Lithium ion secondary battery

Abstract

This invention relates to the field of battery technology, specifically to a lithium-ion secondary battery. It includes a positive electrode and an electrolyte. The positive electrode includes a positive current collector, a positive active coating, and a surface coating. The positive active coating is located between the positive current collector and the surface coating. The surface coating includes first inorganic particles with a median particle size Dv50 of d1, where d1 is 1 nm–1000 nm. The electrolyte includes a carboxylic acid ester and a first nitrile additive. The mass content of the first nitrile additive in the electrolyte is c1, where c1 is 0.2%–3%. d1 and c1 satisfy the condition: d1 / c1 = 15–5000, where d1 is in nm and c1 is in %. The lithium-ion secondary battery of this invention can balance furnace temperature safety, high-temperature cycle stability, and high-temperature storage safety.

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 rechargeable battery technology, especially in applications targeting high-voltage systems, furnace temperature test pass rate has become a key performance indicator to ensure the battery's safety under abuse conditions such as high temperatures. Currently, the conventional approach to improve the furnace temperature test pass rate is to add a safety coating to the surface of the positive electrode active layer to reduce side reactions between the electrolyte and the positive electrode active material, thereby improving the furnace temperature test pass rate. This is because the safety coating on the surface of the positive electrode can effectively build a protective barrier, inhibit oxygen evolution at the positive electrode, block heat conduction, and suppress interfacial side reactions, significantly improving battery thermal safety and furnace temperature stability. However, in practical applications, it has been found that this setup degrades the battery's high-temperature cycling and high-temperature storage performance.

[0003] Therefore, while ensuring that the battery has a good furnace temperature test pass rate, it is necessary to improve the battery's high-temperature cycle performance and high-temperature storage performance. Summary of the Invention

[0004] 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) constructs a physical barrier by forming a surface coating containing first inorganic particles with a median particle size of 1nm-1000nm on the surface of the positive electrode active layer. Simultaneously, a first nitrile additive with a mass content of 0.2%-3% is introduced into the electrolyte, and the ratio of the median particle size of the first inorganic particles to the content of the first nitrile additive is limited to a certain range. The battery of this invention achieves a dynamic balance between the physical isolation function of the surface coating and the chemical repair function of the additive in the electrolyte. The first nitrile additive can target and repair defects such as porosity and interfacial microcracks in the surface coating, while the solvent system ensures the lithium-ion transport efficiency, synergistically improving the battery's furnace temperature safety, high-temperature cycle stability, and high-temperature storage performance.

[0005] In related technologies, while the surface coating of lithium-ion secondary batteries passively provides physical isolation to the surface of the positive electrode active material under high-temperature conditions, there is an inherent contradiction between the density of the surface coating and the lithium-ion transport efficiency. When the density of the surface coating is high, although it can temporarily prevent large-area contact between the electrolyte and the positive electrode active material, it significantly increases the transport resistance of lithium ions at the electrode interface. Especially in high-temperature environments, the ion transport resistance is further amplified, leading to intensified interfacial polarization, increased internal resistance of the battery, accelerated capacity decay, and a significant reduction in the battery's high-temperature cycle stability and high-temperature storage performance.

[0006] Analysis revealed that the primary reason for the high-temperature performance degradation caused by the surface coating is that it cannot fundamentally eliminate trace amounts of harmful substances (such as HF or trace moisture) that have penetrated its pores. Micropores, edge cracks, or residual active sites inevitably exist in the surface coating during its preparation and use; these defects constitute channels for the penetration of harmful substances. Under high temperatures and charge-discharge cycle stress, the surface coating may also crack or peel off, further amplifying existing defects. Furthermore, the surface coating is a static protective layer on the positive electrode surface, lacking the ability to self-repair these micro-damages during long-term cycling. This leads to the continuous accumulation of damage at the electrode interface with increasing cycle count, ultimately resulting in the degradation of the battery's high-temperature cycle performance and high-temperature storage performance.

[0007] To address the aforementioned problems, this invention proposes a synergistic system for a positive electrode and an electrolyte. Specifically, this synergistic system includes a surface coating layer disposed on the surface of the active coating layer of the positive electrode and an electrolyte with a specific formulation. The surface coating layer comprises first inorganic particles, the median particle size of which is limited to the range of 1 nm to 1000 nm. The electrolyte comprises a first nitrile additive, and the mass content of the first nitrile additive in the electrolyte is 0.2% to 3%. The solvent component in the electrolyte comprises a carboxylic acid ester. The key to achieving synergistic effects between the physical coating and the chemical additive lies in the specific numerical relationship between the median particle size d1 of the first inorganic particles and the mass content c1 of the first nitrile additive, i.e., d1 / c1 falling within the range of 15 to 5000.

[0008] In this synergistic system, the surface coating acts as the primary physical barrier, reducing the permeation flux of most harmful substances to a low level. The first nitrile additive in the electrolyte performs a chemical repair function: its cyano functional groups improve the wettability of the electrolyte on the dense surface coating, compensating for the wetting hysteresis defects caused by the nanoscale structure of the coating and ensuring unobstructed liquid-phase ion transport channels at the interface. Simultaneously, this nitrile additive can target the pore walls, edge cracks, or residual sites on the surface of the positive electrode active material of the surface coating, chemically anchoring or participating in the construction of a denser electrode / electrolyte interface film, forming a chemical repair layer for these micro-defects. Compared to carbonate solvents, carboxylic acid ester solvents have lower viscosity, which helps to regulate the overall solvation structure of the electrolyte, reducing the transport resistance of lithium ions in the electrolyte liquid phase and interface film, thus ensuring ion transport efficiency without compromising the physical barrier function of the surface coating. By limiting the ratio of d1 to c1 to within the range of 15 to 5000, a proper balance can be achieved between the density of the first inorganic particles in the coating layer, the wetting ability of the electrolyte in the coating layer, and the interfacial chemical repair ability of the first nitrile additive. When the ratio is too small, the coating layer may be too dense and the additives may be excessive, leading to a surge in interfacial impedance, which is detrimental to the high-temperature cycle stability and high-temperature storage performance of the battery. When the ratio is too large, the physical barrier effect of the coating layer may be insufficient and the additives may be relatively scarce, making it difficult to achieve effective chemical repair, which will also lead to a deterioration in high-temperature cycle stability and high-temperature storage performance.

[0009] Based on this, the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode and an electrolyte; the positive electrode comprises a positive current collector, a positive active coating, and a surface coating, the positive active coating being located between the positive current collector and the surface coating, the surface coating comprising first inorganic particles, the median particle size Dv50 of the first inorganic particles being d1, where d1 is 1nm-1000nm; the electrolyte comprises a carboxylic acid ester and a first nitrile additive; the first nitrile additive comprises... , , and At least one of the following; the mass content of the first nitrile additive in the electrolyte is c1, where c1 is 0.2%-3%; d1 (in nm) and c1 (in %) satisfy: d1 / c1 is 15-5000.

[0010] Through the above technical solution, the present invention has at least the following advantages compared with the prior art: the lithium-ion secondary battery of the present invention can take into account furnace temperature safety performance, high temperature cycle stability and high temperature storage safety performance.

[0011] 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

[0012] Figure 1 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

[0013] 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.

[0014] This invention provides a lithium-ion secondary battery, comprising a positive electrode and an electrolyte. The positive electrode comprises a positive current collector, a positive active coating, and a surface coating. The positive active coating is located between the positive current collector and the surface coating. The surface coating comprises first inorganic particles with a median particle size Dv50 of d1, where d1 is 1 nm-1000 nm, for example, 1 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm. The electrolyte comprises a carboxylic acid ester and a first nitrile additive; the first nitrile additive includes… (I-1) (I-2) (I-3) and At least one of (I-4). The mass content of the first nitrile additive in the electrolyte is c1, where c1 is 0.2%-3%, for example, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5% or 3%. d1 (in nm) and c1 (in %) satisfy: d1 / c1 is 15-5000, for example, 15, 18, 20, 30, 40, 45, 47, 50, 60, 65, 70, 80, 90, 100, 150, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, 4000 or 5000.

[0015] It is understandable that when calculating d1 / c1, the units of d1 and c1 are not included in the calculation, and only the numerical part is selected for calculation. For example, in Example 1, d1 is 219nm and c1 is 1.1%, then d1 / c1 is 219 / 1.1=199.09.

[0016] The inventors discovered that, in related technologies, a safety coating is often applied to the surface of the positive electrode to improve the furnace temperature test pass rate of lithium-ion secondary batteries. This type of coating is typically composed of inorganic oxides, and its main mechanism of action is to form a physical barrier, preventing direct contact between the electrolyte and the positive electrode active material over a large area, thereby suppressing oxygen evolution and interfacial side reactions of the positive electrode material at high temperatures. However, this single physical barrier has inherent performance contradictions. On the one hand, to achieve better barrier effects, the safety coating needs to be highly dense, but this severely hinders lithium-ion transport, increases the battery's internal resistance, and especially degrades the battery's high-temperature cycling and storage performance. On the other hand, even the densest safety coating still has microscopic pores and defects within it, making it impossible to completely eliminate the penetration of trace amounts of harmful substances (HF and water) from the electrolyte. During long-term high-temperature cycling, the positive electrode active material develops microcracks due to repeated volume strain, and a single physical coating cannot repair these dynamically generated defects, thus limiting its long-term protective capability.

[0017] The inventors of this invention discovered that the root of this problem lies in treating the physical isolation of the safety coating and the chemical repair of the electrolyte as two independent and separate protective directions. By constructing a synergistic system comprising a specific surface coating and a specific electrolyte, a synergistic effect of physical isolation and chemical repair can be achieved. The core of this system is to set a surface coating containing first inorganic particles with a median particle size in the range of 1 nm to 1000 nm on the surface of the positive electrode, while introducing a first nitrile additive with a mass content of 0.2% to 3% into the electrolyte, and strictly controlling the ratio of the median particle size of the first inorganic particles to the mass content of the first nitrile additive to be between 15 and 5000.

[0018] In this synergistic system, the surface coating acts as the main body of the physical barrier, and the selection of the median particle size d1 of its first inorganic particles is crucial. The size of d1 directly affects the pore structure of the surface coating and its wettability to the electrolyte. When d1 is too small (e.g., less than 1 nm), the first inorganic particles are too fine, resulting in a sharp increase in specific surface area. This requires a large amount of binder to ensure the cohesion of the surface coating and its adhesion to the active coating. This can lead to excessive coating of the first inorganic particles by the binder, which severely reduces the porosity of the surface coating, hinders electrolyte wetting and lithium-ion transport, and exacerbates concentration polarization. Conversely, when d1 is too large (e.g., greater than 1000 nm), the first inorganic particles are too large, and the surface coating formed by their accumulation exhibits a hollow structure with excessively large internal pore sizes. Harmful substances such as HF can easily enter, leading to the failure of the physical barrier. Therefore, d1 is limited to 1 nm-1000 nm.

[0019] It is understood that the median particle size Dv50, or d1, of the first inorganic particles has a conventional meaning in the art, referring to the particle size corresponding to a cumulative distribution percentage of 50% based on the volume distribution of the particle group. It can be determined by laser particle size analysis, for example, using a Malvern laser particle size analyzer.

[0020] The first nitrile additive in the electrolyte plays a crucial role in defect repair. This first nitrile additive molecule contains three cyano functional groups, exhibiting strong polarity. Its mechanism of action manifests in three aspects. First, wettability compensation. The cyano functional groups can reduce the contact angle of the electrolyte on the dense surface coating, improving the wetting hysteresis problem caused by the low porosity of the nanoscale surface coating and ensuring unobstructed ion transport channels in the liquid phase. Second, interfacial chemical anchoring. The cyano groups can coordinate with metal atoms on the surface of the surface coating particles or transition metal sites on the surface of the positive electrode active material, forming a chemical graft layer in situ at the interface between the surface coating and the electrolyte. This grafting is not a simple physical adsorption, but rather a permanent anchoring at the interface through stable coordination bonds, making it difficult to desorb under high-temperature conditions and endowing the interface with long-term stability. Third, enhancement of the positive electrode electrolyte interface film. The first nitrile additive participates in the construction of the solid electrolyte interface (CEI) film on the positive electrode surface, forming a dense film layer rich in cyano coordination structures, further enhancing the chemical stability of the interface and inhibiting the continuous oxidative decomposition of electrolyte components. The mass content (c1) of the first nitrile additive is limited to 0.2%-3%. When c1 is low (e.g., less than 0.2%), the amount of the first nitrile additive is relatively insufficient, failing to form a complete and effective chemical anchoring and repair layer on the surface of the coating pores, microcracks, and active sites. It also lacks the ability to complex protic acids such as HF, resulting in insignificant improvement in high-temperature performance. Conversely, when c1 is high (e.g., greater than 3%), the first nitrile additive is relatively excessive, leading to the continuous formation of an excessively thick CEI film on the cathode surface. This results in a sharp increase in interfacial impedance and intensified polarization, which in turn degrades the battery's high-temperature cycle stability and high-temperature storage performance.

[0021] In this invention, the mass content c1 of the first nitrile additive in the electrolyte can be obtained by gas chromatography (GC).

[0022] Crucially, d1 / c1 is controlled within the range of 15-5000. This ratio reflects the matching relationship between the microstructural characteristics of the physical barrier (surface coating) and the concentration of the chemical repair agent (first nitrile additive). The inventors of this invention discovered that when this ratio is within the range of 15-5000, the density of the surface coating and its wettability to the electrolyte are balanced, and the concentration of the first nitrile additive is just sufficient to precisely chemically repair the pore walls, edge cracks, and residual sites on the active surface of the surface coating, filling the defects of the physical barrier, forming a protective layer, and achieving the optimal balance between physical isolation and chemical repair. When d1 / c1 is small (e.g., less than 15), it indicates that the median particle size of the first inorganic particles is too small or the content of the first nitrile additive is too high. This will lead to a serious decrease in the porosity of the surface coating and an increase in impedance. At the same time, an excessively thick CEI film will further increase the interfacial impedance, degrading the high-temperature cycle stability and high-temperature storage performance of the battery. When d1 / c1 is large (e.g., greater than 5000), it indicates that the median particle size of the first inorganic particles is too large or the content of the first nitrile additive is too low. This will lead to excessively large pores in the surface coating, failure of the physical barrier function, and insufficient repair of defects and complexation of harmful substances by the first nitrile additive, thus failing to effectively suppress side reactions.

[0023] Meanwhile, the first nitrile additives shown in I-1, I-2, I-3 and I-4 are short-chain (e.g., the number of C atoms on the carbon chain is less than or equal to 4) trinitriles, which have stronger diffusion capabilities and can penetrate to the lattice defects not covered by the long-chain trinitrile adsorption layer. They react with trace amounts of HF to generate amide-acyl fluoride oligomers, which fill the microcracks caused by the escape of oxygen from the lattice, thus repairing the cathode structure and constructing a low-impedance lithium conduction channel.

[0024] In one example, d1 ranges from 100 nm to 500 nm. Further limiting the median particle size of the first inorganic particles within this range allows for a more optimized density and pore structure in the surface coating. With this relatively narrow particle size distribution, the pore size and tortuosity formed between the first inorganic particles are more uniform, ensuring that the surface coating has a sufficiently high physical barrier capability while also providing ample, but not excessive, channels for electrolyte wetting and lithium-ion transport, thereby further optimizing the balance between physical barrier and ion transport.

[0025] In one example, c1 is 0.5%-1.5%. By further limiting the mass content of the first nitrile additive within this range, the concentration of the first nitrile additive can be more precisely matched to a narrower range of the median particle size of the first inorganic particles. This concentration is sufficient to form a continuous, thin, and dense chemical anchoring layer on the inner walls of the surface coating pores and at microcracks, effectively repairing the interface while avoiding the formation of an excessively thick, high-resistivity interfacial film due to an excessive amount of the first nitrile additive.

[0026] In one example, d1 / c1 ranges from 65 to 1000. Within this range, the microstructure of the surface coating and the amount of the first nitrile additive achieve a more refined match. A better balance is achieved between the density of the surface coating and the wettability of the electrolyte, as well as the film-forming and anchoring effects of the first nitrile additive. This more effectively suppresses interfacial side reactions at high temperatures, reduces interfacial impedance, and thus further improves the battery's high-temperature cycle life and thermal safety.

[0027] In one instance, d1 / c1 is 150-350.

[0028] <Positive Electrode Tablets> In one example, the thickness of the surface coating is 0.5 μm-5 μm, for example, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm. The thickness of the surface coating can be tested using conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the positive electrode sheet, immersing it in dimethyl carbonate (DMC) solvent for 12 hours; then rinsing with DMC solvent to remove lithium salts adhering to the positive electrode sheet, cutting the positive electrode sheet along the thickness direction using an argon ion mill (CP), and measuring the thickness of the surface coating at at least 20 randomly selected points on the surface coating using a scanning electron microscope (SEM), and taking the average value.

[0029] In this invention, the positive current collector has a first surface and a second surface disposed opposite to each other. The positive electrode sheet includes at least a first region and a second region, the first region including the positive current collector and the positive active coating located on both sides (the first surface and the second surface) of the positive current collector, and the second region including the positive current collector and the positive active coating located on the first surface of the positive current collector.

[0030] In one example, the first inorganic particle comprises at least one of silicon dioxide, zirconium oxide, lithium silicate, lithium iron phosphate, magnesium oxide, boehmite, aluminum oxide, lithium lanthanum zirconium oxide (LLZO), lithium titanium aluminum phosphate (LATP), and lithium lanthanum titanium oxide (LLTO).

[0031] In one example, the surface coating further includes a first adhesive. The first adhesive includes at least one selected from polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, and perfluorosulfonic acid ionomer.

[0032] In one example, the positive electrode further includes a base coating. The base coating is located between the positive current collector and the positive active coating.

[0033] In one example, the base coating comprises second inorganic particles. The second inorganic particles comprise at least one of silica, zirconium oxide, lithium silicate, lithium iron phosphate, magnesium oxide, boehmite, alumina, LLZO, LATP, and LLTO.

[0034] In one example, the median particle size Dv50 of the second inorganic particle is d2, which is 1 nm to 1000 nm, for example, 10 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm. d2 can be determined by laser particle size analysis, for example, using a Malvern laser particle size analyzer.

[0035] When the median particle size Dv50 of the second inorganic particles is within the above range, it is beneficial to accumulate more particles under the same thickness conditions, thereby further improving the furnace temperature safety performance of the battery.

[0036] In one example, the base coating further includes a second adhesive comprising at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, and perfluorosulfonic acid ionomer.

[0037] In one example, the base coating further includes a second conductive agent. The second conductive agent includes, for example, at least one selected from conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, and carbon fiber.

[0038] In one example, the thickness of the base coating is 0.5 μm-5 μm, for example, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm. The thickness of the base coating can be tested with reference to the top coating, which will not be described in detail here.

[0039] In one example, the second region of the positive electrode further includes a ceramic layer. The ceramic layer is located on the second surface of the positive current collector. Figure 1The diagram shows a cross-sectional view of the positive electrode sheet along its thickness in an embodiment of the present invention. As can be seen from the diagram, the positive electrode sheet includes a positive current collector 10, a base coating 11, a positive active coating 12, a surface coating 13, and a ceramic layer 14. The positive current collector 10 has a first surface and a second surface disposed opposite to each other. The positive active coating 12 is located between the positive current collector 10 and the surface coating 13. The positive current collector 10 includes at least a first region and a second region. The first region includes the positive current collector 10 and the positive active coating 12 located on both sides of the positive current collector. The second region includes the positive current collector 10 and the positive active coating 12 located on the first surface of the positive current collector. The surface coating 13 is disposed on the outer surface of the positive active coating 12 located in the second region. The base coating 11 is located between the positive current collector 10 and the positive active coating 12. The ceramic layer 14 is located on the second surface of the positive current collector 10 in the second region.

[0040] In one example, the thickness of the ceramic layer is greater than the thickness of the base coating. The thickness of the ceramic layer is also greater than the thickness of the top coating. This configuration improves safety performance in needle penetration and heavy impact applications.

[0041] In one example, the ceramic layer includes a third inorganic particle, which includes at least one of silicon dioxide, zirconium oxide, lithium silicate, magnesium oxide, boehmite, and aluminum oxide.

[0042] In this invention, the positive electrode active coating comprises a positive electrode active material. The positive electrode active material includes, for example, at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium-rich manganese-based materials.

[0043] In one example, the positive electrode active material includes the lithium cobalt oxide.

[0044] In one example, the lithium cobalt oxide contains 55.2%-64.5% by mass, for example, 55.2%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, or 64.5%. The lithium cobalt oxide contains 6.1%-7.9% by mass, for example, 6.1%, 6.3%, 6.5%, 6.7%, 6.9%, 7.1%, 7.3%, 7.5%, 7.7%, or 7.9%.

[0045] In one example, the median particle size Dv50 of the lithium cobalt oxide is 2μm-25μm, for example, 2μm, 5μm, 10μm, 15μm, 20μm, or 25μm. The median particle size Dv50 of the lithium cobalt oxide can be determined by laser particle size analysis, for example, using a Malvern laser particle size analyzer. Controlling the median particle size of the lithium cobalt oxide within this range ensures close packing of particles in the positive electrode active coating, improving compaction density and volumetric energy density. Simultaneously, a suitable median particle size also helps form a smooth ion and electron transport network, reducing polarization during charge and discharge. If Dv50 is less than 2μm, the lithium cobalt oxide particles are too fine, resulting in a significantly increased specific surface area and too many active sites in contact with the electrolyte. This can accelerate side reactions such as transition metal dissolution and electrolyte oxidative decomposition, and may also lead to insufficient compaction density. If Dv50 is greater than 25μm, the lithium cobalt oxide particles are too coarse, the lithium ion diffusion path inside a single particle becomes longer, the rate performance decreases, the gap between particles becomes larger, the contact becomes worse, and large particles are more likely to crack due to stress concentration during cycling.

[0046] In one example, the median particle size Dv50 of the lithium cobalt oxide is greater than the median particle size Dv50 of the first inorganic particles.

[0047] In one example, the median particle size Dv50 of the lithium cobalt oxide is greater than the median particle size Dv50 of the second inorganic particles.

[0048] In one example, the specific surface area of ​​the lithium cobalt oxide is 0.125 m². 2 / g-0.225m 2 / g, for example, 0.125m 2 / g, 0.13m 2 / g, 0.14m 2 / g, 0.15m 2 / g, 0.16m 2 / g, 0.17m 2 / g, 0.18m 2 / g, 0.19m 2 / g, 0.2m 2 / g, 0.21m 2 / g, 0.22m 2 / g or 0.225m 2 / g. The specific surface area of ​​the lithium cobalt oxide can be obtained by conventional methods in the art, such as using a McMurray TriStar II 3020 Plus high-throughput specific surface area and pore size analyzer.

[0049] A specific surface area of ​​lithium cobalt oxide within the above-mentioned range allows the electrolyte to fully contact the active sites, ensuring smooth ion insertion and extraction. If the specific surface area is less than 0.125 m²,... 2If the specific surface area is less than 0.225 m² / g, there will be insufficient active sites, inadequate contact between the electrolyte and the positive electrode, reduced ion insertion / extraction efficiency, and decreased battery capacity and cycle performance; if the specific surface area is greater than 0.225 m² / g... 2 If the ratio is less than 1 / g, the contact area between lithium cobalt oxide and the electrolyte will be too large, resulting in an increase in highly active sites on the positive electrode surface, an increase in the amount of transition metals dissolved, and an intensified oxidative decomposition reaction of the electrolyte, releasing more heat. This will not only accelerate the decay of cycle capacity but also reduce the battery furnace temperature safety performance.

[0050] In one example, the lithium cobalt oxide contains element Al at a mass concentration of 5000 ppm to 15000 ppm, for example, 5000 ppm, 6000 ppm, 7000 ppm, 8000 ppm, 9000 ppm, 10000 ppm, 11000 ppm, 12000 ppm, 13000 ppm, 14000 ppm, or 15000 ppm. Element Al effectively stabilizes the layered crystal structure of lithium cobalt oxide. Under high voltage (e.g., ≥4.53V) delithiation conditions, layered lithium cobalt oxide is prone to unfavorable phase transitions accompanied by lattice oxygen precipitation. Element Al can be dissolved into the crystal structure, acting as a pinning agent for the layered structure and suppressing phase transitions and lattice oxygen precipitation. Simultaneously, element Al enhances the mechanical strength of the crystal particles, reducing the risk of stress-induced cracking during cycling. Furthermore, elemental aluminum (Al) can synergistically work with the first nitrile additive in the electrolyte to further suppress the dissolution of cobalt ions from the cathode material and improve the stability of the cathode interface. If the mass content of elemental aluminum in lithium cobalt oxide is less than 5000 ppm, its stabilizing effect on the crystal structure is not significant, and the risk of phase transition and oxygen release under high voltage is relatively high. If the mass content of elemental aluminum in lithium cobalt oxide is greater than 15000 ppm, excessive aluminum atoms may occupy the lithium migration channels, hindering the insertion and extraction of lithium ions, leading to a decrease in the reversible capacity of the battery.

[0051] In this invention, the mass content of elements Al, Co and Li in the lithium cobalt oxide can be obtained by methods conventional in the art, such as inductively coupled plasma mass spectrometry (ICP-MS).

[0052] Electrolyte In one example, the carboxylic acid ester includes ethyl fluorocarbonate; the ethyl fluorocarbonate in the carboxylic acid ester contains more than 50% by mass, for example, 51%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%. The carboxylic acid ester solvent in the electrolyte contains more than 50% ethyl fluorocarbonate by mass. Ethyl fluorocarbonate is a low-viscosity solvent, and using it as the main solvent can further reduce the overall viscosity of the electrolyte and improve ion transport efficiency. Especially in the presence of a dense surface coating, the low-viscosity solvent helps the electrolyte to wet the nanoscale pores of the surface coating more quickly and fully, ensuring the connectivity of the ion transport pathway. At the same time, this solvent system can also synergize with the first nitrile additive to form a solvation structure that is conducive to preferential film formation at the electrode interface, ensuring the full utilization of the chemical repair function. Through the synergistic effect of the above-mentioned physical barrier and chemical repair, the furnace temperature safety performance, high-temperature cycle performance, and high-temperature storage performance of the battery are further improved.

[0053] In one example, the electrolyte further includes a second nitrile additive, which includes at least one of 1,3,6-hexanetrionitrile (HTCN), succinic anhydride (SN), adiponitrile (ADN), and ethylene glycol bis(propionitrile) ether (DENE).

[0054] In one example, the electrolyte further includes a sulfur-containing additive. The sulfur-containing additive includes at least one of 1,3-propanesulfonate lactone (PS), propenyl-1,3-sulfonate lactone (PST), vinyl sulfate (DTD), erythritol sulfate (BDTD), pentaerythritol bicyclic sulfate (TDT), and mannitol carbonate sulfate (BDD).

[0055] In one example, the mass content c2 of the sulfur-containing additive in the electrolyte is 0.3%-6%, for example, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, or 6%. c2 can be obtained by GC testing.

[0056] By combining a primary nitrile additive with a specific amount of sulfur-containing additive, synergistic film formation at the positive and negative electrode interfaces can be achieved. The sulfur-containing additive preferentially forms an interface film rich in active intermediates such as sulfonates and thiols on the electrode surface through ring-opening or reduction reactions, prior to other solvent components in the electrolyte. This interface film forms quickly and provides comprehensive coverage, but may not be dense enough. The primary nitrile additive then anchors itself to this interface film through cyano coordination, filling and modifying any loose or defective areas. Together, they generate a denser, chemically more stable composite solid electrolyte interface film. This composite film can more effectively suppress side reactions between the electrolyte and the positive and negative electrode interfaces, particularly inhibiting the corrosion of electrode materials by HF generated from the decomposition of hexafluorophosphate, thereby improving the high-temperature cycle stability of the battery. When C2 is low (e.g., less than 0.3%), the synergistic film formation effect with the primary nitrile additive is insufficient, and the overall density improvement of the interface film is not significant. When c2 is large (e.g., greater than 6%), the accumulation of decomposition products of sulfur-containing additives may increase the impedance of the interfacial film and trigger unnecessary side reactions at high temperatures, thereby leading to poor high-temperature cycling stability.

[0057] In one example, the ethyl fluoroacetate comprises 2,2-difluoroethyl acetate (DFEA) and / or 2,2-difluoroethyl acetate.

[0058] In one example, the mass content (c3) of ethyl fluoroacetate in the electrolyte is 5%-60%, for example, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%. c3 can be obtained through GC testing. Controlling the ethyl fluoroacetate content within this range allows it to interact with other components to form an optimized solvation structure. Specifically, the electrolyte typically contains carbonate solvents, such as propylene carbonate (PC), which has a high dielectric constant, facilitating lithium salt dissociation, but also a high viscosity. By introducing a specific amount of low-viscosity ethyl fluoroacetate, the electrolyte can be diluted, reducing the overall viscosity and improving ionic conductivity. Simultaneously, in the formed lithium-ion solvation layer, ethyl fluoroacetate and some primary nitrile additives can be distributed on the outer layer, while solvents such as PC are distributed on the inner layer. This structure helps the PC in the inner layer to preferentially reduce or oxidize on the electrode surface to form a basic interfacial film, while the primary nitrile additives in the outer layer anchor and modify this film at appropriate times, and the ethyl fluoroacetate ensures rapid ion migration throughout the process.

[0059] In this invention, the electrolyte may further comprise a lithium salt. The lithium salt includes, for example, at least one selected from lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobis(oxalate) phosphate, lithium tetrafluoro(oxalate) phosphate, lithium oxalate phosphate, lithium bis(oxalate) borate, lithium difluoro(oxalate) borate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(fluorosulfonyl)imide.

[0060] In one example, the electrolyte may further include a fluorinated additive. The fluorinated additive may include, for example, fluoroethylene carbonate (FEC). The fluorinated additive is present in the electrolyte at a mass content of 5%-20%, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%. The mass content of the fluorinated additive in the electrolyte can be determined by GC testing.

[0061] In one example, the carboxylic acid ester may also include at least one of ethyl propionate (EP), propyl propionate (PP), ethyl acetate (EA), and ethyl butyrate (EB).

[0062] In one example, the electrolyte may also comprise a carbonate. The carbonate may include, for example, at least one of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0063] Negative electrode film In one example, the mass content of elemental Si in the negative electrode active coating is 5%-40%, for example, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, or 40%. If the mass content of elemental Si in the negative electrode active coating is too low (e.g., less than 5%), the improvement in battery energy density will be limited; if it is too high (e.g., greater than 40%), the volume expansion of silicon-carbon materials will intensify, easily damaging the SEI film, thereby increasing side reactions and deteriorating the cycle stability and furnace temperature safety performance of the battery.

[0064] In this invention, the mass content of element Si in the negative electrode active coating can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and soaked in dimethyl carbonate (DMC) solvent for 12 hours, then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active coating can then be peeled off from the negative electrode current collector, and the negative electrode active coating is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature (25°C) to 900°C at a rate of 10°C / min, and held at 900°C for 40 minutes, so that the non-silicon components in the negative electrode active coating volatilize while the silicon is fully oxidized to silicon dioxide. The remaining substance is the ash content of the negative electrode active coating. The mass content of silicon in the negative electrode active material layer can be calculated based on the mass of ash. The calculation formula is as follows: based on the total mass of the negative electrode active coating, the mass content of element Si = 7 × mass of ash / (15 × mass of test sample).

[0065] In one example, the silicon-carbon material comprises a porous carbon matrix and silicon material located within the pores of the porous carbon matrix. This structure, which confines the silicon material within the pores of the porous carbon matrix, offers significant advantages in furnace temperature safety compared to other silicon-carbon materials (such as simple physical mixing or surface coating of silicon particles with graphite). This is because the volume expansion of silicon confined within the porous carbon pores is constrained by the rigid pore walls of the carbon framework, forcing the expansion force to be released into the pores, thus preventing macroscopic damage to the overall structure of the negative electrode. Simultaneously, the carbon matrix, as a stable conductive and thermally conductive framework, not only maintains the integrity of the electronic pathways of the negative electrode but also rapidly conducts and disperses localized heat. This structure protects the SEI film on the surface of the negative electrode from frequent rupture and regeneration, significantly reducing heat accumulation caused by side reaction heat generation, thereby further improving the furnace temperature safety performance of the battery.

[0066] In one example, the mass content of elemental Si in the silicon-carbon material is 30%-80%. The mass content of elemental Si in the silicon-carbon material can be determined using conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode is disassembled and removed, its cross-section is polished using an argon-ion polisher, and the silicon-carbon material particles are observed using a scanning electron microscope (SEM) in backscattered imaging mode, magnifying it as much as possible. An energy dispersive spectroscopy (EDS) instrument is used to scan the cross-section of the silicon-carbon material particles, 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 elemental Si is then calculated. At least 10 silicon-carbon material particles are selected for measurement, and the average value is taken.

[0067] In one example, the median particle size Dv50 of the silicon-carbon material is 1 μm-15 μm, for example, 1 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, or 15 μm. The median particle size Dv50 of the silicon-carbon material can be measured by methods conventional in the art, such as using a Malvern laser particle size analyzer.

[0068] When the median particle size Dv50 of silicon-carbon materials is less than 1 μm, the silicon-carbon particles are close to the nanoscale, resulting in a sharp increase in specific surface area. This leads to a large amount of SEI film formed during the initial lithium intercalation, and the particles are prone to agglomeration. During lithium intercalation expansion, they are more likely to pulverize, severely damaging the anode structure and exacerbating side reactions. When the median particle size Dv50 of silicon-carbon materials is greater than 15 μm, the absolute value of the volume expansion is large. Expansion stress concentration easily leads to particle cracking, and the lithium-ion diffusion path becomes longer, increasing impedance. Furthermore, it may cause uneven deposition of the anode active coating, affecting the structural stability and compaction density of the anode sheet.

[0069] In one example, the sphericity of the silicon-carbon material is 0.6-1, for example, 0.6, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1. The sphericity of the silicon-carbon material can be tested using conventional methods in the art, such as SEM. Using image processing software (such as Image Pro Plus), the images of each silicon-carbon material particle in the SEM image at a certain magnification are analyzed to obtain the perimeter and area of ​​each particle. The equivalent radius r1 of the perimeter and the equivalent radius r2 of the area of ​​each particle are calculated respectively. Then, the sphericity S of each particle is S=r2 / r1. The sphericity of each particle (for example, 10 particles) is then weighted and averaged to obtain the sphericity of the silicon-carbon material.

[0070] When the sphericity of silicon-carbon materials is 0.6-1, the surface curvature of the silicon-carbon material particles is more uniform, with point contact being the main feature between particles, and the pores are regularly distributed and of moderate size. This is beneficial for the electrolyte to uniformly wet the negative electrode active coating, forming a continuous ion transport network without sharp dead corners. This induces the formation of a uniform thickness and dense structure of the SEI film on the surface of the silicon-carbon material particles, reducing film rupture caused by localized sharp discharge or stress concentration, and improving film stability and cycle life.

[0071] In this invention, the negative electrode active coating further includes a carbon-based material. The carbon-based material includes, for example, at least one selected from artificial graphite, natural graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

[0072] 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.

[0073] 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.

[0074] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0075] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0076] Example 1 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet Boehmite (d2 = 221 nm), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were mixed evenly in a mass ratio of 8:0.5:0.5:1. N-methylpyrrolidone (NMP) was then added and stirred evenly to obtain a base coating slurry. Boehmite and polyvinylidene fluoride were mixed evenly in a mass ratio of 8:2. NMP was then added and stirred evenly to obtain a ceramic layer slurry. The base coating slurry and the ceramic layer slurry were coated on the surface of aluminum foil and baked to obtain an aluminum foil coated with both the base coating and the ceramic layer. Lithium cobalt oxide (median particle size Dv50 of 11.2 μm; containing element Al, with a mass content of 12745 ppm in lithium cobalt oxide), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were mixed evenly in a mass ratio of 97.6:0.675:0.675:1.05. NMP was then added and stirred evenly to obtain an active material slurry. The active material slurry was then coated onto the surface of the base coating and baked. Boehmite (d1 is 219 nm), conductive carbon black, carbon nanotubes and polyvinylidene fluoride are mixed evenly in a mass ratio of 8:0.5:0.5:1, and NMP is added and stirred evenly to obtain a surface coating slurry. The above surface coating slurry is coated on the outer surface of the positive electrode active coating located in the second region, and the positive electrode sheet is obtained after baking and rolling. The thickness of the top coating is 2.1 μm, the thickness of the base coating is 2.2 μm, and the thickness of the ceramic layer is greater than the thickness of both the top coating and the base coating.

[0077] (2) Preparation of electrolyte In an argon glove box with a water content of <0.1ppm and an oxygen content of <0.1ppm, PC, PP and DEFA are mixed evenly, and the first nitrile additive shown in I-1, the sulfur-containing additive (the mass ratio of PS to BDD is 3:4), FEC, lithium hexafluorophosphate and LiTFSI are added to form a homogeneous solution. After passing the physical property test, the electrolyte is obtained. Of these, c1 is 1.1%, c2 is 3.5%, and c3 is 47.2%; based on the total mass of the electrolyte, the contents of the following substances are: FEC is 13.5%, lithium hexafluorophosphate is 15.2%, LiTFSI is 4.9%, PP is 5.2%; the remainder is PC, and the mass content of DEFA in the carboxylic acid ester is 90.08%; d1 / c1 is 199.09.

[0078] (3) Preparation of negative electrode sheet Artificial graphite, silicon-carbon material (porous carbon matrix and silicon material located in the pores inside the porous carbon matrix), sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and carbon nanotubes were mixed evenly in a mass ratio of 71.5:23:2.5:1.5:1:0.5. Deionized water was added and the mixture was stirred evenly to obtain a negative electrode active slurry. The negative electrode active slurry was coated on the surface of copper foil. It was then dried at room temperature and then transferred to an 80°C oven for 10 hours. After cold pressing and slitting, the negative electrode sheet was obtained. The silicon-carbon material contains 55% Si by mass, has a median particle size Dv50 of 7.8 μm, and a sphericity of 0.98; the negative electrode active coating contains 12.6% Si by mass.

[0079] (4) Battery preparation The positive electrode sheet, separator (a 5 μm thick polyethylene film + a 2 μm burlite coating) and negative electrode sheet prepared in step (1) are stacked in sequence to ensure that the separator is between the positive and negative electrode sheets and plays a role in isolation. Then, the unfilled core is obtained by winding. The core is placed in the outer packaging foil, and the electrolyte prepared in step (2) is injected into the dried core. After vacuum sealing, standing, formation, shaping and sorting, the battery is obtained.

[0080] Example 2 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet Boehmite (d2 = 488 nm), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were mixed evenly in a mass ratio of 8:0.5:0.5:1. N-methylpyrrolidone (NMP) was then added and stirred evenly to obtain a base coating slurry. Boehmite and polyvinylidene fluoride were mixed evenly in a mass ratio of 8:2. NMP was then added and stirred evenly to obtain a ceramic layer slurry. The base coating slurry and the ceramic layer slurry were coated on the surface of aluminum foil and baked to obtain an aluminum foil coated with both the base coating and the ceramic layer. Lithium cobalt oxide (median particle size Dv50 of 2.3 μm; containing element Al, with a mass content of 5784 ppm in lithium cobalt oxide), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were mixed evenly in a mass ratio of 97.6:0.675:0.675:1.05. NMP was then added and stirred evenly to obtain an active material slurry. The active material slurry was then coated onto the surface of the base coating and baked. Silica (d1 is 103 nm) and polyvinylidene fluoride are mixed evenly at a mass ratio of 9:1, and NMP is added and stirred evenly to obtain a surface coating slurry. The above surface coating slurry is coated on the outer surface of the positive electrode active coating located in the second region, and the positive electrode sheet is obtained after baking and rolling. The thickness of the top coating is 3.8 μm, the thickness of the base coating is 4.8 μm, and the thickness of the ceramic layer is greater than the thickness of both the top coating and the base coating.

[0081] (2) Preparation of electrolyte In an argon glove box with a water content of <0.1ppm and an oxygen content of <0.1ppm, PC, PP and DEFA are mixed evenly, and the first nitrile additive shown in I-1, the sulfur-containing additive (the mass ratio of PS and BDD is 14:15), FEC, lithium hexafluorophosphate and LiTFSI are added to form a homogeneous solution. After passing the physical property test, the electrolyte is obtained. Of these, c1 is 0.5%, c2 is 5.8%, and c3 is 30.6%. Based on the total mass of the electrolyte, the contents of the following substances are as follows: FEC is 13%, lithium hexafluorophosphate is 14.9%, LiTFSI is 4.8%, and PP is 24.3%. The remainder is PC, and the mass content of DEFA in the carboxylic acid ester is 55.76%. The d1 / c1 ratio is 206.

[0082] (3) Preparation of negative electrode sheet Artificial graphite, silicon-carbon material (porous carbon matrix and silicon material located in the pores inside the porous carbon matrix), sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and carbon nanotubes were mixed evenly in a mass ratio of 71.5:23:2.5:1.5:1:0.5. Deionized water was added and the mixture was stirred evenly to obtain a negative electrode active slurry. The negative electrode active slurry was coated on the surface of copper foil. It was then dried at room temperature and then transferred to an 80°C oven for 10 hours. After cold pressing and slitting, the negative electrode sheet was obtained. The silicon-carbon material contains 55% Si by mass, has a median particle size Dv50 of 1.2 μm, and a sphericity of 0.86; the negative electrode active coating contains 12.6% Si by mass.

[0083] (4) Battery preparation The positive electrode sheet, separator (a 5 μm thick polyethylene film + a 2 μm burlite coating) and negative electrode sheet prepared in step (1) are stacked in sequence to ensure that the separator is between the positive and negative electrode sheets and plays a role in isolation. Then, the unfilled core is obtained by winding. The core is placed in the outer packaging foil, and the electrolyte prepared in step (2) is injected into the dried core. After vacuum sealing, standing, formation, shaping and sorting, the battery is obtained.

[0084] Example 3 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet Boehmite (d2 = 132 nm), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were mixed evenly in a mass ratio of 8:0.5:0.5:1. N-methylpyrrolidone (NMP) was then added and stirred evenly to obtain a base coating slurry. Boehmite and polyvinylidene fluoride were mixed evenly in a mass ratio of 8:2. NMP was then added and stirred evenly to obtain a ceramic layer slurry. The base coating slurry and the ceramic layer slurry were coated on the surface of aluminum foil and baked to obtain an aluminum foil coated with both the base coating and the ceramic layer. Lithium cobalt oxide (median particle size Dv50 of 24.6 μm; containing element Al, with a mass content of 13660 ppm in lithium cobalt oxide), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were mixed evenly in a mass ratio of 97.6:0.675:0.675:1.05. NMP was then added and stirred evenly to obtain an active material slurry. The active material slurry was then coated onto the surface of the base coating and baked. LATP (d1 is 486nm), conductive carbon black, carbon nanotubes and polyvinylidene fluoride are mixed evenly in a mass ratio of 8:0.5:0.5:1, and NMP is added and stirred evenly to obtain a surface coating slurry. The above surface coating slurry is coated on the outer surface of the positive electrode active coating located in the second region, and the positive electrode sheet is obtained after baking and rolling. The thickness of the top coating is 1.1 μm, the thickness of the base coating is 0.5 μm, and the thickness of the ceramic layer is greater than the thickness of both the top coating and the base coating.

[0085] (2) Preparation of electrolyte In an argon glove box with a water content of <0.1ppm and an oxygen content of <0.1ppm, PC, PP and DEFA are mixed evenly, and the first nitrile additive, sulfur-containing additive (PST), FEC and lithium hexafluorophosphate shown in I-1 are added. The mixture is mixed to form a homogeneous solution, and after passing the physical property test, the electrolyte is obtained. Of these, c1 is 1.5%, c2 is 0.3%, and c3 is 52.8%. Based on the total mass of the electrolyte, the contents of the following substances are: FEC is 13%, lithium hexafluorophosphate is 15.2%, PP is 6.6%, and the remainder is PC. The mass content of DEFA in the carboxylic acid ester is 88.83%. The d1 / c1 ratio is 324.

[0086] (3) Preparation of negative electrode sheet Artificial graphite, silicon-carbon material (porous carbon matrix and silicon material located in the pores inside the porous carbon matrix), sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and carbon nanotubes were mixed evenly in a mass ratio of 71.5:23:2.5:1.5:1:0.5. Deionized water was added and the mixture was stirred evenly to obtain a negative electrode active slurry. The negative electrode active slurry was coated on the surface of copper foil. It was then dried at room temperature and then transferred to an 80°C oven for 10 hours. After cold pressing and slitting, the negative electrode sheet was obtained. The silicon-carbon material contains 55% Si by mass, has a median particle size Dv50 of 14.7 μm, and a sphericity of 0.92; the negative electrode active coating contains 12.6% Si by mass.

[0087] (4) Battery preparation The positive electrode sheet, separator (a 5 μm thick polyethylene film + a 2 μm burlite coating) and negative electrode sheet prepared in step (1) are stacked in sequence to ensure that the separator is between the positive and negative electrode sheets and plays a role in isolation. Then, the unfilled core is obtained by winding. The core is placed in the outer packaging foil, and the electrolyte prepared in step (2) is injected into the dried core. After vacuum sealing, standing, formation, shaping and sorting, the battery is obtained.

[0088] Example 4 group This set of examples is used to verify the impact of changing the median particle size Dv50 d1 of the first inorganic particle.

[0089] This set of embodiments is based on Embodiment 1, except that d1 is changed, specifically: Example 4a, d1 is 52nm, d1 / c1 is 47.27, and the thickness of the surface coating is 0.5μm; Example 4b, d1 is 997 nm, d1 / c1 is 906.36, and the thickness of the surface coating is 4.9 μm.

[0090] Example 5 group This set of examples is used to verify the impact of changing the "mass content c1 of the first nitrile additive in the electrolyte".

[0091] This set of embodiments is based on Embodiment 1, except that c1 is changed, specifically: Example 5a, c1 is 0.2%, d1 / c1 is 1095; Example 5b, c1 is 3%, d1 / c1 is 73.

[0092] Example 6 group This set of examples is used to verify the effect of changing the "mass content of ethyl fluoroacetate in carboxylic acid esters".

[0093] This set of examples is based on Example 1, except that the mass content of ethyl fluoroacetate in the carboxylic acid ester is changed. Specifically: Example 6a, PP was replaced with the same mass of DFEA, that is, all carboxylic acid esters were ethyl fluoroacetate; In Example 6b, the mass content of ethyl fluorophosphate in the carboxylic acid ester was less than 50%. The specific electrolyte composition was as follows: C1 was 1.1%, C2 was 3.5%, and C3 was 25.3%. Based on the total mass of the electrolyte, the contents of the following substances were: FEC was 13.5%, lithium hexafluorophosphate was 15.2%, LiTFSI was 4.9%, PP was 31.4%, and the remainder was PC. The mass content of DFEA in the carboxylic acid ester was 44.62%.

[0094] Example 7 This was used to verify the impact of the change in the "second inorganic particle type".

[0095] The procedure was carried out in accordance with Example 1, except that boehmite was replaced with a combination of silica and LATP of the same particle size, wherein the mass ratio of silica to LATP was 1:1.

[0096] Example 8 group This set of examples is used to verify the impact of changes to "d1 / c1".

[0097] This set of embodiments refers to embodiments 2, 3, 5a and 5b, respectively, except that the median particle size Dv50 d1 of the first inorganic particle is changed, as follows: Example 8a was performed in accordance with Example 2, except that d1 was 486 nm and d1 / c1 was 972. Example 8b was performed in accordance with Example 3, except that d1 was 103 nm and d1 / c1 was 68.67. Example 8c is performed with reference to Example 5a, except that d1 is 1000 nm and d1 / c1 is 5000. Example 8d was carried out with reference to Example 5b, except that d1 was 54nm and d1 / c1 was 18.

[0098] Example 9 group This set of examples is used to verify the impact of changing the median particle size Dv50 of lithium cobalt oxide.

[0099] This set of embodiments is based on Embodiment 1, except that the median particle size Dv50 of lithium cobalt oxide is changed, as follows: Example 9a: The median particle size Dv50 of lithium cobalt oxide is 1.3 μm; In Example 9b, the median particle size Dv50 of lithium cobalt oxide was 28.9 μm.

[0100] Example 10 group This set of examples is used to verify the impact of changing the "mass content of element Si in the negative electrode active coating".

[0101] This set of embodiments refers to Embodiment 1, except that the mass content of elemental Si in the negative electrode active coating is controlled by changing the mass content of elemental Si in the silicon-carbon material and / or the mass content of silicon-carbon material in the negative electrode active slurry, as detailed below: In Example 10a, the mass content of elemental Si in the negative electrode active coating is 5.3%; In Example 10b, the mass content of elemental Si in the negative electrode active coating is 23.6%; In Example 10c, the mass content of element Si in the negative electrode active coating is 38.9%.

[0102] Example 11 Used to verify the effects of "not setting a base coat".

[0103] The same procedure was followed as in Example 1, except that no undercoating was provided in the positive electrode.

[0104] Comparative Example 1 The procedure is the same as in Example 1, except that d1 / c1 is adjusted by changing c1 and d1. Specifically: Comparative Example 1a: c1 is 3%, d1 is 32nm, and d1 / c1 is 10.67; Comparative Example 1b: c1 is 0.2%, d1 is 1260nm, and d1 / c1 is 6300.

[0105] Test case (1) High temperature cycling test The batteries prepared in the examples and comparative examples were subjected to charge-discharge cycles at 45°C with a charge rate of 2C and a discharge rate of 1C within the charge-discharge cutoff voltage range (3V-4.55V). The discharge capacity Q2 of the first cycle and the discharge capacity Q of the 500th cycle were recorded. 500 500-cycle capacity retention rate = Q 500 / Q2×100%, and record the results in Table 1.

[0106] (2) Furnace temperature safety test The batteries prepared in the examples and comparative examples were charged to 4.55V at 0.7C, and then charged at a constant voltage until the cutoff current was 0.05C. They were then placed in a hot chamber for testing. The temperature was increased from room temperature to 135°C at a rate of 5°C / min and held for 60 minutes. The batteries were considered to have passed if they did not catch fire or explode. Ten sets of tests were performed for each example or comparative example. The results are expressed as n / 10, where n represents the number of times the test was passed. The results are recorded in Table 1.

[0107] (3) High-temperature storage test The batteries prepared in the examples and comparative examples were charged to 4.55V at 0.7C (cutoff current of 0.25C), and after being left to stand for 2 hours, the storage thickness B1 was tested. The batteries were then stored in an oven (temperature of 85℃±2℃) for 8 hours. After the batteries returned to room temperature (25℃±2℃), the final thickness B2 was tested. The high-temperature storage expansion rate (%) = (B2-B1) / B1×100%. The test results are recorded in Table 1.

[0108] Table 1 As can be seen from Table 1, the battery of the present invention, compared with the comparative example, can balance furnace temperature safety performance, high-temperature cycle stability, and high-temperature storage safety performance. Among them, compared with Example 1, Example 5b has an increased mass content of the first nitrile additive in the electrolyte, which further improves the high-temperature cycle stability and high-temperature storage safety performance of the battery, but it will degrade the kinetic performance to a certain extent.

[0109] 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 a positive electrode and an electrolyte; The positive electrode sheet includes a positive current collector, a positive active coating, and a surface coating. The positive active coating is located between the positive current collector and the surface coating. The surface coating includes first inorganic particles with a median particle size Dv50 of d1, where d1 is 1 nm to 1000 nm. The electrolyte includes carboxylic acid esters and primary nitrile additives; The first nitrile additive includes , , and At least one of them; The mass content of the first nitrile additive in the electrolyte is c1, where c1 is 0.2%-3%. d1 and c1 satisfy: d1 / c1 is 15-5000, where the unit of d1 is nm and the unit of c1 is _____.

2. The lithium-ion secondary battery according to claim 1, wherein, d1 / c1 is 65-1000; And / or, d1 is 100nm-500nm; And / or, c1 is 0.5%-1.5%.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The thickness of the surface coating is 0.5μm-5μm; And / or, the first inorganic particles include at least one of silicon dioxide, zirconium oxide, lithium silicate, lithium iron phosphate, magnesium oxide, boehmite, aluminum oxide, lithium lanthanum zirconium oxide, lithium titanium aluminum phosphate, and lithium lanthanum titanium oxide. And / or, the surface coating further includes a first adhesive; the first adhesive includes at least one selected from polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, and perfluorosulfonic acid ionomer.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, The positive electrode further includes a base coating layer, which is located between the positive electrode current collector and the positive electrode active coating layer, and the base coating layer includes second inorganic particles; Preferably, the median particle size Dv50 of the second inorganic particle is d2, where d2 is 1nm-1000nm; Preferably, the second inorganic particles include at least one of silicon dioxide, zirconium oxide, lithium silicate, lithium iron phosphate, magnesium oxide, boehmite, aluminum oxide, lithium lanthanum zirconium oxide, lithium titanium aluminum phosphate, and lithium lanthanum titanium oxide. Preferably, the base coating further includes a second adhesive, the second adhesive comprising at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, and perfluorosulfonic acid ionomer; Preferably, the base coating further includes a second conductive agent; the second conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, and carbon fiber. Preferably, the thickness of the base coating is 0.5μm-5μm.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein, The positive current collector has a first surface and a second surface disposed opposite to each other; the positive electrode sheet includes at least a first region and a second region, the first region including the positive current collector and the positive active coating located on both sides of the positive current collector, and the second region including the positive current collector and the positive active coating located on the first surface of the positive current collector; The positive electrode further includes a ceramic layer; the ceramic layer is located on the second surface of the positive current collector in the second region; the ceramic layer includes third inorganic particles, the third inorganic particles including at least one of silicon dioxide, lithium silicate, magnesium oxide, zirconium oxide, boehmite and aluminum oxide.

6. The lithium-ion secondary battery according to claim 1 or 2, wherein, The carboxylic acid ester includes ethyl fluorophosphate; the ethyl fluorophosphate content in the carboxylic acid ester is greater than 50% by mass. And / or, the electrolyte further includes sulfur-containing additives; Preferably, the sulfur-containing additive includes at least one of 1,3-propanesulfonate lactone, propenyl-1,3-sulfonate lactone, vinyl sulfate, erythritol sulfate, pentaerythritol bicyclic sulfate, and mannitol carbonate sulfate. Preferably, the mass content (c2) of the sulfur-containing additive in the electrolyte is 0.3%-6%.

7. The lithium-ion secondary battery according to claim 6, wherein, The fluoroethyl acetate includes 2,2-difluoroethyl acetate and / or 2,2-difluoroethyl acetate; Preferably, the mass content (c3) of the ethyl fluoroacetate in the electrolyte is 5%-60%.

8. The lithium-ion secondary battery according to claim 1 or 2, wherein, The positive electrode active coating includes a positive electrode active material, which includes at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium-rich manganese-based materials. Preferably, the positive electrode active material includes the lithium cobalt oxide; More preferably, the median particle size Dv50 of the lithium cobalt oxide is 2μm-25μm; More preferably, the lithium cobalt oxide contains element Al, and the mass content of element Al in the lithium cobalt oxide is 5000ppm-15000ppm.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein, The lithium-ion secondary battery further includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active coating located on at least one side of the surface of the negative electrode current collector. The negative electrode active coating includes a negative electrode active material, which includes silicon-carbon materials and / or graphite materials. Preferably, the mass content of element Si in the negative electrode active coating is 5%-40%.

10. The lithium-ion secondary battery according to claim 9, wherein, The negative electrode active material includes the silicon-carbon material; Preferably, the silicon-carbon material comprises a porous carbon matrix and silicon material located in the pores within the porous carbon matrix; Preferably, the mass content of element Si in the silicon-carbon material is 30%-80%; Preferably, the median particle size Dv50 of the silicon-carbon material is 1μm-15μm; Preferably, the sphericity of the silicon-carbon material is 0.6-1.

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