Lithium ion battery and method of manufacturing the same

Abstract

The application relates to the technical field of batteries, and discloses a lithium ion battery and a preparation method thereof. The lithium ion battery comprises a positive electrode, a negative electrode and an electrolyte. The negative electrode comprises a negative electrode active material, and the negative electrode active material comprises a graphite matrix and a fluorine-containing interface layer covering the surface of the graphite matrix. The electrolyte comprises a lithium salt, an organic solvent and a film-forming agent, the film-forming agent is a fluorine-containing oxylithium compound salt, the fluorine-containing oxylithium compound is suitable for decomposition in the formation process of the lithium ion battery, and the decomposition product comprises lithium fluoride and a nonmetal-oxygen-fluorine compound. According to the application, the fluorine-containing interface layer is pre-fixed on the negative electrode active material, and the fluorine-containing oxylithium film-forming agent is used, so that an integrated composite interface film is constructed in the first charging process, the consumption of active lithium is reduced, the first coulomb efficiency is improved, the volume change stress is buffered by virtue of the high mechanical strength and flexibility of the fluorine-containing oxylithium film-forming agent, the repeated rupture and repair of the SEI film are avoided, and the cycle stability is significantly improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a lithium-ion battery and its preparation method. Background Technology

[0002] Lithium-ion batteries, due to their high energy density, long cycle life, and lack of memory effect, have been widely used in consumer electronics, electric vehicles, and energy storage systems. As application scenarios increasingly demand higher energy density and longer cycle life, improving the initial coulombic efficiency and cycle stability of lithium-ion batteries has become a hot research topic in the industry. In lithium-ion batteries, the interface film is a passivation layer formed during the initial charging process by the reduction and decomposition of electrolyte components on the negative electrode surface. This interface film effectively prevents the continuous reaction between the electrolyte and the negative electrode material, and has a decisive impact on the battery's initial efficiency, rate performance, and cycle life.

[0003] Currently, the interface film of commercial lithium-ion batteries mainly relies on the electrolyte solvent and lithium salt being at a low potential during the first charge of the battery (graphite anode is typically below 0.8V vs. Li). + The interface film is formed by the natural reduction and decomposition of lithium (Li). This in-situ formed interface film has the following inherent defects: 1) During the first charge, the lithium ions consumed in the decomposition of the electrolyte to form the interface film come from the positive electrode material, leading to a reduction in the active lithium used for energy storage in the battery. This significantly reduces the initial coulombic efficiency (discharge capacity / charge capacity), thus affecting the battery's initial capacity and subsequent cycle capacity. 2) The interface film formed by traditional methods has a loose structure, many defects, and insufficient bonding strength with the negative electrode material. During battery cycling, the negative electrode material undergoes significant volume changes due to the insertion and extraction of lithium ions. The interface film is prone to rupture during repeated volume expansion and contraction. The fresh negative electrode surface exposed after rupture reacts again with the electrolyte to form a new interface film. This "rupture-repair" cycle continuously consumes electrolyte and active lithium, leading to continuous capacity decay and severely limiting the cell's cycle life. Summary of the Invention

[0004] This application provides a lithium-ion battery and a method for preparing the same, in order to at least partially solve the above-mentioned technical problems.

[0005] To achieve the above objectives, according to a first aspect of this application, a lithium-ion battery is provided, comprising: positive electrode; The negative electrode includes a negative electrode active material, which includes a graphite matrix and a fluorine-containing interface layer coated on the surface of the graphite matrix. The electrolyte includes lithium salts, organic solvents, and film-forming agents. The film-forming agent is a fluorinated lithium oxide compound salt, which is suitable for decomposition during the formation of lithium-ion batteries. The decomposition products include lithium fluoride and non-metallic oxygen-fluorine compounds.

[0006] By pre-forming a fluorine-containing interface layer on the graphite anode surface and adding a film-forming agent containing a fluorinated lithium oxide compound salt to the electrolyte, during the first charge of the lithium-ion battery, the fluorinated lithium oxide compound salt preferentially reduces and decomposes to generate nascent lithium fluoride and a non-metallic oxygen-fluorine compound. The nascent lithium fluoride grows epitaxially on the fluorine-containing interface layer, and the non-metallic oxygen-fluorine compound forms lithium-oxygen-non-metallic covalent bonds with the lithium fluoride, thereby constructing a dense, integrated composite interface film. In this scheme, the fluorine-containing interface layer reduces the consumption of active lithium on the cathode during the first charge, effectively improving the first coulombic efficiency. The integrated structure formed by chemical bonding possesses both mechanical strength and flexibility, effectively buffering the stress caused by volume changes in the anode and avoiding the problem of continuous electrolyte and active lithium consumption caused by repeated repairs due to rupture of traditional SEI films, thus significantly improving cycle stability.

[0007] Optionally, the fluorinated lithium oxide compound includes at least one of lithium difluorophosphate, lithium tetrafluorooxalate phosphate, lithium difluorosulfonylimide, and lithium difluorooxalate borate.

[0008] By limiting the use of a variety of fluorinated lithium oxide compounds as film-forming agents, a flexible balance between cost and performance can be achieved, ensuring preferential decomposition and chemical bonding at higher electrode potentials while broadening the industrial application scenarios.

[0009] Optionally, the mass fraction of the film-forming agent is between 0.5% and 2.0% based on the total mass of the electrolyte.

[0010] By controlling the mass fraction of the film-forming agent within this range, it is possible to ensure the formation of a uniform and dense integrated composite interface film on the negative electrode surface. If the amount added is too low, the film formation will be incomplete, while if it is too high, it will increase the interfacial resistance and cause unnecessary waste of raw materials.

[0011] Optionally, the specific surface area of ​​the graphite matrix is ​​1.5 m². 2 / g -3.0m 2 Between / g.

[0012] By controlling the specific surface area of ​​the graphite matrix within this range, it is possible to ensure sufficient reactive sites to form a uniform fluorine-containing interface layer while avoiding excessive growth of the interface film or an increase in side reactions due to an excessively high specific surface area, thereby balancing interface stability and the first coulombic efficiency of the battery.

[0013] Optionally, the mass fraction of the fluorine-containing interface layer is between 5% and 12% based on the total mass of the graphite anode.

[0014] By controlling the mass fraction of the fluorine interface layer within the above range, it is possible to ensure that the interface layer completely covers the graphite matrix to play a pre-film formation role, while avoiding the decrease in the specific capacity of the negative electrode or the obstruction of ion transport due to excessively thick interface layer, thereby achieving both the improvement of the first coulombic efficiency and the energy density of the battery.

[0015] Optionally, the fluorinated interface layer includes lithium fluoride.

[0016] The lithium fluoride in the fluorine-containing interface layer provides an epitaxial growth template for the newly formed lithium fluoride, forming a highly crystalline integrated composite layer that significantly improves the strength of the interface film; the fluorocarbons, on the other hand, utilize their chemical inertness to help suppress side reactions, and the two work together to enhance cycle stability.

[0017] Optionally, the non-metallic oxygen-fluorine compound includes lithium oxyfluoride compounds containing phosphorus and lithium oxyfluoride compounds containing boron.

[0018] Both lithium fluoride-phosphorus compounds and lithium fluoride-boron compounds can form stable chemical bonds with lithium fluoride. Among them, lithium fluoride-phosphorus compounds focus on improving the mechanical strength and density of the interfacial film, while lithium fluoride-boron compounds are more conducive to reducing interfacial impedance and enhancing flexibility, providing a variety of performance optimization paths for technical solutions.

[0019] Alternatively, the organic solvent may include ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0020] Ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate, as mixed organic solvents, can provide a moderate dielectric constant and low viscosity, promoting the full dissociation and uniform dispersion of the film-forming agent in the electrolyte. This ensures that the film-forming agent is preferentially and uniformly reduced and decomposed in the voltage range of 1.5V to 2.0V during the first charging process, thereby forming a dense and stable integrated composite interface film.

[0021] According to a second aspect of this application, a method for preparing a lithium-ion battery is provided, comprising: providing a positive electrode, a negative electrode, and an electrolyte; wherein the negative electrode includes a negative electrode active material, the negative electrode active material includes a graphite substrate and a fluorine-containing interface layer coated on the surface of the graphite substrate; the electrolyte includes a lithium salt, an organic solvent, and a film-forming agent, the film-forming agent being a fluorine-containing lithium oxide compound, the fluorine-containing lithium oxide compound being suitable for decomposition during the lithium-ion battery formation process, the decomposition products including lithium fluoride and a non-metallic oxygen-fluorine compound; and assembling the positive electrode, the negative electrode, and the electrolyte into a lithium-ion battery.

[0022] By directly assembling a negative electrode with a pre-formed fluorine-containing interface layer on its surface with an electrolyte containing a specific film-forming agent, there is no need to add complex pre-lithiation or special formation processes after battery assembly. During the first charge, the high-level preferential decomposition of the film-forming agent can chemically bond with the fluorine-containing interface layer, spontaneously constructing an integrated composite interface film. This simplifies the process, facilitates industrialization, and effectively improves the first coulombic efficiency and cycle stability of lithium-ion batteries.

[0023] Optionally, the preparation method of the negative electrode active material includes: A graphite matrix is ​​heat-treated in a non-oxidizing atmosphere containing fluorine gas to form a fluorine-containing interface layer on the surface of the graphite matrix.

[0024] By adding fluorine-containing gas to the graphite matrix for heat treatment in a non-oxidizing atmosphere, a uniform and dense fluorine-containing interface layer can be formed by the controlled reaction between the fluorine-containing gas and the graphite surface without introducing oxygen elements and causing interface contamination. This provides an ideal active template for subsequent chemical bonding with the decomposition products of the film-forming agent in the electrolyte.

[0025] Optionally, the non-oxidizing atmosphere includes inert gases and fluorine-containing gases, wherein the volume concentration of fluorine-containing gases is between 0.1% and 0.5% based on the total mass of the non-oxidizing atmosphere, and the remainder is inert gases.

[0026] By controlling the volume concentration of fluorine-containing gas within the above range, a uniform and dense fluorine-containing interface layer can be formed on the graphite surface. If the concentration is too low, the reaction will be insufficient, resulting in insufficient coverage of the interface layer. If the concentration is too high, the graphite surface may be over-etched or an excessively thick interface layer may be generated, affecting the electrochemical performance of the negative electrode.

[0027] Optionally, the fluorine-containing gas includes at least one of phosphorus pentafluoride, nitrogen trifluoride, carbon tetrafluoride, and sulfur hexafluoride.

[0028] By limiting the fluorine-containing gases to the aforementioned types, flexible selection can be made based on process and cost considerations, effectively releasing active fluorine species and ensuring the uniform and stable formation of the fluorine-containing interface layer.

[0029] Optionally, the inert gas includes at least one of argon, helium, and neon.

[0030] By limiting the inert gas to the above range, it is possible to choose flexibly according to cost, and its chemical inertness can be used to prevent graphite from oxidizing during high-temperature processing, ensuring the pure formation of the fluorine-containing interface layer. Optionally, heat treatment of the graphite matrix under a non-oxidizing atmosphere containing fluorine gas includes: The graphite matrix was placed in a non-oxidizing atmosphere containing fluorine gas and kept at a temperature between 800℃ and 1000℃ for 1-2 hours.

[0031] By limiting the heat treatment process to the above range, it is possible to ensure that the fluorine-containing gas reacts fully with the graphite matrix to form a dense and stable fluorine-containing interface layer, thus balancing reaction efficiency and the integrity of the graphite structure.

[0032] Optionally, the preparation method of the negative electrode active material includes: The graphite substrate is immersed in a fluorine-containing solution to form a fluorine-containing interface layer on the surface of the graphite substrate.

[0033] By soaking a graphite substrate in a fluorine-containing solution, a fluorine-containing interface layer can be easily and efficiently formed on its surface under mild conditions. The process is simple to operate, low in cost, and easy to scale up for production.

[0034] Optionally, the fluorinated solution includes at least one of ammonium fluoride solution, ammonium bifluoride solution, lithium fluoride solution, and sodium fluoride solution.

[0035] By limiting the fluoride-containing solution to the above range, it is possible to flexibly select according to different costs and process requirements, effectively provide fluoride ions under mild conditions, and ensure the uniform and controllable formation of the fluoride-containing interface layer.

[0036] Other features and advantages of this application will be described in detail in the following detailed description section. Detailed Implementation The present invention will be further described in detail below through specific embodiments. In the following embodiments, many details are described to facilitate a better understanding of this application. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other materials or methods. In some cases, certain operations related to this application are not shown or described in the specification. This is to avoid obscuring the core parts of this application with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0037] Furthermore, the features, operations, or characteristics described in the specification can be combined in any suitable manner to form various embodiments. At the same time, the steps or actions in the method description can be rearranged or adjusted in a manner obvious to those skilled in the art. Therefore, the various orders in the specification are only for clearly describing a particular embodiment and do not imply a necessary order, unless otherwise stated that a particular order must be followed.

[0038] To further illustrate this application, the following describes in detail the lithium-ion battery and its preparation method provided in this application with reference to the embodiments. However, it should be understood that these embodiments are implemented under the premise of the technical solution of this application, and detailed implementation methods and specific operation processes are given. They are only for further illustrating the features and advantages of this application, and are not intended to limit the claims of this application. The scope of protection of this application is not limited to the following embodiments.

[0039] This application provides a lithium-ion battery, including a positive electrode, a negative electrode, and an electrolyte.

[0040] The negative electrode includes a negative electrode active material, which comprises a graphite matrix and a fluorine-containing interface layer coated on the surface of the graphite matrix. The electrolyte includes a lithium salt, an organic solvent, and a film-forming agent. The film-forming agent is a fluorine-containing lithium oxide compound, which is suitable for decomposition during the lithium-ion battery formation process. The decomposition products include lithium fluoride and non-metallic oxygen-fluorine compounds.

[0041] By pre-positioning a fluorine-containing interface layer on the surface of a graphite substrate and adding a fluorinated lithium oxide compound as a film-forming agent to the electrolyte, during the first charge of a lithium-ion battery, the fluorinated lithium oxide compound salt preferentially undergoes reductive decomposition over the carbonate solvent, generating nascent lithium fluoride and a non-metallic oxygen-fluorine compound. The nascent lithium fluoride undergoes epitaxial growth on the surface of the pre-positioned fluorine-containing interface layer, forming a highly crystalline, low-defect lithium fluoride composite layer. Simultaneously, the non-metallic oxygen-fluorine compound forms lithium-oxygen-non-metallic covalent bonds with the lithium fluoride, firmly bonding the two interface films together, thus constructing a dense and stable integrated composite interface film on the negative electrode surface. In this scheme, the pre-positioned fluorine-containing interface layer itself is a component of the interface film, reducing the consumption of positive electrode active lithium during the interface film formation during the first charge, thereby effectively improving the first coulombic efficiency. In addition, the integrated structure formed by chemical bonding has both good mechanical strength and flexibility, which can effectively buffer the volume change stress caused by lithium ion insertion and extraction during the cycling process of the graphite anode. This avoids the problem of continuous consumption of electrolyte and active lithium caused by repeated cracking and repair due to insufficient mechanical properties of traditional SEI films, thus significantly improving the cycle stability of the battery.

[0042] In some embodiments of this application, the fluorinated lithium oxide compound includes at least one of lithium difluorophosphate, lithium tetrafluorooxalate phosphate, lithium difluorosulfonylimide, and lithium difluorooxalate borate.

[0043] By limiting a variety of preferred fluorinated lithium oxide compounds to be used as film-forming agents, flexible selection can be made according to different cost and performance requirements. While ensuring that the high-level decomposition is preferential and chemically bonded to the pre-existing fluorinated interface layer, the economic efficiency and applicability of the solution are taken into account, thus broadening the industrial application scenarios.

[0044] In some embodiments of this application, the mass fraction of the film-forming agent is between 0.5% and 2.0% based on the total mass of the electrolyte. Exemplarily, the mass fraction of the film-forming agent is 0.5%, 1%, 1.5%, or 2.0% based on the total mass of the electrolyte.

[0045] Controlling the mass fraction of the film-forming agent within this range ensures the formation of a uniform and dense integrated composite interfacial film on the negative electrode surface. If the addition amount is less than 0.5%, the amount of film-forming agent decomposed is insufficient, making it difficult to form a completely covered interfacial film on the negative electrode surface, resulting in exposed areas and triggering side reactions. If the addition amount is greater than 2.0%, excessive film-forming agent decomposition products will deposit excessively, increasing the thickness and density of the interfacial film, leading to a prolonged lithium-ion transport path and increased interfacial impedance, and also causing unnecessary waste of film-forming agent raw materials. Therefore, limiting the mass fraction of the film-forming agent to between 0.5% and 2.0% can ensure the integrity and density of the interfacial film while balancing the rate performance and cost control of the battery.

[0046] In some embodiments of this application, the specific surface area of ​​the graphite matrix is ​​1.5 m². 2 / g -3.0m 2 Between / g. For example, the specific surface area of ​​the graphite matrix is ​​1.5 m². 2 / g, 2.0 m 2 / g, 2.5 m 2 / g or 3.0 m 2 / g.

[0047] Controlling the specific surface area of ​​the graphite matrix within this range serves two purposes. First, it provides a sufficient number of reactive sites, ensuring that fluorinated gases or solutions react fully and uniformly with the graphite matrix surface during pretreatment, forming a complete and uniformly thick fluorinated interface layer. This lays a solid foundation for subsequent chemical bonding with the decomposition products of the film-forming agent. Second, this specific surface area range avoids problems such as excessive interfacial film growth, increased side reactions due to excessive active sites, and increased irreversible capacity loss during the first charge-discharge cycle caused by excessively high specific surface areas (e.g., exceeding 3.0 m² / g). Therefore, limiting the specific surface area of ​​the graphite matrix to between 1.5 m² / g and 3.0 m² / g effectively suppresses side reactions while ensuring the uniform formation of the fluorinated interface layer, thus balancing interfacial stability and the battery's initial coulombic efficiency.

[0048] In some embodiments of this application, the mass fraction of the fluorine-containing interface layer is between 5% and 12% based on the total mass of the graphite anode. Exemplarily, the mass fraction of the fluorine-containing interface layer is 5%, 6%, 8%, or 10% based on the total mass of the graphite anode.

[0049] By controlling the mass fraction of the fluorine interface layer within the aforementioned range, two advantages are achieved. First, this range ensures a complete and continuous coverage of the fluorine-containing interface layer on the graphite substrate surface, thereby fully leveraging the pre-formed film-forming effect. This provides a sufficient and uniform active template for the subsequent epitaxial growth and chemical bonding of the film-forming agent decomposition products, effectively reducing the consumption of positive electrode active lithium during the first charge and improving the first coulombic efficiency. Second, this mass fraction range avoids problems such as a decrease in the proportion of negative electrode active material (i.e., a decrease in negative electrode specific capacity) and an extended lithium-ion transport path and increased interface impedance caused by an excessively thick fluorine-containing interface layer, thus guaranteeing the battery's energy density and rate performance. Therefore, limiting the mass fraction of the fluorine-containing interface layer to between 5% and 12% can improve the first coulombic efficiency while simultaneously considering the battery's energy density and ion transport capability.

[0050] In some embodiments of this application, the fluorinated interface layer includes lithium fluoride.

[0051] When the fluorine-containing interface layer includes lithium fluoride: lithium fluoride has high mechanical strength and electrochemical stability, and can provide a crystal template for the epitaxial growth of the newly generated LiF after the decomposition of the film-forming agent, forming an integrated LiF composite layer with high crystallinity and few defects, which significantly improves the structural strength and cycle stability of the interface film.

[0052] In some embodiments of this application, the non-metallic oxygen-fluorine compound includes phosphorus-containing lithium oxyfluorine compound and boron-containing lithium oxyfluorine compound.

[0053] Both phosphorus-containing lithium fluoride oxyphosphate and boron-containing lithium fluoride oxyphosphate compounds can form stable chemical bonds with lithium fluoride, but their effects differ. The Li-OP bond formed by phosphorus-containing lithium fluoride oxyphosphate compounds has a high bond energy, which can firmly bond the pre-existing fluorine-containing interface layer with the film-forming agent decomposition product layer, forming a dense, high-mechanical-strength integrated composite interface film that effectively resists the volume change stress of the negative electrode. The Li-OB bond formed by boron-containing lithium fluoride oxyphosphate compounds, on the other hand, helps reduce interfacial impedance, and its good flexibility can buffer volume change stress, preventing interface film brittleness. Therefore, selecting different compounds according to performance requirements allows for flexible optimization, prioritizing either mechanical strength or low impedance.

[0054] In some embodiments of this application, the organic solvent includes ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate. The volume ratio of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate is 3:5:2.

[0055] In this mixed organic solvent system, each component plays a different synergistic role: ethylene carbonate (EC) has a high dielectric constant (approximately 89.6), which effectively promotes the full dissociation of lithium salt and film-forming agent, providing sufficient free ions, but its viscosity is high (approximately 1.9 cP) and melting point is high; diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) have lower viscosity (approximately 0.75 cP and 0.65 cP, respectively) and lower melting point, which can significantly reduce the overall viscosity of the electrolyte, improve ion transport kinetics, and broaden the liquid temperature range of the electrolyte. When the three are mixed in an appropriate ratio, EC provides a high dielectric constant to ensure the full dissociation of the salt, while DEC and EMC reduce viscosity, promote rapid ion migration, and improve the uniform dispersion of the film-forming agent in the electrolyte. Thus, they synergistically ensure that the film-forming agent can preferentially and uniformly reduce and decompose during the first charging process, forming a dense and stable integrated composite interface film.

[0056] In some embodiments of this application, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(oxalate borate) (LiBOB), lithium difluorooxalate borate (LiDFOB), and lithium bis(fluorosulfonyl)imide (LiFSI).

[0057] This application also provides a method for preparing a lithium-ion battery, comprising: providing a positive electrode, a negative electrode, and an electrolyte; wherein the negative electrode includes a negative electrode active material, the negative electrode active material includes a graphite substrate and a fluorine-containing interface layer coated on the surface of the graphite substrate; the electrolyte includes a lithium salt, an organic solvent, and a film-forming agent, the film-forming agent being a fluorine-containing lithium oxide compound, the fluorine-containing lithium oxide compound being suitable for decomposition during the lithium-ion battery formation process, the decomposition products including lithium fluoride and non-metallic oxygen-fluorine compounds; and assembling the positive electrode, negative electrode, and electrolyte into a lithium-ion battery.

[0058] This lithium-ion battery fabrication method directly assembles a negative electrode with a pre-formed fluorine-containing interface layer on its surface with an electrolyte containing a specific film-forming agent. This eliminates the need for complex pre-lithiation treatments or formation adjustments (such as low-current, long-term pre-formation, or stepped formation) after battery assembly. During the first charge, the film-forming agent undergoes preferential reduction and decomposition in the 1.5V–2.0V voltage range. Its decomposition products spontaneously construct an integrated composite interface film through epitaxial growth and chemical bonding with the pre-formed fluorine-containing interface layer on the negative electrode surface. This process fully utilizes the synergistic effect between negative electrode pretreatment and electrolyte formulation, achieving in-situ construction of a high-performance interface film without adding extra steps or costs. This significantly simplifies the production process, improves industrial feasibility, and effectively enhances the initial coulombic efficiency and cycle stability of the lithium-ion battery.

[0059] In some embodiments of this application, the method for preparing the negative electrode active material includes: heat-treating a graphite substrate under a non-oxidizing atmosphere containing fluorine gas to form a fluorine-containing interface layer on the surface of the graphite substrate.

[0060] By adding fluorine-containing gas to a graphite matrix for heat treatment in a non-oxidizing atmosphere, a controllable fluorine-containing interface layer can be constructed without introducing oxygen. Specifically, a non-oxidizing atmosphere (such as an inert gas or nitrogen environment) eliminates the interference of oxygen on the high-temperature treatment process, avoiding oxidation side reactions or the formation of oxygen-containing impurities (such as CO, C=O, and other polar functional groups) on the graphite surface, thus ensuring the purity and structural consistency of the fluorine-containing interface layer. Simultaneously, the fluorine-containing gas decomposes at high temperature, releasing active fluorine species that undergo a controllable fluorination reaction with the graphite surface, forming a uniform, dense, and firmly bonded fluorine-containing interface layer. This interface layer serves as an ideal active template, with uniformly distributed active sites and a crystal structure matching nascent lithium fluoride. It provides a good structural basis for the epitaxial growth and chemical bonding of the film-forming agent decomposition products during the subsequent first charging process, ensuring the successful construction of the integrated composite interface film.

[0061] In some embodiments of this application, the non-oxidizing atmosphere includes an inert gas and a fluorine-containing gas. The volume concentration of the fluorine-containing gas is between 0.1% and 0.5% based on the total mass of the non-oxidizing atmosphere, with the remainder being inert gases. Exemplarily, the volume concentration of the fluorine-containing gas is 0.1%, 0.2%, 0.3%, 0.4%, or 0.5% based on the total mass of the non-oxidizing atmosphere, with the remainder being inert gases.

[0062] By controlling the volume concentration of fluorine-containing gas within the aforementioned range, a balance can be achieved between the quality of the fluorine-containing interface layer and the protection of the graphite matrix. When the concentration is below 0.1%, the supply of active fluorine species is insufficient, and the fluorination reaction is incomplete, resulting in insufficient coverage of the interface layer or exposed defects. Subsequent decomposition products of the film-forming agent cannot obtain sufficient epitaxial growth templates and chemical bonding sites, affecting the integrity of the integrated composite interface film. When the concentration is above 0.5%, excessive active fluorine species will over-etch the graphite surface, destroying its ordered layered structure and affecting the lithium intercalation capacity. Simultaneously, an excessively thick interface layer is formed, increasing lithium-ion transport resistance and reducing the proportion of anode active material, adversely affecting energy density and rate performance. Therefore, controlling the volume concentration of fluorine-containing gas between 0.1% and 0.5% can ensure sufficient and uniform formation of the interface layer while avoiding negative impacts on the graphite matrix and electrochemical performance.

[0063] In some embodiments of this application, the fluorine-containing gas includes at least one of phosphorus pentafluoride, nitrogen trifluoride, carbon tetrafluoride, and sulfur hexafluoride.

[0064] All of the aforementioned fluorine-containing gases can effectively decompose and release active fluorine species at high temperatures, undergoing a fluorination reaction with the graphite substrate surface. Among them, phosphorus pentafluoride exhibits high reactivity, suitable for rapid film formation; nitrogen trifluoride and carbon tetrafluoride have moderate decomposition temperatures, facilitating process control; and sulfur hexafluoride is chemically stable, resulting in a milder treatment process. The costs of different gas sources also vary, allowing for selection based on the economic requirements of large-scale production. Regardless of the choice of one or a combination of these gases, sufficient and stable active fluorine species can be released in a non-oxidizing atmosphere, ensuring a uniform and controllable fluorination reaction. This results in a complete, uniformly thick, and structurally stable fluorine-containing interface layer on the graphite substrate surface, providing a reliable foundation for the subsequent construction of an integrated composite interface film.

[0065] In some embodiments of this application, the inert gas includes at least one of argon, helium, and neon.

[0066] The aforementioned inert gases all possess high chemical stability and do not react chemically with the graphite matrix during high-temperature heat treatment. They effectively eliminate oxygen in the reaction environment, preventing graphite from undergoing oxidation and ablation or generating oxygen-containing functional group impurities at high temperatures. By constructing a pure, non-oxidizing atmosphere, it is ensured that the active fluorine species released from the decomposition of fluorine-containing gases only undergo the expected fluorination reaction with the graphite surface, avoiding the introduction of oxidation byproducts. This guarantees the purity and structural consistency of the formed fluorine-containing interface layer, providing a clean and uniform active template for the subsequent construction of an integrated composite interface film.

[0067] In some embodiments of this application, heat treatment of the graphite matrix in a non-oxidizing atmosphere containing fluorine gas includes: placing the graphite matrix in a non-oxidizing atmosphere containing fluorine gas and holding it at a temperature between 800°C and 1000°C for 1-2 hours. Exemplarily, the temperature can be 800°C and the holding time can be 2 hours; or, the temperature can be 900°C and the holding time can be 1.5 hours; or, the temperature can be 1000°C and the holding time can be 1 hour.

[0068] When the temperature is below 800℃ or the time is less than 1 hour, the fluorine-containing gas decomposes insufficiently, the fluorination reaction is incomplete, the interface layer is difficult to form a complete cover, and the bonding strength with the graphite matrix is ​​insufficient. When the temperature is above 1000℃ or the time exceeds 2 hours, excessive reaction may damage the graphite surface structure, destroy the ordered layered structure, and affect its lithium intercalation capacity and cycle stability. Therefore, limiting the heat treatment process within the above range can ensure the density and stability of the fluorine-containing interface layer while protecting the structural integrity of the graphite matrix, thus balancing reaction efficiency and anode electrochemical performance.

[0069] In some embodiments of this application, the method for preparing the negative electrode active material includes: soaking a graphite substrate in a fluorine-containing solution to form a fluorine-containing interface layer on the surface of the graphite substrate.

[0070] This process requires no high-temperature equipment or special atmosphere control; it only involves conventional soaking, washing, and drying steps, resulting in low equipment investment and energy costs. The operation is simple and requires minimal technical expertise from operators. Furthermore, the solution soaking method facilitates batch processing, allowing for the simultaneous processing of large quantities of graphite anode materials, demonstrating excellent process scalability and production efficiency. Therefore, this approach significantly reduces preparation costs and process complexity while ensuring the formation of the fluorine-containing interface layer, making it easily scalable for industrial production.

[0071] In some embodiments of this application, the fluorinated solution includes at least one of ammonium fluoride solution, ammonium hydrogen fluoride solution, lithium fluoride solution, and sodium fluoride solution.

[0072] Ammonium fluoride and ammonium bifluoride exhibit good water solubility and high fluoride ion release efficiency, making them suitable for rapid film formation. Lithium fluoride can directly provide a lithium source, which is beneficial for forming a LiF-rich interfacial layer. Sodium fluoride has a lower cost and is suitable for large-scale production. All of the above fluorinated solutions can effectively release fluoride ions under mild conditions (room temperature to approximately 60°C), avoiding potential damage to the graphite structure caused by high-temperature treatment. By controlling the solution concentration and soaking time, the degree of fluorination reaction can be precisely controlled, ensuring the formation of a uniform, controllable-thickness, and fully covered fluorinated interfacial layer on the graphite substrate surface, providing a reliable foundation for the subsequent construction of integrated composite interfacial films.

[0073] Example 1 1. Preparation of electrolyte In an argon-filled glove box (water and oxygen content both less than 1 ppm), 1.0 M lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) in a volume ratio of EC:DEC:EMC = 3:5:2. The solution was stirred until homogeneous to obtain the basic electrolyte. Then, 1.0% (w / w) lithium difluorophosphate (LiPO2F2) was added to this basic electrolyte as a film-forming agent, and the mixture was stirred until homogeneous to obtain the final electrolyte.

[0074] 2. Preparation of prefluorinated negative electrode Artificial graphite powder with a specific surface area of ​​2 m² / g was selected and placed in a tube furnace. A mixed gas consisting of argon (Ar) and phosphorus pentafluoride (PF5), with a PF5 volume concentration of 0.5%, was introduced. The furnace was heat-treated at 1000℃ for 1 hour to form a fluorine-containing interface layer on the graphite surface. After treatment, the graphite was allowed to cool naturally to room temperature to obtain the pre-fluorinated negative electrode.

[0075] 3. Preparation of the positive electrode Lithium iron phosphate (LiFePO4) was used as the positive electrode active material. It was mixed with conductive agent Super P and binder PVDF at a mass ratio of 95:2.5:2.5. An appropriate amount of N-methylpyrrolidone (NMP) was added and stirred into a uniform slurry. The slurry was coated on aluminum foil and then dried, rolled, and slit to obtain the positive electrode sheet.

[0076] 4. Battery Assembly The positive electrode sheet, polypropylene separator, and prefluorinated negative electrode sheet prepared above are stacked in sequence to form a battery cell, and the electrolyte prepared above is injected. After processes such as encapsulation, standing, formation, and aging, a lithium-ion battery is obtained.

[0077] Example 2 The difference from Example 1 lies in the preparation of the prefluorinated carbon composite electrode, wherein the volume concentration of PF5 is 0.3% and the electrode is treated at 900°C for 1.5 h.

[0078] Example 3 The difference from Example 2 lies in the preparation of the electrolyte, with a mass fraction of 0.5% for LiPO2F2.

[0079] Example 4 The difference from Example 2 lies in the preparation of the electrolyte, with a LiPO2F2 mass fraction of 2.0%.

[0080] Example 5 The difference from Example 1 lies in the preparation of the prefluorinated carbon composite electrode and the preparation of the electrolyte, wherein the volume concentration of PF5 is 0.1% and it is treated at 800°C for 2 hours; the mass fraction of LiPO2F2 is 1.5%.

[0081] Example 6 Electrolyte preparation (LiDFOB) In an argon-filled glove box (with water and oxygen content both less than 1 ppm), 1.0 M lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) in a volume ratio of EC:DEC:EMC = 3:5:2. The solution was stirred until homogeneous to obtain the basic electrolyte. Then, 1.0% (w / w) of lithium difluorooxalate borate (LiDFOB) was added to this basic electrolyte as a film-forming additive, and the solution was stirred until homogeneous to obtain the final electrolyte.

[0082] 2. Preparation of prefluorinated negative electrode Artificial graphite powder with a specific surface area of ​​2 m² / g was selected and placed in a tube furnace. A mixed gas consisting of argon (Ar) and phosphorus pentafluoride (PF5), with a PF5 volume concentration of 0.5%, was introduced. The furnace was heat-treated at 1000℃ for 1 hour to form a fluorine-containing interface layer (mainly LiF with some CF bonds) on the graphite surface. After treatment, the surface was naturally cooled to room temperature to obtain the pre-fluorinated negative electrode.

[0083] 3. Preparation of the positive electrode Lithium iron phosphate (LiFePO4) was used as the positive electrode active material. It was mixed with conductive agent SuperP and binder PVDF at a mass ratio of 95:2.5:2.5. An appropriate amount of N-methylpyrrolidone (NMP) was added and stirred into a uniform slurry. The slurry was coated on aluminum foil and then dried, rolled, and slit to obtain the positive electrode sheet.

[0084] 4. Battery Assembly The positive electrode sheet, polypropylene separator, and prefluorinated negative electrode sheet prepared above are stacked in sequence to form a battery cell, and the electrolyte prepared above is injected. After processes such as encapsulation, standing, formation, and aging, a lithium-ion battery is obtained.

[0085] Comparative Example 1 The difference from Example 1 is that the negative electrode active material was not heat-treated, and the film-forming agent LiPO2F2 was not added to the electrolyte.

[0086] Comparative Example 2 The difference from Example 1 is that the negative electrode active material was not heat-treated.

[0087] Comparative Example 3 The difference from Example 1 is that the film-forming agent LiPO2F2 was not added to the electrolyte.

[0088] test 1. The initial coulombic efficiency of the batteries in Examples 1-6 and Comparative Examples 1-3 was tested. The test method included: charging at a constant current rate of 0.5C to the charging cutoff voltage of 3.65V, letting it stand for 10 minutes, and recording its charging capacity; then discharging at a constant current rate of 0.5C to the discharging cutoff voltage of 2.5V, letting it stand for 10 minutes, and recording its discharging capacity. The initial coulombic efficiency was calculated according to the following formula (the results are shown in Table 1): Initial coulombic efficiency = (discharge capacity / charge capacity) × 100% 2. The capacity retention rate of the batteries in Examples 1-5 and Comparative Examples 1-3 after 500 cycles was tested. The test methods included: (1) Charge at 0.5C constant current to the charging cutoff voltage of 3.65V, let stand for 10min; discharge at 0.5C constant current to the discharge cutoff voltage of 2.5V, let stand for 10min, and record its discharge capacity. (2) Repeat step (1) 500 times, and calculate the capacity retention rate of the 500th cycle according to the following formula (the results are shown in Table 1): 500-cycle capacity retention rate = (500th cycle discharge capacity / 1st cycle discharge capacity) × 100%.

[0089] Table 1

[0090] Comparing Examples 1-5 with Comparative Examples 1-3, it is evident that Examples 1-5, which simultaneously employ a pre-fluorinated negative electrode (i.e., a fluorine-containing interface layer pre-placed on the graphite surface) and add LiPO2F2 film-forming agent, exhibit significantly better initial coulombic efficiency (91.2%~93.5%) and 500-cycle capacity retention (89.5%~94.2%) than the comparative examples. In particular, Comparative Example 1 (untreated graphite + no additives) showed an initial efficiency of only 86.3% and a 500-cycle capacity retention of only 70.2%, indicating that it is difficult to achieve ideal interfacial performance using only conventional negative electrodes and conventional electrolytes.

[0091] Comparing Example 6 with Comparative Example 1, it can be seen that Example 6, which uses a pre-fluorinated negative electrode and adds LiDFOB film-forming agent, has a first coulombic efficiency of 90.8% and a 500-cycle capacity retention of 86.3%, both of which are better than Comparative Example 1. Among them, Comparative Example 1 has a first efficiency of only 86.3% and a 500-cycle capacity retention of only 70.2%. This shows that using a pre-fluorinated negative electrode and adding LiDFOB film-forming agent can significantly improve the first coulombic efficiency and cycle stability of the battery.

[0092] Comparative Example 2 only added 1.0% LiPO2F2 film-forming agent, but did not pre-fluorinate the graphite anode. Its initial coulombic efficiency was 89.2%, and its 500-cycle capacity retention was 78.0%. Example 1, while also adding 1.0% LiPO2F2, pre-fluorinated the graphite anode (PF50.5%, 1000℃). Its initial coulombic efficiency increased to 93.5%, a 4.3% improvement over Comparative Example 2; its 500-cycle capacity retention increased to 94.2%, a 16.2% improvement over Comparative Example 2. This indicates that, under the premise of using the same 1.0% LiPO2F2 film-forming agent, pre-fluorination of the graphite anode can significantly improve the initial coulombic efficiency and cycle stability of the battery, and a synergistic effect exists between the pre-fluorination treatment and the film-forming agent.

[0093] In Comparative Example 3, no LiPO2F2 film-forming agent was added, and its anode treatment conditions were the same as in Example 1. Compared with Example 1, its initial coulombic efficiency decreased by 5.2%, and its 500-cycle capacity retention decreased by 18.2%. However, compared with Comparative Example 1, both the initial coulombic efficiency and the 500-cycle capacity retention of Comparative Example 3 were improved. This indicates that, under the premise that the graphite anode undergoes the same fluorination treatment, the addition of LiPO2F2 film-forming agent can significantly improve the initial coulombic efficiency and cycle stability of the battery. The pre-fluorination treatment and the film-forming agent produced a synergistic effect.

[0094] Comparing Examples 2-4, it can be seen that under the same pre-fluorination treatment conditions (PF50.3%, 900℃), when the LiPO2F2 addition amount is 1.0% (Example 2), the first-cycle efficiency is 93.0% and the 500-cycle capacity retention rate is 93.0%, showing the best overall performance. When the addition amount is reduced to 0.5% (Example 3), the first-cycle efficiency and the 500-cycle capacity retention rate decrease to 91.2% and 89.5%, respectively, indicating that insufficient film-forming agent addition will lead to incomplete interfacial film formation. When the addition amount is increased to 2.0% (Example 4), the first-cycle efficiency and the 500-cycle capacity retention rate are 92.5% and 91.8%, respectively, which are also lower than those of Example 2, indicating that excessive film-forming agent may lead to an excessively thick interfacial film and increased impedance, which will have an adverse effect on performance.

[0095] Comparing Examples 1 and 2, with the same LiPO2F2 addition, the pre-fluorination treatment conditions affected both the initial coulombic efficiency and cycle performance. Example 1 (PF5 0.5%, 1000℃) showed superior overall performance. This is likely because the higher PF5 concentration and heat treatment temperature facilitate the formation of a denser and more stable fluorinated interface layer, thereby further improving battery performance.

[0096] The above are merely preferred embodiments of this application and are not intended to limit this application in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of this application without departing from the scope of the technical solution of this application shall still fall within the scope of the technical solution of this application.

Claims

1. A lithium-ion battery, characterized in that, include: positive electrode; The negative electrode includes a negative electrode active material, which includes a graphite matrix and a fluorine-containing interface layer coated on the surface of the graphite matrix. The electrolyte comprises a lithium salt, an organic solvent, and a film-forming agent, wherein the film-forming agent is a fluorinated lithium oxide compound salt, the fluorinated lithium oxide compound being suitable for decomposition during the formation process of the lithium-ion battery, and the decomposition products include lithium fluoride and non-metallic oxygen-fluorine compounds.

2. The lithium-ion battery according to claim 1, characterized in that, The fluorinated lithium oxide compound includes at least one of lithium difluorophosphate, lithium tetrafluorooxalate phosphate, lithium difluorosulfonylimide, and lithium difluorooxalate borate.

3. The lithium-ion battery according to claim 1, characterized in that, The mass fraction of the film-forming agent is between 0.5% and 2.0% based on the total mass of the electrolyte.

4. The lithium-ion battery according to any one of claims 1-3, characterized in that, The specific surface area of ​​the graphite matrix is ​​1.5 m². 2 / g -3.0m 2 Between / g.

5. The lithium-ion battery according to claim 4, characterized in that, Based on the total mass of the negative electrode, the mass fraction of the fluorine-containing interface layer is between 5% and 12%. And / or, the fluorinated interface layer includes lithium fluoride; And / or, the nonmetallic oxygen-fluorine compound includes phosphorus-containing lithium oxyfluorine compounds and boron-containing lithium oxyfluorine compounds; And / or, the organic solvent includes ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate.

6. A method for preparing a lithium-ion battery, characterized in that, include: A positive electrode, a negative electrode, and an electrolyte are provided; wherein the negative electrode comprises a negative electrode active material, the negative electrode active material comprising a graphite matrix and a fluorine-containing interface layer coated on the surface of the graphite matrix; the electrolyte comprises a lithium salt, an organic solvent, and a film-forming agent, the film-forming agent being a fluorine-containing lithium oxide compound, the fluorine-containing lithium oxide compound being suitable for decomposition during the formation process of the lithium-ion battery, the decomposition products including lithium fluoride and non-metallic oxygen-fluorine compounds; the positive electrode, the negative electrode, and the electrolyte are assembled into a lithium-ion battery.

7. The preparation method according to claim 6, characterized in that, The preparation method of the negative electrode active material includes: The graphite matrix is ​​heat-treated in a non-oxidizing atmosphere containing fluorine gas to form a fluorine-containing interface layer on the surface of the graphite matrix.

8. The preparation method according to claim 7, characterized in that, The non-oxidizing atmosphere includes inert gases and fluorine-containing gases; based on the total volume of the non-oxidizing atmosphere, the volume concentration of the fluorine-containing gases is between 0.1% and 0.5%, with the remainder being inert gases.

9. The preparation method according to claim 8, characterized in that, The fluorine-containing gas includes at least one of phosphorus pentafluoride, nitrogen trifluoride, carbon tetrafluoride, and sulfur hexafluoride; And / or, the inert gas includes at least one of argon, helium, and neon.

10. The preparation method according to claim 7, characterized in that, The heat treatment of the graphite matrix under a non-oxidizing atmosphere containing fluorine gas includes: The graphite matrix is ​​placed in a non-oxidizing atmosphere containing fluorine gas and kept at a temperature between 800℃ and 1000℃ for 1-2 hours.

11. The preparation method according to claim 6, characterized in that, The preparation method of the negative electrode active material includes: The graphite substrate is immersed in a fluorine-containing solution to form a fluorine-containing interface layer on the surface of the graphite substrate.

12. The preparation method according to claim 11, characterized in that, The fluorine-containing solution includes at least one of ammonium fluoride solution, ammonium hydrogen fluoride solution, lithium fluoride solution, and sodium fluoride solution.

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