Composite modified positive electrode material for sulfide solid electrolyte and preparation method thereof

Abstract

This invention discloses a composite modified cathode material for sulfide solid electrolytes and its preparation method, belonging to the field of lithium battery technology. The composite modified cathode material prepared by this invention perfectly matches the sulfide solid electrolyte system, effectively breaking through the technical bottleneck of the cathode side of sulfide all-solid-state lithium batteries, significantly improving the cycle stability and rate performance of the battery, and providing key material support and technical solutions for the industrialization of high-safety, high-energy-density sulfide all-solid-state lithium batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a composite modified cathode material for sulfide solid electrolytes and its preparation method. Background Technology

[0002] With the rapid development of new energy vehicles, large-scale energy storage power stations, and high-end portable electronic devices, the market has placed higher demands on the energy density, cycle stability, and safety performance of energy storage devices. Traditional liquid lithium-ion batteries, due to defects such as easy leakage, flammability, and poor high-temperature performance of the electrolyte, face significant bottlenecks in energy density improvement and safe application, making them unable to meet the needs of high-end energy storage scenarios. All-solid-state lithium batteries, which replace traditional liquid electrolytes with inorganic solid electrolytes, possess advantages such as non-flammability, excellent thermal stability, and great potential for energy density improvement, becoming the core development direction for next-generation lithium-ion batteries. Among them, sulfide solid electrolytes, with room temperature ionic conductivity reaching 10... -2 With a S / cm ratio comparable to liquid electrolytes and excellent mechanical ductility and low-temperature processing properties, it has become the solid electrolyte system with the greatest potential for commercial application.

[0003] High-nickel ternary cathode materials, due to their advantages such as high specific capacity, high operating voltage, and readily available raw materials, are ideal cathode materials for matching sulfide solid electrolytes to achieve high energy density in all-solid-state lithium batteries. Among them, high-nickel ternary materials with a nickel content higher than 80% have a theoretical specific capacity exceeding 200 mAh / g, making them a key focus of research and industrialization. However, when high-nickel ternary cathode materials are directly applied to sulfide all-solid-state battery systems, serious interface problems and bulk structure degradation are encountered, resulting in poor battery cycle stability, poor rate performance, and persistently high interfacial impedance. These issues have become the core technical bottlenecks restricting the industrialization of this system, specifically manifested in the following aspects: 1. Bulk structure is prone to degradation: During the charge and discharge process, high-nickel ternary materials undergo multiple phase transformations (such as H1→H2→H3), accompanied by drastic contraction and expansion of the lattice volume, which leads to cracking of material particles; at the same time, under high voltage, lattice oxygen is easily deintercalated, causing the material surface structure to collapse, and transition metal ions are easily dissolved, which further aggravates the destruction of the bulk structure and reduces the cycle life of the material.

[0004] 2. Poor compatibility of the positive / solid electrolyte interface: There is chemical incompatibility between high-nickel ternary cathode materials and sulfide solid electrolytes. When they come into contact, interfacial side reactions easily occur, generating reaction products with high impedance (such as Li2S, metal sulfides, etc.), which significantly increases the interfacial impedance and hinders the transport of lithium ions and electrons. At the same time, the physical contact between the two is poor. There are many interfacial gaps between the rigid cathode particles and the solid electrolyte, which cannot form a continuous ion transport channel, further reducing the rate performance and cycle performance of the battery.

[0005] 3. Existing modification methods have limitations: Currently, the main modification methods for high-nickel ternary cathode materials include element doping and surface coating. Although single element doping can stabilize the bulk structure to a certain extent, it cannot solve the interfacial compatibility problem. Although single oxide surface coating can isolate the cathode from direct contact with the solid electrolyte, it is prone to lithium-ion transport obstruction due to the low ionic conductivity of the coating layer itself and poor bonding force with the substrate. Moreover, traditional coating layers have a uniform structure and cannot meet the dual requirements of interfacial stability and ion transport efficiency.

[0006] Therefore, developing a composite modified cathode material that combines a stable bulk structure, excellent interfacial compatibility, and efficient ion / electron transport performance to achieve efficient matching with sulfide solid electrolytes has become a key technical problem that urgently needs to be solved in the field of sulfide all-solid-state lithium batteries. Summary of the Invention

[0007] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a composite modified cathode material for sulfide solid electrolyte and its preparation method, which perfectly matches the sulfide solid electrolyte system, effectively breaks through the technical bottleneck of the cathode side of sulfide all-solid-state lithium battery, significantly improves the cycle stability and rate performance of the battery, and provides key material support and technical solutions for the industrialization of high-safety, high-energy-density sulfide all-solid-state lithium battery.

[0008] The technical solution of this invention is as follows: On one hand, the present invention provides a method for preparing a composite modified cathode material for sulfide solid electrolytes, comprising the following steps: Preparation of S1 precursor: Prepare a mixed salt solution of nickel, cobalt, zirconium and titanium; use NaOH as precipitant and ammonia as complexing agent, and carry out a co-precipitation reaction at 53-57℃ and pH=10.5-11.5 to obtain the precursor; S2 Sintering and Doping: The above precursor is mixed with a lithium source and pre-fired in an oxygen stream at 450-550℃ for 4-6 hours. Then the temperature is raised to 745-755℃ and held for 9-10 hours. The temperature is then raised to 770-790℃ and held for 2-3 hours to obtain a double-doped matrix material. S3 precursor coating: Add barium acetate aqueous dispersion to ethanol dispersion of dual-doped matrix material, then add tetrabutyl titanate ethanol solution dropwise, adjust pH=9-10, react at 55-65℃ for 3-5h to obtain BaTiO3 precursor coated material; S4 gradient oxygen vacancy construction: The above material was heated to 680-720℃ in a reducing atmosphere of Ar and H2 and held for 2-4 hours; then the atmosphere was switched to Ar and oxygen was introduced while cooling down to 400℃, and the oxygen partial pressure was gradually increased so that the outer oxygen vacancy was gradually filled, forming a gradient oxygen vacancy concentration distribution with high concentration inside and low concentration outside. S5 Electric field polarization treatment: Place the material obtained in step S4 between parallel plate electrodes and heat it to 140-160℃ (so that BaTiO3 is in the paraelectric phase); apply a DC electric field of 5-6kV / cm and hold for 30-50min; cool to room temperature while maintaining the electric field, so that the domains freeze in the polarization direction to form permanent polarization, and finally obtain a double-doped ternary cathode material with a polarized BaTiO3 coating layer with a gradient oxygen vacancy concentration.

[0009] Preferably, in step S1, the molar ratio of Ni, Co, Zr, and Ti in the mixed salt solution is 0.83:0.12:0.03:0.02.

[0010] Preferably, in step S1, the nickel salt used in the mixed salt solution is nickel sulfate, the cobalt salt is cobalt sulfate, the zirconium salt is zirconium sulfate, and the titanium salt is titanium sulfate.

[0011] Preferably, in step S1, the concentration of the mixed salt solution is 2-3 mol / L.

[0012] Preferably, in step S2, the lithium source is lithium hydroxide, which is added at a Li:Me molar ratio of 1.05:1.

[0013] Preferably, in step S3, the molar ratio of the dual-doped matrix material to Ba and Ti is 1:0.02:0.02.

[0014] Preferably, in step S4, the volume ratio of Ar to H2 in the reducing atmosphere is 95:5.

[0015] Preferably, in step S4, the oxygen partial pressure is controlled to increase at the following gradient: 700-600°C, oxygen partial pressure = 10. -5 atm; 600-500℃, oxygen partial pressure = 10 -4 atm; 500-400℃, oxygen partial pressure = 10 -3 atm; below 400℃, switch to pure Ar.

[0016] On the other hand, the present invention provides a composite modified cathode material for sulfide solid electrolytes, which is prepared by the above-described method for preparing composite modified cathode materials for sulfide solid electrolytes.

[0017] This invention achieves performance enhancement of cathode materials from multiple dimensions, including bulk structure, interfacial compatibility, and ion / electron transport efficiency, through synergistic design of Zr-Ti dual-doping bulk phase modification, gradient oxygen vacancy BaTiO3 coating layer construction, and electric field polarization modification. It is perfectly adapted to sulfide solid electrolyte systems and effectively solves the core problems of high-nickel ternary cathode materials in sulfide all-solid-state batteries, such as bulk structure degradation, high interfacial impedance, and poor cycle stability. Compared with existing technologies, it has the following significant advantages: 1. This invention employs Zr and Ti dual-element bulk doping of a high-nickel ternary matrix, where the two elements complement each other and synergistically modify the structure, fundamentally suppressing the bulk structural degradation of the high-nickel ternary material: (1) Zr 4+ With a larger ionic radius than transition metal ions, doping can effectively expand the lithium interlayer spacing, providing a smoother channel for lithium ion diffusion and improving the bulk transport efficiency of lithium ions. At the same time, the bond energy of Zr-O bond is much higher than that of traditional transition metal-oxygen bond, which can firmly bind lattice oxygen, significantly suppress the insertion and extraction of lattice oxygen under high voltage, and avoid the collapse of the material surface structure due to oxygen loss.

[0018] (2) Ti 4+ It possesses reversible valence change capability, and can change its valence through Ti during charging and discharging. 4+ / Ti 3+ The redox reaction achieves electronic compensation, alleviating the volume strain of the crystal lattice caused by lithium ion insertion / extraction; at the same time, it can effectively suppress the harmful H2-H3 crystal phase transformation during the charging and discharging process of high-nickel ternary materials, avoid material particle cracking caused by phase transformation, and greatly improve the structural stability and cycle durability of the material.

[0019] The synergistic effect of dual doping enables the cathode material to maintain its complete bulk structure during repeated charge and discharge processes, and it has excellent internal stability even without the protection of the coating layer, laying the foundation for the long cycle performance of the battery.

[0020] 2. This invention abandons the traditional design concept of a uniform structure coating layer, and constructs a BaTiO3 coating layer with a high internal and low external oxygen vacancy distribution through a reduction-gradient oxidation process. This achieves "precise matching" between the cathode and the sulfide solid electrolyte interface, while taking into account both interface stability and ion transport efficiency. (1) The outer layer of the coating has a low oxygen vacancy concentration and a complete crystal structure, which can effectively isolate the direct contact between the high-nickel ternary cathode and the sulfide solid electrolyte, suppress the chemical side reaction between the two from the source, avoid the generation of high-impedance by-products, and significantly reduce the interfacial contact impedance; at the same time, the complete BaTiO3 crystal structure has good chemical stability, which can prevent transition metal ions from dissolving into the sulfide electrolyte and further improve the interfacial stability.

[0021] (2) The high concentration of oxygen vacancies in the inner layer of the coating provides abundant fast channels for lithium-ion transport, effectively improving the cross-interface transport efficiency of lithium-ions between the cathode substrate and the coating. At the same time, the oxygen vacancies in the inner layer can adjust the electronic conductivity of the coating, reduce the resistance to electron transport, and achieve efficient synergistic transport of ions and electrons.

[0022] The gradient oxygen vacancy design solves the technical problem of the traditional coating layer's inability to achieve both interface stability and ion transport, enabling the coating layer to have both excellent interface isolation effect and ensure rapid lithium ion transport.

[0023] 3. This invention applies high-temperature electric field polarization followed by field-maintaining cooling to the graded oxygen vacancy BaTiO3 coating layer, causing the domains of the BaTiO3 coating layer to freeze in the polarization direction, forming permanent ferroelectricity, resulting in two major performance improvements: (1) The permanently polarized BaTiO3 coating layer generates a built-in electrostatic field. This electric field can accelerate the migration of lithium ions in the coating layer and at the interface through electrostatic driving, significantly improving the diffusion coefficient of lithium ions, enabling the material to have excellent high-rate discharge performance and meet the requirements of high-current charging and discharging.

[0024] (2) Electric field polarization causes the electric domains of BaTiO3 to align in an oriented manner, which reduces the grain boundary resistance during lithium ion transport and enhances the bonding force between the coating layer and the positive electrode substrate, preventing the coating layer from falling off during charging and discharging and ensuring the long-term effectiveness of the modification.

[0025] 4. The three modification methods of this invention—Zr-Ti dual doping, gradient oxygen vacancy BaTiO3 coating, and electric field polarization—are not simply superimposed, but form a synergistic effect mechanism across the entire chain of bulk phase, interface, and transport: dual doping stabilizes the bulk phase structure, providing a good substrate for interface modification; the gradient oxygen vacancy coating layer solves the interface compatibility problem and simultaneously builds an ion transport bridge between the bulk phase and the electrolyte; electric field polarization further activates the ion transport capability of the coating layer, ensuring smooth lithium ion transport throughout the bulk phase, coating layer, and electrolyte.

[0026] This synergistic design enables the cathode material to simultaneously possess the characteristics of high electronic conductivity, high lithium-ion diffusion coefficient, and low interfacial impedance. The assembled sulfide all-solid-state battery exhibits high initial discharge specific capacity, excellent high-rate performance, and long-cycle stability. For example, at a 1C rate, the capacity retention rate can reach more than 92% after 200 cycles, and the interfacial impedance can be reduced to below 30Ω. Its overall performance is far superior to that of cathode materials that are single-doped, single-coated, or unpolarized.

[0027] In summary, the composite modified cathode material prepared by this invention is perfectly matched with the sulfide solid electrolyte system, effectively breaking through the technical bottleneck of the cathode side of sulfide all-solid-state lithium batteries, significantly improving the cycle stability and rate performance of the battery, and providing key material support and technical solutions for the industrialization of high-safety, high-energy-density sulfide all-solid-state lithium batteries. Detailed Implementation

[0028] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1 The method for preparing the composite modified cathode material for sulfide solid electrolytes in this embodiment includes the following steps: S1 precursor preparation Weigh out 436.3 g of nickel sulfate hexahydrate, 67.5 g of cobalt sulfate heptahydrate, 21.3 g of zirconium sulfate tetrahydrate, and 9.6 g of titanium sulfate in a molar ratio of Ni:Co:Zr:Ti = 0.83:0.12:0.03:0.02. Dissolve these salts in deionized water and bring the volume to 1 L to prepare a mixed salt solution A with a metal ion concentration of 2 mol / L. Separately weigh out 192 g of sodium hydroxide, dissolve it in deionized water, and bring the volume to 1.2 L to prepare a 4 mol / L sodium hydroxide solution B as a precipitant. Measure out 100 mL of 25% ammonia water as a complexing agent.

[0030] Add 2L of deionized water as a base solution to a 5L reactor, and purge with nitrogen and stir for 30 minutes to remove dissolved oxygen. Control the reaction temperature at 55±2℃ and the stirring speed at 500rpm. Add the mixed salt solution, precipitant, and complexing agent to the reactor in a parallel stream, maintaining the pH at 11±0.1. Control the feeding time at 4 hours, and continue stirring and aging for 12 hours after the feeding is complete.

[0031] The obtained slurry was filtered, and the filter cake was washed three times with deionized water, 1 L of water each time. The filter cake was placed in a vacuum drying oven and dried at 120℃ for 12 h to obtain Ni. 0.83 Co 0.12 Zr 0.03 Ti 0.02 (OH)2 precursor.

[0032] S2 Lithium Mixture and Primary Sintering 190g of the above precursor and 88.1g of lithium hydroxide monohydrate were placed in a mixer and mixed thoroughly for 30min. The mixture was transferred to an alumina crucible and placed in a tube furnace for sintering under an oxygen atmosphere (flow rate 0.5L / min): the temperature was increased to 500℃ at 2℃ / min and held for 5h; then increased to 750℃ at 1℃ / min and held for 10h; then increased to 780℃ at 1℃ / min and held for 2h. After sintering, the mixture was allowed to cool naturally to room temperature. The product was then removed, gently ground in a mortar, and passed through a 200-mesh sieve to obtain the dual-doped matrix material LiNi. 0.83 Co 0.12 Zr 0.03 Ti 0.02 O2.

[0033] S3 BaTiO3 precursor coating Take 195g of the matrix material obtained in step S2, disperse it in 300mL of anhydrous ethanol, and sonicate for 30min. This is recorded as suspension A. Weigh 4.32g of barium acetate and dissolve it in 50mL of deionized water. This is recorded as solution B. Weigh 5.75g of tetrabutyl titanate and dissolve it in 150mL of anhydrous ethanol. Add 2mL of glacial acetic acid to inhibit hydrolysis, stir well, and this is recorded as solution C.

[0034] While stirring, solution B was added dropwise to suspension A, and stirring continued for 30 minutes. Then, solution C was added dropwise to the above mixture. After the addition was complete, the pH was adjusted to 9 with ammonia. The system was heated to 60°C and stirred for 4 hours to allow the BaTiO3 precursor to precipitate uniformly on the matrix surface.

[0035] After the reaction was completed, the solid product was separated by centrifugation and washed three times with 100 mL of anhydrous ethanol each time. The filter cake was placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the material coated with BaTiO3 precursor.

[0036] S4 gradient oxygen vacancy construction The material obtained in step S3 is placed in a corundum boat and then placed in a tubular atmosphere furnace. Heat treatment is then performed according to the following procedure: Reduction Stage: An Ar / H2 mixture (95% Ar + 5% H2) is introduced at a flow rate of 200 mL / min, and the temperature is increased to 700℃ at a rate of 2℃ / min, and held for 2 hours. This stage generates oxygen vacancies in the BaTiO3 layer, while the Ti layer on the substrate surface... 4+ Partially restored to Ti 3+ .

[0037] Gradient formation stage: After the holding period, switch the atmosphere to high-purity Ar at a flow rate of 200 mL / min. Cool at a rate of 0.5 °C / min, while gradually introducing O2 during the cooling process to control the oxygen partial pressure to increase according to the following gradient: 700-600℃: Oxygen partial pressure = 10 -5 atm; 600-500℃: Oxygen partial pressure = 10 -4 atm; 500-400℃: Oxygen partial pressure = 10 -3 atm; Below 400℃: Switch to pure Ar.

[0038] By combining programmed temperature control with oxygen partial pressure control, a gradient distribution of high oxygen vacancy concentration in the inner layer and low oxygen vacancy concentration in the outer layer is formed in the BaTiO3 layer. After cooling to room temperature, the material is removed, yielding a BaTiO3 coating material with a gradient oxygen vacancy concentration.

[0039] S5 electric field polarization treatment The material obtained in step S4 is placed in a parallel plate electrode fixture with an electrode spacing of 2 mm. The fixture is placed in an oven and heated to 150°C. After the temperature stabilizes, an electric field of 5 kV / cm is applied through a high-voltage DC power supply and maintained for 30 min.

[0040] While maintaining the electric field, the material is cooled to room temperature at a rate of 1 °C / min, causing the domains of BaTiO3 to freeze in the polarization direction, forming permanent polarization. The electric field is then turned off, and the material is removed, yielding the final dual-doped ternary cathode material with a polarized BaTiO3 coating layer exhibiting a gradient oxygen vacancy concentration.

[0041] Example 2 The method for preparing the composite modified cathode material for sulfide solid electrolytes in this embodiment includes the following steps: S1 precursor preparation Same as Example 1.

[0042] S2 Lithium Mixture and Primary Sintering 190g of the above precursor and 88.1g of lithium hydroxide monohydrate were placed in a mixer and mixed thoroughly for 30 min. The mixture was transferred to an alumina crucible and placed in a tube furnace for sintering under an oxygen atmosphere (flow rate 0.5 L / min): the temperature was increased to 450℃ at 2℃ / min and held for 6 h; then increased to 745℃ at 1℃ / min and held for 10 h; then increased to 770℃ at 1℃ / min and held for 3 h. After sintering, the mixture was allowed to cool naturally to room temperature. The product was then removed, gently ground in a mortar, and passed through a 200-mesh sieve to obtain the dual-doped matrix material LiNi. 0.83 Co 0.12 Zr 0.03 Ti 0.02 O2.

[0043] S3 BaTiO3 precursor coating Take 195g of the matrix material obtained in step S2, disperse it in 300mL of anhydrous ethanol, and sonicate for 30min. This is recorded as suspension A. Weigh 4.32g of barium acetate and dissolve it in 50mL of deionized water. This is recorded as solution B. Weigh 5.75g of tetrabutyl titanate and dissolve it in 150mL of anhydrous ethanol. Add 2mL of glacial acetic acid to inhibit hydrolysis, stir well, and this is recorded as solution C.

[0044] While stirring, solution B was added dropwise to suspension A, and stirring continued for 30 minutes. Then, solution C was added dropwise to the above mixture. After the addition was complete, the pH was adjusted to 9 with ammonia. The system was heated to 55°C and stirred for 5 hours to allow the BaTiO3 precursor to precipitate uniformly on the matrix surface.

[0045] After the reaction was completed, the solid product was separated by centrifugation and washed three times with 100 mL of anhydrous ethanol each time. The filter cake was placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the material coated with BaTiO3 precursor.

[0046] S4 gradient oxygen vacancy construction The material obtained in step S3 is placed in a corundum boat and then placed in a tubular atmosphere furnace. Heat treatment is then performed according to the following procedure: Reduction stage: An Ar / H2 mixture (95% Ar + 5% H2) is introduced at a flow rate of 200 mL / min, and the temperature is increased to 680℃ at a rate of 2℃ / min, and held for 4 hours. This stage generates oxygen vacancies in the BaTiO3 layer, while the Ti layer on the substrate surface... 4+ Partially restored to Ti 3+ .

[0047] Gradient formation stage: After the holding period, switch the atmosphere to high-purity Ar at a flow rate of 200 mL / min. Cool at a rate of 0.5 °C / min, while gradually introducing O2 during the cooling process to control the oxygen partial pressure to increase according to the following gradient: 700-600℃: Oxygen partial pressure = 10 -5 atm; 600-500℃: Oxygen partial pressure = 10 -4 atm; 500-400℃: Oxygen partial pressure = 10 -3 atm; Below 400℃: Switch to pure Ar.

[0048] By combining programmed temperature control with oxygen partial pressure control, a gradient distribution of high oxygen vacancy concentration in the inner layer and low oxygen vacancy concentration in the outer layer is formed in the BaTiO3 layer. After cooling to room temperature, the material is removed, yielding a BaTiO3 coating material with a gradient oxygen vacancy concentration.

[0049] S5 electric field polarization treatment The material obtained in step S4 is placed in a parallel plate electrode fixture with an electrode spacing of 2 mm. The fixture is placed in an oven and heated to 140°C. After the temperature stabilizes, an electric field of 5 kV / cm is applied through a high-voltage DC power supply and maintained for 40 min.

[0050] While maintaining the electric field, the material is cooled to room temperature at a rate of 1 °C / min, causing the domains of BaTiO3 to freeze in the polarization direction, forming permanent polarization. The electric field is then turned off, and the material is removed, yielding the final dual-doped ternary cathode material with a polarized BaTiO3 coating layer exhibiting a gradient oxygen vacancy concentration.

[0051] Example 3 The method for preparing the composite modified cathode material for sulfide solid electrolytes in this embodiment includes the following steps: S1 precursor preparation Same as Example 1.

[0052] S2 Lithium Mixture and Primary Sintering 190g of the above precursor and 88.1g of lithium hydroxide monohydrate were placed in a mixer and mixed thoroughly for 30min. The mixture was transferred to an alumina crucible and placed in a tube furnace for sintering under an oxygen atmosphere (flow rate 0.5L / min): the temperature was increased to 550℃ at 2℃ / min and held for 4h; then increased to 755℃ at 1℃ / min and held for 9h; and then increased to 790℃ at 1℃ / min and held for 2h. After sintering, the mixture was allowed to cool naturally to room temperature. The product was then removed, gently ground in a mortar, and passed through a 200-mesh sieve to obtain the dual-doped matrix material LiNi. 0.83 Co 0.12 Zr 0.03 Ti 0.02 O2.

[0053] S3 BaTiO3 precursor coating Take 195g of the matrix material obtained in step S2, disperse it in 300mL of anhydrous ethanol, and sonicate for 30min. This is recorded as suspension A. Weigh 4.32g of barium acetate and dissolve it in 50mL of deionized water. This is recorded as solution B. Weigh 5.75g of tetrabutyl titanate and dissolve it in 150mL of anhydrous ethanol. Add 2mL of glacial acetic acid to inhibit hydrolysis, stir well, and this is recorded as solution C.

[0054] While stirring, solution B was added dropwise to suspension A, and stirring continued for 30 minutes. Then, solution C was added dropwise to the above mixture. After the addition was complete, the pH was adjusted to 9 with ammonia. The system was heated to 65°C and stirred for 3 hours to allow the BaTiO3 precursor to precipitate uniformly on the matrix surface.

[0055] After the reaction was completed, the solid product was separated by centrifugation and washed three times with 100 mL of anhydrous ethanol each time. The filter cake was placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the material coated with BaTiO3 precursor.

[0056] S4 gradient oxygen vacancy construction The material obtained in step S3 is placed in a corundum boat and then placed in a tubular atmosphere furnace. Heat treatment is then performed according to the following procedure: Reduction Stage: An Ar / H2 mixture (95% Ar + 5% H2) is introduced at a flow rate of 200 mL / min, and the temperature is increased to 720℃ at a rate of 2℃ / min, and held for 2 hours. This stage generates oxygen vacancies in the BaTiO3 layer, while the Ti layer on the substrate surface... 4+ Partially restored to Ti 3+ .

[0057] Gradient formation stage: After the holding period, switch the atmosphere to high-purity Ar at a flow rate of 200 mL / min. Cool at a rate of 0.5 °C / min, while gradually introducing O2 during the cooling process to control the oxygen partial pressure to increase according to the following gradient: 700-600℃: Oxygen partial pressure = 10 -5 atm; 600-500℃: Oxygen partial pressure = 10 -4 atm; 500-400℃: Oxygen partial pressure = 10 -3 atm; Below 400℃: Switch to pure Ar.

[0058] By combining programmed temperature control with oxygen partial pressure control, a gradient distribution of high oxygen vacancy concentration in the inner layer and low oxygen vacancy concentration in the outer layer is formed in the BaTiO3 layer. After cooling to room temperature, the material is removed, yielding a BaTiO3 coating material with a gradient oxygen vacancy concentration.

[0059] S5 electric field polarization treatment The material obtained in step S4 is placed in a parallel plate electrode fixture with an electrode spacing of 2 mm. The fixture is placed in an oven and heated to 160°C. After the temperature stabilizes, an electric field of 6 kV / cm is applied through a high-voltage DC power supply and maintained for 50 min.

[0060] While maintaining the electric field, the material is cooled to room temperature at a rate of 1 °C / min, causing the domains of BaTiO3 to freeze in the polarization direction, forming permanent polarization. The electric field is then turned off, and the material is removed, yielding the final dual-doped ternary cathode material with a polarized BaTiO3 coating layer exhibiting a gradient oxygen vacancy concentration.

[0061] Comparative Example 1 The difference from Example 1 is that the material coated with the BaTiO3 precursor prepared in step S3 is directly heat-treated in air at 700°C for 2 hours, without subsequent steps S4 and S5.

[0062] Comparative Example 2 The difference from Example 1 is that the electric field polarization treatment in step S5 is not performed.

[0063] The positive electrode materials obtained in Examples 1-3 and Comparative Examples 1-2 were mixed with sulfide solid electrolyte and conductive carbon black at a mass ratio of 70:28:2 to prepare positive electrode sheets. A sulfide all-solid-state battery was assembled using lithium-indium alloy as the negative electrode. Charge-discharge tests were conducted at a 1C rate within a voltage range of 2.8-4.3V, and the results are shown in Table 1. Table 1 Performance test results of assembled batteries in Examples 1-3 and Comparative Examples 1-2 As shown in Table 1, Comparative Example 1 only completed the coating of the BaTiO3 precursor, without gradient oxygen vacancy construction and electric field polarization treatment, and was used directly after air heat treatment. Its performance was significantly different from that of Example 1. The reasons are analyzed as follows: Comparative Example 1 shows a significant decrease in electronic conductivity because the absence of a reducing atmosphere creates oxygen vacancies. The BaTiO3 coating layer has a pure crystalline phase structure with low intrinsic electronic conductivity, and the lack of oxygen vacancies provides electron transport channels. Furthermore, the absence of Ti on the substrate surface... 4+ →Ti 3+The change in valence cannot improve the overall electronic conductivity of the material, and the resistance to electron transport increases significantly. The lithium-ion diffusion coefficient of Comparative Example 1 is reduced because the "ion channel" effect of the lack of gradient oxygen vacancies and the low intrinsic ionic conductivity of the uniform crystalline phase BaTiO3 coating layer act as an "insulating layer" for lithium-ion transport between the matrix and the sulfide electrolyte, severely hindering the cross-interface migration of lithium ions. The initial discharge specific capacity of Comparative Example 1 at 0.5C is 197.1 mAh / g, which is 11.1 mAh / g lower than that of Example 1. This is because the electron / lithium-ion transport efficiency is low. During the charging and discharging process, the active sites inside the positive electrode and at the interface cannot fully participate in the electrochemical reaction, and some lithium ions are difficult to intercalate / deintercalate, resulting in a significant decrease in the utilization rate of active materials. The 5C discharge specific capacity of Comparative Example 1 was 144.6 mAh / g, a decrease of 19.9 mAh / g compared to Example 1. The capacity loss was more severe at higher rates because high-current charging and discharging places higher demands on the rapid transport of ions / electrons. The defects of low conductivity and low diffusion coefficient are further amplified at high rates, preventing lithium ions from responding to electrode reactions in a timely manner, leading to a sharp drop in rate performance. The capacity retention rate of Comparative Example 1 after 200 1C cycles was only 77.8%, a decrease of 14.3 percentage points compared to Example 1. This is because the BaTiO3 coating layer without gradient oxygen vacancies did not achieve true interfacial stability: on the one hand, the coating layer after air heat treatment had poor adhesion to the substrate and was easily detached during charging and discharging, failing to effectively isolate the positive electrode from the sulfide electrolyte. The two continuously underwent chemical side reactions, generating high-resistivity byproducts; on the other hand, the lack of interfacial protection led to the continuous dissolution of transition metal ions, exacerbating substrate structural collapse and electrolyte degradation. These two factors resulted in rapid capacity decay during cycling. The interfacial impedance of Comparative Example 1 reached 73.2Ω, which is 2.49 times that of Example 1. The core reason is chemical incompatibility and physical transport obstruction: the coating layer without gradient oxygen vacancies cannot suppress the side reactions at the positive / solid electrolyte interface, and high-impedance byproducts continue to accumulate; at the same time, the coating layer itself hinders ion / electron transport, forming a "double high-impedance layer", which becomes the main obstacle to the electrochemical reaction of the battery.

[0064] Comparative Example 2 successfully constructed gradient oxygen vacancies but lacked electric field polarization treatment. Its performance was better than Comparative Example 1, but still significantly worse than Example 1. The reasons are analyzed as follows: The electronic conductivity of Comparative Example 2 decreased because, although the gradient oxygen vacancies provided some electron transport channels (and were therefore better than Comparative Example 1), the lack of electric field polarization caused the BaTiO3 domains to be arranged in a disordered manner, increasing grain boundary resistance and reducing the electron transport efficiency within the coating layer. Therefore, the overall electronic conductivity could not reach the level of Example 1. The lithium-ion diffusion coefficient of Comparative Example 2 decreased by 29.6% compared to Example 1. This is the core reason for the performance degradation of Comparative Example 2: the lack of electric field polarization prevented the BaTiO3 coating layer from forming permanent ferroelectricity. Lacking the "electrostatic driving" effect of the built-in electrostatic field, lithium ions relied solely on oxygen vacancy channels for passive diffusion. The migration rate was far lower than the dual effect of "oxygen vacancy channels + electrostatic driving" in Example 1, resulting in a significant decrease in lithium-ion cross-interface transport efficiency. The initial discharge specific capacity at 0.5C for Comparative Example 2 was 201.7 mAh / g, a decrease of 6.5 mAh / g compared to Example 1. This was due to the reduced lithium-ion diffusion coefficient, which prevented some lithium ions from quickly intercalating / deintercalating at low rates. Although the utilization rate of the active material was higher than that of Comparative Example 1, it was still lower than that of Example 1. The discharge specific capacity at 5C for Comparative Example 2 was 156.2 mAh / g, a decrease of 8.3 mAh / g compared to Example 1. This was because at high rates, the lithium-ion diffusion rate without electrostatic drive could not meet the requirements of high-current reactions, and ion transport lagged behind electron transport, resulting in insufficient electrode reactions and a decrease in rate performance. The capacity retention rate of Comparative Example 2 after 200 cycles at 1C was 80.3%, a decrease of 11.8 percentage points compared to Example 1. This is due to two reasons: First, the lack of electric field polarization caused disordered BaTiO3 domains, resulting in weak bonding between the coating layer and the substrate. During charge and discharge, lattice volume changes easily led to microcracks in the coating layer, allowing a small amount of sulfide electrolyte to directly contact the positive electrode, causing minor side reactions. Second, the low lithium-ion diffusion efficiency led to localized lithium deposition on the electrode surface, exacerbating interface degradation and accelerating the capacity decay rate under long-term cycling. The interface impedance of Comparative Example 2 was 53.9 Ω, an increase of 83.3% compared to Example 1. This is because although the non-electrically polarized coating layer suppressed some side reactions with gradient oxygen vacancies (thus superior to Comparative Example 1), the grain boundary resistance caused by domain disorder and the minor side reactions caused by microcracks in the coating layer still resulted in a higher interface transport resistance than in Example 1, failing to effectively reduce the interface impedance.

Claims

1. A method for preparing a composite modified cathode material for sulfide solid electrolytes, characterized in that, Includes the following steps: Preparation of S1 precursor: Prepare a mixed salt solution of nickel, cobalt, zirconium and titanium; use NaOH as precipitant and ammonia as complexing agent, and carry out a co-precipitation reaction at 53-57℃ and pH=10.5-11.5 to obtain the precursor; S2 Sintering and Doping: The above precursor is mixed with a lithium source and pre-fired in an oxygen stream at 450-550℃ for 4-6 hours. Then the temperature is raised to 745-755℃ and held for 9-10 hours. The temperature is then raised to 770-790℃ and held for 2-3 hours to obtain a double-doped matrix material. S3 precursor coating: Add barium acetate aqueous dispersion to ethanol dispersion of dual-doped matrix material, then add tetrabutyl titanate ethanol solution dropwise, adjust pH=9-10, react at 55-65℃ for 3-5h to obtain BaTiO3 precursor coated material; S4 gradient oxygen vacancy construction: The above material was heated to 680-720℃ in a reducing atmosphere of Ar and H2 and held for 2-4 hours; then the atmosphere was switched to Ar and oxygen was introduced while cooling down to 400℃ and the oxygen partial pressure was gradually increased. S5 Electric field polarization treatment: The material obtained in step S4 is placed between parallel plate electrodes and heated to 140-160℃; a DC electric field of 5-6kV / cm is applied and held for 30-50min; the material is cooled to room temperature while maintaining the electric field, and finally a double-doped ternary cathode material with a polarized BaTiO3 coating layer with gradient oxygen vacancy concentration is obtained.

2. The method for preparing the composite modified cathode material for sulfide solid electrolytes as described in claim 1, characterized in that, In step S1, the molar ratio of Ni, Co, Zr, and Ti in the mixed salt solution is 0.83:0.12:0.03:0.

02.

3. The method for preparing the composite modified cathode material for sulfide solid electrolytes as described in claim 1, characterized in that, In step S1, the nickel salt used in the mixed salt solution is nickel sulfate, the cobalt salt is cobalt sulfate, the zirconium salt is zirconium sulfate, and the titanium salt is titanium sulfate.

4. The method for preparing the composite modified cathode material for sulfide solid electrolytes as described in claim 1, characterized in that, In step S1, the concentration of the mixed salt solution is 2-3 mol / L.

5. The method for preparing the composite modified cathode material for sulfide solid electrolytes as described in claim 1, characterized in that, In step S2, the lithium source is lithium hydroxide, which is added at a Li:Me molar ratio of 1.05:

1.

6. The method for preparing the composite modified cathode material for sulfide solid electrolytes as described in claim 1, characterized in that, In step S3, the molar ratio of the dual-doped matrix material to Ba and Ti is 1:0.02:0.

02.

7. The method for preparing the composite modified cathode material for sulfide solid electrolytes as described in claim 1, characterized in that, In step S4, the volume ratio of Ar to H2 in the reducing atmosphere is 95:

5.

8. The method for preparing the composite modified cathode material for sulfide solid electrolytes as described in claim 1, characterized in that, In step S4, the oxygen partial pressure is controlled to increase at the following gradient: 700-600℃, oxygen partial pressure = 10 -5 atm; 600-500℃, oxygen partial pressure = 10 -4 atm; 500-400℃, oxygen partial pressure = 10 -3 atm; below 400℃, switch to pure Ar.

9. A composite modified cathode material for sulfide solid electrolytes, characterized in that, It is prepared by the method for preparing composite modified cathode material for sulfide solid electrolyte as described in any one of claims 1-8.

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