Lithium ion secondary battery

Abstract

This invention relates to the field of battery technology, specifically to a lithium-ion secondary battery. The lithium-ion secondary battery includes an electrode assembly comprising a positive electrode, a separator, and a negative electrode, stacked and wound together. The negative electrode includes a negative current collector and a negative active layer, the negative active layer comprising a silicon-carbon material, wherein the mass content of elemental Si (C1) in the negative active layer is 1%-70%. The separator has a protrusion on at least one outer surface, the height h of which is 1μm-15μm; the protrusion at least faces the positive electrode. The positive electrode includes a positive current collector, a positive active layer, and a surface coating layer located on the surface of the positive active layer away from the positive current collector. The surface coating layer comprises a solid electrolyte, the thickness t of which is 0.5μm-5μm; h and t satisfy: h / t is 0.3-25. The lithium-ion secondary battery of this invention ensures low-temperature discharge performance while suppressing lithium deposition in the arc region of the wound structure.

Description

Technical Field

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

[0002] In the field of lithium-ion batteries, to improve energy density, silicon-based materials are often introduced into the negative electrode active layer to utilize their theoretical specific capacity, which is far higher than that of traditional graphite. However, in wound-structured batteries, due to the different geometric constraints between the curved and straight regions of the wound structure, the significant volume expansion of the silicon-based material during charging and discharging is subject to stronger lateral restraint at the curved areas. This leads to stress accumulation in these areas, ultimately causing interfacial side reactions and active lithium deposition, known as the "curved lithium deposition" problem, which severely restricts the cycle life of the battery. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a lithium-ion secondary battery. The lithium-ion secondary battery of this invention (hereinafter referred to as the battery) provides a buffer space for the expansion of the silicon negative electrode by setting a protruding structure in the separator to release stress, and constructs a high-efficiency ion transport channel by setting a solid electrolyte surface coating on the surface of the positive electrode; furthermore, by adjusting the relationship between the thickness of the surface coating and the height of the protrusions, it improves lithium plating and enhances the low-temperature discharge performance of the battery.

[0004] In related technologies, when the negative electrode active layer contains silicon, lithium plating is prone to occur in the arc-shaped region of the wound structure during battery cycling. Through in-depth research, the inventors of this invention discovered that the main reason for this problem is that silicon undergoes significant volume expansion during lithium intercalation. The arc-shaped region of the wound structure, due to its different geometric constraints compared to the flat region, makes it difficult to release the stress generated by the expansion, resulting in localized stress concentration. This stress concentration disrupts the uniform contact of the electrode interface, leading to uneven charge distribution and subsequently triggering lithium plating in the arc-shaped region.

[0005] To alleviate the aforementioned stress accumulation problem, a protruding structure facing the positive electrode can be formed on one side of the separator to create a compressible gap, thereby providing buffer space for the volume expansion of the negative electrode and directionally releasing stress in the arc region. However, the inventors found that while this gap-setting method alleviates the lithium plating problem at the physical level, it inevitably increases the ion transport distance between the positive and negative electrodes. Especially at low temperatures, the electrolyte viscosity increases and the ionic conductivity decreases. The elongated transport path further deteriorates the diffusion and migration kinetics of lithium ions at the electrode interface, making it difficult to guarantee the low-temperature discharge performance of the battery.

[0006] Based on the above findings, this invention adds a surface coating layer to the surface of the positive electrode active layer of the positive electrode sheet away from the positive electrode current collector. This surface coating layer contains a solid electrolyte, and its thickness is set in the range of 0.5 μm to 5 μm. This solid electrolyte surface coating layer can establish a fast and continuous ion transport channel on the surface of the positive electrode sheet, thereby compensating for the additional ion transport resistance caused by the gaps in the separator protrusions and the changes in the pore structure of the negative electrode due to silicon expansion, maintaining or even improving the interfacial ion transport efficiency. More importantly, the aforementioned separator protrusions and solid electrolyte surface coating layer are not isolated, but are functionally matched by limiting the ratio of the protrusion height h to the surface coating thickness t. Specifically, h / t is limited to the range of 0.3 to 25. Through this parameter setting, the size of the buffer space provided by the separator for silicon expansion can be adapted to the strength of the solid electrolyte channel constructed on the surface of the positive electrode sheet to accelerate ion transport, thereby simultaneously achieving the effects of suppressing circular arc lithium plating and ensuring the low-temperature discharge performance of the battery.

[0007] Based on this, the inventors of this invention propose the following solution: This invention provides a lithium-ion secondary battery, comprising an electrode assembly including a positive electrode, a separator, and a negative electrode stacked and wound together; the negative electrode includes a negative current collector and a negative active layer located on at least one side surface of the negative current collector, the negative active layer comprising silicon-carbon material, wherein the mass content of element Si (C1) in the negative active layer is 1%-70%; the separator has a protrusion on at least one side outer surface, the height h of the protrusion being 1μm-15μm; the protrusion at least faces the positive electrode; the positive electrode includes a positive current collector, a positive active layer located on at least one side surface of the positive current collector, and a surface coating layer located on the positive active layer away from the surface of the positive current collector; the surface coating layer comprises a solid electrolyte, the thickness t of the surface coating layer being 0.5μm-5μm; h and t satisfy: h / t is 0.3-25.

[0008] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: In wound lithium-ion batteries using silicon-containing anodes, the significant volume expansion of silicon-carbon materials during charging and discharging leads to severe stress concentration in the arc-shaped region, inducing lithium plating and hindering ion transport pathways. The lithium-ion secondary battery provided by this invention achieves a synergistic match between the separator protrusion facing the positive electrode and a surface coating containing a solid electrolyte on the surface of the positive electrode active layer. This is achieved by limiting the ratio of the protrusion height *h* to the surface coating thickness *t* to within the range of 0.3 to 25. The separator protrusion provides a controllable buffer space for the volume expansion of the anode, directionally releasing stress accumulation in the arc-shaped region of the wound structure, thereby effectively suppressing lithium plating in this region. The solid electrolyte surface coating on the positive electrode surface constructs an efficient ion transport channel, compensating for and enhancing the interfacial ion transport capability deteriorated by the protrusion. By limiting h / t, the stress-relieving structure (the gap constructed by the protrusions) and the ion transport enhancement structure (the surface coating) are adapted to each other, thereby achieving the technical effect of simultaneously suppressing circular arc lithium plating and ensuring the low-temperature discharge performance of the battery.

[0009] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0010] Figure 1 The figure shown is a cross-sectional schematic diagram of the diaphragm along the thickness direction in an embodiment of the present invention.

[0011] Figure 2 The figure shown is a cross-sectional schematic diagram of the positive electrode sheet along the thickness direction in an embodiment of the present invention. Detailed Implementation

[0012] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0013] This invention provides a lithium-ion secondary battery, comprising an electrode assembly including a positive electrode, a separator, and a negative electrode stacked and wound together. The positive electrode, the separator, and the negative electrode are stacked and wound to form the electrode assembly. In this electrode assembly, the protrusions on the separator at least face the positive electrode. In the arc-shaped region formed by winding, the difference in the radius of curvature between the outer positive electrode and the inner negative electrode causes uneven compression of the separator, resulting in the protrusions on the separator surface being preferentially compressed in this region, providing a directional stress release space for the outward expansion of the negative electrode active layer.

[0014] In this invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side of the surface of the negative electrode current collector. The negative electrode active layer comprises a silicon-carbon material. The mass content (C1) of elemental Si in the negative electrode active layer is 1%-70%, for example, 3%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%. The value of C1 determines, to a certain extent, the degree of total volume expansion of the negative electrode during charging and discharging. Although a higher silicon content can provide higher capacity, it leads to drastic volume changes and greater stress output, thus requiring stronger stress buffering and ion transport compensation measures. When C1 exceeds 70%, the electronic conductivity network and structural integrity of the negative electrode sheet are prone to rapid collapse during cycling due to excessive volume changes, resulting in a sharp decline in the cycle life of the battery.

[0015] In this invention, the method for testing the mass content C1 of elemental Si in the negative electrode active layer can be as follows: After discharging the battery to 0% SOC, disassemble and remove the negative electrode sheet, soak it in dimethyl carbonate solvent for 12 hours, rinse it, and then immerse it in deionized water for 20 seconds to sonicate it, causing the negative electrode active layer to peel off from the negative electrode current collector. Collect the negative electrode active layer and dry it at 60°C. Using a thermogravimetric analyzer, take a sample of 5mg to 15mg, and in an air or oxygen atmosphere, heat it from room temperature to 900°C at a heating rate of 10°C / min and hold it at that temperature for 40 minutes to allow the non-silicon components to volatilize and the silicon to be completely oxidized to silicon dioxide. The residue is the ash content, and C1 is calculated according to the following formula: C1 = 7 × mass of ash / (15 × mass of test sample).

[0016] In this invention, the separator has a protrusion on at least one outer surface. The term "protrusion" broadly refers to any three-dimensional structure disposed on at least one outer surface of the separator that can form a physical gap facing the positive electrode to provide a compressible space. For example, dot-like protrusions formed by large particles of adhesive in the adhesive layer; or, for example, regularly patterned protrusions formed by embossing or molding processes. This protrusion, disposed on the side of the separator facing the positive electrode, allows the top of the protrusion to directly contact the solid electrolyte surface coating on the surface of the positive electrode. This contact interface allows the surface coating to form a tight, low-resistance ion channel between the surface coating, the positive electrode active material, and the separator, thereby improving interfacial ion transport efficiency.

[0017] Compared to having the protrusion face the negative electrode, having it face the positive electrode creates a compressible gap directly between the positive electrode and the separator in the arc region of the wound structure. When the silicon negative electrode expands, this gap absorbs and directionally releases the compressive stress transmitted from the negative electrode side, thus precisely alleviating stress concentration in the arc region and effectively suppressing lithium plating in that area. If the protrusion faces the negative electrode, it will directly embed into or compress the negative electrode active layer, potentially damaging the negative electrode structure and hindering ion transport. Furthermore, the protrusion's buffering function will be overwhelmed by the expansion and deformation of the negative electrode, losing its precise stress release effect.

[0018] In this invention, the height h of the protrusion is 1μm-15μm, for example, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 12μm, 14μm, or 15μm. The protrusion at least faces the positive electrode sheet. When h is less than 1μm, the compressible space reserved by the protrusion for the volume expansion of the silicon-carbon material is too small, insufficient to effectively release the stress concentration in the arc region of the wound structure, and cannot solve the lithium plating problem in this region. When h is greater than 15μm, it will lead to excessively large gaps inside the battery, which will not only significantly reduce the volumetric energy density of the battery, but also make it difficult to fully compensate for the excessively elongated ion transport path by other means, resulting in a decrease in the overall dynamic performance of the battery.

[0019] In this invention, the height of the protrusion refers to the vertical distance from the apex of the protrusion to the surface of the separator (the part without protrusion). The method for testing the height of the protrusion can be as follows: after discharging the battery to 0% SOC, disassemble and remove the separator, soak it in dimethyl carbonate solvent for 12 hours, and then rinse it with dimethyl carbonate solvent to remove the lithium salt attached to the separator. Take one fold of the separator, use a 3D profilometer to test the flatness of the separator, and measure the height of the protrusion of the separator through image analysis. Take the average height of all the protrusions on this fold of the separator.

[0020] The protrusions on the separator serve a crucial function: creating structured buffer spaces. The inventors discovered a fundamental difference in the stress environment between the flat and curved sections of a wound battery. In the curved section, due to the different radii of curvature between the outer positive electrode and the inner negative electrode, coupled with external constraints, the lateral binding force on the silicon-carbon material during lithium intercalation and expansion is significantly greater than in the flat section. This causes stress to accumulate preferentially, ultimately leading to uneven lithium ion deposition on the negative electrode surface, i.e., lithium plating. This protruding structure creates tiny gaps when the electrode assembly is not under pressure. When the negative electrode expands, these gaps are compressed, thus directionally and controllably absorbing the expansion stress in the curved region, mitigating the stress accumulation process leading to lithium plating.

[0021] In this invention, the positive electrode includes a positive current collector, a positive active layer located on at least one side of the positive current collector, and a surface coating layer located on the positive active layer away from the surface of the positive current collector. The surface coating layer includes a solid electrolyte, and the thickness t of the surface coating layer is 0.5 μm-5 μm, for example, 0.5 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm. When t is less than 0.5 μm, the surface coating layer is difficult to form a continuous and uniform ion transport channel, resulting in insufficient compensation for interfacial ion transport, especially at low temperatures, where the reduction effect on charge transfer impedance is limited. When t is greater than 5 μm, the electronic insulation properties of the surface coating layer itself significantly increase the overall internal resistance of the positive electrode, affecting the effective conduction of electrons, thereby reducing the rate performance and energy density of the battery.

[0022] The mechanism of the surface coating of the solid electrolyte lies in its role as a lithium-ion conductor, constructing an independent and efficient ion transport path on the surface of the positive electrode active layer. In wound electrode assemblies, the membrane protrusions introduced to alleviate the volume expansion of silicon-carbon materials increase the electrode gap, lengthening the diffusion path of lithium ions in the electrolyte. The surface coating of the solid electrolyte can compensate for this deteriorated ion transport path. Since the conduction velocity of lithium ions in the solid electrolyte bulk phase is higher than that in the electrolyte under low temperature or lean electrolyte conditions, the surface coating can effectively reduce the charge transfer impedance at the interface and improve the lithium-ion extraction and transport kinetics at low temperatures. At the same time, the surface coating acts as a physical barrier, preventing direct contact between the electrolyte and the positive electrode active material, inhibiting the oxidative decomposition of the electrolyte and the growth of complex and disordered interfacial films during charge and discharge, and maintaining the stability and low impedance state of the interface.

[0023] In this invention, the thickness t of the surface coating can be determined using the following method: After discharging the lithium-ion secondary battery to 0% SOC, disassemble it and remove the positive electrode sheet. Immerse it in dimethyl carbonate solvent for 12 hours and rinse. After cutting the positive electrode sheet using an argon ion mill to expose the cross-section, measure the thickness of the surface coating using SEM. Select 10 sites on the surface coating for thickness measurement and take the average value.

[0024] In this invention, h and t satisfy the following: h / t is 0.3-25, for example, 0.3, 0.5, 1, 2, 5, 10, 15, 20, or 25. This ratio is key to achieving the synergistic effect of the stress buffer structure and the ion transport compensation structure. When h / t is less than 0.3, it means that the physical buffer space is too small relative to the thicker solid electrolyte surface coating. In this case, stress release is insufficient, the problem of circular lithium plating still exists, and the excessively thick surface coating increases electron transport resistance. When h / t is greater than 25, it means that the physical gap is too large relative to the thinner surface coating, resulting in a significantly longer ion transport path, exceeding the compensation limit of the surface coating's ion transport capacity, and the battery's kinetic and low-temperature performance will deteriorate. Limiting h / t to between 0.3 and 25 allows the compressible space provided by the separator to be compatible to a certain extent with the fast ion channels on the surface of the positive electrode, effectively solving the problem of circular lithium plating while ensuring the overall low-temperature discharge performance of the battery.

[0025] In one instance, h / t is 0.5-15.

[0026] In one instance, h / t is 1-10.

[0027] In one example, h / t is 2-4.

[0028] In one example, C1 and t (in μm) satisfy the following: t / C1 is 0.5-170, for example, 0.5, 0.625, 1, 10, 20, 40, 60, 80, 100, 120, 140, 150, 160, 166.7, or 170. When C1 is high, the negative electrode expands significantly, requiring a thicker solid electrolyte coating to compensate for the additional ion transport losses caused by the larger buffer space and more severe damage to the negative electrode structure. When t / C1 is less than 0.5, it means that in a high-silicon content system, the thickness of the solid electrolyte coating is insufficient, and the efficient ion channels provided are insufficient to meet the requirements, resulting in an unsatisfactory effect on improving the low-temperature discharge performance of the battery. When t / C1 is greater than 170, it means that an excessively thick solid electrolyte coating is used in a low-silicon content system. Although it has excellent ion conductivity, the excessively thick electronic insulating layer will affect the electron conduction rate, potentially affecting the rate performance of the battery, and is also detrimental to improving the battery's energy density.

[0029] In one instance, t / C1 is 0.75-150.

[0030] In one instance, t / C1 is 2-65.

[0031] In one instance, t / C1 is 5-20.

[0032] Negative electrode film In one instance, C1 is 5%-50%.

[0033] In one example, the silicon-carbon material comprises a porous carbon matrix and silicon particles located within the pores of the porous carbon matrix. In this silicon-carbon material, the porous carbon matrix provides a rigid framework structure with internal voids. When nanoscale or microscale silicon particles undergo volume expansion during lithium intercalation, their expansion space is confined within the pre-designed pores of the carbon matrix. This partially absorbs and transforms the macroscopic volume change that would otherwise be conducted to the entire electrode into microscopic local strain within the material. This stress buffering at the source significantly reduces the stress transmitted to the separator and the entire electrode, and, in conjunction with the protruding structure of the separator, further improves the problem of circular arc lithium plating.

[0034] In one example, the mass content of elemental Si in the silicon-carbon material is 30%-80%, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80%. The mass content of elemental Si in the silicon-carbon material determines the buffering capacity within a single silicon-carbon particle. There is a matching relationship between the internal porosity of the porous carbon matrix and the silicon content. Within this range, the silicon content ensures sufficient carbon framework space to accommodate volume changes in silicon while maintaining the integrity of the overall particle structure.

[0035] In this invention, the mass content of elemental Si in the silicon-carbon material can be tested using the following method: Discharge the battery to 0% SOC, disassemble and remove the negative electrode, polish the cross-section of the negative electrode using an argon-ion polisher, and observe it using SEM in backscattered imaging mode. In this mode, the contrast of the silicon-carbon material is brighter, allowing for clear identification. Use an energy dispersive spectroscopy (EDS) instrument to perform a surface scan of the cross-section of the silicon-carbon particles. The scanned area should be no less than 50% of the particle's cross-section, and the scan range must be completely within the particle's cross-section. The mass content of elemental silicon is then calculated. At least 10 particles must be randomly selected for measurement, and the average value is taken.

[0036] In one example, the average particle size of the silicon-carbon material is 1 μm-20 μm, for example, 1 μm, 3 μm, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, or 20 μm. When the average particle size of the silicon-carbon material is less than 1 μm, the specific surface area of ​​the silicon-carbon material is large, which leads to an increase in side reactions with the electrolyte, resulting in a lower initial coulombic efficiency of the battery. When the average particle size of the silicon-carbon material is greater than 20 μm, it not only increases the solid-phase diffusion distance of lithium ions within the particles but also reduces low-temperature discharge performance. A suitable particle size range can ensure good dispersion of silicon-carbon material particles in the electrode slurry and uniformity of the coating, constructing a uniform and stable electrode structure.

[0037] In this invention, the average particle size of the silicon-carbon material can be tested using the following method: After discharging the battery to 0% SOC, the negative electrode sheet is disassembled and soaked in dimethyl carbonate solvent for 12 hours, then rinsed to remove lithium salt. The negative electrode sheet is then cut using an argon ion milling machine, and observed using SEM in backscatter imaging mode. At 5000x magnification, at least 10 silicon-carbon material particles with significant differences in particle size are randomly selected, and the particle size of each particle is measured, and the average value is taken. When the particle in the image is a regular circle, its particle size is the diameter of that circle; when the particle is an irregular circle, any two points on the edge of the particle are connected to form a straight line segment inside the particle, and the longest straight line segment is selected as the particle size. If the number of particles at 5000x magnification is less than 10, the field of view for image acquisition and measurement needs to be increased until all 10 particles are measured.

[0038] In one example, the outer surface of the negative electrode has a recess. Creating this recess on the negative electrode surface using methods such as laser etching provides space for the dramatic volume expansion of the silicon-carbon material. If the negative electrode surface is a continuous plane, the volume expansion of the silicon-carbon material will generate in-plane compressive stress across the entire electrode surface. This stress is transmitted to the positive electrode through the separator and interface, creating continuous and irregular compressive and bending moments on the solid electrolyte coating layer of the positive electrode. Over long periods, the coating layer may develop microcracks or even peel off from the positive electrode active layer, leading to a decrease in the long-term performance of the coating layer. The recess on the negative electrode surface can decompose this in-plane stress and direct it into the recess for release, greatly reducing the magnitude of the force transmitted to the positive electrode, thereby protecting the solid electrolyte coating layer.

[0039] In one example, the depth H2 of the recess is 2μm-50μm, for example, 2μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm or 50μm.

[0040] In one instance, H2 is 5μm-30μm.

[0041] In this invention, the depth H2 of the recess refers to the vertical distance from the lowest point of the recess to the surface of the negative electrode active layer. H2 can be tested by the following method: using a 3D profilometer, test the depth of all or at least 5 recesses on the surface of the negative electrode sheet, and take the average value.

[0042] In one example, H2 (in μm) and t (in μm) satisfy the following: H2×t is 1-250, for example, 1, 5, 10, 20, 50, 80, 100, 150, 200, or 250. When H2×t is less than 1, it means the recess is too shallow or the solid electrolyte coating is too thin. In this case, the buffer space provided by the recess is insufficient to fully dissipate the stress transmitted to the positive electrode, which may cause the coating to bear excessive mechanical load during long-term use, reducing its long-term effectiveness. When H2×t is greater than 250, it means the recess is too deep or the solid electrolyte coating is too thick. An excessively deep recess may damage the electronic conductivity network structure of the negative electrode itself, forming local current concentration points at the edges of the recess, significantly increasing the risk of lithium deposition in these high curvature, high current density regions. Lithium deposition consumes active lithium, leading to an increase in the internal resistance of the entire cell, offsetting the performance advantages brought by the coating. At the same time, an excessively thick solid electrolyte coating will also occupy additional thickness space, resulting in a loss of battery energy density.

[0043] In one instance, H2×t is 5-90.

[0044] In one instance, H2×t is 10-40.

[0045] In one example, the recess includes a recessed hole and / or a groove. The recess can be a linear groove or a recessed hole. The groove can be a continuous line extending in one direction, or it can be formed by multiple lines arranged and spliced ​​together in the same direction.

[0046] In one example, the recess includes the groove. The width of the groove is 10μm-500μm, for example, 10μm, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, 150μm, 200μm, 250μm, 300μm, 350μm, 400μm, 450μm, or 500μm. The spacing between the grooves is 0.5mm-5mm, for example, 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, or 5mm. The width of the groove has a conventional meaning in the art. The orthogonal projection of the groove onto the surface of the negative electrode includes two long sides, and the width of the groove refers to the average distance from one long side to the other in the length or width direction of the negative electrode. Fifty points are randomly selected along a long side, and the width corresponding to each point is measured. The average value is taken to obtain the width of the groove. The width of the groove can be determined by, for example, using a 3D profilometer to test the width of all grooves or at least five grooves on the surface of the negative electrode sheet, and taking the average value. The groove spacing has a conventional meaning in the art, referring to the average distance between the two adjacent long sides of two adjacent grooves in the length or width direction of the negative electrode sheet. Fifty points are randomly selected along a long side, and the width corresponding to each point is measured. The average value is taken to obtain the spacing. The groove spacing can be determined by, for example, using a 3D profilometer to test the spacing of all grooves or at least five grooves on the surface of the negative electrode active coating, and taking the average value.

[0047] In one example, the negative electrode active layer further includes a carbon-based material. The carbon-based material includes, for example, at least one of artificial graphite, natural graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

[0048] In this invention, the negative electrode active layer further includes a negative electrode conductive agent and / or a negative electrode binder. The negative electrode conductive agent includes, for example, at least one of superconducting carbon, acetylene black, carbon black, carbon dots, carbon nanotubes (including single-walled carbon nanotubes and / or multi-walled carbon nanotubes), graphene, and carbon nanofibers. The negative electrode binder includes, for example, at least one of polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and polytetrafluoroethylene.

[0049] In this invention, based on the total mass of the negative electrode active layer, the mass content of the negative electrode material is 80%-99.8%, the mass content of the negative electrode conductive agent is 0.1%-10%, and the mass content of the negative electrode binder is 0.1%-10%.

[0050] <Septum> In one example, the protrusion comprises polymethyl methacrylate (PMMA) and / or polyvinylidene fluoride (PVDF).

[0051] In one instance, h is 1μm-15μm.

[0052] In one instance, h is 3μm-10μm.

[0053] In one example, the diameter of the orthographic projection of the protrusion onto the substrate layer is 0.5 μm-20 μm, for example, 0.5 μm, 1 μm, 5 μm, 10 μm, 15 μm, or 20 μm. The diameter of the orthographic projection of the protrusion onto the substrate layer can be tested using the following method: Obtain an SEM image of the membrane cross-section using SEM scanning; take a 100 μm × 100 μm area on the surface and measure the diameter of the orthographic projection of 20 protrusions in this area onto the substrate layer; repeat the above operation 5 times, and take the average of the 5 test results as the diameter of the orthographic projection of the protrusion onto the substrate layer. If the number of protrusions in the 100 μm × 100 μm area is less than 20, continue taking 100 μm × 100 μm areas until 20 protrusions are measured. When the orthographic projection of the protrusion onto the substrate layer is a regular circle, the diameter of the orthographic projection of the protrusion onto the substrate layer is the diameter of the regular circle; when the orthographic projection of the protrusion onto the substrate layer is not a "regular circle", the diameter of the orthographic projection of the protrusion onto the substrate layer is the equivalent diameter of a circle with the same area as the "irregular circle".

[0054] In one example, the coverage of the protrusions on the membrane surface is 5%-60%, for example, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%. The coverage of the protrusions on the substrate layer surface can be tested using the following method: obtain an SEM image of the membrane surface using SEM scanning, take a 100μm × 100μm area on the surface, measure and calculate the proportion of the area covered by the protrusions relative to the substrate layer within this area as the coverage rate; repeat the above operation 5 times and take the average value.

[0055] In one example, the diaphragm includes a substrate layer, a functional coating located on at least one surface of the substrate layer, and an adhesive layer located on at least one outer surface of the diaphragm. The substrate layer includes, for example, polyethylene (PE) and / or polypropylene.

[0056] In one example, the functional coating comprises ceramic particles and / or nitrogen-containing organic matter. The ceramic particles include, for example, at least one selected from boehmite, alumina, aluminum hydroxide, magnesium oxide, magnesium hydroxide, silicon dioxide, and titanium dioxide. The nitrogen-containing organic matter comprises at least one selected from cyano, isocyanate, isocyanate, and amino groups.

[0057] In one example, the nitrogen-containing organic compound includes at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, melamine cyanurate, and melamine trithiocyanate.

[0058] In one example, the functional coating includes the nitrogen-containing organic compound. This nitrogen-containing organic compound can diffuse to the surface of the positive electrode active material and stabilize its crystal structure. The organic compound may contain at least one functional group selected from cyano, isocyano, isocyanate, and amino groups. During battery cycling, these substances containing specific functional groups gradually dissolve or diffuse from the functional coating and are chemically adsorbed or located on the surface of the positive electrode active material (e.g., lithium cobalt oxide). By stabilizing metal atoms and lattice oxygen, they effectively suppress the release of reactive oxygen species and irreversible structural phase transitions, thereby improving the cycle stability of the battery.

[0059] In one example, the functional coating faces the positive electrode.

[0060] In one example, the adhesive layer comprises polymethyl methacrylate and / or polyvinylidene fluoride.

[0061] In one example, the adhesive layer includes a first adhesive layer and a second adhesive layer. The diaphragm includes the substrate layer, the functional coating located on one surface of the substrate layer, the first adhesive layer located on the outer surface of the functional coating, and the second adhesive layer located on the other surface of the substrate layer; the protrusion is located at least on the outer surface of the first adhesive layer. Figure 1 The diagram shows a cross-sectional view of the diaphragm along its thickness in an embodiment of the present invention. As can be seen from the diagram, the diaphragm includes a substrate layer 1, a functional coating 3, an adhesive layer 2, and protrusions 4. The adhesive layer 2 includes a first adhesive layer 21 and a second adhesive layer 22. The functional coating 3 is located on one side surface of the substrate layer 1, the first adhesive layer 21 is located on the outer surface of the functional coating 3, the second adhesive layer 22 is located on the other side surface of the substrate layer 1, and the protrusions 4 are located on the outer surface of the first adhesive layer 21.

[0062] When the thickness of the solid electrolyte coating on the positive electrode surface increases, it may hinder the escape of trace gases generated by side reactions inside the electrode assembly. If these trace gases cannot be discharged in time, they will accumulate at the interface, forming air gaps, thus leading to poor cycle stability of the battery. To improve the above problem, the gas permeability value 's' of the separator is further limited to 100 secs / 100cc-500 secs / 100cc, for example, 100 secs / 100cc, 150 secs / 100cc, 200 secs / 100cc, 250 secs / 100cc, 300 secs / 100cc, 350 secs / 100cc, 400 secs / 100cc, 450 secs / 100cc, or 500 secs / 100cc. The lower the gas permeability value of the separator, the better its gas permeability. When s is less than 100 secs / 100cc, the membrane porosity is high, which may lead to insufficient mechanical strength and a risk of short circuit. When s is greater than 500 secs / 100cc, the membrane permeability is slightly poor, which may result in gas not being able to escape effectively, thus leading to poor cycle stability.

[0063] In one instance, s is 100secs / 100cc - 350secs / 100cc.

[0064] In one example, s (in secs / 100cc) and t (in μm) satisfy the following: s×t is 50-3505, for example, 50, 100, 300, 500, 800, 1000, 1500, 2000, 2500, 3000, or 3505. If s×t is less than 50, it may mean that the coating is too thin or the separator is too permeable, in which case the separator loses its basic mechanical barrier or ion transport regulation function. If s×t is greater than 3505, it indicates that the gas blocking effect caused by the density and thickness of the coating may exceed the venting capacity of the separator's high permeability, failing to effectively prevent the formation of interfacial gaps, leading to poorer cycle stability of the battery.

[0065] In one instance, s×t is 100-1100.

[0066] In one instance, s×t is 300-450.

[0067] In this invention, the air permeability value s of the diaphragm can be measured using a conventional air permeability tester in the art. Its unit is seconds per 100 cubic centimeters, which refers to the time required for 100 cubic centimeters of air to pass through a unit area of ​​the diaphragm under a specific pressure. The smaller this value, the faster the air permeability and the better the air permeability.

[0068] <Positive Electrode Tablets> In one example, t is 0.5 μm–3 μm. At this relatively thin thickness, the surface coating can form a dense and uniform fast ion transport channel, sufficient to significantly reduce charge transfer impedance at low temperatures. Simultaneously, the negative impacts on electron conduction and volumetric energy density are minimized.

[0069] In this invention, the solid electrolyte refers to any material that is solid at the battery operating temperature, has ion conductivity, and is electronically insulating. It is used to construct an artificial interface layer on the positive electrode surface to reduce charge transfer resistance.

[0070] In one example, the solid electrolyte includes at least one of oxide solid electrolytes, sulfide solid electrolytes, phosphate solid electrolytes, and borate solid electrolytes.

[0071] In one example, the oxide-based solid electrolyte includes at least one of lithium oxynitride, lithium lanthanum zirconium gallium oxide, lithium lanthanum titanate (LLTO), lithium lanthanum zirconium oxide (LLZO), lithium aluminum titanium oxyphosphide (LATP), and lithium silicate-based oxides. The sulfide-based solid electrolyte includes Li₂S–P₂S₅ glass, Li₂S–P₂S₅ glass ceramics, and Li₂S–P₂S₅ glass. 10 GeP2S 12 At least one of the following. The phosphate solid electrolyte includes at least one of Li3PO4 and / or LiPO3. The borate solid electrolyte includes Li3B7O. 12 To further improve the interfacial performance of batteries under low-temperature conditions, the solid electrolyte material constituting the surface coating can be selected from specific materials with high ionic conductivity and a wide electrochemical window. These materials can form a functional layer that is both a lithium-ion conductor and an electronic insulator, physically isolating the positive electrode active material from the electrolyte, and fundamentally inhibiting the decomposition of the electrolyte on the surface of the charged positive electrode and the continuous growth of the disordered interfacial film at low temperatures.

[0072] In one example, the positive electrode sheet has a first surface and a second surface disposed opposite to each other; the length of the positive electrode active layer located on the first surface is greater than the length of the positive electrode active layer located on the second surface. The surface coating is located on the outer surface of the positive electrode active layer disposed on the first surface and / or on the outer surface of the positive electrode active layer disposed on the second surface.

[0073] In one example, the surface coating is located on the outer surface of the positive electrode active layer disposed on the first surface. For example... Figure 2 The figure shows a cross-sectional schematic diagram of the positive electrode sheet along the thickness direction in an embodiment of the present invention. As can be seen from the figure, the positive electrode sheet includes a positive current collector 5, a positive active layer 6 located on both sides of the positive current collector 5, and a surface coating 7 disposed on the outer surface of the positive active layer 6 located on the first surface.

[0074] In this invention, when both sides of the positive electrode sheet are provided with positive active layers, and the lengths of the active layers on both sides are inconsistent, there exists a long surface with a longer active layer and a short surface with a shorter active layer. In this case, the surface coating can be preferentially disposed on the long surface. In the wound electrode assembly, the long surface usually corresponds to the inner winding, which has a relatively small radius of curvature. The inner winding is subject to more severe stress compression, and this surface is usually the working surface with higher current density and is also the main working surface for maximizing capacity. Disposing the surface coating on the long surface can maximize its effect of reducing interfacial impedance and improving low-temperature ion extraction and transport dynamics.

[0075] In one example, the first surface of the positive electrode has several protrusions, and the second surface of the positive electrode has several concave portions, with the protrusions and concave portions corresponding to each other. The protrusions on the first surface of the positive electrode serve as contact conduction areas, closely adhering to the separator, thus acting as channels for lithium-ion transport; while the concave portions on the second surface of the positive electrode serve as stress concentration areas, absorbing and bearing the expansion and compression stress generated by the electrode during charging and discharging. Based on this structure, coating the outer surface of the protrusions with a solid electrolyte surface coating allows the ion conductor coating to form a tight and continuous surface contact with the separator, further enhancing the continuity of the ion channel.

[0076] In one example, the depth H1 of the recess is 2μm-50μm, for example, 2μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm or 50μm.

[0077] In one instance, H1 is 5μm-30μm.

[0078] In one example, H1 and t satisfy the following condition: H1 / t is 0.1-100, for example, 0.1, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100. If H1 / t is less than 0.1, it indicates that the surface of the positive electrode is relatively flat and the protrusion features are not obvious. In this case, the effect of further improving the lithium-ion transport efficiency is not significant. If H1 / t is greater than 100, it means that the protrusion is too steep, which may cause the solid electrolyte surface coating slurry to be too thin at the top of the protrusion and too thick at its root. This uneven surface coating thickness also has no significant effect on further improving the lithium-ion transport efficiency. An appropriate H1 / t ratio ensures that the thickness of the surface coating transitions smoothly with the morphology, which is conducive to the further formation of uniform fast ion channels.

[0079] In one instance, H1 / t is 0.5-25.

[0080] In one instance, H1 / t is 5-15.

[0081] In one example, the diameter of the recess is 0.5mm-10mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm. The spacing between the recesses is 0.5mm-10mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm.

[0082] In one example, the positive electrode active layer comprises a positive electrode active material. The positive electrode active material includes, for example, at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate.

[0083] In this invention, the positive electrode active layer further includes a positive electrode conductive agent and / or a positive electrode binder. The positive electrode conductive agent includes, for example, at least one of superconducting carbon, acetylene black, carbon black, carbon dots, carbon nanotubes (including single-walled carbon nanotubes and / or multi-walled carbon nanotubes), graphene, and carbon nanofibers. The positive electrode binder includes, for example, at least one of polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and polytetrafluoroethylene.

[0084] In this invention, based on the total mass of the positive electrode active layer, the mass content of the positive electrode active material is 80%-99.8%, the mass content of the positive electrode conductive agent is 0.1%-10%, and the mass content of the positive electrode binder is 0.1%-10%.

[0085] In this invention, the battery further includes an electrolyte. The electrolyte includes at least one of a lithium salt, an organic solvent, and an additive. The lithium salt, the organic solvent, and the additive may include substances conventionally used in the art.

[0086] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0087] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0088] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0089] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0090] Example 1 The battery is prepared according to the following method: (1) Preparation of negative electrode sheet Artificial graphite, silicon carbon material, conductive carbon, lithium carboxymethyl cellulose, and polyacrylic acid were mixed in a mass ratio of 65.2:32:0.05:0.35:2.4, and deionized water was added to prepare a negative electrode slurry. The negative electrode slurry was coated on both sides of a carbon-coated copper foil, and after baking, rolling, and welding of negative electrode tabs, a negative electrode sheet with a thickness of 110 μm was obtained. Then, grooves were made on the surface of the negative electrode sheet using a laser. The groove has a depth H2 of 17.8 μm, a width of 80 μm, and a spacing of 1.2 mm. The silicon-carbon material consists of a porous carbon matrix and silicon particles located in the pores of the porous carbon matrix. The mass content of elemental Si in the silicon-carbon material is 56%, and the average particle size of the silicon-carbon material is 8.5 μm. The mass content of elemental Si in the negative electrode active layer is 18% (C1).

[0091] (2) Preparation of positive electrode sheet Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 97:1:2, and N-methylpyrrolidone (NMP) was added and stirred until homogeneous to prepare a positive electrode slurry. This positive electrode slurry was then coated onto both the front and back surfaces of an aluminum foil (the positive electrode slurry was baked to form the positive electrode active layer); Li7La3Zr2O 12 (LLZO) and polyvinylidene fluoride are mixed at a mass ratio of 97:3, and NMP is added and stirred evenly to prepare a solid electrolyte slurry. The solid electrolyte slurry is then coated on the outer surface of the positive electrode active layer on the first surface (the solid electrolyte slurry forms a surface coating after baking). After drying and rolling, a positive electrode sheet is obtained. Then, it is slit and processed using a special roller with protrusions to obtain a positive electrode sheet with several concave parts on the first surface and several convex parts on the second surface. The thickness t of the surface coating is 2.1 μm, and H1 / t is 8.57. The depth H1 of the recess is 18μm, the diameter is 2mm, and the spacing is 3.8mm.

[0092] (3) Preparation of electrolyte In an argon glove box with a water content of <0.1ppm and an oxygen content of <0.1ppm, ethylene carbonate, ethylene carbonate, and diethyl carbonate were mixed in a weight ratio of 1:3:6 as an organic solvent. Then, fluoroethylene carbonate and LiPF6 were dissolved in the organic solvent to obtain an electrolyte. Based on the total mass of the electrolyte, the content of LiPF6 was 12.5% ​​and the content of fluoroethylene carbonate was 15%.

[0093] (4) Battery preparation The negative electrode sheet, separator, and positive electrode sheet prepared in step (2) are stacked in sequence to ensure that the separator is between the positive and negative electrode sheets to play a role in isolation; then, the electrode assembly without liquid injection is obtained by winding; the electrode assembly is placed in the outer packaging foil, and the electrolyte prepared in step (3) is injected into the dried electrode assembly. After vacuum sealing, standing, formation, shaping, sorting and other processes, the battery is obtained. The separator comprises a 5μm thick PE substrate layer, a 2μm thick functional coating (including 1,3,5-triazine-2,4,6-triamine) disposed on one side of the substrate layer, a 2μm thick first adhesive layer (including PMMA and PVDF) disposed on the surface of the functional coating, and a 0.5μm thick second adhesive layer (including PMMA) disposed on the other side of the substrate layer; protrusions (formed by the adhesives (PMMA and PVDF) in the first adhesive layer) are located on the outer surface of the first adhesive layer; the height h of the protrusions is 6.2μm; the protrusions face the positive electrode; the air permeability s of the separator is 207secs / 100cc; h / t is 2.95; t / C1 is 11.67; H2×t is 37.38; s×t is 434.7.

[0094] Example 2 The procedure is carried out in accordance with Example 1, except that step (1) involves preparing the negative electrode sheet, and step (2) involves preparing the positive electrode sheet and the separator, as detailed below: (1) Preparation of negative electrode sheet Artificial graphite, silicon carbon material, conductive carbon, lithium carboxymethyl cellulose, and polyacrylic acid were mixed in a mass ratio of 65.2:32:0.05:0.35:2.4, and deionized water was added to prepare a negative electrode slurry. The negative electrode slurry was coated on both sides of a carbon-coated copper foil, and after baking, rolling, and welding of negative electrode tabs, a negative electrode sheet with a thickness of 110 μm was obtained. Then, grooves were made on the surface of the negative electrode sheet using a laser. The groove has a depth H2 of 29.3 μm, a width of 452 μm, and a spacing of 5 mm. The silicon-carbon material consists of a porous carbon matrix and silicon particles located in the pores of the porous carbon matrix. The mass content of elemental Si in the silicon-carbon material is 56%, and the average particle size of the silicon-carbon material is 6.2 μm. The mass content of elemental Si in the negative electrode active layer is 18% (C1).

[0095] (2) Preparation of positive electrode sheet Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 97:1:2, and N-methylpyrrolidone (NMP) was added and stirred until homogeneous to prepare a positive electrode slurry. This positive electrode slurry was then coated onto both the front and back surfaces of an aluminum foil (the positive electrode slurry was baked to form the positive electrode active layer). Li... 10GeP2S 12 The mixture is prepared by mixing polyvinylidene fluoride and NMP at a mass ratio of 97:3, and stirring until homogeneous. The solid electrolyte slurry is then coated onto the outer surface of the positive electrode active layer on the first surface (the solid electrolyte slurry forms a surface coating after baking). The positive electrode sheet is obtained by drying and rolling. It is then slit and processed using a special roller with protrusions to obtain a positive electrode sheet with several concave parts on the first surface and several convex parts on the second surface. The thickness t of the surface coating is 1 μm, and H1 / t is 5. The depth H1 of the recess is 5μm, the diameter is 0.5mm, and the spacing is 0.5mm.

[0096] (4) Battery preparation The separator comprises a 5 μm thick PE substrate layer, a 2 μm thick functional coating (including melamine trithiocyanate) disposed on one side of the substrate layer, a 2 μm thick first adhesive layer (including PMMA and PVDF) disposed on the surface of the functional coating, and a 0.5 μm thick second adhesive layer (including PMMA) disposed on the other side of the substrate layer; protrusions (formed by the adhesives (PMMA and PVDF) in the first adhesive layer) are located on the outer surface of the first adhesive layer; the height h of the protrusions is 3.3 μm; the protrusions face the positive electrode; the air permeability s of the separator is 336 secs / 100cc; h / t is 3.3; t / C1 is 5.56; H2×t is 29.3; s×t is 336.

[0097] Example 3 The procedure is carried out in accordance with Example 1, except that step (1) involves preparing the negative electrode sheet, and step (2) involves preparing the positive electrode sheet and the separator, as detailed below: (1) Preparation of negative electrode sheet Artificial graphite, silicon carbon material, conductive carbon, lithium carboxymethyl cellulose, and polyacrylic acid were mixed in a mass ratio of 65.2:32:0.05:0.35:2.4, and deionized water was added to prepare a negative electrode slurry. The negative electrode slurry was coated on both sides of a carbon-coated copper foil, and after baking, rolling, and welding of negative electrode tabs, a negative electrode sheet with a thickness of 110 μm was obtained. Then, grooves were made on the surface of the negative electrode sheet using a laser. The groove has a depth H2 of 5.5 μm, a width of 17 μm, and a spacing of 0.5 mm. The silicon-carbon material consists of a porous carbon matrix and silicon particles located in the pores of the porous carbon matrix. The mass content of element Si in the silicon-carbon material is 56%, and the average particle size of the silicon-carbon material is 12 μm. The mass content of element Si in the negative electrode active layer is 18% (C1).

[0098] (2) Preparation of positive electrode sheet Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 97:1:2, and N-methylpyrrolidone (NMP) was added and stirred until homogeneous to prepare a positive electrode slurry. This positive electrode slurry was then coated onto both the front and back surfaces of an aluminum foil (the positive electrode slurry was baked to form the positive electrode active layer); Li3B7O 12 The mixture is prepared by mixing polyvinylidene fluoride and NMP at a mass ratio of 97:3, and stirring until homogeneous. The solid electrolyte slurry is then coated onto the outer surface of the positive electrode active layer on the first surface (the solid electrolyte slurry forms a surface coating after baking). The positive electrode sheet is obtained by drying and rolling. It is then slit and processed using a special roller with protrusions to obtain a positive electrode sheet with several concave parts on the first surface and several convex parts on the second surface. The thickness t of the surface coating is 2.9 μm, and H1 / t is 10.34. The depth H1 of the recess is 30μm, the diameter is 10mm, and the spacing is 10mm.

[0099] (4) Battery preparation The separator comprises a 5μm thick PE substrate layer, a 2μm thick functional coating (including borosilicate) disposed on one side of the substrate layer, a 2μm thick first adhesive layer (including PVDF) disposed on the surface of the functional coating, and a 0.5μm thick second adhesive layer (including PMMA) disposed on the other side of the substrate layer; protrusions (formed by the adhesive (PVDF) in the first adhesive layer) are located on the outer surface of the first adhesive layer; the height h of the protrusions is 9.5μm; the protrusions face the positive electrode; the air permeability s of the separator is 109secs / 100cc; h / t is 3.28; t / C1 is 16.11; H2×t is 15.95; s×t is 316.1.

[0100] Example 4 group This set of examples is used to verify the impact of changing the "mass content C1 of element Si in the negative electrode active layer".

[0101] This set of embodiments is based on Embodiment 1, except that C1 is controlled by changing the mass content of element Si in the silicon-carbon material and / or the mass content of silicon-carbon material in the negative electrode slurry, as detailed below: In Example 4a, the mass content of elemental Si in the silicon-carbon material was 35%; the mass ratio of artificial graphite, silicon-carbon material, conductive carbon, lithium carboxymethyl cellulose, and polyacrylic acid was 87.7:9.5:0.05:0.35:2.4; and the content of Cl was 3.3%. Example 4b: The mass ratio of artificial graphite, silicon carbide material, conductive carbon, lithium carboxymethyl cellulose, and polyacrylic acid was 87.2:10:0.05:0.35:2.4; the C1 content was 5.6%. Example 4c: The mass ratio of artificial graphite, silicon carbide material, conductive carbon, lithium carboxymethyl cellulose, and polyacrylic acid was 30.2:67:0.05:0.35:2.4; C1 was 37.5%. In Example 4d, the mass content of elemental Si in the silicon-carbon material was 63%; the mass ratio of artificial graphite, silicon-carbon material, conductive carbon, lithium carboxymethyl cellulose, and polyacrylic acid was 20.7:76.5:0.05:0.35:2.4; and the content of C1 was 48.2%. In Example 4e, the mass content of element Si in the silicon-carbon material is 78%; the mass ratio of artificial graphite, silicon-carbon material, conductive carbon, lithium carboxymethyl cellulose and polyacrylic acid is 16.6:80.6:0.05:0.35:2.4; and the content of C1 is 62.9%.

[0102] Example 5 group This set of examples is used to verify the impact of changing the "height h of the protrusion".

[0103] This set of embodiments is based on Embodiment 1, except that h is changed, as follows: Example 5a, h is 1.2 μm; Example 5b, h is 14.7 μm.

[0104] Example 6 group This set of examples is used to verify the effect of changing the thickness t of the surface coating.

[0105] This set of embodiments is based on Embodiment 1, except that t is changed, as follows: Example 6a, t is 0.5 μm; Example 6b, t is 4.9 μm.

[0106] Example 7 group This set of examples is used to verify the impact of changes to "h / t".

[0107] This set of embodiments refers to Embodiments 2 and 3, the difference being that h / t is adjusted by changing t, as detailed below: Example 7a was carried out with reference to Example 2, except that t was 2.9 μm and h / t was 1.14; Example 7b was carried out with reference to Example 3, except that t was 1 μm and h / t was 9.5.

[0108] Example 8 group This set of examples is used to verify the impact of changes to "t / C1".

[0109] This set of embodiments is based on Embodiment 1, except that t / C1 is adjusted by changing t and C1, as follows: Example 8a, C1 is 3.3%, t is 4.9 μm, t / C1 is 148.48; Example 8b: C1 is 62.9%, t is 0.5 μm, and t / C1 is 0.79.

[0110] Example 9 group This set of examples is used to verify the impact of changes in the "average particle size of silicon-carbon materials".

[0111] This set of embodiments is based on Embodiment 1, except that the average particle size of the silicon-carbon material is changed, as follows: Example 9a: The average particle size of the silicon-carbon material is 1.3 μm; In Example 9b, the average particle size of the silicon-carbon material was 18.7 μm.

[0112] Example 10 group This set of examples is used to verify the impact of changes to "H2×t".

[0113] This set of embodiments follows the same procedure as Embodiment 1, except that H2×t is controlled by changing H2 and t, as detailed below: Example 10a, H2 is 2.3 μm, t is 0.5 μm; H2×t is 1.15; Example 10b, H2 is 45.2 μm, t is 4.9 μm; H2×t is 221.48.

[0114] Example 11 The procedure was carried out in accordance with Example 1, except that a laser was used to create recesses on the surface of the negative electrode instead of grooves. Specifically, the depth of the recesses was 17.8 μm, the diameter of the recesses was 100 μm, and the spacing was 60 μm.

[0115] Example 12 This was used to verify the impact of changes in the "permeability value s" of the diaphragm.

[0116] The procedure was carried out in accordance with Example 1, except that s was 488 secs / 100cc.

[0117] Example 13 group This set of examples is used to verify the impact of changing the "position of the surface coating".

[0118] This set of embodiments is based on Embodiment 1, except that the location of the surface coating layer is changed, as follows: Example 13a: The surface coating is located on the outer surface of the positive electrode active layer on the second surface; Example 13b: The coating layer is located on the outer surface of the positive active layer on the first and second surfaces.

[0119] Example 14 group This set of examples is used to verify the impact of changes to "H1 / t".

[0120] This set of embodiments is based on Embodiment 1, except that H1 / t is adjusted by changing H1 and / or t, as follows: Example 14a, H1 is 2 μm, H1 / t is 0.95; Example 14b, H1 is 50 μm, H1 / t is 23.81; Example 14c, H1 is 2 μm, t is 4.9 μm, H1 / t is 0.41; Example 14b, H1 is 50 μm, t is 0.5 μm, H1 / t is 100.

[0121] All of the above embodiments satisfy the following conditions: the diameter of the protrusion projected onto the substrate layer is 0.5μm-20μm; the coverage of the protrusion on the membrane surface is 5%-60%.

[0122] Comparative Example 1 The procedure was carried out in accordance with Example 1, except that the diaphragm surface had no protrusions and the positive electrode surface did not have a coating.

[0123] Comparative Example 2 The procedure was carried out in accordance with Example 1, except that no surface coating was applied to the surface of the positive electrode.

[0124] Comparative Example 3 Groups The procedure is the same as in Example 1, except that h and t are changed, as follows: Comparative Example 3a, h is 15 μm, t is 0.5 μm, and h / t is 30; Comparative Example 3b: h = 1 μm, t = 5 μm, h / t = 0.2.

[0125] Comparative Example 4 The procedure was carried out in accordance with Example 4e, except that no surface coating was provided on the surface of the positive electrode.

[0126] Test case (1) Volumetric energy density The volumetric energy density of the batteries prepared in the examples and comparative examples was tested. The specific testing methods are as follows: The formula for the volumetric energy density of a battery is ED = E / V, where E is the discharge energy of the battery and V is the volume of the battery.

[0127] The test method is as follows: charge the battery with a current of 0.2C to the upper limit voltage (4.55V), then charge it with a constant voltage until the current drops to 0.02C; then discharge it with a current of 0.2C until it reaches 3.0V, and the energy of the discharge is E; measure the thickness, width and length of the battery and calculate the product of the three to obtain the volume V of the battery, and record the results in Table 1.

[0128] (2) Loop testing The batteries prepared in the examples and comparative examples were subjected to cycle tests. The specific test methods are as follows: The battery was placed in a constant temperature room at 25℃ for 2 hours, then charged at a constant current of 1C to the upper limit voltage of 4.55V, and then charged at a constant voltage until the current dropped to 0.05C. The thickness of the battery at this time was recorded as h1. After standing for 10 minutes, it was discharged at 0.5C to 3.0V, with a capacity of c1, and cycled for 1000 times. Then, it was charged at a constant current of 1C to the upper limit voltage of 4.55V, and then charged at a constant voltage until the current dropped to 0.05C. The thickness of the battery at this time was recorded as h2. Then, it was discharged at 0.5C to 3.0V, with a capacity of c2. h2 / h1 is the expansion rate after 1000 cycles, and c2 / c1 is the capacity retention rate after 1000 cycles. The results are recorded in Table 1.

[0129] (3) Low-temperature discharge test The batteries prepared in the examples and comparative examples were subjected to low-temperature discharge tests. The specific test methods are as follows: In a 25℃ environment, the battery was charged at a constant current of 0.2C to the upper limit voltage of 4.55V, and then charged at a constant voltage until the current dropped to 0.02C. After standing for 5 minutes, it was discharged at 0.2C to 3.0V, and the discharge capacity c3 was recorded. The battery was then charged at a constant current of 0.2C to the upper limit voltage of 4.55V, and then charged at a constant voltage until the current dropped to 0.02C to fully charge. The fully charged battery was placed in a -20℃ constant temperature room and left to stand for 2 hours. After being discharged at 0.2C to 3.0V, the discharge capacity c4 was recorded. c4 / c3 is the low-temperature discharge capacity retention rate at -20℃. The results are recorded in Table 1.

[0130] (4) Circular arc lithium plating test The batteries prepared in the examples and comparative examples were subjected to a circular arc lithium plating test. The specific test method is as follows: The battery was kept in a constant temperature chamber at 25℃ for 2 hours, charged at a constant current of 1C to the upper limit voltage of 4.55V, then charged at a constant voltage until the current dropped to 0.05C, left to stand for 10 minutes, and then discharged at 0.5C to 3.0V. This cycle was repeated 30 times. The battery was then charged at a constant current of 1C to the upper limit voltage of 4.55V, and then charged at a constant voltage until the current dropped to 0.05C. After being fully charged, the battery was disassembled to check the interface condition of the arc area. If there was no lithium plating at all, it was recorded as "no lithium plating"; if only dot-like gray lithium plating appeared, it was recorded as "slight lithium plating"; if strip-like silver lithium plating appeared, it was recorded as "lithium plating"; if lithium plating was clearly observed not only at the arc area but also around the arc, that is, if lithium plating spread from the arc area to the periphery, it was recorded as "severe lithium plating". The results are recorded in Table 1.

[0131] Table 1 As can be seen from Table 1, compared with the comparative example, the battery of the present invention can suppress lithium plating in the arc region while ensuring low-temperature discharge performance.

[0132] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes an electrode assembly, which includes a positive electrode sheet, a separator, and a negative electrode sheet that are stacked and wound together. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side of the surface of the negative electrode current collector. The negative electrode active layer includes silicon-carbon material, and the mass content of elemental Si (C1) in the negative electrode active layer is 1%-70%. The diaphragm has a protrusion on at least one outer surface, and the height h of the protrusion is 1μm-15μm; the protrusion at least faces the positive electrode plate. The positive electrode sheet includes a positive current collector, a positive active layer located on at least one side of the positive current collector, and a surface coating layer located on the positive active layer away from the surface of the positive current collector; the surface coating layer includes a solid electrolyte, and the thickness t of the surface coating layer is 0.5 μm-5 μm; h and t satisfy: h / t is 0.3-25.

2. The lithium-ion secondary battery according to claim 1, wherein, h / t is 0.5-15; preferably, h / t is 1-10; And / or, C1 and t satisfy: t / C1 is 0.5-170, where the unit of t is μm; preferably, t / C1 is 2-65.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, C1 is 5%-50%; And / or, the silicon-carbon material comprises a porous carbon matrix and silicon particles located in the pores of the porous carbon matrix; And / or, the mass content of element Si in the silicon-carbon material is 30%-80%; And / or, the average particle size of the silicon-carbon material is 1μm-20μm.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, The diaphragm includes a substrate layer, a functional coating located on at least one surface of the substrate layer, and an adhesive layer located on at least one outer surface of the diaphragm; Preferably, the functional coating comprises ceramic particles and / or nitrogen-containing organic matter, wherein the ceramic particles comprise at least one of boehmite, alumina, aluminum hydroxide, magnesium oxide, magnesium hydroxide, silicon dioxide, and titanium dioxide; and the nitrogen-containing organic matter comprises at least one of cyano, isocyanate, isocyanate, and amino groups; more preferably, the nitrogen-containing organic matter comprises at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, melamine cyanurate, and melamine trithiocyanate. Preferably, the functional coating faces the positive electrode sheet; Preferably, the adhesive layer comprises polymethyl methacrylate and / or polyvinylidene fluoride; Preferably, the adhesive layer includes a first adhesive layer and a second adhesive layer, and the diaphragm includes the substrate layer, the functional coating located on one side of the substrate layer, the first adhesive layer located on the outer surface of the functional coating, and the second adhesive layer located on the other side of the substrate layer; the protrusion is located at least on the outer surface of the first adhesive layer.

5. The lithium-ion secondary battery according to claim 4, wherein, The protrusions include polymethyl methacrylate and / or polyvinylidene fluoride; And / or, the diameter of the protrusion projected onto the substrate layer is 0.5μm-20μm; And / or, the protrusions cover 5%-60% of the surface of the diaphragm.

6. The lithium-ion secondary battery according to claim 1 or 2, wherein, The air permeability value s of the diaphragm is 100secs / 100cc-500secs / 100cc; Preferably, 100 secs / 100cc-350 secs / 100cc; Preferably, s and t satisfy: s×t is 50-3505, where the unit of s is secs / 100cc and the unit of t is μm; more preferably, s×t is 100-1100.

7. The lithium-ion secondary battery according to claim 1 or 2, wherein, t is 0.5μm-3μm; And / or, the solid electrolyte includes at least one of oxide solid electrolytes, sulfide solid electrolytes, phosphate solid electrolytes, and borate solid electrolytes; Preferably, the oxide-based solid electrolyte includes at least one of lithium oxynitride phosphate, lithium lanthanum zirconium gallium oxide, lithium lanthanum titanate, lithium lanthanum zirconium oxide, lithium aluminum titanium oxyphosphide, and lithium silicate-based oxides; the sulfide-based solid electrolyte includes Li₂S–P₂S₅ glass, Li₂S–P₂S₅ glass ceramic, and Li₂S–P₂S₅ glass. 10 GeP2S 12 At least one of the following; the phosphate solid electrolyte includes at least one of Li3PO4 and / or LiPO3; the borate solid electrolyte includes Li3B7O 12 .

8. The lithium-ion secondary battery according to claim 1 or 2, wherein, The positive electrode sheet has a first surface and a second surface disposed opposite to each other; the length of the positive electrode active layer located on the first surface is greater than the length of the positive electrode active layer located on the second surface; The surface coating is located on the outer surface of the positive electrode active layer disposed on the first surface and / or on the outer surface of the positive electrode active layer disposed on the second surface; Preferably, the coating layer is located on the outer surface of the positive electrode active layer disposed on the first surface.

9. The lithium-ion secondary battery according to claim 8, wherein, The first surface of the positive electrode has a plurality of protrusions, and the second surface of the positive electrode has a plurality of concave portions; Preferably, the depth H1 of the recess is 2μm-50μm; Preferably, the depths H1 and t of the recess satisfy the following: H1 / t is 0.1-100; more preferably, it is 0.5-25. Preferably, the diameter of the recess is 0.5mm-10mm, and the spacing between the recesses is 0.5-10mm.

10. The lithium-ion secondary battery according to claim 1 or 2, wherein, The outer surface of the negative electrode sheet has a recessed portion, which includes a recessed hole and / or a groove. Preferably, the depth of the recess, H2, is 2μm-50μm; Preferably, the depths H2 and t of the recess satisfy the following: H2×t is 1-250, where H2 is in μm and t is in μm; more preferably, H2×t is 5-90. Preferably, the recessed portion includes the groove, the width of the groove is 10μm-500μm, and the spacing between the grooves is 0.5mm-5mm.

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