lithium-ion batteries

Abstract

The present application relates to a lithium-ion battery. The lithium-ion battery includes a positive electrode plate and an electrolyte, wherein the positive electrode plate includes a lithium cobalt oxide positive electrode material, and the lithium cobalt oxide positive electrode material includes a substrate and a nitrogen-doped carbon coating coated on the surface of the substrate; the electrolyte includes a lithium salt, an additive, and a solvent, wherein the additive includes lithium difluorophosphate, and the lithium difluorophosphate can react with the nitrogen-doped carbon coating of the lithium cobalt oxide positive electrode material to form a composite interface. The solution provided in the present application can effectively inhibit the dissolution of Co during the cycle, improve the interface stability, reduce the generation of interface side reactions and side reaction products such as HF, so that the battery has both high energy density and long cycle life, and is suitable for energy storage device applications in the fields of consumer electronics, new energy vehicle power batteries, energy storage equipment, etc.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion batteries, and particularly to a lithium-ion battery. Background Art

[0002] A lithium battery is an electrochemical energy storage device that converts electrochemical energy and electrical energy into each other, and realizes the transfer and exchange of energy through the injection and extraction of lithium ions and electrons in electrode materials. Along with the development of the lithium-ion battery industry, the O3-phase lithium cobaltate material with advantages such as ultra-high tap density and volumetric energy density has gradually become the preferred cathode material for lithium-ion batteries used in portable consumer electronic products.

[0003] However, the actual capacity of traditional O3-phase lithium cobaltate (170 mAh / g, 4.45V) is significantly lower than its theoretical capacity (274 mAh / g), which limits the potential improvement of the energy density of the LiCoO2 material; adopting the method of increasing the working voltage can extract more lithium ions, which is an effective strategy to promote LiCoO2 to obtain a higher energy density. However, at high voltages (≥4.5V), the O3-phase lithium cobaltate faces problems such as irreversible harmful phase transitions (such as O3→H1-3→O1), lattice oxygen evolution, and serious surface side reactions, which directly damage the stability of its lithium storage structure and lead to capacity decay. Summary of the Invention

[0004] To solve or partially solve the problems existing in the related art, this application provides a lithium-ion battery, which can effectively inhibit the dissolution of Co during the cycling process, improve the interface stability, reduce the generation of interface side reactions and side reaction products such as HF, and enable the battery to have both high energy density and long cycle life, and is suitable for applications in fields such as consumer electronic products, power batteries for new energy vehicles, and energy storage devices.

[0005] In the first aspect of this application, a lithium-ion battery is provided, including a positive electrode plate and an electrolyte. The positive electrode plate includes a lithium cobaltate positive electrode material, and the lithium cobaltate positive electrode material includes a matrix and a nitrogen-doped carbon coating layer coated on the surface of the matrix;

[0006] The electrolyte includes a lithium salt, an additive, and a solvent, wherein the additive includes lithium difluorophosphate, and the lithium difluorophosphate can react with the nitrogen-doped carbon coating layer of the lithium cobaltate positive electrode material to form a composite interface.

[0007] In some embodiments, the matrix of the lithium cobaltate positive electrode material is O2-phase lithium cobaltate, and its chemical general formula is Li x Co 1-z M z O2; wherein, 0.95≤x≤1, 0<z≤0.1, and M is at least one of Mg, Al, Ti, La, Y, Zr, and Ce.

[0008] In some embodiments, the average particle size Dv50 of the matrix of the lithium cobalt oxide positive electrode material is 3 μm to 10 μm, and the specific surface area is 0.5 m 2 / g~4.6 m 2 / g.

[0009] In some embodiments, the CN bond binding energy of the nitrogen-doped carbon coating layer of the lithium cobalt oxide positive electrode material is (398.5±0.3) eV, and the C=O bond binding energy is (531.2±0.3) eV.

[0010] In some embodiments, the nitrogen doping amount in the nitrogen-doped carbon coating layer of the lithium cobalt oxide positive electrode material is 2 at % to 5 at %.

[0011] In some embodiments, the thickness of the nitrogen-doped carbon coating layer of the lithium cobalt oxide positive electrode material is 5 nm to 50 nm.

[0012] In some embodiments, according to TOF-SIMS testing, in the composite interface on the surface of the positive electrode sheet, when the nucleus-mass ratio m / z is 63, PO 2- Fragment (m / z=63) signal intensity ≥1×10 4 counts; when the nucleus-cytoplasm ratio m / z is 19, F - Fragment (m / z=19) signal intensity ≤5×10 3 counts.

[0013] In some embodiments, the composite interface of the positive electrode sheet surface is PO 2- Fragment / F - The signal intensity ratio of the fragments was ≥2:1.

[0014] In some embodiments, the method for preparing the lithium cobalt oxide positive electrode material comprises: mixing O2-phase lithium cobalt oxide with a nitrogen-containing polymer, and carbonizing the mixture to obtain a nitrogen-doped carbon-coated lithium cobalt oxide positive electrode material.

[0015] In some embodiments, the nitrogen-containing polymer is selected from at least one of polyacrylonitrile, polyimide, polyaniline, and polypyrrole.

[0016] In some embodiments, the nitrogen-containing polymer is selected from polyacrylonitrile.

[0017] In some embodiments, the O2-phase lithium cobalt oxide and the nitrogen-containing polymer are mixed in a mass ratio of 100:(1-5).

[0018] In some embodiments, the carbonization conditions are: carbonization at 150° C. to 250° C. for 2 h to 6 h under an inert atmosphere.

[0019] In some embodiments, the lithium salt includes lithium bis(fluorosulfonyl)imide at a concentration of 1.0M to 1.5M.

[0020] In some embodiments, the concentration of lithium difluorophosphate in the electrolyte is 0.05M~0.2M.

[0021] In some embodiments, the additive further comprises fluoroethylene carbonate at a concentration of 0.05M to 0.5M.

[0022] In some embodiments, the solvent includes ethylene carbonate and ethyl methyl carbonate.

[0023] In some embodiments, in the solvent, the volume ratio of ethylene carbonate to ethyl methyl carbonate is (1-7):(3-9).

[0024] The technical solution provided by this application may include the following beneficial results: a lithium cobalt oxide positive electrode material with a nitrogen-doped carbon coating layer can inhibit Co dissolution at high voltage, reduce interfacial side reactions, and improve interfacial stability. The nitrogen-doped carbon coating on the surface of the lithium cobalt oxide positive electrode material cooperates with the electrolyte containing a di-phosphite additive to form a LiF-Li3PO4 composite interface at the positive electrode interface during battery cycling, reducing charge transfer impedance, thereby further improving the positive and negative electrode interface stability, improving cycle stability and lithium ion conductivity. At the same time, it can also synergistically inhibit the rapid decomposition of the electrolyte during charge and discharge cycles at high voltage, reduce the generation of side reactions such as HF, thereby reducing damage to the interface by side reaction products, protecting the positive and negative electrode interfaces and improving electrolyte stability, thereby improving the battery's overall cycle performance and extending the battery life (800 cycle capacity retention rate ≥90%) while also having a high energy density (≥800Wh / L). The use of O2 phase lithium cobalt oxide and the doping of elements such as Mg, Al, Ti, La, Y, Zr, and Ce can further improve the stability of the bulk structure, giving it better cycle stability when used under high voltage.

[0025] The PAN carbonization method is used to generate a nitrogen-doped carbon coating on the surface of O2-phase lithium cobalt oxide. PAN can generate a stable nitrogen-doped carbon coating under relatively low conditions, which makes the lithium cobalt oxide positive electrode material have good thermal stability and chemical stability. At the same time, the PAN synthesis and processing costs are low, and it can also effectively reduce the production cost of the high-voltage lithium-ion battery, which is conducive to the large-scale application of the lithium-ion battery in consumer electronics, new energy vehicle power batteries and many other fields.

[0026] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present application. DETAILED DESCRIPTION

[0027] The embodiments of the present application will be described in more detail below. It should be understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present application more thorough and complete, and to fully convey the scope of the present application to those skilled in the art.

[0028] The terms used in this application are for the purpose of describing specific embodiments only and are not intended to limit the application. Unless otherwise defined, all terms used herein have the same meaning as those generally understood by those of ordinary skill in the art to which the present invention belongs. Although any methods and materials equivalent to the methods and materials described herein can also be used in the implementation or testing of the present invention, preferred methods and materials are now described.

[0029] It should be understood that although the terms "first", "second", "third", etc. may be used in this application to describe various information, this information should not be limited to these terms. These terms are only used to distinguish information of the same type from each other. Features defined as "first" or "second" may explicitly or implicitly include one or more of these features. The singular forms "a", "said", and "the" used in this application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0030] Where a numerical range is provided, it is understood that each intervening value between the upper and lower limits of the range and any other specified or intervening values ​​in the specified range is encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included in the smaller range and are also encompassed within the present invention, subject to any explicitly excluded limits in the specified range. Where a specified range includes one or two limits, ranges excluding either or both of those included limits are also encompassed within the present invention. In the description of this application, the meaning of "multiple" is two or more, unless otherwise specifically defined.

[0031] The actual capacity of traditional O3 phase lithium cobalt oxide (170 mAh / g, 4.45 V) is significantly lower than its theoretical capacity (274 mAh / g), limiting the potential energy density improvement of LiCoO2 materials. Increasing the operating voltage can release more lithium ions, which is an effective strategy to promote LiCoO2 to achieve higher energy density. However, at high voltage (≥4.5V), O3 phase lithium cobalt oxide faces irreversible harmful phase transitions (such as O3→H1-3→O1), lattice oxygen precipitation and severe surface side reactions, which directly destroy the stability of its lithium storage structure, leading to capacity attenuation and safety risks.

[0032] In response to the above problems, the embodiments of the present application provide a lithium-ion battery that can effectively improve the shortcomings of O3 phase lithium cobalt oxide, such as irreversible phase transition, lattice oxygen precipitation and serious surface side reactions under high voltage, so that the battery has the advantages of high energy density, long cycle performance and high safety performance, which is conducive to its application in high-end consumer electronic products, new energy vehicle power batteries, energy storage equipment and other fields.

[0033] The lithium-ion battery provided in the embodiment of the present application includes a positive electrode plate, an electrolyte, a negative electrode plate and a separator.

[0034] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer located on the surface of the positive electrode current collector. The positive electrode material layer is formed by coating the positive electrode slurry on the surface of the positive electrode current collector.

[0035] The positive electrode current collector mentioned in the embodiments of this application is not particularly limited, as long as it is conductive and does not cause adverse chemical changes in the battery. It can be any material known to be suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector can be a metal material such as aluminum, stainless steel, nickel plating, titanium, tantalum, or a carbon material such as carbon cloth or carbon paper; preferably, aluminum foil.

[0036] The positive electrode material layer comprises a lithium cobalt oxide positive electrode material, that is, the positive electrode active material in the positive electrode material layer comprises a lithium cobalt oxide positive electrode material. The lithium cobalt oxide positive electrode material provided in the embodiments of the present application comprises a substrate and a nitrogen-doped carbon coating layer coated on the surface of the substrate, wherein the nitrogen-doped carbon coating layer is formed by carbonizing a nitrogen-containing polymer.

[0037] The electrolyte contains lithium salts, additives, and solvents. The additives include at least lithium difluorophosphate. During the charge and discharge process of the assembled battery, the lithium difluorophosphate in the electrolyte reacts with the nitrogen-doped carbon coating of the lithium cobalt oxide positive electrode material in the positive electrode material layer to form a LiF-Li3PO4 composite interface, which serves as the interface protective film on the surface of the positive electrode sheet.

[0038] In the embodiment of the present application, active sites are introduced through the nitrogen-doped carbon coating layer on the surface of the lithium cobalt oxide positive electrode material, which is conducive to the chemical bonding between the surface of the positive electrode material layer and the decomposition products of the electrolyte to form a stable interface protective film, reduce the charge transfer impedance, improve the interface stability and lithium ion conductivity, and at the same time effectively inhibit the rapid decomposition of the electrolyte during the charge and discharge cycle at high voltage, reduce the generation of harmful side reaction products such as HF, thereby further improving the structural stability of the positive electrode sheet and the stability of the electrolyte, improving the cycle capacity retention rate of the battery, and extending the battery life.

[0039] In some specific embodiments, the matrix of the lithium cobalt oxide positive electrode material is O2 phase lithium cobalt oxide, and its chemical formula is Li x Co1-z M z O₂; where 0.95 ≤ x ≤ 1, 0 < z ≤ 0.1, and M is at least one of Mg, Al, Ti, La, Y, Zr, and Ce.

[0040] The O₂-phase lithium cobaltate has a lamellar structure. Further, the main peak of its XRD test (Cu-Kα ray) is located at (18.6 ± 0.1)°, and the interlayer spacing c-axis is 1.42 nm to 1.45 nm, which is different from that of the O₃-phase lithium cobaltate (c-axis ≈ 1.40 nm).

[0041] It can be understood that the XRD (X-ray Diffraction) test is a test method that uses the interaction between X-rays and crystalline substances to obtain information such as the crystal structure, phase composition, and grain size of materials by analyzing the diffraction pattern. The Cu-Kα ray is one of the commonly used X-ray sources.

[0042] Due to the different oxygen layer stacking sequences (ABBA), the O₂-phase lithium cobaltate has higher structural stability and reversible phase change ability, and is more suitable for applications in high-voltage environments. By carbonizing and coating the lithium cobaltate cathode material, a nitrogen-doped carbon coating layer is constructed on the surface of the O₂ phase, thereby inhibiting the dissolution of Co; also, through the special design of the electrolyte composition, lithium difluorophosphate is used as an electrolyte additive component, which can react with the nitrogen-doped carbon coating layer on the surface of the O₂-phase lithium cobaltate to form a composite interface film, improving the interfacial ion conductivity. Therefore, it can solve the problems that the electrolyte is easily decomposed to produce HF side reaction products at high voltage, corrode the surface of the O₂-phase lithium cobaltate, cause the dissolution of Co, and accelerate the thickening of the negative electrode SEI film. It can also effectively inhibit the gas generation problem caused by the oxidation side reaction between the active sites on the surface of the O₂-phase lithium cobaltate and the electrolyte solvent, improve the battery cycle capacity retention rate, extend the battery life, and at the same time improve the safety performance of the battery. Meanwhile, by doping elements such as Mg, Al, Ti, La, Y, Zr, Ce, etc. into the O₂-phase lithium cobaltate matrix, the bulk phase structure stability can be further improved, making it have better cycle stability when used at high voltage.

[0043] In some specific embodiments, the average particle size Dv50 of the matrix of the lithium cobaltate cathode material is 3 μm to 10 μm. Specifically, for example, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc., or any value within the above range.

[0044] In some specific embodiments, the specific surface area of the matrix of the lithium cobaltate cathode material is 0.5 m 2 / g to 4.6 m 2 / g. Specifically, for example, 0.5 m 2 / g, 0.8 m 2 / g, 1 m 2 / g, 1.5m 2 / g, 2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g、4m 2 / g, 4.6m 2 / g, etc., or any value within the above range.

[0045] Appropriate specific surface area and particle size can optimize the electrochemical performance of lithium cobalt oxide cathode materials. When the average particle size of the matrix of lithium cobalt oxide cathode materials is 3μm~10μm and the specific surface area is 0.5m 2 / g~4.6m 2 / g range, with a smaller particle size and appropriate specific surface area, which can increase the contact area between the positive electrode material and the electrolyte and improve the transmission efficiency of lithium ions; at the same time, it can also reduce the defect sites on the surface of the positive electrode active material, reduce the occurrence of interfacial side reactions, and help improve the battery's rate performance and cycle stability.

[0046] In some specific embodiments, the CN bond binding energy of the nitrogen-doped carbon coating layer of the lithium cobalt oxide positive electrode material is (398.5±0.3) eV, the C=O bond binding energy is (531.2±0.3) eV, and the surface of the lithium cobalt oxide positive electrode material is nitrogen-doped. The nitrogen element doped in the carbon layer is beneficial to improving the conductivity and chemical stability of the carbon layer.

[0047] In some specific embodiments, the nitrogen doping amount in the nitrogen-doped carbon coating of the lithium cobalt oxide cathode material is 2 at% to 5 at%, for example, 2 at%, 2.5 at%, 3 at%, 3.5 at%, 4 at%, 4.5 at%, 5 at%, or any value within the foregoing range. A nitrogen doping amount within the range of 2 at% to 5 at% is beneficial for enhancing the conductivity and interfacial stability of the carbon layer. However, excessively high doping amounts can easily lead to structural defects in the carbon layer, affecting electrochemical performance.

[0048] In some specific embodiments, the thickness of the nitrogen-doped carbon coating layer of the lithium cobalt oxide cathode material is 5 nm to 50 nm, such as 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, or any value within the aforementioned range. A coating that is too thin may not effectively inhibit Co dissolution, while a coating that is too thick may hinder lithium ion transport.

[0049] The bond energy, nitrogen doping level, and thickness of the nitrogen-doped carbon coating of the lithium cobalt oxide cathode material can be determined using XPS testing. The XPS spectrum indicates that the carbon coating of the lithium cobalt oxide cathode material retains the chemical characteristics of the nitrogen-containing polymer, thereby introducing active sites into the carbon coating, which facilitates the reaction between the cathode material and the electrolyte to form a highly stable composite interfacial protective film.

[0050] It can be understood that XPS (X-ray Photoelectron Spectroscopy) testing refers to a testing method based on the photoelectric effect, which analyzes the elemental composition, valence state and chemical environment by detecting the energy of photoelectrons emitted by atoms on the surface of the material after being excited by X-rays.

[0051] Nitrogen doping of the carbon coating on the surface of the lithium cobalt oxide positive electrode material will reduce its conductivity. The higher the nitrogen doping, the more significant the number of catalytic active sites in the carbon coating, and the more significant the reduction in conductivity; the thicker the coating, the more significant the reduction in conductivity. In the embodiments of the present application, by regulating parameters such as the nitrogen doping amount and the thickness of the nitrogen-doped carbon coating on the surface of the lithium cobalt oxide positive electrode material, the active sites and number of the carbon coating are balanced with the conductivity. The nitrogen-doped carbon coating constructed on the surface of the O2 phase lithium cobalt oxide effectively inhibits Co dissolution and reduces interfacial side reactions, while ensuring that the lithium cobalt oxide positive electrode material has better conductivity.

[0052] Moreover, the embodiment of the present application also introduces lithium difluorophosphate into the electrolyte to form a composite interface with the positive electrode material, thereby improving the interfacial lithium ion conductivity and synergistically improving the conductive properties of the material.

[0053] In some specific embodiments, according to TOF-SIMS testing, the composite interface of the positive electrode sheet is PO 2- Fragment (m / z=63) signal intensity ≥1×10 4 counts, F - Fragment (m / z=19) signal intensity ≤5×10 3 counts.

[0054] As you can understand, TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) is a highly sensitive surface analysis technique used to examine the chemical composition and elemental distribution of material surfaces and interfaces. It analyzes the surface composition of a sample by emitting a high-energy primary ion beam to ionize atoms or molecules on the sample surface. The technique then measures the flight time of these secondary ions to determine their mass-to-charge ratio (m / z).

[0055] In the embodiment of the present application, PO in the composite interface of the positive electrode surface 2-The fragments are derived from lithium phosphate and other compounds generated by the decomposition of lithium difluorophosphate in the electrolyte. These compounds are expressed as PO in TOF-SIMS analysis. 2- Fragments exist. When the composite interface of the positive electrode surface is 2- Fragment (m / z=63) signal intensity ≥1×10 4 When the number of counts reaches 3, the surface of the positive electrode sheet is formed with a higher content of lithium phosphate, which can improve the lithium ion conductivity of the interface and reduce the interface impedance. The high content of lithium phosphate helps to form a stable composite interface, that is, to form a stable interface film, which can inhibit the decomposition reaction of the electrolyte under high voltage and reduce Co dissolution, thereby improving the cycle stability and safety of the battery under high voltage.

[0056] In the embodiment of the present application, F - The fragments mainly come from the decomposition of lithium salts and fluorine-containing compounds in the electrolyte, LiF, which is represented by F in TOF-SIMS analysis. - Fragments exist. When the composite interface of the positive electrode surface is - The signal intensity of the fragment (m / z=19) is ≤5×10 3 When the number of counts is 2, a relatively low content of lithium fluoride is formed on the surface of the positive electrode. Lithium fluoride can improve the stability of the interface, and a lower content of lithium fluoride can ensure that the interface has excellent flexibility, avoiding the increase in brittleness of the interface film due to excessive lithium fluoride and the deterioration of the overall performance of the interface; at the same time, a lower F - The signal intensity indicates that the electrolyte, especially lithium salt, decomposes less at high voltage, which helps to reduce side reactions at high voltage and reduce the generation of harmful byproducts such as HF, thereby improving the safety performance and cycle life of the battery.

[0057] Furthermore, the composite interface of PO 2- Fragment / F - The signal intensity ratio of the fragments is ≥2:1. 2- Fragment / F - The signal intensity ratio of the fragments reflects the relative content of lithium phosphate and lithium fluoride in the interface film formed on the surface of the positive electrode. The higher the ratio, the higher the lithium phosphate content in the interface film, which helps to form a more stable interface and improve the cycle stability and safety performance of the battery.

[0058] Specifically, in the composite interface film on the surface of the positive electrode sheet, R ct ≤8Ω·cm 2 Among them, R ctCharge transfer resistance (Charge Transfer Resistance) is an important parameter in electrochemistry that describes the resistance of the charge transfer process at the electrode / solution interface and reflects the speed of the electrochemical reaction kinetics.

[0059] Therefore, by controlling the PO 2- Fragments and F - The fragment signal intensity can ensure that the battery has excellent electrochemical performance at high voltage.

[0060] In some specific embodiments, the positive electrode slurry also includes a binder and a conductive agent. Among them, the binder is a component that helps the positive electrode material to adhere to the conductive agent and to the current collector. The binder can be polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, styrene-butadiene rubber and fluororubber. The conductive agent can be acetylene black, Ketjen black, Super P, graphite fiber, carbon fiber, graphene or metal fiber. A solvent, such as N-methyl-2-pyrrolidone, can also be included in the positive electrode slurry to obtain the desired viscosity after mixing the positive electrode material and the optional binder and conductive agent.

[0061] A stable positive electrode slurry is prepared by mixing the positive electrode material, binder, conductive agent and solvent, and then the slurry is coated on the positive electrode current collector to form a positive electrode material layer. The slurry coating surface density is 20 mg / cm 2 ~25mg / cm 2 , for example 20 mg / cm 2 , 21mg / cm 2 , 22mg / cm 2 , 23mg / cm 2 , 24mg / cm 2 , 25mg / cm 2 etc., or any value within the above range. The compacted density is 3.2 g / cm 3 ~4.3g / cm 3 , for example 3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 、3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 、3.8g / cm 3 、3.9g / cm 3 , 4.0g / cm 3 , 4.1g / cm3 , 4.2 g / cm 3 , 4.3 g / cm 3 , etc., or any value within the above range.

[0062] The embodiment of the present application also provides a preparation method of the lithium cobalt oxide cathode material, which obtains a nitrogen-doped carbon-coated lithium cobalt oxide cathode material by mixing O2-phase lithium cobalt oxide with a nitrogen-containing polymer and carbonizing.

[0063] Specifically, the chemical general formula of the O2-phase lithium cobalt oxide is Li x Co 1-z M z O2; where 0.95 ≤ x ≤ 1, 0 < z ≤ 0.1, and M is at least one of Mg, Al, Ti, La, Y, Zr, Ce. After doping the above elements into the lithium cobalt oxide material, the bulk phase structure stability can be further improved, making it have better cycle stability when used at high voltages.

[0064] The synthesis method of the O2-phase lithium cobalt oxide includes: mixing a sodium source, a cobalt source, and a doping element M source in a stoichiometric ratio and sintering to obtain a P2-phase precursor; mixing the P2-phase precursor with a lithium source for molten salt ion exchange to obtain the O2-phase lithium cobalt oxide.

[0065] The sodium source is, for example, one or more of sodium acetate, sodium carbonate, sodium hydroxide, sodium nitrate, sodium chloride, sodium sulfate, sodium bromide. The cobalt source is, for example, one or more of cobaltous oxide, cobalt tetroxide, cobalt hydroxide, cobalt oxyhydroxide, cobalt carbonate, cobalt acetate, cobalt sulfate, cobalt nitrate, cobalt chloride. The doping element is one or more of oxides, hydroxides, carbonates, acetates, sulfates, nitrates, chlorides, and oxyacids of Mg, Al, Ti, La, Y, Zr, Ce, such as aluminum oxide. The sintering conditions are sintering at 700°C to 1000°C for 10 h to 72 h. The lithium source is, for example, lithium nitrate, lithium chloride, etc.

[0066] The nitrogen-containing polymer can be selected from at least one of polyacrylonitrile, polyimide, polyaniline, and polypyrrole. Through the nitrogen-containing polymer, the coating layer on the surface of the lithium cobalt oxide cathode material is a nitrogen-doped carbon layer, thereby introducing active sites into the carbon layer. These active sites increase the surface energy and chemical activity of the carbon layer, enhance the interaction between the carbon layer and the electrolyte, enabling it to form a chemical bond with the decomposition products of the electrolyte to form a stable composite interface on the surface of the positive electrode sheet, improving the stability of the positive electrode interface and the lithium ion conduction performance; at the same time, the nitrogen-doped carbon layer can also effectively inhibit the decomposition reaction of the electrolyte at high voltages, reduce the generation of harmful side reaction substances such as HF, reduce gas production, protect the positive electrode material, extend the battery life, and improve the battery safety performance.

[0067] In some preferred embodiments, the nitrogen-containing polymer is selected from polyacrylonitrile.

[0068] Among them, polyimide has excellent thermal stability and mechanical strength and is suitable for use under high temperature conditions.

[0069] While polyaniline (PAN) can form a stable carbon layer at high temperatures, its synthesis and processing conditions are relatively demanding and its cost is high. Polyaniline exhibits excellent electrical conductivity and electrochemical activity, with high conductivity, which can improve interfacial electron transport properties. However, its chemical stability is poor and it is easily degraded in electrolytes. Polypyrrole (PPY) has high electrical conductivity and good electrochemical properties. It can be synthesized chemically or electrochemically, but its stability in electrolytes needs to be improved. Polyacrylonitrile, on the other hand, can form a stable nitrogen-doped carbon coating after carbonization, imparting excellent thermal and chemical stability to lithium cobalt oxide cathode materials, making them less susceptible to electrolyte decomposition or corrosion. The PAN-derived coating exhibits good compatibility with electrolyte additives such as lithium difluorophosphate, facilitating the formation of a stable composite interface. Furthermore, PAN can achieve high nitrogen doping levels of 2 at% to 5 at% during carbonization, with uniform nitrogen distribution, further improving interfacial stability and lithium-ion conductivity. Furthermore, PAN's low synthesis and processing costs make it suitable for large-scale production.

[0070] In some embodiments, the O2-phase lithium cobalt oxide and the nitrogen-containing polymer are mixed in a mass ratio of 100:(1-5). Specific examples include 100:1, 100:2, 100:3, 100:4, 100:5, or any value within the aforementioned range. Excessively high mass of the nitrogen-containing compound can increase SEI film resistance, affect lithium ion transport performance, and reduce the battery's rate capability and cycling stability.

[0071] In some specific embodiments, the carbonization conditions are: carbonization at 150°C to 250°C for 2h to 6h under an inert atmosphere. The inert atmosphere can be a chemically inactive gas such as argon, which will not react chemically with the positive electrode material or nitrogen-containing polymer at high temperatures, preventing side reactions such as oxidation of the material at high temperatures, ensuring the uniformity of the carbonization process and the performance of the final product, and forming a uniform nitrogen-doped carbon coating. The temperature can be, for example, 150°C, 175°C, 180°C, 185°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, or any value within the above range. The time can be, for example, 2h, 3h, 4h, 5h, 6h, or any value within the above range.

[0072] In some specific embodiments, the lithium salt includes one or more of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium difluorobisoxalatophosphate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonylimide), lithium bis(fluorosulfonylimide), and lithium difluorophosphate. Preferably, the lithium salt includes lithium bis(fluorosulfonylimide).

[0073] Furthermore, the lithium salt is selected from lithium bis(fluorosulfonyl)imide at a concentration of 1.0M to 1.5M, preferably 1.2M to 1.3M. Specific examples include 1.0M, 1.1M, 1.2M, 1.25M, 1.3M, 1.4M, 1.5M, or any value within the aforementioned range. The concentration of the lithium salt herein refers to the concentration of the lithium salt in the electrolyte.

[0074] In some specific embodiments, the concentration of lithium difluorophosphate in the electrolyte is 0.05M to 0.2M, preferably 0.1M to 0.15M, for example, 0.05M, 0.06M, 0.07, 0.08M, 0.09M, 0.1M, 0.11M, 0.12M, 0.13M, 0.14M, 0.15M, 0.16M, 0.17M, 0.18M, ​​0.19M, 0.2M, etc., or any value within the above range.

[0075] In some specific embodiments, the additive further comprises one or more of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, propene sultone, methylene disulfonate, pentafluoroethoxyphosphazene, dicyclohexylcarbonate, trimethyl imide phosphate, and hexamethylene diisocyanate.

[0076] Preferably, the additive is selected from fluoroethylene carbonate and lithium difluorophosphate. Among them, fluoroethylene carbonate can preferentially form a film on the negative electrode, reducing the SEI film impedance, while lithium difluorophosphate decomposes to form a composite interface film on the positive electrode, inhibiting Co dissolution, and synergistically improving the battery cycle stability.

[0077] Specifically, the concentration of fluoroethylene carbonate in the electrolyte is 0.05M to 0.5M, preferably 0.1M to 0.25M. Specific examples include 0.05M, 0.1M, 0.15M, 0.2M, 0.25M, 0.3M, 0.35M, 0.4M, 0.45M, 0.5M, etc., or any value within the above range. The concentration of fluoroethylene carbonate herein refers to the concentration of fluoroethylene carbonate in the electrolyte.

[0078] The fluoroethylene carbonate in the electrolyte can also be expressed as a mass fraction, and its addition amount can be 1wt% to 3wt%; for example, 1wt%, 1.2wt%, 1.5wt%, 1.8wt%, 2wt%, 2.5wt%, 3wt%; preferably 2wt% to 2.5wt%.

[0079] In the embodiments of the present application, the lithium salt is selected from lithium bis(fluorosulfonyl)imide, which, upon decomposition, generates lithium fluoride on the surface of the positive electrode. This, together with the lithium phosphate generated by the decomposition of the additive lithium difluorophosphate, forms a stable composite interface film, thereby effectively inhibiting the decomposition reaction of the electrolyte under high pressure, reducing Co dissolution, and improving the lithium ion conductivity of the interface. This, together with the nitrogen-doped carbon coating on the surface of the O2-phase lithium cobalt oxide, synergizes with the nitrogen-doped carbon coating on the surface of the O2-phase lithium cobalt oxide to improve the problem of decreased battery conductivity caused by the coating formed on the surface of the O2-phase lithium cobalt oxide. Lithium bis(fluorosulfonyl)imide is used as the lithium salt, and fluoroethylene carbonate and lithium difluorophosphate are used as additives. The decomposition products can form an interface film on the surface of the electrode that is highly stable and can improve the lithium ion conductivity of the interface, thereby reducing the charge transfer impedance and inhibiting the oxidative decomposition of the electrolyte, so that the battery has both high energy density and long cycle life.

[0080] In some specific embodiments, the solvent includes one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethyl propionate, propyl propionate, ethyl fluoroacetate, ethyl methyl fluorocarbonate, dimethyl fluorocarbonate, propylene fluorocarbonate, γ-butyrolactone, sulfolane, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, methyl propionate, methyl butyrate, ethyl n-butyrate, methyl acrylate, and ethyl acrylate.

[0081] Preferably, the solvent is selected from ethylene carbonate and ethyl methyl carbonate. More preferably, in the solvent, the volume ratio of ethylene carbonate to ethyl methyl carbonate is (1-7):(3-9), specifically for example 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, etc., or any ratio within the above range.

[0082] In the embodiments of the present application, the ethylene carbonate solvent has a high dielectric constant, can effectively dissolve lithium salts, and provide good ion conductivity; ethyl methyl carbonate has a low viscosity, can improve the overall ion conductivity of the electrolyte, and ensure the rapid transmission of lithium ions during the charge and discharge process. The two are coordinated in an appropriate ratio to balance the high dielectric constant and low viscosity of ethylene carbonate, ensuring that the electrolyte has both good ion conductivity and will not affect battery performance due to excessive viscosity. It also has good thermal stability at high temperatures, improving the safety performance of the battery. In addition, the combination of the two can make the solvent have good compatibility with the positive and negative electrode materials, help to form a stable SEI film and positive electrode interface film at the positive and negative electrode interfaces, and improve the battery cycle stability.

[0083] In some specific embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode material layer located on the surface of the negative electrode current collector. The negative electrode current collector mentioned in the embodiments of the present application is not particularly limited, as long as it is conductive and does not cause adverse chemical changes in the battery. Typical enriched current collectors can be, for example, copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector, preferably copper foil.

[0084] In some specific embodiments, a negative electrode active material, a binder, a conductive agent, a solvent, and the like are mixed to prepare a stable negative electrode slurry, which is then coated onto a negative electrode current collector to form a negative electrode material layer.

[0085] The negative electrode active material may include a compound capable of reversibly intercalating / deintercalating lithium ions, including a carbon-based active material, a silicon-based active material, or a mixture thereof. Examples include artificial graphite, natural graphite, acetylene black, carbon nanotubes, graphene, elemental silicon, silicon-carbon composite materials, etc. The negative electrode active material used in the embodiment of the present application is preferably SiO y -C composite material (silicon content y = 2% to 30%). The binder can be polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, styrene-butadiene rubber, and fluororubber. The solvent can include water or an organic solvent such as N-methyl-2-pyrrolidone, and the amount used can be such that the desired viscosity is achieved when the negative electrode active material and optional binder and conductive agent are included.

[0086] In the embodiment of the present application, lithium difluorophosphate in the electrolyte also participates in the negative electrode film formation to form a composite interface film, which helps to improve the stability of the negative electrode and increase the battery cycle life.

[0087] In some specific embodiments, according to EDS analysis, the phosphorus (P) content in the composite interface film on the surface of the negative electrode plate is 0.5 at % to 1.0 at %, for example, 0.5 at %, 0.6 at %, 0.7 at %, 0.8 at %, 0.9 at %, 1.0 at %, or any value within the aforementioned range.

[0088] It can be understood that EDS (Energy Dispersive X-ray Spectroscopy) is a technology that analyzes the elemental composition of materials based on X-ray energy distribution.

[0089] In some specific embodiments, the amount of Co dissolved from the surface of the negative electrode after cycling is ≤500 ppm according to ICP-MS testing.

[0090] As can be understood, ICP-MS (Inductively Coupled Plasma Mass Spectrometry) is an elemental analysis technology that combines the high efficiency ionization capability of inductively coupled plasma (ICP) with the high sensitivity and high resolution advantages of mass spectrometry (MS). It is mainly used for qualitative and quantitative detection of trace to ultra-trace elements and isotope ratio analysis.

[0091] In some embodiments, the surface SEI film impedance of the negative electrode after cycling is ≤3Ω·cm according to EIS test. 2 .

[0092] As you can understand, EIS (electrochemical impedance spectroscopy) is a method used to study the electrochemical processes between electrodes and electrolytes, as well as the electron and ion transfer dynamics in active electrode materials. Through Nyquist plots and equivalent circuit fitting, the impedance characteristics of materials can be analyzed from multiple dimensions, including charge transfer resistance (Rct), equivalent series resistance (Rs), Warburg diffusion impedance (Zw), and surface capacitance behavior.

[0093] In some specific embodiments, the separator of the battery can be a porous polymer film prepared from a polyolefin polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer). Preferably, the separator used in the embodiment of the present application is a ceramic-coated polyolefin separator with a thickness of 5μm~15μm, a ceramic coating component of Al2O3 or SiO2, a coating thickness of 1μm~5μm, a porosity of 40%~60%, and a puncture strength of ≥500gf. The separator is used in conjunction with the above-mentioned electrolyte, positive electrode sheet and negative electrode sheet to further improve the thermal stability of the battery, suppress the short circuit risk caused by the shrinkage of the separator under high voltage, and improve the safety performance of the battery.

[0094] In the lithium-ion battery described herein, a separator is disposed between the positive and negative electrodes to prevent short circuits. The battery preparation process may include the following steps: overlapping the positive and negative electrode sheets via the separator, winding and folding them as needed, and then placing them within a housing; injecting an electrolyte into the housing and encapsulating the housing. Furthermore, overcurrent protection elements, guide plates, and the like may be placed within the housing as needed to prevent pressure buildup and overcharging and discharging within the electrochemical device.

[0095] In the lithium-ion battery of the embodiment of the present application, the formation condition after the battery is packaged is 0.1C charging to 4.6V.

[0096] The application fields of the lithium-ion batteries of the embodiments of the present application are not particularly limited, and can be used in consumer electronics, new energy vehicles, energy storage and other fields.

[0097] To make the present invention easier to understand, the present invention will be further described in detail below with reference to the following examples. These examples are merely illustrative and do not limit the scope of application of the present invention. Unless otherwise specified, the raw materials or components used in the present invention can be obtained through commercial channels or conventional methods.

[0098] Example 1

[0099] 1) O2 phase lithium cobalt oxide synthesis:

[0100] The sodium source (Na2CO3), cobalt source (Co3O4), and doping element M source (Al2O3) were mixed at a molar ratio of Na / Co / Al = 0.75:0.95:0.05, sintered at 800 ° C for 15 h, and cooled to obtain a P2 phase precursor;

[0101] The above-mentioned P2 phase precursor was added to a lithium-containing molten salt system composed of LiNO3 / LiCl (the mass ratio of LiNO3 and LiCl was 3:7), and the molten salt ion exchange reaction was carried out at 200℃ for 1.5h to supplement lithium, wash and dry to obtain O2 phase Li 0.98 Co 0.95 Al 0.05 O2.

[0102] 2) PAN coating:

[0103] The O2-phase lithium cobalt oxide material was mixed with polyacrylonitrile (PAN) in a mass ratio of 100:2 and carbonized at 220°C for 4 hours under an Ar atmosphere to form a lithium cobalt oxide cathode material with a nitrogen-doped carbon coating. The coating thickness of the lithium cobalt oxide cathode material was measured to be (25±5) nm.

[0104] 3) The lithium cobalt oxide positive electrode material is mixed with the conductive agent conductive carbon black and the binder PVDF (polyvinylidene fluoride) in a mass ratio of 96:2:2, and the mixture is thoroughly stirred in N-methylpyrrolidone solvent to form a uniform positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and then cut into positive electrode sheets with a diameter of 12 mm after drying, rolling, compacting and other processes.

[0105] 4) The negative electrode active material (SiO y A negative electrode slurry was prepared by mixing a 10% silicon-carbon composite material with a binder (SBR-CMC) and a conductive agent (carbon black) in a weight ratio of 95:3.5:1.5. The mixture was then added to water as a solvent. The slurry was then coated onto a copper foil current collector. After drying, roller pressing, and compaction, the slurry was cut into negative electrode sheets with a diameter of 12 mm.

[0106] 5) In an argon-filled glove box with a water content of <10 ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 3:7 to prepare a solvent. The electrolyte was then prepared by adding lithium salt lithium bis(fluorosulfonyl)imide (LiFSI), additives fluoroethylene carbonate (FEC), and lithium difluorophosphate (LiDFP) according to the required concentrations and mixing thoroughly. The concentration of LiFSI was 1.2 M, FEC was 0.2 M, and LiDFP was 0.12 M.

[0107] 6) A polyolefin porous membrane coated with inorganic particles (Al2O3) is used as the membrane.

[0108] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The stacked electrode sheets and separator are then wound to form a battery cell. The cell is then placed in a battery case, electrolyte is injected, and the battery is packaged. The finished lithium-ion battery is then formed (charged to 4.6V at 0.1C) and aged.

[0109] Example 2

[0110] The difference from Example 1 lies in the doping element of the O2 phase lithium cobalt oxide positive electrode material in step 1).

[0111] Example 2A:

[0112] Step 1) O2 phase lithium cobalt oxide synthesis:

[0113] The sodium source (Na2CO3), cobalt source (Co3O4), and doping element M source (Al2O3) were mixed at a molar ratio of Na / Co / Al = 0.75:0.9:0.1, sintered at 800 ° C for 15 h, and cooled to obtain the P2 phase precursor; after molten salt ion exchange treatment, the O2 phase Li 0.98 Co 0.9 Al 0.1 Other operations and parameters are the same as those in Example 1.

[0114] Example 2B:

[0115] Step 1) O2 phase lithium cobalt oxide synthesis:

[0116] The sodium source (Na2CO3), cobalt source (Co3O4), and doping element M source (MgCO3, TiO2) were mixed at a molar ratio of Na / Co / Mg / Ti = 0.75:0.9:0.05:0.05, sintered at 800℃ for 15h, and cooled to obtain the P2 phase precursor; after molten salt ion exchange treatment, the O2 phase Li 0.98 Co 0.9 Mg 0.05 Ti 0.05Other operations and parameters are the same as those in Example 1.

[0117] Example 2C:

[0118] Step 1) O2 phase lithium cobalt oxide synthesis:

[0119] The sodium source (Na2CO3), cobalt source (Co3O4), and doping element M source (La2CO3, CeO2) were mixed at a molar ratio of Na / Co / La / Ce=0.75:0.9:0.05:0.05, sintered at 800℃ for 15h, and cooled to obtain the P2 phase precursor; after molten salt ion exchange treatment, the O2 phase Li 0.98 Co 0.9 La 0.05 Ce 0.05 Other operations and parameters are the same as those in Example 1.

[0120] Example 2D:

[0121] Step 1) O2 phase lithium cobalt oxide synthesis:

[0122] The sodium source (Na2CO3), cobalt source (Co3O4), and doping element M source (Y2O3, ZrO2) were mixed at a molar ratio of Na / Co / Y / Zr = 0.75:0.9:0.05:0.05, sintered at 800℃ for 15h, and cooled to obtain the P2 phase precursor; after molten salt ion exchange treatment, the O2 phase Li 0.98 Co 0.9 Y 0.05 Zr 0.05 Other operations and parameters are the same as those in Example 1.

[0123] Example 3

[0124] The difference from Example 1 lies in the thickness of the nitrogen-doped carbon coating layer in the lithium cobalt oxide positive electrode material in step 2).

[0125] Example 3A: In step 2) of Example 1, the carbonization temperature is increased to 250° C., and the coating layer thickness of the obtained lithium cobalt oxide positive electrode material is 50 nm. Other operations and parameters are the same as those of Example 1.

[0126] Example 3B: In step 2) of Example 1, the carbonization temperature is lowered to 180° C., and the coating layer thickness of the obtained lithium cobalt oxide positive electrode material is 5 nm. Other operations and parameters are the same as those of Example 1.

[0127] Example 4

[0128] The difference from Example 1 lies in the ratio of the O2 phase lithium cobalt oxide material to polyacrylonitrile PAN in step 2).

[0129] Example 4A: In step 2) of Example 1, the mass ratio of the O2 phase lithium cobalt oxide material to polyacrylonitrile PAN is adjusted to 100:1, and other operations and parameters are the same as in Example 1.

[0130] Example 4B: In step 2) of Example 1, the mass ratio of the O2 phase lithium cobalt oxide material to polyacrylonitrile PAN is adjusted to 100:5, and other operations and parameters are the same as in Example 1.

[0131] Example 4C: In step 2) of Example 1, the mass ratio of the O2 phase lithium cobalt oxide material to polyacrylonitrile PAN is adjusted to 100:6, and other operations and parameters are the same as in Example 1.

[0132] Comparative Example 1

[0133] The difference from Example 1 is that the PAN coating step in step 2) is omitted, and the positive electrode plate uses an uncoated O2 phase lithium cobalt oxide positive electrode material. Other operations and parameters are the same as Example 1.

[0134] Comparative Example 2

[0135] The difference from Example 1 is that the base material used is O3 phase lithium cobalt oxide material, which is coated with PAN. Other operations and parameters are the same as Example 1.

[0136] Comparative Example 3

[0137] The difference from Example 1 is that the electrolyte composition used is different. The lithium salt in the electrolyte prepared in step 5 is changed to 1.2M LiPF6, and does not contain additives FEC and LiDFP. Other operations and parameters are the same as Example 1.

[0138] The lithium-ion batteries prepared in the above examples and comparative examples were subjected to performance tests to observe the effect of the combination of the lithium cobalt oxide positive electrode material and the electrolyte on the battery performance.

[0139] Test Method

[0140] 1) First-effect test

[0141] Equipment: Land CT2001A test system

[0142] Conditions: Voltage range: 3.0V~4.6V (full battery); Charge and discharge rate: 0.1C; Temperature: (25±1)℃

[0143] Calculation: First efficiency = (first discharge capacity / first charge capacity) × 100%

[0144] 2) Gram capacity test

[0145] Equipment: Land CT2001A test system

[0146] Conditions: 0.1C constant current charge to 4.6V, constant voltage to current ≤ 0.05C; then 0.1C constant current discharge to 3.0V

[0147] Calculation: First discharge capacity or gram capacity (mAh / g) = discharge capacity / positive electrode material mass

[0148] 3) Cycle retention rate test

[0149] Equipment: Neware Battery Testing System (Neware BTS-4000)

[0150] Conditions: Cycle number: 800 times; Charge and discharge rate: 1C; Voltage range: 3.0V~4.6V; Temperature: (25±1)℃

[0151] Calculation: Cycle retention rate = (800th discharge capacity / first discharge capacity) × 100%

[0152] 4) Co dissolution test

[0153] ICP-MS (inductively coupled plasma mass spectrometry) was used to test the amount of Co dissolved in the negative electrode after the battery cycle test (800 charge and discharge cycles).

[0154] 5) SEI film impedance test

[0155] EIS (electrochemical impedance spectroscopy) was used to test the SEI film impedance of the negative electrode after the battery cycle test (800 charge and discharge cycles) (conditions: constant temperature of 25°C, amplitude of 10 mV, frequency range of 100 kHz~0.01 Hz).

[0156] 6) Nitrogen doping test

[0157] XPS (X-ray photoelectron spectroscopy) was used to test the nitrogen doping content of the positive electrode after the battery cycle test (800 charge and discharge cycles) was completed.

[0158] 7) Volume expansion rate test

[0159] Equipment: Scanning electron microscope (model ZEISS Gemini 500 SEM)

[0160] Samples before cycling: The uncycled electrode (positive or negative electrode) was cut into 10 mm × 10 mm squares, washed with DMC (dimethyl carbonate) three times to remove electrolyte residues, and dried in an argon glove box at 60 °C under vacuum for 12 h.

[0161] Samples after cycling: After the battery cycling test is completed (800 charge and discharge cycles), the battery is disassembled to remove the electrodes and then processed with the same cleaning and drying process.

[0162] SEM cross-section method: measuring the change in diameter of a single particle (at least 50 particles are counted)

[0163]

[0164] Where D0 is the particle diameter before cycling, and D1 is the particle diameter after cycling (assuming isotropic expansion).

[0165] 8) TOF-SIMS test

[0166] Instrument: Time-of-flight secondary ion mass spectrometer (model: ION-TOF5 produced by ION-TOF, Germany)

[0167] Test mode: sputtering depth of 50 nm on the cathode surface

[0168] Fragment signal: PO 2- (m / z=63), F - (m / z=19).

[0169] Table 1

[0170]

[0171] According to the above test data, the full-chain innovation of the positive electrode material, negative electrode material, electrolyte and process described in the embodiments of the present application solves the phase change of O3 phase lithium cobalt oxide at high voltage and the interface degradation of O2 phase lithium cobalt oxide at high voltage. After long cycling (800 times) at a high voltage of 4.6V, the XPS test on the positive electrode surface shows that the CN bond binding energy is stable at (398.5±0.3) eV; the ICP-MS detection of the Co dissolution amount on the negative electrode surface is ≤500ppm, and the EIS test of the SEI film impedance (Rsei) is ≤3Ω·cm 2 , and the P content was detected by EDS, ranging from 0.5at% to 1.0at%; the electrolyte was analyzed by HPLC, showing that the FEC residue was ≥80% and the LiDFP decomposition rate was ≤20%, that is, it had quantifiable interface chemical parameters (such as XPS bond energy, TOF-SIMS fragmentation ratio, ICP dissolution amount, etc.) and a capacity retention rate of ≥90% after long cycles. This lithium-ion battery has both high energy density (≥800Wh / L) and long cycle life (800 times ≥90%).

[0172] Example 1 is a lithium-ion battery made using a standard process. The lithium cobalt oxide positive electrode material of its positive electrode has a nitrogen-doped coating layer. Compared with the lithium-ion battery in Comparative Example 1 that has not been coated, the first efficiency, initial gram capacity, and cycle capacity retention rate of Example 1 are all much higher than those of the comparative example. The lithium cobalt oxide positive electrode material described in the examples of this application is used in conjunction with the electrolyte to achieve both excellent energy density and long cycle performance. Moreover, the lithium cobalt oxide positive electrode material that has not been subjected to nitrogen-doped carbon coating has a Co dissolution rate of up to 600ppm, an SEI film impedance increased to 3.1Ω·cm², and a volume expansion rate greater than 3.0%. It can be seen that the nitrogen-doped carbon coating layer can effectively protect the electrode interface and improve the stability of the electrode interface, thereby improving the battery's service life and safety performance at high voltages.

[0173] The matrix used in Example 1 is O2 phase lithium cobalt oxide. Compared with the O3 phase lithium cobalt oxide in Comparative Example 2, the cycle capacity retention rate of Example 1 is 91.5%, while the capacity retention rate of Comparative Example 2 is only 73.6% after 800 cycles. In addition, the Co dissolution amount of Comparative Example 2 is as high as 580ppm, the SEI film impedance increases to 4Ω·cm², and the volume expansion rate is greater than 3.5%. TOF-SIMS test of PO 2- / F - The ratio is only 0.7:1, which shows that the intrinsic structure of the O3 phase lithium cobalt oxide matrix is ​​unstable and cannot be completely repaired by coating treatment; while the lithium cobalt oxide positive electrode material with a nitrogen-doped coating layer formed by O2 phase lithium cobalt oxide and PAN coating treatment has excellent structural stability, which significantly improves the material stability and the electrochemical performance of the battery.

[0174] The composite electrolyte of Example 1 containing LiFSI / LiDFP / FEC is used, while the LiPF6-based electrolyte is used in Comparative Example 3. After 800 cycles, the capacity retention rate of Comparative Example 3 is only 76.2%, while the capacity retention rate of Example 1 is as high as 91.5%. 2- / F - The ratio dropped to 0.8:1, Co dissolution increased to 458 ppm, and SEI film resistance increased to 3 Ω·cm². This demonstrates the irreplaceable synergy between the LiFSI / LiDFP / FEC composite electrolyte and the nitrogen-doped carbon coating, effectively suppressing Co dissolution, reducing electrolyte consumption and damage to the positive and negative electrodes by side reaction products, and significantly improving the battery's cycling stability at high voltages.

[0175] Example 2A uses a high Al-doped O2 phase lithium cobalt oxide matrix, Example 2B uses a Mg and Ti-doped O2 phase lithium cobalt oxide matrix, Example 2C uses a La and Ce-doped O2 phase lithium cobalt oxide matrix, and Example 2D uses a Y and Zr-doped O2 phase lithium cobalt oxide matrix. It can be seen that the doping of the M element can further improve the bulk structural stability of the O2 phase lithium cobalt oxide matrix, thereby further reducing Co dissolution, inhibiting high-voltage phase transition, further improving the volume expansion of the battery after cycling, and improving the cycle capacity retention rate of the battery at high voltage.

[0176] The thickness of the lithium cobalt oxide positive electrode material coating layer of Example 3A is 50 nm. Compared with 25 nm in Example 1, its Co dissolution amount increases from 218 ppm to 300 ppm, and the cycle retention rate decreases from 91.5% to 89.7%. It can be seen that the thick coating layer may hinder the diffusion of lithium ions, the nitrogen doping amount is reduced (3.2→2.8 at%), and the interface bonding effect is weakened; the thickness of the lithium cobalt oxide positive electrode material coating layer of Example 3B is 5 nm. Compared with 25 nm in Example 1, its Co dissolution amount increases from 218 ppm to 380 ppm, and the cycle retention rate decreases from 91.5% to 86.5%. It can be seen that a too thin coating layer cannot effectively inhibit Co dissolution, the interface stability is slightly poor, and the battery cycle performance is slightly poor.

[0177] In the coating process of Example 4A, the mass ratio of the O2-phase lithium cobalt oxide material to polyacrylonitrile PAN was adjusted to 100:1. Compared with 100:2 in Example 1, the amount of PAN was reduced, the nitrogen doping amount was reduced from 3.2 at% to 1.8 at%, the Co dissolution amount increased significantly (218→372 ppm), and the cycle retention rate decreased by 6.2%. It can be seen that the decrease in PAN content will lead to discontinuity of the coating layer, affecting the long-cycle performance of the battery. In the coating process of Example 4B, the mass ratio of the O2-phase lithium cobalt oxide material to polyacrylonitrile PAN was adjusted to 100:5. Compared with 100:2 in Example 1, the amount of PAN was increased, the nitrogen doping amount was reduced from 3.2 at% to 1.8 at%, the Co dissolution amount was significantly increased (218→372 ppm), and the cycle retention rate decreased by 6.2%. When the PAN content increased from 2at% to 3.5at%, the Co dissolution amount decreased (218→200ppm), and the cycle retention rate increased to 92.0%; when the PAN dosage was further increased, the nitrogen doping amount increased from 3.2at% to 3.7at%, the Co dissolution amount decreased (200→194ppm), the cycle retention rate was 91.4%, the first efficiency dropped to 92.1%, and the initial gram capacity dropped to 229mAh / g. It can be seen that high PAN content can form a thicker and more uniform coating layer, and improve the interface stability; however, excessive PAN dosage significantly increases the thickness of the coating layer, which will lead to the obstruction of lithium ion transmission, and worsen the battery rate performance and cycle stability.

[0178] The embodiments of the present application have been described above. The above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is selected to best explain the principles of the embodiments, their practical applications, or improvements to the technology in the market, or to enable other persons skilled in the art to understand the embodiments disclosed herein.

Claims

1. A lithium-ion battery comprising a positive electrode and an electrolyte, characterized in that: The positive electrode plate includes a lithium cobalt oxide positive electrode material, and the lithium cobalt oxide positive electrode material includes a matrix and a nitrogen-doped carbon coating layer coated on the surface of the matrix; wherein, the matrix of the lithium cobalt oxide positive electrode material is O2-phase lithium cobalt oxide, and its chemical general formula is Li x Co 1-z M z O2; wherein, 0.95 ≤ x ≤ 1, 0 < z ≤ 0.1, and M is at least one of Mg, Al, Ti, La, Y, Zr, and Ce; The electrolyte comprises a lithium salt, an additive and a solvent, wherein the additive comprises lithium difluorophosphate, and the lithium difluorophosphate can react with the nitrogen-doped carbon coating layer of the lithium cobalt oxide positive electrode material to form a composite interface; according to TOF-SIMS testing, in the composite interface on the surface of the positive electrode sheet, when the nucleus-mass ratio m / z is 63, PO 2- Fragment signal intensity ≥1×10 4 counts, when the nucleus-cytoplasm ratio m / z is 19, F - Fragment signal intensity ≤5×10 3 counts, PO in the composite interface on the surface of the positive electrode 2- Fragment / F - The signal intensity ratio of the fragments was ≥2:

1.

2. The lithium-ion battery according to claim 1, wherein: The average particle size Dv50 of the matrix of the lithium cobalt oxide positive electrode material is 3 μm to 10 μm, and the specific surface area is 0.5 m 2 / g~4.6 m 2 / g.

3. The lithium-ion battery according to claim 1, wherein: The CN bond binding energy of the nitrogen-doped carbon coating layer of the lithium cobalt oxide positive electrode material is (398.5±0.3) eV, and the C=O bond binding energy is (531.2±0.3) eV; And / or, the nitrogen doping amount in the nitrogen-doped carbon coating layer of the lithium cobalt oxide positive electrode material is 2at% to 5at%; And / or, the thickness of the nitrogen-doped carbon coating layer of the lithium cobalt oxide positive electrode material is 5 nm to 50 nm.

4. The lithium-ion battery according to claim 1, wherein The preparation method of the lithium cobalt oxide positive electrode material comprises: mixing O2-phase lithium cobalt oxide with a nitrogen-containing polymer, and carbonizing the mixture to obtain a nitrogen-doped carbon-coated lithium cobalt oxide positive electrode material.

5. The lithium-ion battery according to claim 4, wherein: The nitrogen-containing polymer is selected from at least one of polyacrylonitrile, polyimide, polyaniline and polypyrrole.

6. The lithium-ion battery according to claim 5, wherein: The nitrogen-containing polymer is selected from polyacrylonitrile.

7. The lithium-ion battery according to claim 4, wherein: The O2 phase lithium cobalt oxide and the nitrogen-containing polymer are mixed in a mass ratio of 100:(1-5).

8. The lithium-ion battery according to claim 4, wherein The carbonization conditions are: carbonization at 150° C. to 250° C. for 2 h to 6 h under an inert atmosphere.

9. The lithium-ion battery according to claim 1, wherein: The lithium salt includes lithium bis(fluorosulfonyl)imide with a concentration of 1.0M to 1.5M; and / or, the concentration of lithium difluorophosphate in the electrolyte is 0.05M~0.2M; And / or, the additive further comprises fluoroethylene carbonate at a concentration of 0.05M to 0.5M; and / or, the solvent comprises ethylene carbonate and ethyl methyl carbonate; And / or, the solvent is a combination of ethylene carbonate and ethyl methyl carbonate in a volume ratio of (1-7):(3-9).

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