport path, resulting in increased ionic conductivity and significantly reduced interface impedance. Therefore, the battery cell provided by this invention, due to the transition layer, achieves a synergistic improvement in side reaction suppression, low impedance, and high cycle stability.

[0024] It should be noted that the thickness of the transition layer should be within the range required by this invention. If the thickness is too low, it will not be able to fully cover the surface of the negative electrode, resulting in insufficient protection and easy to cause side reactions and mechanical damage, thereby reducing the cycle life and safety of the battery. If the thickness is too high, it will increase the volume of the transition layer, resulting in an excessively long transport path between the negative electrode and the separator, thereby affecting the diffusion efficiency of ions, increasing the internal resistance of the battery, and also affecting the cycle life of the battery.

[0025] Optionally, in some preferred embodiments of the present invention, a is 1.8~6μm, the thickness of the dense region is 0.4~1.5μm, the porosity is 10~25%, and the average pore size is 10~22nm; the thickness of the transition region is 0.8~3μm, the porosity is 31~46%, and the average pore size is 32~48nm; the thickness of the high-porosity region is 0.6~1.5μm, the porosity is 53~70%, and the average pore size is 52~100nm.

[0026] Optionally, the active material of the surface active layer of the negative electrode sheet is one or more of artificial graphite, natural graphite, silicon-based negative electrode, hard carbon, and soft carbon.

[0027] The method for preparing a battery cell provided in this embodiment of the present invention includes: A first pre-pressing material is provided, comprising a negative electrode sheet and a first protective layer of a thickness bonded to the surface of the negative electrode sheet, the first protective layer having a thickness of 0.5~4μm, a porosity of 10%~35%, and an average pore size of 10~35nm; A second pre-compression material is provided, the second pre-compression material comprising a diaphragm and a second protective layer bonded to the surface of the diaphragm, the second protective layer having a thickness of 1~4μm, a porosity of 52%~77%, and an average pore size of 55~155nm; The materials forming the first and second protective layers are protective materials, and the protective materials are selected from at least one of PVDF (polyvinylidene fluoride) and PMMA (polymethyl methacrylate); Following the method of first protective layer facing second protective layer, the first pre-pressed material, the second pre-pressed material and the positive electrode sheet are wound or stacked, and hot-pressed at 80~140℃ and 0.5~2MPa for 1~10min, so that the molecular chains of the first protective layer and the second protective layer diffuse and entangle with each other, and the two layers fuse to form a transition layer.

[0028] The preparation method provided in this invention involves assembling a negative electrode sheet with a first protective layer having a small average pore size and low porosity and a separator with a second protective layer having a large average pore size and high porosity, and then hot-pressing them together. During the hot-pressing process, the molecular chains of the first and second protective layers diffuse and entangle with each other, forming a transition layer with a gradient pore structure and no abrupt interface. The average pore size of this transition layer changes continuously in the thickness direction, and the transition from the dense region to the high-porosity region is natural, achieving a synergistic improvement in side reaction suppression, low impedance, and high cycle stability.

[0029] Furthermore, the preparation method provided by this invention enhances the physical protection of the negative electrode surface by forming a first PVDF protective layer on the surface of the negative electrode, effectively isolating friction and dust impact, thus significantly reducing the scratch rate of the negative electrode and significantly improving the process yield. This method only requires adding a spraying and baking unit to the existing coating and baking line, and the separator coating can use existing coating equipment. Since no major equipment modification is required, it can be directly applied to water-based, water-based or oil-based negative electrode mass production lines, making it highly practical.

[0030] Specifically, the preparation method is as follows: S1. Preparation of the first pre-compression material There are two methods for preparing the first pre-compression material: (1) Provide a negative electrode slurry. The negative electrode slurry is the negative electrode slurry used in the preparation of conventional negative electrode sheets. The negative electrode slurry can be an aqueous or oil-based slurry (i.e., the solvent is NMP), or it can be an aqueous slurry (i.e., the solvent is water). The negative electrode slurry is coated onto the surface of the negative electrode current collector and dried to obtain the negative electrode sheet.

[0031] A first protective liquid containing protective material is applied to the surface of the negative electrode to form a wet film, which is then baked at 60~140℃ to cure the solvent and evaporate. Then, the solution is applied at a concentration of 1.3~1.8 g / cm³. 3 Compact the material by rolling at room temperature to obtain the first pre-compressed material.

[0032] The protective material is selected from at least one of PVDF and PMMA.

[0033] (2) Provide negative electrode slurry, which is the negative electrode slurry used in the preparation of conventional negative electrode sheets. The negative electrode slurry is an oil-based slurry (i.e., the solvent is NMP). The negative electrode slurry is coated onto the surface of the negative electrode current collector. Then, a first protective liquid containing a protective material is coated, followed by baking and curing at 60~140℃ to allow the solvent to evaporate.

[0034] That is, when the negative electrode slurry is an oil-based slurry, the first protective liquid can be applied directly after the negative electrode slurry is coated without drying, and then dried after the first protective liquid is applied.

[0035] The thickness of the first protective layer is 0.5~4μm, the porosity is 10%~30%, and the average pore size is 10~30nm. Preferably, the thickness of the first protective layer is 1~3μm, the porosity is 12~26%, and the average pore size is 11~25nm.

[0036] Alternatively, the coating method of the negative electrode slurry is no different from the manufacturing method used in the existing negative electrode sheet manufacturing process.

[0037] Optionally, the first protective layer can be applied by atomized spraying, slot extrusion, transfer coating, etc.

[0038] Optionally, the concentration of the protective material in the first protective solution is 5-20 wt%. Optionally, the solvent in the first protective solution is selected from at least one of DMF, NMP, THF, and ethyl acetate.

[0039] S2. Preparation of the second pre-compression material A second protective liquid containing protective material and pore-forming agent is applied to the surface of the diaphragm to obtain an intermediate product containing a wet coating. The intermediate product is then placed in a coagulation bath to allow the protective material to solidify and form a porous structure. The coagulation bath is a mixture of water and alcohol in a volume ratio of 1:1 to 30, the temperature is 5 to 40°C, and the treatment time in the coagulation bath is 5 to 15 minutes.

[0040] During the phase transition, the solvent (such as NMP, DMF, or DMAc) in the coating solution continuously diffuses outward, while the non-solvent (water + methanol / ethanol) in the coagulation bath penetrates in the reverse, creating a strong solvent / non-solvent exchange driving force. As the solvent is extracted and the non-solvent enters, the PVDF / PMMA polymer chains rapidly transition from a homogeneous state to a two-phase separated state due to the abrupt change in environmental properties. The polymer-enriched phase gradually solidifies to form a continuous framework, while the non-solvent-enriched phase forms the subsequent pore spaces. The pore-forming agent added to the coating system will be dissolved or removed during the phase transition and subsequent water washing, thereby forming additional pores and interconnected channels within the PVDF / PMMA framework.

[0041] The thickness of the second protective layer is 1~4 μm, the porosity is 50%~75%, and the average pore size is 50~150 nm. Preferably, the thickness of the second protective layer is 1.5~4 μm, the porosity is 55~73%, and the average pore size is 60~140 nm.

[0042] Alternatively, a conventional diaphragm can be used, such as a polyolefin diaphragm, or more specifically, a PP (polypropylene) diaphragm.

[0043] Alternatively, the second protective layer can be formed by spraying, electrospinning, stenciling, roller coating, etc.

[0044] Optionally, in some embodiments of the present invention, the alcohol is methanol. The reason for choosing methanol is its good non-solvent properties and rapid solvent exchange capability.

[0045] Optionally, in some embodiments of the present invention, the concentration of the protective material in the second protective liquid is 5-20 wt%, the mass ratio of the pore-forming agent to the protective material is 2-10:100, preferably 3-9:100, and the pore-forming agent is selected from at least one of polyethylene glycol and polyvinylpyrrolidone.

[0046] When the concentration of the protective material, the concentration of the pore-forming agent, and the specific substances of the pore-forming agent in the second protective liquid are within the above range, it can be ensured that the porosity and average pore size of the formed second protective layer are within a more suitable range, thereby producing a battery cell with better performance.

[0047] Optionally, the solvent in the second protective solution is selected from at least one of DMF, NMP, THF, and ethyl acetate.

[0048] S3, Cell Assembly and Hot-Pressure Fusion The first pre-pressed material, the second pre-pressed material, and the positive electrode sheet are sequentially wound or stacked. The positive electrode sheet used is a conventional positive electrode sheet.

[0049] Hot pressing at 80~140℃ and 0.5~2MPa for 1~10min causes the molecular chains of the first protective layer and the second protective layer to diffuse and entangle with each other, and the two layers fuse to form a transition layer.

[0050] It should be noted that the hot-pressing temperature, pressure, and time must be within the range specified in this invention. If the temperature is too high, the protective layer will soften excessively, flow, or even degrade and carbonize, resulting in the absence of dense areas, transition areas, or highly porous areas; the micropores of the diaphragm base membrane may thermally close, become blocked, or even shrink, leading to obstructed ion transport, increased internal resistance, and impact on rate and cycle performance. In severe cases, the diaphragm may melt, shrink, or rupture, even causing short circuits and other safety risks; it may also cause the electrode to crack and lose charge due to thermal stress, resulting in black spots on the battery cell. If the temperature is too low, the protective layer will not soften sufficiently, and the first and second protective layers may not fuse effectively, or a transition area may not be formed. This can easily lead to delamination during subsequent processes or applications, increased impedance, longer transport paths, and impact on rate and cycle performance. Excessive hot-pressing pressure can cause the diaphragm micropores to collapse and close, reducing permeability and hindering ion transport, increasing internal resistance, and affecting rate capability and cycling performance. Over-pressing of the electrode can lead to active material detachment or even electrode breakage. The protective layer may be excessively extruded, or even absent, resulting in adhesion failure, delamination, and a longer ion transport path. Insufficient hot-pressing pressure can lead to incomplete fusion, ineffective formation of a transition layer (partially or entirely), affecting ion transport efficiency, increasing interfacial impedance and internal resistance, potentially causing lithium plating and impacting cycling performance. Excessive hot-pressing time can cause aging and degradation of the protective layer, breakage of protective layer branches, and accelerated deformation, pore closure, and embrittlement of the diaphragm and electrode due to thermal stress. It can also lengthen cycle time and reduce efficiency. Insufficient hot-pressing time can result in inadequate heat transfer, insufficient softening of the protective layer, and insufficient or non-existent interfacial bonding strength, all of which affect ion transport efficiency.

[0051] Then, the battery cell is obtained according to the conventional battery cell manufacturing process, such as hot pressing and fusion followed by adhesive bonding and fixing → welding of electrode tabs → insertion of aluminum-plastic film → edge sealing → baking → liquid injection and wetting → formation → sealing and degassing → shaping and capacity testing, to obtain the battery cell.

[0052] The present invention provides a battery comprising a battery cell provided in the present invention, or a battery cell prepared by the preparation method provided in the present invention.

[0053] Example 1 A negative electrode slurry is provided, which has a solid content of 50% and is water as the solvent. It includes graphite, SBR, CMC and conductive agent (SP) in a mass ratio of 94:2:2:2. The negative electrode slurry was coated onto the negative electrode current collector (copper foil), coated, and baked to obtain an average areal density of 16 mg / cm³. 2 The negative electrode plate.

[0054] The first protective liquid was continuously coated on both sides of the negative electrode sheet at a walking speed of 40 m / min and then baked and cured at 110℃ to form a first protective layer on the surface of the negative electrode sheet, at a concentration of 1.4 g / cm³. 3After compaction and cold pressing, the material is rolled to obtain the first pre-compressed material. The first protective liquid is a solution of PVDF dissolved in NMP, wherein the concentration of PVDF is 12%. The thickness of the first protective layer is 2 μm, the porosity is 19%, and the average pore size range is 18 nm.

[0055] The second protective liquid was coated onto one side of a PP diaphragm to obtain an intermediate product. This intermediate product was then placed in a coagulation bath at 20°C (water to methanol volume ratio 1:5) for 1 minute to form a second protective layer on the diaphragm surface, resulting in the second pre-compression material. The second protective liquid was a solution of PVDF and polyvinylpyrrolidone (PVP-K30) dissolved in NMP, with PVDF concentration of 12% wt and a PVP-K30 to PVDF mass ratio of 6:100. The second protective layer had a thickness of 2 μm, a porosity of 65%, and an average pore size range of 100 nm.

[0056] Provides an average areal density of 30 mg / cm³ 2 The positive electrode sheet has an aluminum foil current collector, and its surface active layer includes lithium iron phosphate, PVDF, and conductive agent (SP) in a mass ratio of 96:2:2.

[0057] The first pre-pressed material, the second pre-pressed material, and the positive electrode sheet are stacked sequentially, resulting in the following structure: Figure 2 As shown. Hot pressing at 90℃ and 0.5MPa for 2 minutes causes the molecular chains of the first and second protective layers to diffuse and entangle, fusing the two layers to form a transition layer. At this point, the structure is as follows: Figure 1 As shown.

[0058] The remaining steps of the conventional battery cell manufacturing process are followed to produce the battery cell. Specifically, the electrolyte composition during the electrolyte injection process is: lithium hexafluorophosphate (LiPF6) 13% wt, ethylene carbonate (EC) 25% wt, ethyl methyl carbonate (EMC) 59% wt, and vinylene carbonate (VC) 3% wt.

[0059] The thickness of the transition layer in the fabricated cell is 3 μm, the thickness of the dense region is 0.7 μm, the porosity is 18%, and the average pore size is 15 nm; the thickness of the transition region is 1.6 μm, the porosity is 34%, and the average pore size is 40 nm; the thickness of the high-porosity region is 0.7 μm, the porosity is 60%, and the average pore size is 80 nm.

[0060] The specific methods for detecting the thickness of the high-porosity region, transition region, and dense region are as follows: using the cryogenic fracture method (liquid nitrogen brittle fracture), the sample is rapidly frozen and fractured with liquid nitrogen to expose the interface of the current collector-negative electrode dressing-transition layer-diaphragm base membrane. The cross-section is observed using FESEM and the interface thickness of the high-porosity region, transition region, and dense region is measured. The average pore size of each region is measured by combining the FESEM cross-section with ImageJ image statistical method. The porosity of each region is obtained by calculating the pore area ratio through grayscale binarization of the cross-sectional electron microscope image.

[0061] Example 2 A negative electrode slurry is provided, which has a solid content of 50% and is water as the solvent. It includes graphite, SBR, CMC and conductive agent (SP) in a mass ratio of 94:2:2:2. The negative electrode slurry was coated onto the negative electrode current collector (copper foil), coated, and baked to obtain an average areal density of 16 mg / cm³. 2 The negative electrode plate.

[0062] The first protective liquid was continuously coated on both sides of the negative electrode sheet at a walking speed of 50 m / min and then baked and cured at 120°C to form a first protective layer on the surface of the negative electrode sheet, at a concentration of 1.4 g / cm³. 3 After compaction and cold pressing, the material is rolled to obtain the first pre-compressed material. The first protective liquid is a solution of PVDF dissolved in NMP, wherein the concentration of PVDF is 5%wt. The thickness of the first protective layer is 1μm, the porosity is 26%, and the average pore size range is 25nm.

[0063] The second protective liquid was coated onto one side of a PP membrane to obtain an intermediate product. This intermediate product was then placed in a coagulation bath at 10°C (water and methanol volume ratio 1:1) for 1 minute to form a second protective layer on the membrane surface, resulting in the second pre-compression material. The second protective liquid was a solution of PVDF and a pore-forming agent dissolved in NMP, with PVDF concentration at 18% wt and the pore-forming agent (polyvinylpyrrolidone PVP-K90 + polyethylene glycol PEG-4000 = 1:1) to PVDF mass ratio at 9:100. The second protective layer had a thickness of 1.5 μm, a porosity of 73%, and an average pore size range of 140 nm.

[0064] A positive electrode is provided, the current collector of the positive electrode is aluminum foil, and the active layer on its surface includes lithium iron phosphate, PVDF and conductive agent (SP) in a mass ratio of 96:2:2.

[0065] The first pre-pressed material, the second pre-pressed material, and the positive electrode sheet are stacked sequentially. Hot-pressing at 90℃ and 0.5MPa for 2 minutes causes the molecular chains of the first and second protective layers to diffuse and entangle, fusing the two layers to form a transition layer.

[0066] The remaining steps of the conventional battery cell manufacturing process are followed to produce the battery cell. Specifically, the electrolyte composition during the electrolyte injection process is: lithium hexafluorophosphate (LiPF6) 13%wt, ethylene carbonate (EC) 25%wt, ethyl methyl carbonate (EMC) 59%wt, and vinylene carbonate (VC) 3%wt.

[0067] The thickness of the transition layer in the fabricated cell is 1.8 μm, the thickness of the dense region is 0.4 μm, the porosity is 25%, and the average pore size is 22 nm; the thickness of the transition region is 0.8 μm, the porosity is 46%, and the average pore size is 48 nm; the thickness of the high-porosity region is 0.6 μm, the porosity is 70%, and the average pore size is 100 nm.

[0068] Example 3 A negative electrode slurry is provided, which has a solid content of 50% and is water as the solvent. It includes graphite, SBR, CMC and conductive agent (SP) in a mass ratio of 94:2:2:2. The negative electrode slurry was coated onto the negative electrode current collector (copper foil), coated, and baked to obtain an average areal density of 16 mg / cm³. 2 The negative electrode plate.

[0069] The first protective liquid was continuously coated on both sides of the negative electrode sheet at a walking speed of 40 m / min and then baked and cured at 130℃ to form a first protective layer on the surface of the negative electrode sheet, at a concentration of 1.4 g / cm³. 3 After compaction and cold pressing, the material is rolled to obtain the first pre-compressed material. The first protective liquid is a solution of PVDF dissolved in NMP, wherein the concentration of PVDF is 20%wt. The thickness of the first protective layer is 3μm, the porosity is 12%, and the average pore size range is 11nm.

[0070] The second protective solution was coated onto one side of a PP diaphragm to obtain an intermediate product. This intermediate product was then placed in a coagulation bath at 25°C (water to methanol volume ratio 1:15) for 1.5 minutes to form a second protective layer on the diaphragm surface, resulting in the second pre-compression material. The second protective solution was a solution of PVDF and polyethylene glycol PEG-4000 dissolved in NMP, with PVDF concentration at 12% wt and a PEG-4000 to PVDF mass ratio of 3:100. The second protective layer had a thickness of 4 μm, a porosity of 55%, and an average pore size range of 60 nm.

[0071] A positive electrode is provided, the current collector of the positive electrode is aluminum foil, and the active layer on its surface includes lithium iron phosphate, PVDF and conductive agent (SP) in a mass ratio of 96:2:2.

[0072] The first pre-pressed material, the second pre-pressed material, and the positive electrode sheet are stacked sequentially. Hot-pressing at 90℃ and 0.5MPa for 2 minutes causes the molecular chains of the first and second protective layers to diffuse and entangle, fusing the two layers to form a transition layer.

[0073] The remaining steps of the conventional battery cell manufacturing process are followed to produce the battery cell. Specifically, the electrolyte composition during the electrolyte injection process is: lithium hexafluorophosphate (LiPF6) 13% wt, ethylene carbonate (EC) 25% wt, ethyl methyl carbonate (EMC) 59% wt, and vinylene carbonate (VC) 3% wt.

[0074] The thickness of the transition layer in the fabricated cell is 6 μm, the thickness of the dense region is 1.5 μm, the porosity is 10%, and the average pore size is 10 nm; the thickness of the transition region is 3 μm, the porosity is 31%, and the average pore size is 32 nm; the thickness of the high-porosity region is 1.5 μm, the porosity is 53%, and the average pore size is 52 nm.

[0075] Comparative Example 1 This comparative example is basically the same as Example 1, except that the first protective liquid is replaced with the second protective liquid.

[0076] The transition layer in the fabricated battery cell does not have a structure that transitions between dense regions, transition regions, and high-pore regions.

[0077] Comparative Example 2 This comparative example is basically the same as Example 1, except that the second protective liquid is replaced with the first protective liquid.

[0078] The transition layer in the fabricated battery cell does not have a structure that transitions between dense regions, transition regions, and high-pore regions.

[0079] Comparative Example 3 This comparative example is basically the same as Example 1, except that: after stacking the first pre-pressing material, the second pre-pressing material and the positive electrode sheet in sequence, hot pressing is not performed.

[0080] Comparative Example 4 This comparative example is basically the same as Example 1, except that: The thickness of the first protective layer is 3 μm; No second protective layer is formed on the membrane surface; After stacking the first pre-pressed material, the separator, and the positive electrode in sequence, hot pressing is not performed.

[0081] Comparative Example 5 This comparative example is basically the same as Example 1, except that: The second protective layer formed has a thickness of 3 μm; No first protective layer is formed on the surface of the negative electrode; After stacking the negative electrode, the second pre-pressed material, and the positive electrode in sequence, hot pressing is not performed.

[0082] Comparative Example 6 This comparative example is basically the same as Example 1, except that: The hot pressing temperature is too low, at 60℃.

[0083] The first and second protective layers are not bonded together after hot pressing and are directly separated, with no obvious transition zone formed in the middle of the thickness direction, so a transition layer cannot be formed.

[0084] Comparative Example 7 This comparative example is basically the same as Example 1, except that: The hot pressing temperature is too high, at 170℃.

[0085] The thickness of the transition layer was 1.5 μm. According to the test, the thickness was 0.6 μm near the negative electrode side, the porosity was 12% and the average pore size was 16 nm; the thickness in the middle was 0.3 μm, the porosity was 18% and the average pore size was 27 nm; and the thickness near the membrane side was 0.6 μm, the porosity was 31% and the average pore size was 38 nm.

[0086] The transition layer formed in this comparative example has a significantly lower overall porosity and pore size compared to the transition layer in Example 1.

[0087] Comparative Example 8 This comparative example is basically the same as Example 1, except that: No pressure was applied during the hot pressing process.

[0088] The first and second protective layers are not bonded together after hot pressing and are directly separated, with no obvious transition zone formed in the middle of the thickness direction, so a transition layer cannot be formed.

[0089] Comparative Example 9 This comparative example is basically the same as Example 1, except that: The hot pressing pressure is too high, at 3 MPa.

[0090] The thickness of the transition layer was 2 μm. According to the test, the thickness was 0.7 μm near the negative electrode side, the porosity was 18% and the average pore size was 15 nm; the thickness in the middle was 0.6 μm, the porosity was 29% and the average pore size was 31 nm; and the thickness near the membrane side was 0.7 μm, the porosity was 42% and the average pore size was 47 nm.

[0091] The transition layer formed in this comparative example has a significantly lower overall porosity and pore size compared to the transition layer in Example 1.

[0092] Comparative Example 10 This comparative example is basically the same as Example 1, except that: No first protective layer was formed on the surface of the negative electrode, no second protective layer was formed on the surface of the separator, and hot pressing was not performed.

[0093] That is, this comparison example provides a conventional battery cell.

[0094] Experimental Example The process defect rate and electrochemical performance of the battery cells in each embodiment were measured. The specific test methods are as follows: Negative electrode scratch defect rate: The number of scratches on the surface of the negative electrode is counted by visual inspection and confirmation by 30x magnification of the image instrument. The scratch defect rate is calculated by dividing the weight of the scratched defective electrode by the theoretical weight of the produced electrode (excluding edge material).

[0095] 0℃ charge transfer impedance Rct: After the battery cell is fully charged, it is placed in a constant temperature environment at 0℃. The AC impedance spectrum (EIS) is measured using an electrochemical workstation. The charge transfer impedance Rct at 0℃ is obtained by fitting the equivalent circuit, with the unit being mΩ.

[0096] Capacity retention rate after 500 cycles at 45℃: The capacity retention rate is calculated by dividing the discharge capacity at the 500th cycle by the initial discharge capacity after 500 cycles in a constant temperature chamber at 45℃ using the standard charge and discharge regime for the battery cell.

[0097] Capacity recovery rate at room temperature after 30 cycles at 0℃: The cell is first cycled at 0℃ for 30 cycles, and then the discharge capacity is tested at 25℃ according to the standard charge and discharge regime of the cell. The capacity recovery rate is calculated by dividing the room temperature recovery discharge capacity by the initial room temperature discharge capacity.

[0098] Gas production after 14 days of storage at 60℃: The fully charged cells were stored in a 60℃ constant temperature chamber for 14 days, and the gas production of the cells (volume change before and after storage) was determined by the water displacement method, in mL.

[0099] Record the test results in Table 1.

[0100] Table 1 Electrochemical performance of each embodiment and comparative example

[0101] As can be seen from Table 1, the battery cells prepared in each embodiment of the present invention have significantly better negative electrode sheet process yield and electrochemical performance compared with Comparative Example 10 (conventional battery cell). Comparing Comparative Example 1 with Example 1, the PVDF layer formed between the separator and the negative electrode in Comparative Example 1 has a large average pore size and high porosity near the negative electrode, which results in a lack of protection for the negative electrode during the manufacturing process. In addition, polyvinylpyrrolidone will remain in the cell, which will cause the cell to bulge due to gas production, increase polarization, increase impedance, deteriorate cycle performance, and increase safety risks.

[0102] Comparing Comparative Example 2 with Example 1, the PVDF layer formed between the separator and the negative electrode in Comparative Example 2 has a small average pore size and low porosity near the separator side, which leads to poor ion transport, thereby increasing the impedance of the cell, deteriorating the cycle performance, and making it prone to lithium plating and deterioration during low-temperature cycling. Comparing Comparative Example 3 with Example 1, since Comparative Example 3 does not undergo hot pressing, there is no transition between the first protective layer and the second protective layer, and there is a clear interface. Although it can play a role in protecting the negative electrode sheet during the process, it leads to a longer ion transport path, poor transport, and increased charge transfer impedance, resulting in poor cell cycle. Comparing Comparative Example 4 with Example 1, Comparative Example 4, due to the absence of the second protective layer, makes it difficult for the first protective layer on the surface of the negative electrode to form a uniform and effective bond with the diaphragm, which leads to the obstruction of lithium ion diffusion at the interface and an increase in charge transfer impedance, resulting in poor cell cycle. Comparing Comparative Example 5 with Example 1, Comparative Example 5 has a missing first protective layer in its negative electrode sheet, resulting in a high rate of scratch defects during the manufacturing process. The scratch defects in the negative electrode sheet are prone to lithium deposition when charging at low temperatures. Furthermore, the separator and the negative electrode sheet cannot form a uniform and effective bond or a transition layer, which leads to a longer lithium-ion transport path, poor transport, and increased charge transfer impedance, resulting in poor cell cycle performance. Comparing Comparative Example 6 with Example 1, the hot pressing temperature was too low, which prevented the formation of a uniform and effective bond between the separator and the negative electrode sheet and the formation of a transition layer. This resulted in a longer lithium-ion transport path, poor transport, and increased charge transfer impedance, which in turn led to poor cell cycle performance. Comparing Comparative Example 7 with Example 1, the hot-pressing temperature was too high. After hot-pressing at 170°C, the diaphragm of the bare cell shrank and deformed significantly, wrinkled, and some edges shrank and exposed the electrode sheets. In some areas, the positive and negative electrode sheets could not be effectively isolated, resulting in a high risk of short circuit. The porosity and pore size of the transition layer between the negative electrode sheet and the diaphragm were severely compressed, which hindered the diffusion of lithium ions at the interface and increased the charge transfer impedance, resulting in poor cell cycle performance. Comparing Comparative Example 8 with Example 1, no pressure was applied during the hot pressing process, which prevented the formation of a uniform and effective bond between the separator and the negative electrode sheet, thus hindering the formation of a transition layer. This resulted in a longer lithium-ion transport path, poor transport, and increased charge transfer impedance, leading to poor cell cycle performance. Comparing Comparative Example 9 with Example 1, excessive hot pressing pressure severely compresses the porosity and pore size of the transition layer between the electrode, diaphragm, negative electrode and diaphragm, resulting in hindered diffusion of lithium ions at the interface, increased charge transfer impedance, and thus cell cycle deterioration and increased risk of internal micro-short circuit.

[0103] In summary, the battery cell and its preparation method provided in the embodiments of the present invention have the following characteristics: 1. Significantly reduced electrode scratches and powder shedding. A dense PVDF protective layer is formed on the surface of the negative electrode. As this layer provides physical protection, it effectively isolates friction and dust impact, which greatly reduces the scratch rate of the electrode and significantly improves the process yield.

[0104] 2. Increased ionic conductivity and significantly reduced interfacial impedance. This scheme forms a gradient pore structure by hot-pressing and fusing the dense negative electrode layer and the high-porosity membrane layer. Since the continuous change of the average pore size of the gradient eliminates the interface abruptness in the traditional double-layer structure, the ion transport path is smoother, and the ion conductivity increases instead of decreasing, while the interface impedance decreases significantly.

[0105] 3. Reduced high-temperature cycling expansion rate, extended lifespan. The dense layer near the negative electrode can effectively block the side reactions caused by the electrolyte, suppress the negative electrode side reactions and gas generation, reduce the excessive growth of SEI, significantly reduce the high-temperature cycling expansion rate of the cell, and significantly improve the cycle life.

[0106] 4. Compatible with existing production lines, suitable for large-scale mass production. This solution only requires adding a spraying and baking unit to the existing coating and baking line. The diaphragm coating can use existing coating equipment. Since no major equipment modifications are required, it can be directly applied to water-based, water-based, or oil-based negative electrode mass production lines, making it highly practical.

[0107] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A battery cell, characterized in that, It includes a negative electrode, a separator, and a positive electrode stacked in sequence; A transition layer is disposed between the negative electrode and the separator. The transition layer is made of at least one of PVDF and PMMA. The thickness of the transition layer is 'a'. The thicker region of the transition layer near the negative electrode (10%~30%) is a dense region with a porosity of 10%~30% and an average pore size of 10~30 nm. The middle region (40%~60%) of the transition layer (thicker region a) is a transition region with a porosity of 30%~50% and an average pore size of 30~50 nm. The remaining thick region of the transition layer is a high-porosity region with a porosity of 50%~75% and an average pore size of 50~150 nm. The average pore size of the high-porosity region is greater than the average pore size of the transition region, and the average pore size of the transition region is greater than the average pore size of the dense region. There are no abrupt interfaces between the dense region, the transition region, and the high-porosity region; a ranges from 1 to 7 μm.

2. The battery cell according to claim 1, characterized in that, The active material of the surface active layer of the negative electrode sheet is one or more of artificial graphite, natural graphite, silicon-based negative electrode, hard carbon, and soft carbon. Optionally, a is 1.8~6μm, the thickness of the dense region is 0.4~1.5μm, the porosity is 10~25%, and the average pore size is 10~22nm; the thickness of the transition region is 0.8~3μm, the porosity is 31~46%, and the average pore size is 32~48nm; the thickness of the high-porosity region is 0.6~1.5μm, the porosity is 53~70%, and the average pore size is 52~100nm.

3. The method for preparing a battery cell according to claim 1 or 2, characterized in that, include: A first pre-pressing material is provided, the first pre-pressing material comprising a negative electrode sheet and a first protective layer bonded to the surface of the negative electrode sheet, the first protective layer having a thickness of 0.5~4μm, a porosity of 10%~35%, and an average pore size of 10~35nm; A second pre-compression material is provided, the second pre-compression material comprising a diaphragm and a second protective layer bonded to the surface of the diaphragm, the second protective layer having a thickness of 1~4μm, a porosity of 52%~77%, and an average pore size of 55~155nm; The materials forming the first protective layer and the second protective layer are protective materials, and the protective materials are selected from at least one of PVDF and PMMA; Following the method of the first protective layer facing the second protective layer, the first pre-pressed material, the second pre-pressed material, and the positive electrode sheet are sequentially wound or stacked, and hot-pressed at 80~140℃ and 0.5~2MPa for 1~10min, so that the molecular chains of the first protective layer and the second protective layer diffuse and entangle with each other, and the two layers fuse to form a transition layer having the dense region, the transition region, and the high porosity region.

4. The preparation method according to claim 3, characterized in that, The first protective layer has a thickness of 1~3μm, a porosity of 12~26%, and an average pore size of 11~25nm; the second protective layer has a thickness of 1.5~4μm, a porosity of 55~73%, and an average pore size of 60~140nm.

5. The preparation method according to claim 4, characterized in that, The preparation method of the first pre-compression material includes: The negative electrode sheet is prepared in advance, and a first protective liquid containing the protective material is coated on the surface of the negative electrode sheet to form a wet film. Then, it is baked and cured at 60~140℃ to evaporate the solvent, and finally rolled at room temperature. Alternatively, an aqueous or oil-based negative electrode slurry can be coated onto the surface of the negative electrode current collector, followed by a first protective liquid containing the protective material, and then baked at 60-140°C to cure the solvent.

6. The preparation method according to claim 5, characterized in that, The concentration of the protective material in the first protective solution is 5-20 wt%; optionally, the solvent in the first protective solution is selected from at least one of DMF, NMP, THF and ethyl acetate.

7. The preparation method according to claim 3, characterized in that, The second protective layer is formed on the surface of the diaphragm in the following ways: A second protective liquid containing the protective material and the pore-forming agent is coated on the surface of the diaphragm to obtain an intermediate product. The intermediate product is then placed in a coagulation bath to solidify the protective material and form a porous structure. The coagulation bath is a mixture of water and alcohol with a volume ratio of 1:1 to 30 and a temperature of 5 to 40°C. Optionally, the alcohol is methanol.

8. The preparation method according to claim 7, characterized in that, The concentration of the protective material in the second protective liquid is 5-20 wt%, the mass ratio of the pore-forming agent to the protective material is 2-10:100, and the pore-forming agent is selected from at least one of polyethylene glycol and polyvinylpyrrolidone.

9. The preparation method according to claim 8, characterized in that, The solvent in the second protective solution is selected from at least one of DMF, NMP, THF and ethyl acetate.

10. A battery, characterized in that, This includes the battery cell as described in claim 1 or 2, or the battery cell prepared by the method described in any one of claims 3 to 9.

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