Negative plate and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate and a lithium ion secondary battery comprising the negative plate. The negative plate comprises a negative coating, and the negative coating comprises a negative active layer and a surface coating; the surface coating comprises lithium lanthanum titanium oxide; the negative electrode active layer comprises a silicon-carbon material, and the sphericity degree S of the silicon-carbon material is more than or equal to 0.3 and less than or equal to 0.8; the mass content C1 of the Si element in the negative electrode coating is 8-70%; the molar ratio of Ti to La in the negative electrode coating is 1.6-2; the surface coating is at least partially embedded into the negative electrode active layer, and Ti and / or La are at least partially located on the surface of the silicon carbon material; and the embedding depth d1 of the surface coating layer is 1-8 [mu] m. By improving the negative plate, the K value deterioration phenomenon can be remarkably improved, and meanwhile, the safety performance of the battery is improved; the battery provided by the invention has excellent fast charging performance and low-temperature performance.

Description

Technical Field

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

[0002] Lithium-ion batteries possess advantages such as high specific energy, high operating voltage, and low self-discharge rate, and are widely used in consumer electronics, electric vehicles, and other fields. To achieve higher energy density, silicon-carbon materials have become an inevitable choice. Among them, bulk silicon-carbon materials (e.g., those with a sphericity of ≤0.8) are commonly used as a negative electrode material. Their high packing density allows for a significant increase in the volumetric proportion of active material at the same compaction density, reducing the amount of inactive components such as conductive agents and binders, thereby improving the battery's volumetric energy density. Simultaneously, bulk silicon-carbon materials can increase the contact area between particles, promoting electrolyte wetting and lithium-ion diffusion, improving lithium-ion migration efficiency, and thus enhancing the battery's kinetic performance.

[0003] However, commonly used bulk silicon-carbon materials have two major drawbacks: First, safety and K-value issues: The surface of bulk silicon has sharp edges, which can easily puncture the separator during battery cycling, causing micro-short circuits, severely deteriorating the battery's internal resistance (K-value), and posing serious safety hazards. Second, kinetics and low-temperature lithium plating issues: The conductivity of silicon itself is much lower than that of graphite, resulting in a significant decrease in the ion or electron transport kinetics of the battery when using silicon-carbon materials as the negative electrode, especially during low-temperature charging. This also leads to a deterioration in fast-charging performance, manifested as a wider lithium-ion deposition (lithium plating) window, severely limiting the battery's fast-charging capability and low-temperature performance.

[0004] Therefore, there is a need to invent a lithium-ion secondary battery that can simultaneously improve the degradation of the K value and the degradation of the low-temperature lithium plating window caused by bulk silicon anodes. Summary of the Invention

[0005] The purpose of this invention is to overcome the aforementioned problems in the prior art and to provide a negative electrode and a lithium-ion secondary battery (hereinafter referred to as the battery) including the negative electrode. The negative electrode of this invention can significantly improve the K-value degradation phenomenon; the battery of this invention has excellent fast charging performance and low-temperature performance.

[0006] The first aspect of the present invention provides a negative electrode sheet, characterized in that the negative electrode sheet includes a negative electrode current collector and a negative electrode coating disposed on at least one side of the surface of the negative electrode current collector, the negative electrode coating including a negative electrode active layer and a surface coating located on the surface of the negative electrode active layer away from the surface of the negative electrode current collector; the surface coating includes a solid electrolyte, the solid electrolyte including lithium lanthanum titanium oxide; the negative electrode active layer includes a negative electrode material, the negative electrode material including silicon-carbon material, the sphericity S of the silicon-carbon material satisfying: 0.3≤S≤0.8; the mass content of Si element Cl in the negative electrode coating is 8%-70%; the molar ratio of Ti element to La element in the negative electrode coating is 1.6-2; the surface coating... The silicon-carbon material has at least partially embedded Ti and / or La elements on its surface; the embedding depth d1 of the surface coating is 1 μm-8 μm; the mass content C2 of Ti in the negative electrode coating and the embedding depth d1 of the surface coating satisfy: 500≤C2 / d1≤9000, where d1 is in μm and C2 is in ppm; the embedding depth d1 of the surface coating and the mass content C1 of Si in the negative electrode active layer satisfy: 0.02≤d1 / C1≤1, where d1 is in μm and C1 is in %; the thickness d2 of the surface coating and the embedding depth d1 of the surface coating satisfy: 0.1≤d2 / d1≤1.

[0007] A second aspect of the present invention provides a lithium-ion secondary battery, characterized in that the lithium-ion secondary battery includes the negative electrode sheet described in the first aspect of the present invention.

[0008] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) By improving the negative electrode sheet, the present invention can significantly improve the K value deterioration phenomenon and improve the safety performance of the battery. (2) The present invention can effectively optimize the ion transport and electron transport channels inside the battery and improve the dynamic performance of the battery; the battery of the present invention has excellent fast charging performance and low temperature performance.

[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 image shown is a cross-sectional scanning electron microscope (SEM) image of the negative electrode sheet in an example of the present invention.

[0011] Figure 2 The diagram shown is a schematic diagram of the negative electrode sheet in an example of the present invention.

[0012] Figure 3 The image shown is a SEM image of the silicon-carbon material in an example of the present invention.

[0013] Figure 4 The image shown is a SEM image of the coating thickness in an example of the present invention.

[0014] Figure 5 The image shown is a thermogravimetric analysis (TGA) spectrum of the negative electrode in an example of the present invention.

[0015] Figure 6 The diagram shown is a structural schematic of the core in an example of the present invention.

[0016] Figure 7 The diagram shown is a schematic diagram of the surface coating in the coating area of ​​the negative electrode sheet in an embodiment of the present invention. Detailed Implementation

[0017] 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.

[0018] The first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode coating disposed on at least one side surface of the negative current collector, the negative electrode coating comprising a negative active layer and a surface coating located on the surface of the negative active layer away from the surface of the negative current collector; the surface coating comprising a solid electrolyte, the solid electrolyte comprising lithium lanthanum titanium oxide.

[0019] In this invention, the surface coating is at least partially embedded in the negative electrode active layer; the Ti element and / or the La element are at least partially located on the surface of the silicon-carbon material.

[0020] In this invention, the embedding depth d1 of the surface coating is 1μm-8μm (e.g., 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm or 8μm).

[0021] In this invention, the bulk silicon material comprises bulk silicon particles (sphericity: 0.3 ≤ S ≤ 0.8). These bulk silicon particles exhibit irregular morphology, with sharp angular structures distributed on their surfaces. The particles themselves have areas with large outward curvature, and when adjacent bulk silicon particles are stacked, gaps or recessed areas (also called "valley areas") of varying sizes are formed due to low morphological matching. Based on the morphology of the bulk silicon particles themselves and the stacking characteristics between particles, the K-value deteriorates. In one example, when the surface coating is located in a recessed area, it indicates that the surface coating is at least partially embedded in the negative electrode active layer.

[0022] like Figure 1 The image shown is a cross-sectional scanning electron microscope (SEM) image of the negative electrode sheet in an example of the present invention. Figure 2 The figure shows a schematic diagram of the negative electrode sheet in an embodiment of the present invention. As can be seen from the figure, the negative electrode sheet includes a negative current collector 1 and a negative electrode coating 2 coated on the surface of the negative current collector. The negative electrode coating includes a negative active layer 2-1 and a surface coating 2-2 located on the negative active layer away from the surface of the negative current collector. Along the thickness direction of the negative electrode coating, the surface coating is at least partially embedded in the negative active layer, and the embedding depth of the surface coating is d1.

[0023] In this invention, the molar ratio of Ti to La in the negative electrode coating is 1.6-2 (e.g., 1.6, 1.7, 1.8, 1.9 or 2).

[0024] In one example, the molar ratio of Ti to La in the negative electrode coating is 1.7-1.9.

[0025] In this invention, the mass content C2 of Ti element in the negative electrode coating and the embedding depth d1 of the surface coating satisfy the following condition: 500 ≤ C2 / d1 ≤ 9000 (e.g., 500, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, or 9000), where d1 is in μm and C2 is in ppm. This can be understood as 500 ≤ C2 / d1 ≤ 9000, calculated using the corresponding values ​​when C2 is in ppm and d1 is in μm. For example, when C2 is 8100 ppm and d1 is 4.5 μm, C2 / d1 is 1800.

[0026] In one instance, 1000 ≤ C2 / d1 ≤ 2000.

[0027] In this invention, the negative electrode active layer comprises a negative electrode material, which comprises silicon-carbon material, and the sphericity S of the silicon-carbon material satisfies: 0.3≤S≤0.8 (e.g., 0.3, 0.4, 0.5, 0.6, 0.7 or 0.8); the mass content of Si element C1 in the negative electrode coating is 8%-70% (e.g., 8%, 10%, 20%, 30%, 40%, 50% or 70%).

[0028] like Figure 3The image shown is a SEM image of the silicon-carbon material in an example of this invention. As can be seen from the image, the silicon-carbon material of this invention is bulk silicon (i.e., silicon-carbon material with a sphericity S satisfying: 0.3 ≤ S ≤ 0.8), with an irregular morphology and sharp surface edges. When bulk silicon is used as the negative electrode material, its irregular morphology and sharp surface edges make it prone to piercing the separator during cycling, causing micro-short circuits and increasing the battery's internal resistance (K value). Simultaneously, its massive volume expansion (approximately 300%) leads to the pulverization and shedding of the negative electrode material, repeated rupture and regrowth of the SEI film, and the low conductivity of silicon itself, resulting in poor ion or electron transport dynamics, further causing rapid battery capacity decay and poor fast-charging and low-temperature performance.

[0029] To address the above problems, this invention employs the following solution to synergistically optimize the performance of the negative electrode and the battery: First, by limiting the mass content of Si element in the negative electrode active layer to a specific range, the advantages of high specific capacity of silicon-carbon materials can be fully utilized to improve the energy density of the battery, while also effectively suppressing its volume expansion, reducing the pulverization and shedding of the negative electrode material, and improving the ionic conductivity and structural stability of the negative electrode sheet.

[0030] Secondly, this invention introduces a surface coating containing a solid electrolyte into the negative electrode sheet. This surface coating acts as a hard and dense physical barrier, effectively passivating the sharp edges of the bulk silicon to reduce the risk of separator puncture. Simultaneously, its high mechanical modulus can, to some extent, constrain the volume expansion of the bulk silicon, reducing the shedding of negative electrode material, stabilizing the negative electrode structure, suppressing abnormal increases in the K-value, and improving battery safety performance. Furthermore, the surface coating itself has high ionic conductivity, providing a uniform and rapid transport channel for lithium ions. This not only compensates for the kinetic defects of silicon-carbon materials but also homogenizes the lithium ion flow, eliminates conduction dead zones, effectively suppresses lithium metal deposition, and further improves the battery's low-temperature performance and fast-charging performance. Lithium lanthanum titanium oxide (LLTO), as a high-performance inorganic oxide solid electrolyte, plays an important role in the surface coating, specifically as follows: First, it possesses high lithium-ion conductivity, enabling it to participate in the construction of uniform and efficient lithium-ion transport channels at the negative electrode, significantly improving the insufficient kinetic performance of silicon-carbon materials, promoting uniform lithium-ion insertion and extraction, and thus enhancing the battery's low-temperature and fast-charging performance; second, it exhibits excellent electronic insulation and electrochemical stability, reducing the risk of electronic short circuits and improving the battery's cycle stability under high-voltage conditions (e.g., ≥4.3V); furthermore, its crystal structure is highly selective for lithium ions, effectively preventing transition metal ions from dissolving and migrating to the negative electrode surface, reducing side reactions, and extending the battery's cycle life.

[0031] Third, by controlling the molar ratio of Ti to La in the negative electrode coating within a specific range, this invention not only optimizes the crystal structure of LLTO, thereby enhancing the mechanical strength and density of the surface coating and effectively buffering the volume expansion of silicon-carbon materials, but also promotes the growth of La. 3+ The ordered-disordered arrangement achieves an optimal balance, creating smoother migration channels for Li ions and significantly improving the ionic conductivity of the negative electrode. The surface coating is at least partially embedded in the negative electrode active layer during the rolling process. Therefore, Ti and / or La elements are at least partially located on the surface of the silicon-carbon material, which can further effectively passivate the active sites on the silicon-carbon material surface, reduce side reactions, and optimize the lithium-ion transport channels on the silicon-carbon material surface, reducing interfacial ion transport impedance and synergistically improving the cycle stability and kinetic performance of the battery. If the molar ratio of Ti to La elements in the negative electrode coating is too small (e.g., <1.6), the solid electrolyte material structure is closer to that of the parent La. 2 / 3 TiO3's insufficient crystal structure optimization hinders the effective enhancement of the surface coating's mechanical strength and density, limiting its buffering effect on the volume expansion of silicon-carbon materials. Furthermore, it fails to promote the formation of regular ion transport channels in the surface coating, resulting in a low ionic conductivity of the negative electrode. If the molar ratio of Ti to La in the negative electrode coating is too high (e.g., >2), the Li ion content in the system is relatively high. While this can introduce more lithium sites, the La... 3+ Excessive disorder can also disrupt the continuity and stability of lithium-ion migration channels, leading to a decrease in ionic conductivity.

[0032] Fourth, by further controlling the ratio of the Ti element mass content C2 in the negative electrode coating to the embedding depth d1 of the surface coating to meet a specific range, this invention can effectively improve the kinetic and low-temperature performance of the battery. This is because lithium lanthanum titanium oxide (LLTO) itself has a stable [TiO6] octahedron and [LaO]... 12 The rigid framework structure formed by polyhedra connected by shared vertices, with high-bond-energy Ti-O bonds at its core, endows the surface coating with excellent chemical inertness and air stability. When this surface coating is applied to the negative electrode surface, it forms a robust physical barrier, significantly improving the structural stability of the negative electrode in air. When the ratio of the mass content of Ti (C2) in the negative electrode coating to the embedding depth (d1) of the surface coating is within a specific range, the "Ti-O network" is more firmly embedded in the silicon-carbon material surface, thus more effectively exerting its physical barrier function. This not only effectively blunts particle edges and alleviates volume expansion stress to improve structural stability, but also constructs a continuous and efficient interfacial ion transport channel, further improving the battery's kinetic and low-temperature performance.

[0033] In this invention, the sphericity S of the silicon-carbon material can be tested using conventional methods in the art. For example, it can be determined by image analysis. Specifically, at least 10 two-dimensional projection images of particles are randomly acquired using a scanning electron microscope (SEM), and the projected area (A) and perimeter (P) of each particle are calculated using image analysis software (such as ImageJ). The sphericity (S) of each particle is then calculated using the following formula: S = 4πA / P 2 Finally, the arithmetic mean of the sphericity of all statistically analyzed particles is taken as the sphericity value of the silicon-carbon material.

[0034] In this invention, the mass content C1 of Si element in the negative electrode coating can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC (e.g., discharged to 3V), the negative electrode sheet is disassembled and removed, soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. After calcining the negative electrode sheet in air at 450 degrees Celsius for 2-4 hours, the negative electrode coating is scraped off the negative electrode sheet with a ceramic knife, and the mass content C1 of Si element in the negative electrode coating is obtained by testing with inductively coupled plasma optical emission spectrometry (ICP-OES).

[0035] In this invention, the embedding depth d1 of the surface coating can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC (e.g., discharged to 2.7V), the negative electrode is disassembled and removed, soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then the lithium salt attached to the negative electrode is removed with DMC solvent. After drying, the negative electrode is cut with an argon ion mill (CP), and the cross-section of the negative electrode is observed with a scanning electron microscope (SEM). The embedding area of ​​the surface coating can be clearly observed. Ten test sites are randomly selected on the surface of the area, the thickness of each site is measured, and the average value is taken to obtain the embedding depth of the surface coating.

[0036] In this invention, the embedding depth d1 of the surface coating and the mass content C1 of Si element in the negative electrode active layer satisfy the following condition: 0.02 ≤ d1 / C1 ≤ 1 (e.g., 0.02, 0.1, 0.3, 0.5, 0.7, 0.9, or 1), where d1 is in μm and C1 is in %. This can be understood as 0.02 ≤ d1 / C1 ≤ 1 being calculated using the corresponding values ​​when C1 is in % and d1 is in μm. For example, when C1 is 20% and d1 is 4.5 μm, C2 / d1 is 0.225.

[0037] In one instance, 0.05 ≤ d1 / C1 ≤ 0.45.

[0038] By controlling the ratio of the embedding depth d1 of the surface coating to the mass content C1 of Si in the negative electrode active layer within a specific range, the synergistic optimization of the protective effect of the surface coating and the expansion buffering of the active material can be achieved. This effectively covers and fills the sharp edges and valleys of the bulk silicon particles, mitigating the volume expansion and pulverization of Si during lithium insertion / extraction. It also improves the wettability of the electrolyte, increasing the contact between the bulk silicon material and the electrolyte, enhancing ion and electron transport rates, and thus significantly improving the battery's kinetic performance. If d1 / C1 is too small (e.g., less than 0.02), it means that at higher Si contents, the embedding depth of the surface coating is insufficient, failing to effectively buffer the severe volume expansion of the bulk silicon particles during cycling. This leads to continuous overgrowth and rupture of the solid electrolyte interphase (SEI) film, significantly increasing the interfacial impedance (i.e., increasing the K value), and affecting the battery's cycle performance. If d1 / C1 is too large (e.g., greater than 1), the embedding depth of the surface coating is deeper when the Si content in the negative electrode active layer is low. Although the excessive surface coating can form a thicker barrier layer on the surface of the negative electrode active layer and play a better protective role, it will also prolong the migration path of lithium ions, leading to an increase in battery impedance, which in turn affects the rate performance and charge / discharge efficiency of the battery.

[0039] In this invention, the thickness d2 of the surface coating and the embedding depth d1 of the surface coating satisfy: 0.1≤d2 / d1≤1 (for example, 0.1, 0.3, 0.5, 0.7, 0.9 or 1).

[0040] When the ratio of the thickness d2 of the surface coating to the embedding depth d1 of the surface coating is within a specific range, it can effectively cover the sharp edges of the bulk silicon particles and inhibit the shedding of active materials, while ensuring that the surface coating is properly embedded in the negative electrode active layer. This effectively buffers the stress of silicon volume expansion, increases the wettability of the electrolyte, ensures the efficient transport of lithium ions at the interface, and further improves the cycle performance and kinetic performance of the battery.

[0041] In this invention, the embedding depth d1 of the surface coating can be controlled in the following ways: adjusting the solid content of the surface coating slurry to control its fluidity, thereby changing the permeability of the surface coating in the negative electrode active layer; adjusting the specifications of the surface coating gravure roller to change the application amount and permeation driving force of the surface coating; adjusting the particle size of the solid electrolyte to optimize its filling and permeation ability in the pores of the negative electrode active layer; adjusting the porosity of the negative electrode active layer to adjust the permeable space and channels of the surface coating material, thereby achieving precise control of the embedding depth d1 of the surface coating, etc. It is worth noting that, in addition to the control methods listed above, those skilled in the art can also select other reasonable control methods according to actual process requirements, and this technical solution does not limit the control method to a single one.

[0042] In one example, the average particle size of the solid electrolyte is 150 nm to 1000 nm (e.g., 150 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm or 1000 nm).

[0043] In this invention, the average particle size of the solid electrolyte can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. Alternatively, the negative electrode sheet before soaking in the electrolyte can be directly taken, and the negative electrode sheet can be cut with an argon ion milling machine using a CP laser. Then, the solid electrolyte particles are observed using a scanning electron microscope (SEM). At least 10 solid electrolyte particles are randomly selected, and the particle size of each particle is measured, and the average value is taken. When the particle in the microscope image is a regular circle, the particle size is the diameter of the regular circle. When the particle in the microscope image is not a "regular 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 inside the particle is selected as the particle size.

[0044] In one example, the particle size Dv99 of the solid electrolyte is 300nm-800nm ​​(e.g., 300nm, 400nm, 500nm, 600nm, 700nm or 800nm), which can be measured by a laser particle size analyzer.

[0045] In this invention, the silicon-carbon material comprises a porous carbon matrix and silicon material located in the pores within the porous carbon matrix.

[0046] In this invention, the thickness d2 of the surface coating is 0.5μm-10μm (e.g., 0.5μm, 2μm, 4μm, 6μm, 8μm or 10μm); the average thickness d3 of the negative electrode active layer is 25μm-75μm (e.g., 25μm, 30μm, 40μm, 50μm, 60μm, 70μm or 75μm).

[0047] In one instance, d2 is 0.5μm-5μm.

[0048] In one instance, 10 ≤ d3 / d2 ≤ 100 (e.g., 10, 30, 50, 70 or 100).

[0049] In one instance, 20 ≤ d3 / d2 ≤ 50.

[0050] In this invention, the particle size Dv50 of the lithium lanthanum titanium oxide is 150nm-1000nm (e.g., 150nm, 300nm, 450nm, 600nm, 750nm, 900nm or 1000nm).

[0051] In one example, the particle size Dv50 of the lithium lanthanum titanium oxide is 200nm-600nm.

[0052] In one example, the thickness d2 of the surface coating and the particle size Dv50 of the lithium lanthanum titanium oxide satisfy the following condition: 1≤d2 / Dv50≤100 (e.g., 1, 20, 40, 60, 80 or 100).

[0053] like Figure 4 The image shows an SEM image of the coating thickness in an example of this invention. As can be seen from the image, the thickness of the coating is at the micrometer level. This invention achieves an ultra-thin, dense coating structure at the sub-micrometer to micrometer level by controlling the LLTO particle size Dv50 to the nanometer level. This transforms the failure mode of the coating from the traditional thick coating's tendency to peel off entirely to a stable state dominated by interfacial shear and flexible deformation. This allows the coating to maintain firm adhesion through flexible deformation during drastic volume changes in silicon-carbon materials, preventing coating detachment (powder shedding) and effectively improving the long-term cycle stability of the battery. Furthermore, this invention further limits d3 / d2 within a specific range, maximizing the interfacial bonding force between the coating and the negative electrode active layer. This avoids stress concentration and localized peeling of the coating caused by an excessively thin or uneven negative electrode active layer, while ensuring complete coverage and passivation of the negative electrode material. It prevents problems such as cracking or detachment at the interface and hindering lithium-ion transport caused by excessive coating thickness leading to increased internal stress.

[0054] In this invention, the thickness d2 of the surface coating and the average thickness d3 of the negative electrode active layer are tested using the same method as the embedding depth d1 of the surface coating, and will not be repeated here.

[0055] In this invention, the ionic conductivity of the lithium lanthanum titanium oxide is 0.05 ms / cm to 1 ms / cm (e.g., 0.05 ms / cm, 0.2 ms / cm, 0.4 ms / cm, 0.6 ms / cm, 0.8 ms / cm or 1 ms / cm).

[0056] In this invention, the lithium lanthanum titanium oxide also includes Al and / or Sr elements.

[0057] In one example, the mass content of Ti C2 in the negative electrode coating is 2700ppm-20000ppm (e.g., 2700ppm, 5000ppm, 10000ppm, 15000ppm or 20000ppm).

[0058] In one example, the mass content of Ti (C2) in the negative electrode coating is 7000ppm-8500ppm.

[0059] In one example, the mass content of Li C3 in the negative electrode coating is 45ppm-1500ppm (e.g., 45ppm, 100ppm, 500ppm, 1000ppm or 1500ppm).

[0060] In one example, the mass content of La (C4) in the negative electrode coating is 4500ppm-30000ppm (e.g., 4500ppm, 10000ppm, 15000ppm, 20000ppm, 25000ppm or 30000ppm).

[0061] In one example, the mass content of La (C4) in the negative electrode coating is 10,000 ppm to 15,000 ppm.

[0062] In one example, the mass content of Al C5 in the negative electrode coating is 250ppm-2000ppm (e.g., 250ppm, 500ppm, 1000ppm, 1500ppm or 2000ppm).

[0063] In one example, the mass content of Sr C6 in the negative electrode coating is 500ppm-4000ppm (e.g., 500ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm or 4000ppm).

[0064] This invention allows for elemental doping and / or coating of LLTO. By introducing Al and / or Sr, its crystal structure can be synergistically optimized, thereby further improving its ionic conductivity. The mechanism lies in: Al... 3+ Partially replaces Ti 4+ Introducing appropriate lattice distortion and lithium vacancies into the crystal lattice helps to broaden lithium-ion transport channels and significantly improve bulk ionic conductivity; while Sr 2+ Partially replaces La 3+ This can effectively inhibit La 3+ The orderly arrangement of Al and Sr significantly reduces grain boundary impedance. The introduction of Al and Sr can work together to facilitate the entire lithium-ion transport process from the bulk phase to the interface. This doped and optimized LLTO coating provides a lower impedance and more uniform high-speed transport network for lithium ions at low temperatures, effectively avoiding excessively high local current density and lithium plating problems caused by slow lithium-ion transport. This significantly widens the battery's low-temperature charging safety window and optimizes low-temperature fast charging performance.

[0065] In this invention, Al and / or Sr can exist in LLTO in the form of simple mixed doping or coating, for example, uniformly dispersed in LLTO in the form of particles or existing on the surface of LLTO in the form of coating.

[0066] In this invention, the mass content of Ti, Al, Sr, Li or La in the negative electrode coating can be obtained by conventional methods in the art, such as inductively coupled plasma (ICP) testing.

[0067] In one example, the surface scan area of ​​the surface coating is 256 μm. 2 In the cross-sectional scanning energy spectrum (e.g., 16μm×16μm), the mass content of Ti is 1%-10% (e.g., 1%, 4%, 6%, 8% or 10%), and the mass content of La is 2%-10% (e.g., 2%, 4%, 6%, 8% or 10%).

[0068] In one example, the surface scan area of ​​the surface coating is 4225 μm. 2 In the surface scanning energy spectrum (e.g., 65μm×65μm), the mass content of Ti is 10%-20% (e.g., 10%, 12%, 14%, 16%, 18% or 20%), and the mass content of La is 15%-40% (e.g., 15%, 20%, 25%, 30%, 35% or 40%).

[0069] In this invention, the particle size Dv50 of the silicon carbide material is 2μm-20μm (e.g., 2μm, 5μm, 10μm, 15μm or 20μm); the average particle size of the silicon carbide material is 1μm-20μm (e.g., 1μm, 5μm, 10μm, 15μm or 20μm).

[0070] In one example, the average particle size of the silicon-carbon material is 2 μm-15 μm.

[0071] In this invention, the electrical conductivity of the silicon-carbon material is 0.01 S / cm to 30 S / cm (e.g., 0.01 S / cm, 1 S / cm, 5 S / cm, 10 S / cm, 15 S / cm, 20 S / cm, 25 S / cm or 30 S / cm).

[0072] In one example, the conductivity of the silicon-carbon material is 10 S / cm to 30 S / cm.

[0073] In this invention, the silicon content of the silicon-carbon material is 20%-80% by mass (e.g., 20%, 30%, 40%, 50%, 60% or 80%).

[0074] In one example, the silicon-carbon material has a silicon content of 30%-70% by mass.

[0075] In this invention, the average particle size of the silicon-carbon material can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC (e.g., discharging the battery to 2.7V), the negative electrode sheet is disassembled and removed. After soaking in DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. Alternatively, the negative electrode sheet before soaking in electrolyte can be directly taken, and the negative electrode sheet can be cut using an argon ion milling machine with a CP laser. Then, it can be observed using a scanning electron microscope (SEM) (using backscattered imaging mode). In this mode, the contrast of the silicon-carbon material is brighter (which can be used to distinguish the carbon-based material and conductive agent in the negative electrode active layer). The measurement is performed at 5K magnification. At least 10 silicon-carbon particles are randomly selected, and the particle size of each silicon-carbon particle is measured and the average value is taken. If the number of particles is less than 10 at 5K magnification, another microscopic image is taken until 10 particles are measured. When the particles in the mirror image are regular circles, the particle diameter is the diameter of the regular circle; when the particles in the mirror image are not "regular circles", connect any two points on the edge of the particle to form a straight line segment inside the particle, and select the longest straight line segment inside the particle as the particle diameter.

[0076] In this invention, the particle size Dv50 of the lithium lanthanum titanium oxide and the particle size Dv50 of the silicon-carbon material can be obtained by conventional methods in the art, such as by laser particle size analyzer.

[0077] In this invention, the mass content of element Si in the silicon-carbon material can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC (e.g., discharged to 2.7V), the negative electrode is disassembled and removed, or the negative electrode is taken directly before immersion in electrolyte. The cross-section of the negative electrode is polished using an argon ion milling machine, and the silicon-carbon material is observed in SEM equipment using backscattered imaging mode, magnifying it as much as possible. The cross-section of the silicon-carbon particles is scanned using an energy dispersive spectroscopy (EDS) instrument, with the scanned area not less than 50% of the particle cross-section, and the scanning range should be completely within the particle cross-section. The mass content of element Si is then calculated. At least 10 particles are selected for measurement, and the average value is taken.

[0078] In this invention, the negative electrode active layer includes a first active layer and a second active layer disposed along the thickness direction of the negative electrode sheet, wherein the first active layer is located between the negative electrode current collector and the second active layer; the first active layer includes a first carbon-based material, the second active layer includes the silicon-carbon material; and the second active layer further includes a second carbon-based material.

[0079] In one example, the average particle size of the first carbon-based material is 6 μm-30 μm (e.g., 6 μm, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm); and the average particle size of the second carbon-based material is 2 μm-10 μm (e.g., 2 μm, 4 μm, 6 μm, 8 μm or 10 μm).

[0080] In one example, the first carbon-based material and the second carbon-based material each independently include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

[0081] The negative electrode employs a zoned coating technology, with both the first and second active layers comprising carbon-based materials. The first carbon-based material has a larger particle size, resulting in greater pressure resistance and better chemical stability. During electrode rolling, the larger particles, under stress diffusion, create larger gaps and spaces between them, facilitating electrolyte filling and wetting, and improving electrolyte transport rate. Conversely, the second carbon-based material, located further away from the negative electrode current collector and with a smaller particle size, can directly contact the electrolyte. Its shorter ion transport distance significantly reduces ion diffusion resistance, accelerates electrolyte wetting, improves the charging window, and mitigates the high impedance issue caused by VC film formation, thereby enhancing negative electrode kinetics. Therefore, by controlling the ratio of the particle sizes of the first and second carbon-based materials, the battery can achieve both good cycle stability and kinetic performance.

[0082] In this invention, the average particle size of the first carbon-based material and the average particle size of the second carbon-based material can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. Alternatively, the negative electrode sheet before soaking in electrolyte can be directly taken, and the negative electrode sheet can be cut using an argon ion milling machine with a CP laser. Then, SEM is used for observation. At least 10 particles of the first carbon-based material and the second carbon-based material are randomly selected, and the particle size of each carbon-based material is measured and the average value is taken. When the particles in the mirror image are regular circles, the particle size is the diameter of the regular circle. When the particles in the mirror image are not "regular circles", 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 inside the particle is selected as the particle size.

[0083] In this invention, the solid electrolyte further includes lithium lanthanum phosphate (LLPO), wherein the molar ratio of La to P in the lithium lanthanum phosphate is (0.85-1.15):1 (for example, 0.85:1, 0.9:1, 0.95:1, 1:1, 1.05:1, 1.1:1 or 1.15:1).

[0084] In one example, the molar ratio of La to P in the lithium lanthanum phosphate is (0.9-1.1):1.

[0085] In one example, the mass content Q of lithium lanthanum phosphate in the surface coating is 3.75%-22.5% (e.g., 3.75%, 5%, 10%, 15%, 20% or 22.5%).

[0086] This invention introduces lithium lanthanum phosphate (LLPO), which works synergistically with the ultrathin surface coating to further enhance its flexibility. The low Young's modulus of LLPO effectively reduces the overall rigidity of the composite coating, allowing it to undergo greater elastic deformation under external forces. The ultrathin surface coating itself has low internal stress and strong interfacial adhesion. The introduction of LLPO endows the surface coating with excellent flexible deformation capabilities, enabling it to maintain a lasting "flexible contact" with silicon-carbon particles through minute elastic deformation during the dramatic volume expansion of silicon-carbon materials, rather than relying solely on rigidity to resist stress. This disperses and buffers interfacial stress, effectively suppressing cracking and peeling of the surface coating and significantly improving the cycle stability of the battery.

[0087] In this invention, the surface coating also includes lithium polyacrylate (PAA-Li) and / or styrene-butadiene rubber (SBR), which work synergistically to optimize the mechanical and electrochemical properties of the surface coating. PAA-Li, with its high elastic modulus, effectively buffers the significant volume expansion of silicon-carbon materials during cycling, enhancing the interfacial adhesion between the surface coating and the negative electrode active layer. SBR, with its excellent kinetic properties, can form continuous ion transport auxiliary channels, further improving the ion transport rate while ensuring the surface coating structure adheres firmly, thereby jointly enhancing the battery's cycle stability, fast-charging performance, and low-temperature performance.

[0088] In this invention, the surface coating may not include a conductive agent. Because the surface coating of this invention has an ultra-thin, dense structure and high ionic conductivity, not adding an additional conductive agent can preferentially ensure the intrinsic ionic conductivity and interfacial stability of the LLTO surface coating, avoiding the introduction of conventional conductive agents (such as carbon black) that could form electron leakage pathways or cause side reactions with the electrolyte. However, when the thickness of the surface coating is large (e.g., greater than 5 μm), electron transport reaches a bottleneck. At this point, it is necessary to introduce carbon nanotubes (CNTs) to construct a three-dimensional electronically conductive network to reduce electronic impedance and ensure that the surface coating possesses optimal charge transport performance.

[0089] In this invention, the thermogravimetric analysis test curve of the negative electrode sheet includes at least two weight loss intervals and one weight gain interval in the range of 30℃ to 1000℃. The first weight loss interval is located in the range of 30℃ to 325℃, the second weight loss interval is located in the range of 325℃ to 580℃, and the first weight gain interval is located in the range of 580℃ to 1000℃. The weight loss rate of the first weight loss interval is m1, the weight loss rate of the second weight loss interval is m2, and the weight gain rate of the first weight gain interval is m3. In one instance, 3% ≤ m1 ≤ 5% (e.g., 3%, 3.5%, 4%, 4.5% or 5%), 3% ≤ m2 ≤ 4.5% (e.g., 3%, 3.5%, 4% or 4.5%), and 2% ≤ m3 ≤ 4% (e.g., 2%, 2.5%, 3%, 3.5% or 4%).

[0090] like Figure 5 The figure shows the thermogravimetric analysis (TGA) spectrum of the negative electrode sheet in an example of the present invention. The horizontal axis represents temperature (°C), and the vertical axis represents mass percentage (%). As can be seen from the figure, the TGA curve of the negative electrode sheet in this embodiment includes two weight loss intervals and one high-temperature weight gain interval within the temperature range of 30°C to 1000°C. Specifically, the first weight loss interval is located between 30°C and 325°C, with a weight loss rate of 3.133%; the second weight loss interval is located between 325°C and 580°C, with a weight loss rate of 3.9%; and the first weight gain interval is located between 580°C and 1000°C, with a weight gain rate of 3.64%. In a proportional pair (i.e., the negative electrode coating does not contain a solid electrolyte surface coating), the TGA curve of its negative electrode sheet also includes two weight loss intervals and one high-temperature weight gain interval within the temperature range of 30°C to 1000°C. The first weight loss range is between 30℃ and 331℃, with a weight loss rate of 2.5%; the second weight loss range is between 331℃ and 568℃, with a weight loss rate of 3.336%; and the first weight gain range is between 568℃ and 1000℃, with a weight gain rate of 5.042%.

[0091] The comparison shows that both the examples and the comparative examples exhibit a significant weight loss phase in the low-temperature region (30℃-331℃), which is related to the evaporation of adsorbed water on the surface of the negative electrode. The weight loss rate of the examples (3.133%) is higher than that of the comparative examples (2.5%). This is because the solid electrolyte is composed of nanoscale particles with a high surface area per unit mass or volume, meaning it has high surface energy and chemical activity, providing numerous adsorption sites for water molecules. This weight loss, besides including water evaporation, primarily reflects the removal of adsorbed water or structural hydroxyl groups from the surface coating. Furthermore, the weight loss behavior of the examples and the comparative examples in the high-temperature region (560℃-1000℃) is also significantly different. Since the added surface coating in the examples acts as a hard and dense physical barrier with excellent chemical protection, it effectively suppresses electrolyte side reactions, resulting in a significantly lower weight gain rate (3.64%) in the examples compared to the comparative examples (5.042%) in the high-temperature region. Thermogravimetric analysis data fully confirms that the solid electrolyte surface coating constructed in this invention has been successfully coated on the surface of the negative electrode sheet and has the characteristics of being dense, stable and heat-resistant. It can form an effective physical shield and chemical protection for the internal negative electrode active layer, thereby significantly improving the interface stability of the negative electrode and the cycle stability of the battery.

[0092] In this invention, the negative electrode is subjected to thermogravimetric analysis (TG) test. The specific method is as follows: The test is performed using a thermogravimetric analyzer (TGA). Negative electrode sheets of the same area as those in this invention and the comparative example are collected as test samples. After weighing and recording the data, they are placed in a special crucible and placed inside the instrument. Under a high-purity nitrogen atmosphere of 50 mL / min, the temperature is increased from 30℃ to 1000℃ at a heating rate of 5℃ / min.

[0093] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode, a separator, and a negative electrode as described in the first aspect of the present invention; the positive electrode, the separator, and the negative electrode are stacked and wound to form a core; the lithium-ion secondary battery has an arc-shaped region and a straight region located between the arc-shaped regions. Figure 6 The figure shows a schematic diagram of the core structure in an embodiment of the present invention. As can be seen from the figure, the core includes an arc area 5 and a straight area 6 located between the arc areas.

[0094] In one example, the thickness d4 of the surface coating in the arc region is 0.5μm-10μm (e.g., 0.5μm, 2μm, 4μm, 6μm, 8μm or 10μm).

[0095] In one example, the thickness of the surface coating in the flat area is d5, 0 μm ≤ d5 < 0.2 μm (e.g., 0 μm, 0.1 μm or 0.2 μm).

[0096] In one instance, d4 is 0.5μm-5μm.

[0097] In this invention, the width W of the arc region along the winding direction of the lithium-ion secondary battery is 5mm-15mm (e.g., 5mm, 7mm, 9mm, 11mm, 13mm or 15mm).

[0098] In one instance, 500 ≤ W / d4 ≤ 30000 (e.g., 500, 1000, 5000, 10000, 15000, 20000, 25000 or 30000).

[0099] In one instance, 3000 ≤ W / d4 ≤ 11000.

[0100] like Figure 7The diagram shows a schematic representation of the surface coating layer in the coating area of ​​the negative electrode sheet in an embodiment of the present invention. As can be seen from the diagram, the negative electrode sheet includes an arc region 5 and a straight region 6. Along the winding direction of the lithium-ion secondary battery, the width of the arc region is W. The surface coating layer is applied in a discontinuous, full-coverage manner, selectively coating the arc region 5. The present invention employs a structural design where the arc region is coated, and the straight region is not coated or only coated with an extremely thin layer (e.g., surface coating thickness <0.2 μm). A high-strength, highly flexible surface coating layer is selectively applied to the arc region of the negative electrode sheet, i.e., the recessed areas between bulk silicon particles and areas with large outward curvature of the particles, while the straight region remains uncoated or only coated with an extremely thin layer. The reasons are as follows: Firstly, the arc-shaped area corresponds to the concave area between adjacent blocky silicon-carbon particles and the area with a large outward curvature of the particles. This is the area where the stress generated by the volume expansion of silicon-carbon materials during battery cycling is concentrated, and it is also the critical area with the highest risk of separator puncture. By setting a surface coating in this area, the edges of the blocky silicon can be effectively passivated and the volume expansion stress can be buffered, thereby reducing the risk of separator puncture and improving the structural stability and safety performance of the battery. Secondly, the flat area retains an uncoated or extremely thin coating state, which can avoid the loss of battery volumetric energy density caused by the introduction of a large amount of surface coating. In addition, as a fast channel for electrolyte wetting, the uncoated or extremely thin coating in the flat area can also prevent the surface coating from excessively isolating the electronic conductive network, ensuring ion migration efficiency, so that the battery has both high energy density and excellent cycle stability, safety performance and kinetic performance.

[0101] By further adjusting the ratio of the width W of the arc region to the thickness d4 of the surface coating in the arc region within a specific range, it can be ensured that the surface coating forms a continuous and suitable protective layer in the recessed areas of the blocky silicon particles and the high-curvature convex areas. Its thickness is sufficient to effectively passivate the edges of the silicon particles, absorb the concentrated stress generated by volume expansion during cycling, reduce the risk of the separator being punctured, and ensure the stability and safety performance of the battery structure. It can also maintain the rapid wetting of the electrolyte, improve the lithium-ion migration efficiency, and enable the battery to have both high energy density, excellent cycle stability and kinetic performance. If W / d4 is too large (e.g., greater than 30000), meaning the width W of the arc region is large, corresponding to a larger radius arc structure of the battery, the outer side of this arc will experience more concentrated stress during battery cycle charging and discharging, resulting in a more significant impact on the K value. A relatively thicker surface coating is required to provide sufficient interface protection and stress buffering. If the surface coating thickness is insufficient, it will be difficult to buffer the stress concentration, leading to insufficient improvement in battery cycle stability and K value. If W / d4 is too small (e.g., less than 500), meaning the arc structure of the battery has a smaller radius, the curvature of this area is large, requiring a higher degree of flexibility in the surface coating. A thinner surface coating is required. If the surface coating is too thick, it is prone to peeling (powdering) under repeated cyclic stress, affecting the battery's cycle performance.

[0102] In this invention, the positive electrode sheet includes a positive electrode material, which includes a nickel-cobalt-manganese ternary material and / or a nickel-cobalt-aluminum ternary material.

[0103] In one example, the chemical formula of the nickel-cobalt-manganese ternary material is Li. b Ni c Co d Mn e A f O2, wherein 0.85≤b≤1.05 (e.g., 0.85, 0.9, 0.95, 1 or 1.05), 0.9≤c≤0.98 (e.g., 0.9, 0.92, 0.94, 0.96 or 0.98), 0.01≤d≤0.05 (e.g., 0.01, 0.02, 0.03, 0.04 or 0.05), 0.01≤e≤0.05 (e.g., 0.01, 0.02, 0.03, 0.04 or 0.05), 0≤f≤0.05 (e.g., 0, 0.01, 0.02, 0.03, 0.04 or 0.05), and A includes at least one of Mg, Al, Ti, Zr, W, B, Nb, Ta, Y and F. By doping these elements into the material, the structural stability of the positive electrode can be effectively improved, and the cycle performance of the battery can be improved.

[0104] In one example, the chemical formula of the nickel-cobalt-aluminum ternary material is Li. g Ni h Co j Al k B l O2, wherein 0.85≤g≤1.05 (e.g., 0.85, 0.9, 0.95, 1 or 1.05), 0.9≤h≤0.98 (e.g., 0.9, 0.92, 0.94, 0.96 or 0.98), 0.01≤j≤0.05 (e.g., 0.01, 0.02, 0.03, 0.04 or 0.05), 0.01≤k≤0.05 (e.g., 0.01, 0.02, 0.03, 0.04 or 0.05), 0≤l≤0.05, and B includes at least one of Mg, Ti, Zr, W, Nb, Ta, B, Y and F.

[0105] In one example, the charging cutoff voltage of the lithium-ion secondary battery is ≥4.3V.

[0106] LLTO coatings, with their dense structure and highly lithium-ion-selective lattice channels, can act as an effective physical and chemical barrier, selectively blocking transition metal ions (such as Mn) that dissolve and migrate from the cathode. 2+ Ni 2+In particular, battery systems containing high-nickel cathode materials (e.g., Ni molar content greater than or equal to 0.9%) exhibit more significant transition metal ion dissolution during cycling, preventing them from reaching the silicon-carbon anode surface. This fundamentally inhibits the catalytic decomposition of the solid electrolyte interphase (SEI) film by transition metal ions, reducing active lithium loss and side reaction gas generation, and improving battery cycle stability. Secondly, LLTO possesses excellent electronic insulation and electrochemical stability, reducing the risk of electronic short circuits and further enhancing battery cycle stability under high voltage conditions (e.g., ≥4.3V).

[0107] In this invention, the diaphragm includes a substrate layer, a ceramic layer located on at least one side of the substrate layer, and an adhesive layer located on both outer surfaces of the diaphragm.

[0108] In one example, the substrate layer includes a matrix that may include at least one of polyethylene, polyvinyl chloride, polyoxyethylene, polypropylene, nylon, glass fiber, polyethylene phthalate (PET), polyimide (PI), aramid, cellulose, and nonwoven fabric.

[0109] In one example, the ceramic layer comprises a nitrogen-containing material and / or an inorganic material; the nitrogen-containing material includes melamine, melamine polyphosphate, melamine thiocyanate, melamine cyanurate, formaldehyde polymelamine hydrochloride, melamine polyphosphate, piperazine pyrophosphate, 1,3,5-triazine-2,4,6-triamine, symmetrical triaminotriazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazine-2-yl)guanidine, 2,4-diamino- The inorganic material comprises at least one of 6-dimethylamino-1,3,5-triazine, cyanuric chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-methoxy-1,3,5-triazine; the inorganic material comprises at least one of boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon dioxide, tin dioxide, titanium dioxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, cerium oxide, zirconium titanate, barium titanate, and magnesium fluoride.

[0110] In one example, the adhesive layer comprises at least one of polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), sodium carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li), and polyvinylpyrrolidone (PVP).

[0111] In one example, the porosity of the membrane is 30%-55% (e.g., 30%, 35%, 40%, 45%, 50% or 55%).

[0112] If the membrane porosity is too low (e.g., <30%), it will reduce the electrolyte retention and increase the resistance to ion migration. If the porosity is too high (e.g., >55%), it will be accompanied by an increase in pore size, making it easy to be blocked by trace amounts of LLTO or silicon carbide materials generated during the cycle, resulting in uneven current distribution and exacerbating side reactions and capacity decay. This invention limits the membrane porosity to a specific range so that it can achieve a synergistic effect with the ultra-thin and low-powder-shedding characteristics of the surface coating. The excellent structure of the surface coating can minimize the pulverization and shedding of active materials during the cycle from the source. Even if trace amounts of dust are generated, the membrane can effectively accommodate these particles due to its suitable porosity, avoiding blockage of the pores and causing uneven local current distribution, thus maintaining uniform electrolyte wetting and ion migration.

[0113] In one example, the thermal shrinkage rate of the separator is ≤2% (e.g., 2%, 1.5%, 1%, 0.5%, or 0.1%) perpendicular to the winding direction of the lithium-ion secondary battery, and ≤2% (e.g., 2%, 1.5%, 1%, 0.5%, or 0.1%) along the winding direction of the lithium-ion secondary battery. This ensures the structural stability of the separator in high-temperature environments and effectively prevents the risk of short circuits due to large positive and negative contact areas caused by separator shrinkage.

[0114] In this invention, the lithium-ion secondary battery further includes an electrolyte, which comprises an organic solvent, a conductive lithium salt, and additives. The organic solvent includes carbonate solvents and / or carboxylic acid ester solvents. The carbonate solvents include cyclic carbonate solvents and chain carbonate solvents. The cyclic carbonate solvents include at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and fluoroethylene carbonate (FEC). The chain carbonate solvents include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC). The carboxylic acid ester solvents include at least one of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB). The electrolyte salt comprises at least one selected from lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium di(pentafluoroethylsulfonyl)imide, lithium tri(trifluoromethylsulfonyl)methyl, or lithium di(trifluoromethylsulfonyl)imide. The additive comprises at least one selected from vinylene carbonate, 1,3-propenesulfonyl lactone, vinyl vinyl carbonate, vinyl sulfate, butadionitrile, glutaronitrile, adiponitrile, heptanonitrile, octanoic acid, sebaconitrile, 1,3,6-hexanetrionitrile (HTCN), glycerol trionitrile, and 1,2-bis(2-cyanoethoxy)ethane.

[0115] The batteries can all be assembled in accordance with conventional methods in the field.

[0116] 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.

[0117] 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.

[0118] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0119] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0120] Example 1 Batteries are prepared according to the following method. (1) Preparation of positive electrode sheet LiNi 0.89 Co 0.05 Mn 0.05 Al 0.01 O2, PVDF, acetylene black, and single-walled carbon nanotubes were mixed in a mass ratio of 97.4:1.2:0.5:0.9. N-methylpyrrolidone was added as a solvent and the mixture was stirred under vacuum until it formed a uniform and fluid positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of an aluminum foil with a thickness of 9 μm. After baking, rolling, and slicing, the positive electrode sheet was obtained. (2) Preparation of negative electrode sheet Artificial graphite (average particle size 18.6 μm), sodium carboxymethyl cellulose, polyacrylic acid, styrene-butadiene rubber, Super-P, and single-walled carbon nanotubes were mixed in a mass ratio of 90:1:2:2:1.5:3.5, deionized water was added, and the mixture was stirred under vacuum to prepare the first negative electrode slurry. Then, artificial graphite (average particle size 6.9 μm), silicon carbide (S = 0.5), sodium carboxymethyl cellulose, polyacrylic acid, styrene-butadiene rubber, Super-P, and single-walled carbon nanotubes were mixed in a mass ratio of 50:40:1: The mixture of 2:2:1.5:3.5 was stirred with deionized water under vacuum to prepare the second negative electrode slurry. The first negative electrode slurry was then coated onto both sides of a 6μm thick carbon-coated copper foil and dried. The second negative electrode slurry was then coated onto the dried surfaces of the first negative electrode slurry and dried as well. Finally, using a 200-mesh, 45μm deep gravure roller, a surface coating was applied to the second negative electrode slurry surface (LLTO, PAA-Li, and SBR were mixed in a 95:1:4 mass ratio, and Li was added). 0.1 La 0.9 The negative electrode sheet was obtained by gravure coating with PO4 (Q is 6.25%), baking and drying in an oven at 80℃ for 10 hours, and then slitting. The surface coating layer is at least partially embedded in the negative electrode active layer; the surface scan area of ​​the surface coating layer is 250 μm. 2 In the cross-sectional scanning energy spectrum, the mass content of Ti was 2.4% and the mass content of La was 3.7%; the surface scan area of ​​the coating was 4120 μm. 2 In the surface scanning energy dispersive spectroscopy (SSD), the mass content of Ti was 14.7%, and the mass content of La was 24.5%; the W content was 8.6 mm, d4 was 2.6 μm, d5 was 0.05 μm, and the W / d4 ratio was 3308; the particle size Dv50 of the silicon-carbon material was 11.3 μm, the average particle size was 11.8 μm, and the conductivity was 20.3 S / cm; the mass content of silicon in the anode active layer was 20%; in the thermogravimetric analysis (TGA) test curve of the anode sheet, m1 was 3.133%, m2 was 3.9%, and m3 was 3.64%; (3) Battery preparation The positive electrode sheet, separator (a 5 μm thick polyethylene film coated with a 2 μm thick boehmite ceramic layer on one side of the polyethylene film, and then coated with a 1 μm thick PMMA adhesive layer on both sides) and negative 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 core is obtained by winding. The core is sealed with aluminum-plastic film and baked in vacuum for 24 hours to remove moisture. Then, the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DMC / EMC (volume ratio 1:1:1), 1.5% HTCN and 15% FEC are added, and the concentration of lithium hexafluorophosphate is 1 mol / L) is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained. The membrane has a porosity of 48%, a thermal shrinkage rate of 0.7% perpendicular to the winding direction of the lithium-ion secondary battery, and a thermal shrinkage rate of 0.4% along the winding direction of the lithium-ion secondary battery.

[0121] Example 2 Batteries are prepared according to the following method. (1) Preparation of positive electrode sheet LiNi 0.89 Co 0.05 Mn 0.05 Al 0.01 O2, PVDF, acetylene black, and single-walled carbon nanotubes were mixed in a mass ratio of 97.4:1.2:0.5:0.9. N-methylpyrrolidone was added as a solvent and the mixture was stirred under vacuum until it formed a uniform and fluid positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of an aluminum foil with a thickness of 9 μm. After baking, rolling, and slicing, the positive electrode sheet was obtained. (2) Preparation of negative electrode sheet Artificial graphite (average particle size 6.5 μm), sodium carboxymethyl cellulose, polyacrylic acid, styrene-butadiene rubber, Super-P, and single-walled carbon nanotubes were mixed in a mass ratio of 90:1:2:2:1.5:3.5, deionized water was added, and the mixture was stirred under vacuum to prepare the first negative electrode slurry. Then, artificial graphite (average particle size 2.1 μm), silicon carbide (S = 0.3), sodium carboxymethyl cellulose, polyacrylic acid, styrene-butadiene rubber, Super-P, and single-walled carbon nanotubes were mixed in a mass ratio of 50:40:1: The mixture of 2:2:1.5:3.5 was stirred with deionized water under vacuum to prepare the second negative electrode slurry. The first negative electrode slurry was then coated onto both sides of a 6μm thick carbon-coated copper foil and dried. The second negative electrode slurry was then coated onto the dried surfaces of the first negative electrode slurry and dried as well. Finally, using a 200-mesh, 45μm deep gravure roller, a surface coating was applied to the second negative electrode slurry surface (LLTO, PAA-Li, and SBR were mixed in a 95:1:4 mass ratio, and Li was added). 0.1 La 0.9 The negative electrode sheet was obtained by gravure coating with PO4 and Q (6%), baking and drying in an oven at 80℃ for 10 hours, and then slitting. The surface coating layer is at least partially embedded in the negative electrode active layer; the surface scan area of ​​the surface coating layer is 250 μm. 2 In the cross-sectional scanning energy spectrum, the mass content of Ti was 6.5% and the mass content of La was 9.5%; the surface scan area of ​​the coating was 4120 μm. 2 In the surface scanning energy dispersive spectroscopy (SSD), the mass content of Ti was 10.3%, and the mass content of La was 15.2%; W was 5.1 mm, d4 was 0.5 μm, d5 was 0 μm, and W / d4 was 10200; the particle size Dv50 of the silicon-carbon material was 2.2 μm, the average particle size was 5.2 μm, and the conductivity was 10.2 S / cm, with a silicon content of 20% by mass; in the thermogravimetric analysis (TGA) test curve of the negative electrode, m1 was 4.2%, m2 was 3.2%, and m3 was 2.1%; (3) Battery preparation The positive electrode sheet, separator (a 5 μm thick polyethylene film coated with a 2 μm thick boehmite ceramic layer on one side of the polyethylene film, and then coated with a 1 μm thick PMMA adhesive layer on both sides) and negative 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 core is obtained by winding. The core is sealed with aluminum-plastic film and baked in vacuum for 24 hours to remove moisture. Then, the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DMC / EMC (volume ratio 1:1:1), 1.5% HTCN and 15% FEC are added, and the concentration of lithium hexafluorophosphate is 1 mol / L) is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained. The membrane has a porosity of 32%, a thermal shrinkage rate of 0.7% perpendicular to the winding direction of the lithium-ion secondary battery, and a thermal shrinkage rate of 0.4% along the winding direction of the lithium-ion secondary battery.

[0122] Example 3 Batteries are prepared according to the following method. (1) Preparation of positive electrode sheet LiNi 0.89 Co 0.05 Mn 0.05 Al 0.01 O2, PVDF, acetylene black, and single-walled carbon nanotubes were mixed in a mass ratio of 97.4:1.2:0.5:0.9. N-methylpyrrolidone was added as a solvent and the mixture was stirred under vacuum until it formed a uniform and fluid positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of an aluminum foil with a thickness of 9 μm. After baking, rolling, and slicing, the positive electrode sheet was obtained. (2) Preparation of negative electrode sheet Artificial graphite (average particle size 29.7 μm), sodium carboxymethyl cellulose, polyacrylic acid, styrene-butadiene rubber, Super-P, and single-walled carbon nanotubes were mixed in a mass ratio of 90:1:2:2:1.5:3.5, deionized water was added, and the mixture was stirred under vacuum to prepare the first negative electrode slurry. Then, artificial graphite (average particle size 9.3 μm), silicon carbide (S = 0.75), sodium carboxymethyl cellulose, polyacrylic acid, styrene-butadiene rubber, Super-P, and single-walled carbon nanotubes were mixed in a mass ratio of 50:40:1. The mixture of 2:2:1.5:3.5 was stirred with deionized water under vacuum to prepare the second negative electrode slurry. The first negative electrode slurry was then coated onto both sides of a 6μm thick carbon-coated copper foil and dried. The second negative electrode slurry was then coated onto the dried surfaces of the first negative electrode slurry and dried as well. Finally, using a 200-mesh, 45μm deep gravure roller, a surface coating was applied to the second negative electrode slurry surface (LLTO, PAA-Li, and SBR were mixed in a 95:1:4 mass ratio, and then Li was added). 0.1 La 0.9 The negative electrode sheet was obtained by gravure coating with PO4 (Q is 6.5%), baking and drying in an oven at 80℃ for 10 hours, and then slitting. The surface coating layer is at least partially embedded in the negative electrode active layer; the surface scan area of ​​the surface coating layer is 250 μm. 2 In the cross-sectional scanning energy spectrum, the mass content of Ti was 9.6% and the mass content of La was 5.8%; the surface scan area of ​​the coating was 4120 μm. 2 In the surface scanning energy dispersive spectroscopy (SSD), the mass content of Ti was 18.9%, and the mass content of La was 38.7%; the W value was 14.5 mm, d4 was 4.8 μm, d5 was 0.1 μm, and the W / d4 ratio was 3021; ​​the particle size Dv50 of the silicon-carbon material was 17.9 μm, the average particle size was 14.8 μm, and the conductivity was 26.5 S / cm, with a silicon content of 20% by mass; in the thermogravimetric analysis (TGA) test curve of the negative electrode, m1 was 4.8%, m2 was 3.5%, and m3 was 3.7%; (3) Battery preparation The positive electrode sheet, separator (a 5 μm thick polyethylene film coated with a 2 μm thick boehmite ceramic layer on one side of the polyethylene film, and then coated with a 1 μm thick PMMA adhesive layer on both sides) and negative 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 core is obtained by winding. The core is sealed with aluminum-plastic film and baked in vacuum for 24 hours to remove moisture. Then, the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DMC / EMC (volume ratio 1:1:1), 1.5% HTCN and 15% FEC are added, and the concentration of lithium hexafluorophosphate is 1 mol / L) is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained. The membrane has a porosity of 55%, a thermal shrinkage rate of 0.7% perpendicular to the winding direction of the lithium-ion secondary battery, and a thermal shrinkage rate of 0.4% along the winding direction of the lithium-ion secondary battery.

[0123] The relevant parameters of the negative electrode coating in Example 1 are shown in Tables 1-1 and 1-2. Examples 2-7 were carried out with reference to Example 1, except that the relevant parameters of the negative electrode coating were changed, as shown in Tables 1-1 and 1-2.

[0124] Example 8 group This set of embodiments is based on Embodiment 1, except that the elemental composition of LLTO is changed, as follows: Example 8a: LLTO does not contain Al or Sr elements; Example 8b: LLTO does not contain Al.

[0125] Example 9 This embodiment is based on Embodiment 1, except that the surface coating does not contain LLPO.

[0126] Example 10 This embodiment is based on Embodiment 1, except that the composition of the surface coating is changed, as follows: LLTO, PAA-Li, SBR, and carbon nanotubes were mixed in a mass ratio of 94.5:1:4:0.5, and then Li was added. 0.1 La 0.9 PO4, Q is 6.25%.

[0127] Example 11 This embodiment is based on Embodiment 1, except that the thickness d4 of the surface coating in the arc area is the same as the thickness d5 of the surface coating in the straight area, that is, d5 is 2.6μm.

[0128] Example 12 group This set of embodiments follows the same procedure as Embodiment 1, except that W / d4 is adjusted by changing the width W of the arc region and the thickness d4 of the surface coating in the arc region, as detailed below: Example 12a, W / d4 is 500, W is 5 mm, and d4 is 10 μm; Example 12b, W / d4 is 30000, W is 15mm, and d4 is 0.5μm.

[0129] Unless otherwise stated, the lithium-ion secondary batteries of Examples 1 to 12 all meet the following requirements: the surface coating is at least partially embedded in the negative electrode active layer; the surface scan area of ​​the surface coating is 256 μm. 2 In a cross-sectional energy dispersive spectroscopy (e.g., 16 μm × 16 μm), the mass content of Ti is 1%-10%, and the mass content of La is 2%-10%; the surface scan area of ​​the coating is 4225 μm. 2 In the surface scanning energy spectrum (e.g., 65μm×65μm), the mass content of Ti is 10%-20% and the mass content of La is 15%-40%; the thermogravimetric analysis test curve of the negative electrode includes at least two weight loss intervals and one weight gain interval in the range of 30℃-1000℃. The first weight loss interval is located in the range of 30℃-325℃, the second weight loss interval is located in the range of 325℃-580℃, and the first weight gain interval is located in the range of 580℃-1000℃; the weight loss rate of the first weight loss interval is m1, the weight loss rate of the second weight loss interval is m2, and the weight gain rate of the first weight gain interval is m3, with 3%≤m1≤5%, 3%≤m2≤4.5%, and 2%≤m2≤4%.

[0130] Comparative Example 1 This comparative example is based on Example 1, except that the negative electrode does not contain a coating containing a solid electrolyte.

[0131] Comparative Example 2 This comparative example is based on Example 1, except that the composition of the surface coating is changed, as follows: Lithium aluminum titanium phosphate (LATP), PAA-Li, and SBR were mixed in a mass ratio of 95:1:4, and then Li was added. 0.1 La 0.9 PO4, Q is 6.25%.

[0132] Comparative Examples 3 and 4 were carried out in accordance with Example 1, except that the relevant parameters of the negative electrode coating were changed, as shown in Tables 1-1 and 1-2.

[0133] Test case (1) K-value test K is the voltage decay value of the battery per hour, mV / h; the internal resistance of the voltage was tested at different times, and the results were recorded in Table 2 using the formula K=(OCV1-OCV2) / time interval between two tests.

[0134] (2) Loop testing The batteries prepared in the examples and comparative examples were subjected to cycle tests, and the specific test methods are as follows: The battery was left to stand at 45℃±2℃ for 10 minutes, then discharged at 0.5C to 2.5V and left to stand for 10 minutes. It was then charged at 1.8C to the upper cutoff voltage limit of 4.35V, cut off at 0.05C, and left to stand for 10 minutes. Finally, it was discharged at 3C to 2.5V and left to stand for 10 minutes, yielding the initial capacity C0. This charging and discharging process was repeated until the 300th constant-voltage charging cycle ended, followed by a 10-minute stand. Then, the battery was discharged again at a 3C current density to 2.5V and left to stand for 10 minutes. The discharge capacity at this point was recorded as the post-cycle capacity C1. The cycle capacity retention rate is calculated as C1×100% / C0, and the results are shown in Table 2.

[0135] (3) Lithium plating test (fast charging performance) The lithium-ion batteries prepared in the examples and comparative examples were subjected to lithium plating tests. The specific test methods are as follows: The resulting batteries were charged at 5°C at a rate of 0.5C to a cutoff voltage of 4.35V and a cutoff current of 0.025C. After standing for 5 minutes, they were discharged at a rate of 3C to a cutoff voltage of 2.5V. This charge-discharge cycle was repeated for 20 cycles. After charging at a rate of 0.5C to the cutoff voltage and cutoff current, and standing for 1 hour, the batteries were disassembled to observe the lithium deposition status on the surface of the negative electrode. The criteria for judging the lithium deposition phenomenon on the negative electrode were: 1) No lithium deposition; 2) Lithium deposition on the top, bottom, or creases of the negative electrode, with the total lithium deposition area accounting for less than 10% of the negative electrode, was recorded as slight lithium deposition; 3) Lithium deposition area covering 10%-50% of the entire electrode surface was recorded as lithium deposition; 4) Lithium deposition area covering more than 50% of the entire electrode surface was recorded as severe lithium deposition. The results are shown in Table 2.

[0136] (4) Energy density test The energy density of the lithium-ion secondary batteries prepared in the examples and comparative examples was tested, and the specific testing methods are as follows: At 25℃, the battery mass was measured as m1 kg. The battery was charged at 0.5C to the upper limit voltage of 4.35V and discharged at 0.5C to the lower limit voltage of 2.5V. The above charge and discharge cycle was repeated twice. The energy of the second discharge was selected as the battery discharge energy W1Wh. The energy density was calculated as W1 / m1 Wh / kg. The results are shown in Table 2.

[0137] (5) Low-temperature discharge test The lithium-ion batteries prepared in the examples and comparative examples were subjected to low-temperature discharge tests. The specific test methods are as follows: At 25℃±3℃, a fully charged battery was discharged at 0.5C to the lower limit voltage of 2.5V, left to stand for 10 minutes, and the initial discharge capacity Q0 was recorded. The battery was then placed in a -10℃ constant temperature chamber and fully charged at a constant current of 0.5C, with a cutoff current of 0.02C. After storage for 6 hours, the battery was discharged at a constant current of 0.2C, and the discharge capacity Q1 was recorded. The low-temperature discharge capacity retention rate = Q1 / Q0 × 100%, and the results are shown in Table 2.

[0138] Table 1-1 Table 1-2 Table 2 As can be seen from Table 2, compared with the comparative example, the battery of the present invention can significantly reduce the K value and does not exhibit lithium plating or only slight lithium plating under high-rate charge and discharge conditions, thus significantly improving the fast-charging performance and safety performance of the battery; the battery of the present invention has high energy density as well as excellent cycle performance and low-temperature discharge performance.

[0139] 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 negative electrode sheet, characterized in that, The negative electrode sheet includes a negative current collector and a negative electrode coating disposed on at least one side of the surface of the negative current collector. The negative electrode coating includes a negative active layer and a surface coating located on the surface of the negative active layer away from the surface of the negative current collector. The surface coating includes a solid electrolyte, and the solid electrolyte includes lithium lanthanum titanium oxide. The negative electrode active layer includes a negative electrode material, which includes silicon-carbon material, and the sphericity S of the silicon-carbon material satisfies: 0.3 ≤ S ≤ 0.8; the mass content of Si element (C1) in the negative electrode coating is 8%-70%; The molar ratio of Ti to La in the negative electrode coating is 1.6-2; The surface coating is at least partially embedded in the negative electrode active layer, and the Ti and / or La elements are at least partially located on the surface of the silicon-carbon material; The embedding depth d1 of the surface coating is 1μm-8μm; The mass content C2 of Ti in the negative electrode coating and the embedding depth d1 of the surface coating satisfy the following: 500≤C2 / d1≤9000, where d1 is in μm and C2 is in ppm. The embedding depth d1 of the surface coating and the mass content C1 of Si element in the negative electrode active layer satisfy the following condition: 0.02≤d1 / C1≤1, where d1 is in μm and C1 is in % (%). The thickness d2 of the surface coating and the embedding depth d1 of the surface coating satisfy the condition: 0.1≤d2 / d1≤1.

2. The negative electrode sheet according to claim 1, wherein, The silicon-carbon material includes a porous carbon matrix and silicon material located in the internal channels of the porous carbon matrix; And / or, d2 is 0.5μm-10μm; preferably 0.5μm-5μm; And / or, the thickness d3 of the negative electrode active layer is 25μm-75μm; Preferably, 10 ≤ d3 / d2 ≤ 100; more preferably, 20 ≤ d3 / d2 ≤ 50; And / or, the particle size Dv50 of the lithium lanthanum titanium oxide is 150nm-1000nm; preferably, the particle size Dv50 of the lithium lanthanum titanium oxide is 200nm-600nm. Preferably, the thickness d2 of the surface coating and the particle size Dv50 of the lithium lanthanum titanium oxide satisfy the following condition: 1≤d2 / Dv50≤100.

3. The negative electrode sheet according to claim 1 or 2, wherein, The ionic conductivity of the lithium lanthanum titanium oxide is 0.05 ms / cm-1 ms / cm; And / or, the lithium lanthanum titanium oxide further includes Al and / or Sr elements; And / or, the mass content of Ti (C2) in the negative electrode coating is 2700ppm-20000ppm; And / or, the mass content of Li (C3) in the negative electrode coating is 45ppm-1500ppm; And / or, the mass content of La (C4) in the negative electrode coating is 4500ppm-30000ppm; Preferably, the mass content of Al (C5) in the negative electrode coating is 250 ppm to 2000 ppm; Preferably, the mass content of Sr (C6) in the negative electrode coating is 500ppm-4000ppm; And / or, the surface scan area of ​​the surface coating is 256 μm. 2 In the cross-sectional scanning energy spectrum, the mass content of Ti is 1%-10%, and the mass content of La is 2%-10%. And / or, the surface scan area of ​​the surface coating is 4225 μm. 2 In the surface scanning energy spectrum, the mass content of Ti is 10%-20% and the mass content of La is 15%-40%.

4. The negative electrode sheet according to claim 1 or 2, wherein, The particle size Dv50 of the silicon carbide material is 2μm-20μm; And / or, the average particle size of the silicon-carbon material is 1μm-20μm; preferably 2μm-15μm; And / or, the electrical conductivity of the silicon-carbon material is 0.01 S / cm-30 S / cm; preferably 10 S / cm-30 S / cm; And / or, the silicon content in the silicon-carbon material is 20%-80% by mass; preferably 30%-70%.

5. The negative electrode sheet according to claim 1 or 2, wherein, The negative electrode active layer includes a first active layer and a second active layer disposed along the thickness direction of the negative electrode sheet, wherein the first active layer is located between the negative electrode current collector and the second active layer; the first active layer includes a first carbon-based material, and the second active layer includes the silicon-carbon material. Preferably, the average particle size of the first carbon-based material is 6 μm-30 μm; Preferably, the first carbon-based material includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

6. The negative electrode according to claim 1 or 2, wherein, The solid electrolyte further includes lithium lanthanum phosphate, wherein the molar ratio of La to P in the lithium lanthanum phosphate is (0.85-1.15):1; preferably (0.9-1.1):

1. Preferably, the mass content Q of lithium lanthanum phosphate in the surface coating is 3.75%-22.5%.

7. The negative electrode according to claim 1 or 2, wherein, The thermogravimetric analysis test curve of the negative electrode sheet includes at least two weight loss intervals and one weight gain interval in the range of 30℃ to 1000℃. The first weight loss interval is located in the range of 30℃ to 325℃, the second weight loss interval is located in the range of 325℃ to 580℃, and the first weight gain interval is located in the range of 580℃ to 1000℃. The weight loss rate of the first weight loss interval is m1, the weight loss rate of the second weight loss interval is m2, and the weight gain rate of the first weight gain interval is m3. Preferably, 3%≤m1≤5%, 3%≤m2≤4.5%, and 2%≤m2≤4%.

8. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes a positive electrode, a separator, and a negative electrode as described in any one of claims 1-7; the positive electrode, the separator, and the negative electrode are stacked and wound to form a core; the lithium-ion secondary battery has an arc-shaped region and a straight region located between the arc-shaped regions; Preferably, the thickness d4 of the surface coating in the arc region is 0.5μm-10μm; more preferably, d4 is 0.5μm-5μm. Preferably, the thickness of the surface coating in the flat area is d5, where 0 ≤ d5 < 0.2 μm; Preferably, the width W of the arc region along the winding direction of the lithium-ion secondary battery is 5mm-15mm; More preferably, 500 ≤ W / d4 ≤ 30000; even more preferably, 3000 ≤ W / d4 ≤ 11000.

9. The lithium-ion secondary battery according to claim 8, wherein, The positive electrode sheet includes a positive electrode material, which includes a nickel-cobalt-manganese ternary material and / or a nickel-cobalt-aluminum ternary material. Preferably, the chemical formula of the nickel-cobalt-manganese ternary material is Li. b Ni c Co d Mn e A f O2, wherein 0.85≤b≤1.05, 0.9≤c≤0.98, 0.01≤d≤0.05, 0.01≤e≤0.05, 0≤f≤0.05, and A includes at least one of Mg, Al, Ti, Zr, W, B, Nb, Ta, Y, and F; Preferably, the chemical formula of the nickel-cobalt-aluminum ternary material is Li. g Ni h Co j Al k B l O2, wherein 0.85≤g≤1.05, 0.9≤h≤0.98, 0.01≤j≤0.05, 0.01≤k≤0.05, 0≤l≤0.05, and B includes at least one of Mg, Ti, Zr, W, Nb, Ta, B, Y, and F; Preferably, the charging cutoff voltage of the lithium-ion secondary battery is ≥4.3V.

10. The lithium-ion secondary battery according to claim 8, wherein, The diaphragm includes a substrate layer, a ceramic layer located on at least one side of the substrate layer, and an adhesive layer located on both outer surfaces of the diaphragm. Preferably, the porosity of the diaphragm is 30%-55%; Preferably, the thermal shrinkage rate of the separator is ≤2% perpendicular to the winding direction of the lithium-ion secondary battery and ≤2% along the winding direction of the lithium-ion secondary battery.

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