Optimization method for graphite / hard carbon composite interface of low temperature lithium iron phosphate battery anode

Abstract

This invention discloses a method for optimizing the interface of a graphite / hard carbon composite anode in low-temperature lithium iron phosphate (LiFePO4) batteries. The method first prepares an anode slurry by combining artificial graphite and hard carbon, and then coats it onto copper foil to form a wet film. Next, a self-made fluorinated niobium tungsten titanate composite is dispersed in anhydrous ethanol and sprayed onto the surface of the wet film. Gradient drying is then used to allow the modified material to self-assemble on the surface of the composite anode, forming an artificial interface layer. Finally, the composite anode with optimized interface is obtained by roll pressing. The fluorinated niobium tungsten titanate composite is prepared through a complex four-step process: sol-gel, high-temperature fluorination, molten salt etching, and vapor phase reconstruction, resulting in a porous structure and oxygen vacancy defects. This invention's method is simple, uses readily available raw materials, and the constructed interface layer effectively reduces the lithium-ion desolvation barrier, inhibits lithium dendrite growth at low temperatures, and significantly improves the discharge capacity and cycle stability of LiFePO4 batteries under low-temperature conditions.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a method for optimizing the graphite / hard carbon composite interface of the negative electrode in low-temperature lithium iron phosphate batteries. Background Technology

[0002] Lithium-ion batteries, as one of the most promising energy storage technologies, have been widely used in portable electronic devices, electric vehicles, and large-scale energy storage systems due to their high energy density, long cycle life, and environmental friendliness. Among various cathode material systems, lithium iron phosphate (LFP) is favored for its low cost, high safety, and excellent cycle stability. However, the low-temperature performance bottleneck of LFP batteries has always restricted their widespread application in cold regions. Studies have shown that in temperatures below -20 degrees Celsius, the ion transport kinetics inside the battery deteriorate significantly, mainly manifested as increased electrolyte viscosity, decreased lithium-ion migration rate, and a sharp increase in charge transfer impedance at the electrode / electrolyte interface. Among these, the anode material has a particularly critical impact on low-temperature performance. The most widely used commercially available artificial graphite anode, due to its inherent lithium diffusion kinetics, hinders the insertion / extraction of lithium ions between graphite layers, making it prone to lithium deposition on the graphite surface, forming lithium dendrites. This not only causes irreversible capacity loss but, in more severe cases, can puncture the separator, causing internal short circuits and posing safety hazards.

[0003] To improve the low-temperature performance of anode materials, researchers have attempted to composite graphite with hard carbon. Hard carbon, with its disordered carbon layer structure and large interlayer spacing, provides abundant lithium-ion storage sites and shorter diffusion paths, mitigating the intercalation resistance of lithium ions at low temperatures to some extent. However, simple physical composites of graphite and hard carbon cannot fundamentally solve the interface problem. Lithium ions must undergo a desolvation process before entering the anode material from the electrolyte. This process requires overcoming a high energy barrier, especially at low temperatures where the desolvation kinetics are severely delayed, making it difficult for lithium ions to smoothly cross the electrode / electrolyte interface and enter the anode. In existing technologies, constructing an artificial solid electrolyte interface film on the graphite surface, such as introducing fluoride or phosphate modification layers, can promote the lithium-ion desolvation process and stabilize the interface structure to some extent. However, these modification methods still have many shortcomings: the types of conventional modifying materials are limited, and the effect on reducing the lithium-ion desolvation energy barrier is not significant enough; the bonding force between the modified layer and the graphite substrate is weak, and it is easy to fall off and fail during long-term cycling; some preparation processes are complex, involving non-commercial raw materials or harsh reaction conditions, making it difficult to achieve large-scale production and application.

[0004] To address the aforementioned problems in existing technologies, this invention proposes a novel interface optimization method that effectively promotes lithium-ion desolvation, significantly reduces interfacial transport impedance, and achieves strong bonding with graphite / hard carbon composite substrates. This is of great significance for overcoming the technical bottleneck of lithium iron phosphate batteries in low-temperature applications. From the perspective of interfacial chemical regulation, this invention designs an inorganic modified material with a special composition and structure and introduces it into the surface of a graphite / hard carbon composite anode in a simple and controllable manner. The aim is to construct an artificial interface layer with both high ionic conductivity and a low desolvation barrier, thereby simultaneously improving the battery's discharge capacity, rate performance, and cycle stability in low-temperature environments, providing technical support for the reliable operation of electric vehicles and energy storage systems in cold regions. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method for optimizing the graphite / hard carbon composite interface of the anode in low-temperature lithium iron phosphate batteries.

[0006] A first aspect of the present invention provides a method for optimizing the graphite / hard carbon composite interface of a low-temperature lithium iron phosphate battery anode, comprising the following steps:

[0007] S1. By weight, mix 60-85 parts of artificial graphite and 15-40 parts of hard carbon to obtain a composite powder; mix the composite powder with 1-3 parts of conductive carbon black, 1-2 parts of styrene-butadiene rubber and 1-2 parts of sodium carboxymethyl cellulose, add 80-200 parts of deionized water, stir, and obtain graphite / hard carbon composite negative electrode slurry.

[0008] S2. The graphite / hard carbon composite negative electrode slurry obtained in step S1 is coated on copper foil to obtain copper foil with a wet film on the surface; 2-20 parts of fluorinated niobium tungsten titanate composite are dispersed in 100-500 parts of anhydrous ethanol and sprayed onto the surface of the wet film to obtain the sprayed electrode sheet.

[0009] S3. The coated electrode sheet is subjected to gradient drying: drying at 48-52℃; then drying at 78-82℃; and finally vacuum drying at 115-125℃ to obtain the dried electrode sheet.

[0010] S4. Place the dried electrode sheet in a roller press for compaction to obtain the rolled electrode sheet; place the rolled electrode sheet in a vacuum oven at 58-62℃ and let it stand.

[0011] In this invention, the core of the low-temperature lithium iron phosphate battery anode graphite / hard carbon composite interface optimization method lies in the precise construction and stabilization of the interface layer. Graphite and hard carbon composite powder are mixed with conductive carbon black, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC). CMC dissolves in the aqueous phase to form a three-dimensional network structure, with SBR emulsion particles dispersed within. After drying, this provides a rigid framework and flexibility, synergistically optimizing the electrode's mechanical properties. In the spraying step, the fluorinated niobium tungsten titanate composite is dispersed in ethanol. During spraying on the wet film surface, the ethanol evaporates rapidly, and the fluorinated niobium tungsten titanate composite migrates to the slurry surface due to surface tension differences, avoiding penetration into the electrode interior and ensuring uniform interface modification. Gradient drying is achieved in stages: the low-temperature stage removes moisture and ethanol, and the polar groups on the surface of the fluorinated niobium tungsten titanate composite are initially fixed to the oxygen-containing functional groups of the carbon substrate through hydrogen bonding; the medium-temperature stage promotes chemical bonding, with some hydrogen bonds transforming into stronger chemical bonds, forming a stable interface bonding layer; the high-temperature stage makes the interface layer dense and uniform. After rolling, the battery is left to stand to eliminate internal stress and further remove residual moisture, thus avoiding the adverse effects of moisture on battery performance. This process ultimately forms an interface layer of suitable thickness, which significantly improves the interface compatibility and cycle stability of the negative electrode under low-temperature conditions, providing the battery with excellent electrochemical performance.

[0012] According to a preferred embodiment of the present invention, in step S1, the particle size of the artificial graphite is 15-20 μm.

[0013] According to a preferred embodiment of the present invention, in step S2, the amount of coating applied to the wet film surface is 0.5-1.0 mg / cm².

[0014] According to a preferred embodiment of the present invention, in step S3, the product is dried at 48-52°C for 0.5-1 h; then dried at 78-82°C for 1-1.5 h; and finally dried under vacuum at 115-125°C for 4-6 h.

[0015] According to a preferred embodiment of the present invention, in step S4, the compaction pressure is 8-10 MPa.

[0016] According to a preferred embodiment of the present invention, the preparation steps of the fluorinated niobium tungsten titanate composite include:

[0017] A1. By weight, dissolve 90-100 parts of tetrabutyl titanate, 18-25 parts of niobium pentachloride and 20-80 parts of sodium tungstate in 500-1000 parts of anhydrous ethanol to obtain a mixed solution; stir the mixed solution at 45-50℃ under argon protection, add 30-60 parts of deionized water-ethanol mixture dropwise, and continue stirring to obtain a sol; age the sol in a water bath at 58-62℃ to obtain a wet gel, and then dry it in a vacuum drying oven at 78-82℃ to obtain a precursor powder;

[0018] A2. Place the precursor powder obtained in step A1 in a tube furnace and heat it to 645-655℃ under an argon atmosphere. After naturally cooling to room temperature, sintered powder is obtained. Grind and mix the sintered powder with 0.3-0.8 parts of lithium fluoride to obtain a mixture. Under argon protection, heat the mixture to 845-855℃ and hold it. After the reaction is completed, cool it to 195-205℃ and allow it to cool naturally to room temperature to obtain fluorinated niobium tungsten titanate bulk material.

[0019] A3. Grind and sieve the fluorinated niobium tungsten titanate block obtained in step A2 to obtain powder; mix the powder with a mixed molten salt of 2.5-4 parts potassium chloride and 2.5-4 parts sodium chloride, and place it in a corundum crucible; heat to 745-755℃ in a tube furnace under an argon atmosphere and hold at that temperature; after etching, cool naturally to room temperature to obtain the product; wash the product with deionized water, centrifuge, and finally vacuum dry at 78-82℃ to obtain a porous fluorinated niobium tungsten titanate intermediate;

[0020] A4. The porous fluorinated niobium tungsten titanate intermediate prepared in step A3 is placed in a fluidized bed reactor. A mixture of nitrogen trifluoride and argon is introduced into the argon carrier gas, and the temperature is raised to 545-555℃ for heat treatment. After the heat treatment is completed, pure argon is purged. Then the temperature is raised to 395-405℃, and a mixture of hydrogen and argon is introduced for reduction treatment. Under argon protection, it is naturally cooled to room temperature.

[0021] In this invention, the preparation of the fluorinated niobium tungstate titanate composite is based on the synergistic effect of sol-gel and molten salt etching. First, tetrabutyl titanate, niobium pentachloride, and sodium tungstate are dissolved in anhydrous ethanol. By precisely controlling the hydrolysis rate, a water-ethanol mixture is slowly added dropwise to avoid localized supersaturation precipitation, forming a uniform and transparent sol. After aging and drying, the sol yields a precursor powder. The precursor is mixed with lithium fluoride and sintered at high temperature. Lithium fluoride acts as a fluorine source, reacting with the metal oxide to introduce fluorine. Simultaneously, some lithium ions exist in the material in interstitial form or as a solid solution, significantly improving the surface ionic conductivity. Subsequently, the powder is mixed with molten salt. Under specific high-temperature conditions, the molten salt provides a liquid-phase reaction medium, selectively etching amorphous regions or specific crystal planes through a dissolution-recrystallization mechanism to form abundant nanopores and defect structures. Finally, the intermediate is treated in a fluorine-containing gas environment. Nitrogen trifluoride decomposes at high temperature to generate active fluorine radicals, which undergo a displacement reaction with lattice oxygen to release oxygen and form oxygen vacancies. Hydrogen reduction further regulates the oxygen vacancy concentration, ultimately obtaining a composite with a porous structure and oxygen vacancy defects, ensuring the material's high ion conductivity and structural stability at low temperatures.

[0022] According to a preferred embodiment of the present invention, in step A1, the stirring time at 45-50°C is 2-4 hours.

[0023] According to a preferred embodiment of the present invention, in step A2, the time for holding the temperature at 645-655°C is 4-6 hours.

[0024] According to a preferred embodiment of the present invention, in step A3, the time for holding the temperature at 745-755°C is 3-5 hours.

[0025] According to a preferred embodiment of the present invention, in step A4, the time for the reduction treatment by introducing a mixture of hydrogen and argon is 1-2 hours.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The low-temperature lithium iron phosphate battery negative electrode graphite / hard carbon composite interface optimization method proposed in this invention achieves a comprehensive improvement in electrochemical performance by constructing an artificial interface layer with a novel composition and structure. First, the fluorinated niobium tungsten titanate composite used in this invention has a complex coordination structure formed by niobium, tungsten, and titanium, three transition metal elements, with fluorine and oxygen. After being processed by a four-step process of sol-gel, high-temperature fluorination, molten salt etching, and vapor phase reconstruction, the material has abundant nanopores and oxygen vacancy defects. This unique structural feature gives it excellent ion conductivity and electrochemical activity: on the one hand, the porous structure increases the contact area between the electrode material and the electrolyte, providing more channels for the rapid transport of lithium ions at the interface; on the other hand, oxygen vacancies, as active sites, can significantly reduce the desolvation energy barrier of lithium ions, making it easier for lithium ions to escape from the solvent molecular sheath and enter the interior of the negative electrode material, fundamentally solving the problem of slow interface transport kinetics under low-temperature conditions.

[0028] (2) At the interface optimization process level, this invention adopts a wet film spraying combined with gradient drying method to achieve uniform anchoring and self-assembly of modified materials on the graphite / hard carbon composite negative electrode surface. Compared with traditional coating or impregnation processes, this method has the outstanding advantages of simple operation, mild conditions, and easy large-scale production. During the spraying process, ethanol evaporates rapidly, driving the modified materials to spontaneously migrate to the surface of the slurry due to surface tension differences, avoiding uneven distribution caused by the modifier penetrating deep into the electrode; the subsequent gradient drying process causes hydrogen bonding and stronger chemical bonding between the modified materials and the substrate in sequence, constructing an artificial interface layer with controllable thickness and strong bonding. This interface layer can not only effectively buffer the volume change of the negative electrode material during charging and discharging, suppress the pulverization and damage of the electrode structure, but also act as a physical barrier to prevent direct contact between the electrolyte and the negative electrode surface, reducing the occurrence of side reactions, thereby significantly improving the structural stability and cycle life of the electrode.

[0029] (3) The graphite / hard carbon composite anode optimized by this invention exhibits a significant improvement in low-temperature electrochemical performance. In low-temperature environments, the battery using the method of this invention shows a higher discharge specific capacity retention rate under high-rate discharge conditions, which is significantly improved compared to the unmodified comparative sample; under extreme low-temperature conditions, it can still maintain a high reversible capacity level. In terms of cycle stability, the modified electrode exhibits a significantly higher capacity retention rate than the unmodified sample after long-term cycling in low-temperature environments. In addition, the interface layer constructed by this invention effectively inhibits the growth of lithium dendrites under low-temperature conditions and induces the formation of a stable solid electrolyte interface film rich in lithium fluoride and niobium tungsten titanate components, avoiding repeated rupture and reconstruction of the interface film, and greatly improving the safety and reliability of the battery. In summary, this invention provides an efficient, feasible, and easily industrialized technical solution for overcoming the application bottleneck of lithium iron phosphate batteries in low-temperature environments. Detailed Implementation

[0030] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0031] Example 1

[0032] This embodiment provides a method for optimizing the graphite / hard carbon composite interface of the negative electrode in low-temperature lithium iron phosphate batteries, including the following steps:

[0033] S1. Place 70g of artificial graphite and 30g of hard carbon in a mixer and dry mix at 200rpm for 30min to obtain 100g of uniform composite powder. Place this 100g composite powder, 2g of conductive carbon black, 1.5g of styrene-butadiene rubber, and 1.5g of sodium carboxymethyl cellulose in a planetary mixer, add 140g of deionized water, and stir at 1500rpm for 2h. Adjust the slurry solid content to 45% to obtain a uniform graphite / hard carbon composite negative electrode slurry.

[0034] S2. The graphite / hard carbon composite negative electrode slurry obtained in step S1 is uniformly coated onto a 10 μm thick copper foil with a 200 μm thickness using a doctor blade. The coating speed is controlled at 1.5 m / min to obtain a copper foil with a wet film on the surface. 10 g of fluorinated niobium tungsten titanate composite is dispersed in 300 g of anhydrous ethanol and ultrasonically vibrated for 10 min to prepare a uniform dispersion. The dispersion is then uniformly sprayed onto the wet film surface using an ultrasonic spraying device at a spray rate of 0.2 mL / min. During the spraying process, the substrate temperature is maintained at 40 °C, and the spraying amount is controlled at 0.8 mg / cm². The rapid evaporation of ethanol allows the modified material to be uniformly anchored on the surface of the composite negative electrode, resulting in the coated electrode sheet.

[0035] S3. The coated electrode sheet is placed in a multi-stage oven for gradient drying: first, it is dried at 50℃ for 0.8h to remove most of the moisture and ethanol; then it is dried at 80℃ for 1.2h to promote the chemical bonding between the modified material and the substrate; finally, it is vacuum dried at 120℃ for 5h to form a dense and uniform artificial interface layer on the surface of the composite negative electrode. The thickness of the interface layer is controlled within the range of 100-200nm to obtain the dried electrode sheet.

[0036] S4. The dried electrode sheet is placed in a roller press for compaction. The roller pressing pressure is controlled at 9 MPa, the roller gap is adjusted to 80 μm, and the compaction density reaches 1.55 g / cm³, resulting in the rolled electrode sheet. The rolled electrode sheet is then placed in a 60℃ vacuum oven for 12 hours to eliminate internal stress and remove residual trace moisture. Finally, the electrode sheet is cut into the required size and stored in an argon glove box for later use.

[0037] The preparation steps of the fluorinated niobium tungsten titanate composite are as follows:

[0038] A1. Dissolve 95g of tetrabutyl titanate, 22g of niobium pentachloride, and 50g of sodium tungstate in 750g of anhydrous ethanol to obtain a mixed solution. Place the mixed solution in a reaction vessel and stir in a water bath at 48℃ for 3h under argon protection. Then, slowly add 45g of deionized water-ethanol mixture at a rate of 0.5mL / min with vigorous stirring to control the hydrolysis rate and avoid local supersaturation precipitation. Continue stirring for 4h to form a transparent sol. Age the sol in a water bath at 60℃ for 12h to form a wet gel. Then, transfer the wet gel to a vacuum drying oven at 80℃ and dry for 24h to completely remove the solvent, obtaining the precursor powder.

[0039] A2. The precursor powder obtained in step A1 is placed in a tube furnace and heated to 650°C at a heating rate of 2°C / min under an argon atmosphere, and held at this temperature for 5 hours for pre-crystallization treatment to initially form the titanium-oxygen framework. After natural cooling to room temperature, sintered powder is obtained. The sintered powder is then mixed with 0.5g of lithium fluoride in a mortar and ground thoroughly for 30 minutes to obtain a homogeneous mixture. The mixture is transferred to a nickel crucible and heated to 850°C at 3°C / min under argon protection, and held at this temperature for 6 hours for a high-temperature solid-state reaction. During this process, lithium fluoride acts as a fluorine source to introduce fluorine, which reacts with metal oxides to form fluorides. Simultaneously, some lithium ions remain in the material as interstitial ions or in the form of lithium-niobium / titanium / tungsten-oxygen solid solutions. After the reaction is complete, the mixture is slowly cooled to 200°C at a rate of 1°C / min, and then naturally cooled to room temperature to obtain fluorinated niobium-tungsten titanate bulk material.

[0040] A3. The fluorinated niobium tungsten titanate block obtained in step A2 is ground and passed through a 300-mesh sieve to obtain a uniform powder. 1.2g of this powder is mixed with a molten salt of 3g potassium chloride and 3g sodium chloride at a mass ratio of 1:5 and placed in an alumina crucible. The crucible is placed in a tube furnace, and the temperature is raised to 750℃ at 5℃ / min under an argon atmosphere and held for 4 hours for molten salt etching. Utilizing the eutectic properties of potassium chloride and sodium chloride to provide a liquid-phase reaction medium, the material surface is selectively etched through a dissolution-recrystallization mechanism, preferentially dissolving amorphous regions or specific crystal planes to form nanopores and defect structures. After etching, the material is naturally cooled to room temperature. The product is washed repeatedly with deionized water five times to remove residual molten salt, and centrifuged at 8000rpm for 10 minutes after each wash. Finally, it is dried in a vacuum drying oven at 80℃ for 12 hours to obtain a porous fluorinated niobium tungsten titanate intermediate.

[0041] A4. Take 1g of the porous fluorinated niobium tungsten titanate intermediate prepared in step A3 and place it in a fluidized bed reactor. A mixture of 10% nitrogen trifluoride and 90% argon (by volume) is introduced into the argon carrier gas at a flow rate of 80mL / min. The temperature is increased to 550℃ at 2℃ / min and held for 2h for surface vapor-phase fluorination treatment. During this process, nitrogen trifluoride decomposes at high temperature to generate active fluorine radicals, which undergo a displacement reaction with the oxygen in the material's lattice (O2- + 2F· → 2F- + 1 / 2O2), forming a gradient fluorinated layer on the material surface and introducing oxygen vacancy defects. After treatment, pure argon is used for purging for 30min to remove residual gas. The temperature is then increased to 400℃, and a mixture of 10% hydrogen and 90% argon (by volume) is introduced for reduction treatment for 1.5h to further control the oxygen vacancy concentration. Finally, the mixture is naturally cooled to room temperature under argon protection to obtain the final inorganic modified porous fluorinated niobium tungsten titanate composite.

[0042] Example 2

[0043] The difference between this embodiment and Embodiment 1 is that, in S1, 60g of artificial graphite and 40g of hard carbon are mixed to obtain 100g of composite powder, the composite powder is mixed with 1g of conductive carbon black, 1g of styrene-butadiene rubber and 2g of sodium carboxymethyl cellulose, and 200g of deionized water is added and stirred to obtain graphite / hard carbon composite negative electrode slurry.

[0044] S2. The graphite / hard carbon composite negative electrode slurry obtained in step S1 is coated onto a 10 μm thick copper foil with a 200 μm thickness using a doctor blade at a coating speed of 1.5 m / min to obtain a copper foil with a wet film on the surface. 2 g of fluorinated niobium tungsten titanate composite is dispersed in 100 g of anhydrous ethanol to prepare a dispersion solution, which is then uniformly sprayed onto the wet film surface at a spray rate of 0.2 mL / min. The spraying amount is controlled at 0.5 mg / cm². During the spraying process, the substrate temperature is maintained at 40 °C to obtain the coated electrode sheet.

[0045] S3. Place the coated electrode sheet in a multi-stage oven for gradient drying at 48℃ for 1 hour, then at 78℃ for 1.5 hours, and finally at 115℃ under vacuum for 6 hours to obtain the dried electrode sheet.

[0046] S4. Place the dried electrode sheet in a roller press and compact it under a pressure of 8MPa. Adjust the roller gap to 80μm and the compaction density to 1.5g / cm³ to obtain the roller-pressed electrode sheet. Place the roller-pressed electrode sheet in a vacuum oven at 58℃ and let it stand for 12h. Finally, cut it into the required size and store it in an argon glove box for later use.

[0047] The preparation steps of the fluorinated niobium tungsten titanate composite are as follows:

[0048] A1. Dissolve 90g of tetrabutyl titanate, 18g of niobium pentachloride and 20g of sodium tungstate in 500g of anhydrous ethanol to obtain a mixed solution. Stir the mixed solution at 45℃ for 4h under argon protection. Then, slowly add 30g of deionized water-ethanol mixture at a rate of 0.5mL / min under vigorous stirring and continue stirring for 4h to form a transparent sol. Place the sol in a 58℃ water bath for 12h to age to form a wet gel. Then dry in a 78℃ vacuum drying oven for 24h to remove the solvent and obtain a white loose precursor powder.

[0049] A2. The precursor powder was placed in a tube furnace and heated to 645°C at 2°C / min under an argon atmosphere and held for 6 hours for pre-crystallization. After natural cooling to room temperature, sintered powder was obtained. The sintered powder was thoroughly ground and mixed with 0.3g of lithium fluoride to obtain a mixture. The mixture was placed in a nickel crucible and heated to 845°C at 3°C / min under argon protection for 6 hours for high-temperature solid-state reaction. During this process, LiF was used as a fluorine source to introduce fluorine elements and react with metal oxides to form fluorides. At the same time, some Li⁺ was retained in the material. After the reaction was completed, the mixture was slowly cooled to 195°C at 1°C / min and then naturally cooled to room temperature to obtain fluorinated niobium tungsten titanate bulk.

[0050] A3. The fluorinated niobium tungsten titanate block was ground through a 300-mesh sieve to obtain a uniform powder. 1g of the powder was mixed with 2.5g of potassium chloride and 2.5g of sodium chloride in a mixed molten salt and placed in a corundum crucible. The temperature was raised to 745℃ at 5℃ / min and held for 5h in a tube furnace under an argon atmosphere. The low eutectic properties of the molten salt provided a liquid-phase reaction medium to selectively etch the material surface through a dissolution-recrystallization mechanism. After etching, the material was naturally cooled to room temperature. The product was washed repeatedly with deionized water 5 times to remove residual molten salt. After each wash, the product was centrifuged at 8000rpm for 10min. Finally, the product was vacuum dried at 78℃ for 12h to obtain a porous fluorinated niobium tungsten titanate intermediate.

[0051] A4. Take 1g of porous fluorinated niobium tungsten titanate intermediate and place it in a fluidized bed reactor. In an argon carrier gas, introduce a mixed gas of 5% nitrogen trifluoride and 95% argon at a flow rate of 50mL / min. Heat the mixture to 545℃ at 2℃ / min and hold for 3h for surface vapor-phase fluorination treatment. Nitrogen trifluoride decomposes at high temperature to generate active fluorine radicals, which undergo a displacement reaction with lattice oxygen to form a gradient fluorinated layer and introduce oxygen vacancy defects. After treatment, switch to pure argon gas to purge for 30min to remove residual gas. Then, raise the temperature to 395℃ and introduce a mixed gas of 5% hydrogen and 95% argon for reduction treatment for 2h to further regulate the oxygen vacancy concentration. Finally, cool naturally to room temperature under argon protection to obtain the final inorganic modified material, porous fluorinated niobium tungsten titanate composite.

[0052] Example 3

[0053] The difference between this embodiment and Embodiment 1 is that, in S1, 85g of artificial graphite and 15g of hard carbon are mixed to obtain 100g of composite powder, and the composite powder is mixed with 3g of conductive carbon black, 2g of styrene-butadiene rubber and 1g of sodium carboxymethyl cellulose, and 80g of deionized water is added and stirred to obtain graphite / hard carbon composite negative electrode slurry.

[0054] S2. The graphite / hard carbon composite negative electrode slurry obtained in step S1 is coated onto a 10 μm thick copper foil with a 200 μm thickness using a doctor blade at a coating speed of 1.5 m / min to obtain a copper foil with a wet film on the surface. 20 g of fluorinated niobium tungsten titanate composite is dispersed in 500 g of anhydrous ethanol to prepare a dispersion solution, which is then uniformly sprayed onto the wet film surface at a spray rate of 0.2 mL / min. The spraying amount is controlled at 1.0 mg / cm². During the spraying process, the substrate temperature is maintained at 40 °C to obtain the coated electrode sheet.

[0055] S3. Place the coated electrode sheet in a multi-stage oven for gradient drying at 52℃ for 0.5h, then at 82℃ for 1h, and finally at 125℃ under vacuum for 4h to obtain the dried electrode sheet.

[0056] S4. Place the dried electrode sheet in a roller press and compact it under a pressure of 10MPa. Adjust the roller gap to 80μm and the compaction density to 1.6g / cm³ to obtain the roller-pressed electrode sheet. Place the roller-pressed electrode sheet in a vacuum oven at 62℃ and let it stand for 12h. Finally, cut it into the required size and store it in an argon glove box for later use.

[0057] The preparation steps of the fluorinated niobium tungsten titanate composite are as follows:

[0058] A1. Dissolve 100g of tetrabutyl titanate, 25g of niobium pentachloride and 80g of sodium tungstate in 1000g of anhydrous ethanol to obtain a mixed solution. Stir the mixed solution at 50℃ for 2h under argon protection. Then, slowly add 60g of deionized water-ethanol mixture at a rate of 0.5mL / min under vigorous stirring and continue stirring for 4h to form a transparent sol. Place the sol in a 62℃ water bath and age for 12h to form a wet gel. Then, dry in a vacuum drying oven at 82℃ for 24h to remove the solvent and obtain a white, loose precursor powder.

[0059] A2. The precursor powder was placed in a tube furnace and heated to 655°C at 2°C / min under an argon atmosphere and held for 4 hours for pre-crystallization. After natural cooling to room temperature, sintered powder was obtained. The sintered powder was thoroughly ground and mixed with 0.8g of lithium fluoride to obtain a mixture. The mixture was placed in a nickel crucible and heated to 855°C at 3°C / min under argon protection and held for 4 hours for high-temperature solid-state reaction. During this process, LiF was used as a fluorine source to introduce fluorine elements and react with metal oxides to form fluorides. At the same time, some Li⁺ was retained in the material. After the reaction was completed, the mixture was slowly cooled to 205°C at 1°C / min and then naturally cooled to room temperature to obtain fluorinated niobium tungsten titanate bulk.

[0060] A3. The fluorinated niobium tungsten titanate block was ground through a 300-mesh sieve to obtain a uniform powder. 1.6g of the powder was mixed with 4g of potassium chloride and 4g of sodium chloride in a mixed molten salt and placed in an alumina crucible. The temperature was raised to 755℃ at 5℃ / min and held for 3h in a tube furnace under an argon atmosphere. The eutectic properties of the molten salt provided a liquid-phase reaction medium to selectively etch the material surface through a dissolution-recrystallization mechanism. After etching, the material was naturally cooled to room temperature. The product was washed repeatedly with deionized water 5 times to remove residual molten salt. After each wash, the product was centrifuged at 8000rpm for 10min. Finally, the product was vacuum dried at 82℃ for 12h to obtain a porous fluorinated niobium tungsten titanate intermediate.

[0061] A4. Take 1g of porous fluorinated niobium tungsten titanate intermediate and place it in a fluidized bed reactor. In an argon carrier gas, introduce a mixed gas of 20% nitrogen trifluoride and 80% argon at a flow rate of 100mL / min. Heat to 555℃ at 2℃ / min and hold for 1h for surface vapor-phase fluorination treatment. Nitrogen trifluoride decomposes at high temperature to generate active fluorine radicals, which undergo a displacement reaction with lattice oxygen to form a gradient fluorinated layer and introduce oxygen vacancy defects. After treatment, switch to pure argon to purge for 30min to remove residual gas. Then, raise the temperature to 405℃ and introduce a mixed gas of 20% hydrogen and 80% argon for reduction treatment for 1h to further regulate the oxygen vacancy concentration. Finally, cool naturally to room temperature under argon protection to obtain the final inorganic modified material, porous fluorinated niobium tungsten titanate composite.

[0062] Comparative Example 1

[0063] The difference between this comparative example and Example 1 is that the fluorinated niobium tungsten titanate complex is not added in step S2; the other steps S1, S3, and S4 are the same as in Example 1.

[0064] Comparative Example 2

[0065] The difference between this comparative example and Example 1 is that in step A2 of the preparation of the fluorinated niobium tungsten titanate composite, after grinding and mixing the sintered powder with 0.5g of lithium fluoride, a high-temperature solid-phase reaction of heating to 850℃ and holding is not carried out; other steps, including interface optimization methods S1, S2, S3, S4 and modified material preparation A1, A3, A4, are the same as in Example 1.

[0066] Comparative Example 3

[0067] The difference between this comparative example and Example 1 is that in step A4 of the preparation of the fluorinated niobium tungsten titanate composite, the mixed gas heating treatment of nitrogen trifluoride and argon is not performed; the other steps, including interface optimization methods S1, S2, S3, S4 and modified material preparation A1, A2, A3, are the same as in Example 1.

[0068] In accordance with national and industry standard testing specifications, a series of standardized tests were conducted on the low-temperature lithium iron phosphate battery negative electrode graphite / hard carbon composite interface optimization method described in Examples 1-3 and Comparative Examples 1-3.

[0069] The interface-optimized composite negative electrodes prepared in Examples 1-3 and Comparative Examples 1-3 were used as working electrodes and assembled into CR2032 coin cells with lithium iron phosphate positive electrodes, polyethylene separators and commercial electrolytes in an argon atmosphere glove box. The water and oxygen contents in the glove box were both below 0.1 ppm. The assembled cells were left to stand at room temperature for 12 hours before electrochemical performance testing was performed.

[0070] Constant current charge-discharge tests were conducted using the LAND battery testing system, with the test voltage range being 2.0V to 3.8V.

[0071] The 0.5C discharge specific capacity test at room temperature (25℃) was conducted in a constant temperature chamber. Before the test, the battery was left to stand at 25℃ for 4 hours to reach thermal equilibrium. Then, it was charged at a constant current rate of 0.5C to 3.8V and discharged at a constant current rate of 0.5C to 2.0V. The discharge specific capacity of the first cycle was recorded.

[0072] The 0.5C discharge specific capacity tests at low temperatures of -20℃ and -30℃ were conducted in constant temperature chambers at the corresponding temperatures. Before the test, the battery was left to stand at the target temperature for 4 hours, then charged at a constant current of 0.5C to 3.8V, and then discharged at a constant current of 0.5C to 2.0V. The discharge specific capacity of the first cycle was recorded.

[0073] Low-temperature cycle stability testing was conducted at -20℃. After the battery was placed in a constant temperature chamber at -20℃ for 4 hours, it was subjected to 500 charge-discharge cycles at a rate of 0.5C. In each cycle, the battery was charged at a constant current of 0.5C to 3.8V and then discharged at a constant current of 0.5C to 2.0V. The discharge specific capacity of the first cycle and the 500th cycle was recorded, and the capacity retention rate of the 500th cycle relative to the first cycle was calculated. The retention rate is expressed as a percentage.

[0074] Five batteries were tested in parallel in each group of experiments, and the arithmetic mean of all test results was taken.

[0075] The performance test data above are shown in Table 1.

[0076] Table 1 Performance Test Results

[0077]

[0078] As can be seen from the above, the 0.5C discharge specific capacities of Examples 1-3 at -20℃ and -30℃ reached 278-289mAh / g and 205-218mAh / g, respectively, while the unmodified Comparative Example 1 was only 198mAh / g and 132mAh / g. This indicates that the artificial interface layer constructed by introducing the fluorinated niobium tungsten titanate composite significantly reduced the desolvation energy barrier of lithium ions at low temperatures and promoted the rapid transport of lithium ions at the negative electrode interface.

[0079] Meanwhile, the capacity retention rates of Examples 1-3 after 500 cycles at -20°C were as high as 86.2-88.1%, which is much higher than the 58.3% of Comparative Example 1. This indicates that the interface layer effectively inhibits the growth of lithium dendrites and repeated rupture of the SEI film during low-temperature cycling, and greatly improves the stability of the electrode structure.

[0080] Comparative Example 2 lacked a high-temperature solid-phase reaction step, resulting in insufficient reaction between lithium fluoride and the matrix to form a lithium-containing solid solution. Consequently, its interfacial ionic conductivity was insufficient, and its low-temperature performance (225 mAh / g at -20℃, 156 mAh / g at -30℃) and cycle retention rate (71.6%) were significantly inferior to those of the Example.

[0081] Due to the lack of gas-phase fluorination treatment, Comparative Example 3 failed to introduce sufficient oxygen vacancy defects through fluorine-oxygen replacement, resulting in a reduction of lithium-ion transport active sites at the interface. Although its low-temperature performance (241 mAh / g at -20℃, 168 mAh / g at -30℃) and cycle retention rate (75.4%) were slightly higher than Comparative Example 2, they were still significantly lower than those of the Example.

[0082] The above comparison fully demonstrates that the present invention, through the synergistic effect of four processes—sol-gel, high-temperature fluorination, molten salt etching, and vapor phase reconstruction—successfully constructs a fluorinated niobium tungsten titanate interface layer with a porous structure and oxygen vacancy defects, fundamentally solving key technical problems in the prior art such as difficulty in lithium-ion desolvation at low temperatures, high interface impedance, and easy growth of lithium dendrites.

Claims

1. A method for optimizing the graphite / hard carbon composite interface of the negative electrode in a low-temperature lithium iron phosphate battery, characterized in that, Includes the following steps: S1. By weight, mix 60-85 parts of artificial graphite and 15-40 parts of hard carbon to obtain a composite powder; mix the composite powder with 1-3 parts of conductive carbon black, 1-2 parts of styrene-butadiene rubber and 1-2 parts of sodium carboxymethyl cellulose, add 80-200 parts of deionized water, stir, and obtain graphite / hard carbon composite negative electrode slurry. S2. The graphite / hard carbon composite negative electrode slurry obtained in step S1 is coated on copper foil to obtain copper foil with a wet film on the surface; 2-20 parts of fluorinated niobium tungsten titanate composite are dispersed in 100-500 parts of anhydrous ethanol and sprayed onto the surface of the wet film to obtain the sprayed electrode sheet. S3. The coated electrode sheet is subjected to gradient drying: drying at 48-52℃; then drying at 78-82℃; and finally vacuum drying at 115-125℃ to obtain the dried electrode sheet. S4. Place the dried electrode sheet in a roller press for compaction to obtain the rolled electrode sheet; place the rolled electrode sheet in a vacuum oven at 58-62℃ and let it stand.

2. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to claim 1, characterized in that, In step S1, the particle size of the artificial graphite is 15-20 μm.

3. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to claim 1, characterized in that, In step S2, the amount of coating applied to the wet film surface is 0.5-1.0 mg / cm².

4. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to claim 1, characterized in that, In step S3, the product is dried at 48-52℃ for 0.5-1h; then dried at 78-82℃ for 1-1.5h; and finally dried under vacuum at 115-125℃ for 4-6h.

5. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to claim 1, characterized in that, In step S4, the compaction pressure is 8-10 MPa.

6. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to any one of claims 1-5, characterized in that, The preparation steps of the fluorinated niobium tungsten titanate composite include: A1. By weight, dissolve 90-100 parts of tetrabutyl titanate, 18-25 parts of niobium pentachloride and 20-80 parts of sodium tungstate in 500-1000 parts of anhydrous ethanol to obtain a mixed solution; stir the mixed solution at 45-50℃ under argon protection, add 30-60 parts of deionized water-ethanol mixture dropwise, and continue stirring to obtain a sol; age the sol in a water bath at 58-62℃ to obtain a wet gel, and then dry it in a vacuum drying oven at 78-82℃ to obtain a precursor powder; A2. Place the precursor powder obtained in step A1 in a tube furnace and heat it to 645-655℃ under an argon atmosphere. After naturally cooling to room temperature, sintered powder is obtained. Grind and mix the sintered powder with 0.3-0.8 parts of lithium fluoride to obtain a mixture. Under argon protection, heat the mixture to 845-855℃ and hold it. After the reaction is completed, cool it to 195-205℃ and allow it to cool naturally to room temperature to obtain fluorinated niobium tungsten titanate bulk material. A3. Grind and sieve the fluorinated niobium tungsten titanate block obtained in step A2 to obtain powder; mix the powder with a mixed molten salt of 2.5-4 parts potassium chloride and 2.5-4 parts sodium chloride, and place it in a corundum crucible; heat to 745-755℃ in a tube furnace under an argon atmosphere and hold at that temperature; after etching, cool naturally to room temperature to obtain the product; wash the product with deionized water, centrifuge, and finally vacuum dry at 78-82℃ to obtain a porous fluorinated niobium tungsten titanate intermediate; A4. The porous fluorinated niobium tungsten titanate intermediate prepared in step A3 is placed in a fluidized bed reactor. A mixture of nitrogen trifluoride and argon is introduced into the argon carrier gas, and the temperature is raised to 545-555℃ for heat treatment. After the heat treatment is completed, pure argon is purged. Then the temperature is raised to 395-405℃, and a mixture of hydrogen and argon is introduced for reduction treatment. Under argon protection, it is naturally cooled to room temperature.

7. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to claim 6, characterized in that, In step A1, the stirring time at 45-50℃ is 2-4 hours.

8. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to claim 6, characterized in that, In step A2, the temperature is raised to 645-655℃ and held for 4-6 hours.

9. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to claim 6, characterized in that, In step A3, the temperature is raised to 745-755℃ and held for 3-5 hours.

10. The method for optimizing the graphite / hard carbon composite interface of the low-temperature lithium iron phosphate battery anode according to claim 6, characterized in that, In step A4, the reduction treatment time for introducing a mixture of hydrogen and argon is 1-2 hours.

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