Gradient oxide-coated positive electrode material, preparation method thereof and application of full solid-state battery

Abstract

The application discloses a gradient oxide-coated positive electrode material, a preparation method thereof and application of the material in a full-solid-state battery, and belongs to the field of positive electrode materials of full-solid-state lithium batteries. X Ti X O3 phase, and a preparation method adopts step-by-step precursor deposition and two-stage heat treatment, the material effectively inhibits the interface side reaction between the positive electrode and the sulfide electrolyte, significantly reduces the interface impedance, and improves the cycle stability.

Description

Technical Field

[0001] This invention belongs to the field of cathode materials for all-solid-state lithium batteries, specifically relating to oxide-coated cathode materials with gradient structures and their preparation methods, as well as the application of this material in all-solid-state batteries with sulfide solid electrolyte systems. More specifically, this invention relates to a technical solution for constructing a niobium-titanium gradient oxide coating layer through stepwise solution deposition and two-stage heat treatment, falling within the technical scope of surface modification and interface engineering of electrochemical energy storage materials. Background Technology

[0002] All-solid-state lithium batteries replace traditional organic liquid electrolytes with solid electrolytes, fundamentally eliminating safety hazards such as electrolyte leakage, combustion, and explosion. Simultaneously, the wider electrochemical window of solid electrolytes allows for the use of higher-voltage cathode materials and metallic lithium anodes, potentially pushing battery energy density above 500 Wh / kg, making it a core development direction for next-generation high-energy-density energy storage technology. Among various solid electrolyte systems, sulfide solid electrolytes stand out due to their room-temperature ionic conductivity (up to 10⁻⁶ Wh / kg), comparable to liquid electrolytes. -2 With its properties on the order of S / cm and good mechanical ductility, it is considered the electrolyte material most likely to be the first to achieve industrial application in all-solid-state batteries.

[0003] High-nickel ternary layered cathode material LiNi0.8Co0.1Mn0.1O2 has become one of the preferred cathode materials for all-solid-state batteries due to its high specific capacity (theoretical capacity of about 275 mAh / g) and high operating voltage. However, there is a serious interfacial instability problem between high-nickel cathode materials and sulfide solid electrolytes, mainly manifested in the following three aspects. First, the space charge layer effect: due to the Li in high-nickel cathode materials... + The chemical potential of Li is significantly lower than that of sulfide electrolytes during charging. + A large amount of material migrates from the electrolyte side to the positive electrode side, forming a lithium-poor, high-resistivity space charge layer on the electrolyte side of the interface, leading to a sharp increase in interfacial impedance. Secondly, interfacial chemical side reactions occur: the electrochemical stability window of sulfide electrolytes is relatively narrow (approximately 1.6 to 2.3 V vs. Li). + / Li), at the high charging potential of the high-nickel cathode, S in the electrolyte 2- Oxidation produces insulating byproduct layers such as polysulfides, sulfates, and phosphates, further deteriorating ion transport kinetics. Third, element interdiffusion: During charge-discharge cycles, transition metal ions (Ni...) on the positive electrode side... 2+ Co 3+ Mn 4+The interfacial interdiffusion of P and S elements on the cathode and electrolyte sides leads to an irreversible transformation of the cathode surface structure from a layered to a rock-salt phase, while the conductivity on the electrolyte side decreases due to compositional deviation. The synergistic effect of these interfacial failure mechanisms results in rapid capacity decay and continuous impedance increase in all-solid-state batteries.

[0004] To address the cathode / sulfide electrolyte interface problem, a widely adopted strategy is to coat the cathode particles with an oxide protective layer. Patent CN102017244A discloses a cathode active material coated with an impedance-reducing coating and an all-solid-state lithium secondary battery using it, leveraging the high ionic conductivity of LiNbO3 to reduce the interfacial resistance between the cathode and the sulfide electrolyte. Patent CN103999275A further discloses a composite active material, an all-solid-state battery, and a method for manufacturing the composite active material. Patent CN111864256B discloses a sulfide solid electrolyte and an all-solid-state lithium secondary battery. At the academic research level, researchers have also explored Al2O3, Li2ZrO3, and Li4Ti5O3. 12 Oxides are used as coating materials to modify the cathode surface through processes such as atomic layer deposition (ALD), sol-gel method, and mechanical fusion. While ALD can achieve sub-nanometer thickness control and excellent coating uniformity, the expensive equipment and limited production capacity make it difficult to meet the demands of large-scale power battery production. The sol-gel method is simple and low-cost, but traditional single-step deposition processes can only form single-component coating layers, failing to construct composite structures with gradient functions. Mechanical fusion allows for dry coating, avoiding solvent use, but its coating uniformity and thickness accuracy are inferior to liquid-phase methods.

[0005] Regarding the selection of coating materials, LiNbO3 is one of the most extensively studied cathode coating materials to date, particularly in its amorphous Li... + The ionic conductivity is approximately 10. -6 S / cm, significantly higher than Al2O3 (approximately 10). -14 S / cm) and ZrO2 (approximately 10 -12 Insulating oxides such as LiNbO3 (S / cm). However, the electrochemical oxidation potential of LiNbO3 is approximately 3.8 V vs. Li + Li, even when charged to above 4.3 V using a high-nickel cathode, still faces the risk of oxidative degradation. LiTiO3 series coated materials exhibit better chemical and electrochemical oxidation stability than LiNbO3, but their bulk Li... +The ionic conductivity is low. Therefore, it is difficult to perfectly meet the multidimensional requirements of the cathode interface of all-solid-state batteries by using any single coating material alone. In recent years, the concepts of composite coating and multilayer coating have been proposed to overcome the limitations of single coating materials. However, most existing solutions are simple double-layer stacking or mechanical mixing, which have not yet achieved a truly continuous gradient transition of components from the cathode matrix to the electrolyte interface, and cannot eliminate the chemical potential steps between layers at the molecular level.

[0006] However, existing coating technologies generally suffer from the following limitations: single-component oxide coatings cannot simultaneously meet the requirements of high ionic conductivity and high chemical stability. Although LiNbO3 has high ionic conductivity, its chemical potential difference with sulfide electrolytes remains large, leading to side reactions at the coating-electrolyte interface during long-term cycling. Al2O3 and ZrO2, while chemically stable, have extremely low ionic conductivity, significantly increasing interfacial impedance. Monolayer uniform coating structures cannot satisfy different interfacial requirements on the cathode and electrolyte sides respectively. Furthermore, traditional wet coating processes struggle to precisely control the microstructure and component distribution of the coating layer, resulting in insufficient batch-to-batch stability of the coating effect. From an interface science perspective, an ideal cathode surface protective layer should have a lattice matching degree similar to that of the cathode material and high ionic conductivity on the side adjacent to the cathode substrate to ensure unimpeded charge transport, while simultaneously possessing a wide electrochemical window and high chemical inertness on the electrolyte-facing side to resist oxidative corrosion by the electrolyte. These two functional requirements present a thermodynamic contradiction: high ionic conductivity typically implies a more open crystal structure and lower lattice energy, while high chemical stability requires tight atomic packing and strong chemical bonding. Traditional single-component coatings cannot reconcile this contradiction, and simple double-layer stacking introduces new chemical potential steps at the interface between the two layers. Therefore, there is an urgent need to develop a novel cathode surface modification technology capable of achieving multifunctional gradient distribution within the same coating layer to systematically address the multidimensional failure problem at the cathode / sulfide electrolyte interface in all-solid-state batteries. Summary of the Invention

[0007] To address the technical problem that single-component oxide coatings in existing technologies cannot simultaneously meet the requirements of high ionic conductivity and high chemical stability, and the technical deficiency that traditional coating structures cannot adapt to the differentiated interface requirements of the cathode side and the electrolyte side, the present invention aims to provide an oxide-coated cathode material with a gradient composition structure, its preparation method, and its application in all-solid-state batteries. By constructing a bifunctional gradient coating layer with continuously changing composition from the inside to the outside on the surface of the cathode particles, the invention simultaneously solves three key interface problems: space charge layer suppression, interfacial side reaction blocking, and ion transport channel construction.

[0008] A first aspect of the present invention provides a gradient oxide-coated cathode material, comprising a cathode active material core and a gradient oxide coating layer covering the surface of the core. The cathode active material core is a high-nickel ternary layered cathode material, LiNi. a Co b Mn c O2, where 0.7≤a≤0.9, 0.05≤b≤0.15, 0.05≤c≤0.15, and a+b+c=1. The gradient oxide coating layer consists of an inner transition sublayer near the core and an outer stable sublayer located outside the inner transition sublayer. The thickness of the inner transition sublayer is 2 to 5 nm, mainly composed of Li3NbO4 crystal phase, wherein the content of Li3NbO4 crystal phase is not less than 80% of the total mass of the sublayer. The thickness of the outer stable sublayer is 3 to 15 nm, mainly composed of LiNb 1-x Ti x The structure consists of an O3 crystalline phase, where 0.2 ≤ x ≤ 0.5. The total thickness of the gradient oxide coating layer ranges from 5 to 20 nm.

[0009] The second aspect of this invention provides a method for preparing the above-mentioned gradient oxide-coated cathode material, comprising the following steps: dispersing high-nickel ternary cathode material powder in anhydrous ethanol; sequentially adding a niobium-containing precursor and a titanium-containing precursor under an inert atmosphere for stepwise adsorption deposition; centrifuging and vacuum drying followed by mixing with a lithium source; and performing a two-stage heat treatment under an oxygen-containing atmosphere, wherein the first stage at a lower temperature promotes preferential nucleation of the Li3NbO4 crystal phase in the inner transition sublayer, and the second stage at a higher temperature completes the formation of LiNb in the outer stable sublayer. 1-x Ti x O3 crystallization. The synergistic combination of stepwise precursor deposition and two-stage heat treatment ensured the precise construction of the gradient composition structure. In the stepwise deposition process, the first niobium-containing precursor formed the first adsorption film on the surface of the cathode particles. The subsequently added titanium-containing precursor, due to the niobium-containing species occupying the surface active sites, naturally deposited on the outside of the first layer. Thus, the spatial distribution of niobium inside and titanium outside was achieved by utilizing the kinetic difference of the sequential deposition. The niobium-containing precursor was selected from ammonium niobate or niobium pentaethoxy, and the titanium-containing precursor was selected from tetrabutyl titanate or isopropyl titanate. The niobium source can be uniformly adsorbed at a lower temperature, while the titanium source needs to complete hydrolysis and condensation at a slightly higher temperature. The two-step temperature increment design further enhanced the layered deposition effect. The mixing process of the dried precursor-coated powder and the lithium source was carried out in an inert glove box to avoid the deterioration effect of ambient moisture on the cathode material and the precursor layer. The amount of lithium source added is precisely controlled according to the molar ratio of cathode material to lithium source of 1:0.02 to 1:0.06. Excess lithium can promote the full crystallization of lithium oxide in the coating layer and fill the residual pores, but excessive amount may cause residual Li2O or Li2CO3 impurities to remain on the surface of the coating layer.

[0010] A third aspect of the present invention provides an all-solid-state battery, comprising a positive electrode containing the above-described gradient oxide-coated positive electrode material, a sulfide solid electrolyte, and a negative electrode.

[0011] Compared with the prior art, the present invention has the following beneficial effects. First, the gradient composition design achieves multifunctional synergy: the Li3NbO4 crystal phase in the inner transition sublayer has a high Li content. + Ionic conductivity (not less than 1×10) -6 (S / cm), which can build a fast ion transport channel at the cathode / coating interface, effectively reducing the interfacial charge transfer impedance; LiNb in the outer stable sublayer 1- x Ti x O3 composite crystal phase through Ti 4+ For Nb 5+ Partial substitution significantly improves lattice structure stability and oxidation resistance, with an electrochemical oxidation potential of not less than 4.5 V vs. Li. + / Li effectively blocks the oxidative decomposition and interdiffusion of elements in sulfide electrolytes. Secondly, a continuous chemical potential gradient eliminates the space charge layer: the gradient oxide coating layer achieves Li-based space charge elimination from the core to the outer surface. + The continuous and smooth transition of chemical potential avoids the formation of a high-resistivity space charge layer between a single coating layer and the substrate or electrolyte due to abrupt changes in chemical potential. Compared with the traditional LiNbO3 single-layer coating scheme, the interfacial impedance is reduced by 70% to 85%. Third, a two-stage heat treatment ensures structural controllability: the first stage, a low-temperature treatment, utilizes the relatively low crystallization temperature of Li3NbO4 to preferentially nucleate and grow a dense inner layer on the surface of the cathode particles; the second stage, a high-temperature treatment, promotes the growth of LiNb on the outer side of the already formed inner layer. 1-x Ti x The epitaxial crystallization of O3 and the temperature window difference of the two-stage heat treatment ensure precise control of the bilayer gradient structure. Fourth, excellent electrochemical performance: After assembling an all-solid-state battery with the obtained gradient oxide-coated cathode material and sulfide solid electrolyte, the initial discharge specific capacity at 0.5C rate can reach 188 to 198 mAh / g, and the capacity retention rate after 100 cycles is not less than 92%. The discharge specific capacity at 1C rate can still be maintained above 170 mAh / g, demonstrating excellent rate performance and cycle stability. Fifth, good process economy and suitable for industrial promotion: The stepwise solution deposition process adopted in this invention only requires conventional stirred reactor equipment. The precursor and solvent are both inexpensive and readily available industrial chemicals. The maximum temperature of the two-stage heat treatment does not exceed 550°C, and the energy consumption is much lower than that of traditional high-temperature sintering coating schemes. A single batch can process kilogram-level cathode powder with good batch-to-batch consistency, which has significant cost advantages and industrial promotion potential. Attached Figure Description

[0012] Figure 1The image shown is a HAADF-STEM cross-sectional image of the gradient oxide-coated cathode material prepared in Example 1 of this invention.

[0013] Figure 2 This is an EDS elemental line scan distribution diagram of the gradient oxide coating layer in Embodiment 1 of the present invention.

[0014] Figure 3 Comparison of GIXRD diffraction patterns of the gradient oxide-coated cathode material and the uncoated cathode material prepared in Example 1 of this invention.

[0015] Figure 4 The graph shows a comparison of the first charge-discharge curves of Examples 1 to 4 and Comparative Examples 1 to 4 of the present invention at a rate of 0.5C.

[0016] Figure 5 This is a comparison chart of the cycling performance of Examples 1 to 4 and Comparative Examples 1 to 4 of the present invention at a 0.5C rate.

[0017] Figure 6 The Nyquist plots are the initial electrochemical impedance spectroscopy spectra of the all-solid-state batteries of Example 1 and Comparative Examples 1 to 3 of this invention.

[0018] Figure 7 The images show a comparison of the FIB-SEM cross-sections of the positive electrode composite layers after 100 cycles in Example 1 and Comparative Example 1 of this invention. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0020] Example 1

[0021] This embodiment provides a method for preparing a gradient oxide coated cathode material, using LiNi0.8Co0.1Mn0.1O2 as the core of the cathode active material. The specific preparation process is as follows.

[0022] Take 20 g of commercially available LiNi0.8Co0.1Mn0.1O2 cathode powder (D 50=11 μm, primary particle size approximately 300 nm), transferred to a 500 mL three-necked flask, 250 mL of anhydrous ethanol added, and mechanically stirred under an argon atmosphere to fully disperse the powder and form a uniform suspension with a concentration of approximately 80 g / L. 0.186 g of ammonium niobate (calculated based on niobium accounting for 1.5% of the total molar amount of transition metals in the cathode material) was weighed and dissolved in 20 mL of anhydrous ethanol to prepare a niobium source solution. Under a stirring speed of 300 r / min, the niobium source solution was added to the suspension dropwise at a constant rate. The system temperature was raised to 55°C and stirred for 1.5 h, allowing the niobium-containing precursor molecules to be uniformly anchored to the hydroxyl active sites on the cathode particle surface through van der Waals forces and chemisorption. During this process, the niobium-containing hydroxyl species generated by the hydrolysis of ammonium niobate preferentially accumulate and deposit at defect sites and grain boundaries on the cathode particle surface, forming a niobium-rich precursor inner layer.

[0023] Subsequently, 0.107 g of tetrabutyl titanate (calculated based on titanium accounting for 0.8% of the total molar amount of transition metals in the cathode material) was weighed and dissolved in 15 mL of anhydrous ethanol. The titanium source solution was added dropwise to the above-mentioned niobium-containing precursor suspension at a constant rate, and the temperature was raised to 65°C and stirred for 2.5 h. Tetrabutyl titanate underwent a hydrolysis-condensation reaction in the ethanol solution, forming a titanium-containing precursor outer layer on the outside of the deposited niobium-containing precursor layer. Because the hydrolysis rate of tetrabutyl titanate is lower than that of ammonium niobate, and the niobium-containing precursor had already occupied the preferential adsorption sites on the cathode particle surface upon addition, the titanium-containing precursor naturally deposited on the outside of the niobium-containing precursor layer, fundamentally achieving a gradient precursor distribution of niobium inside and titanium outside.

[0024] The above suspension was centrifuged at 4000 r / min for 15 min, the supernatant was discarded, and the resulting precipitate was transferred to a vacuum drying oven and dried at 80°C for 12 h to obtain the precursor-coated powder. The precursor-coated powder was then uniformly mixed and ground with LiOH·H2O at a molar ratio of cathode material to LiOH·H2O of 1:0.035. The mixing process was carried out in an inert glove box to avoid the introduction of moisture.

[0025] The mixed powder was evenly spread in an alumina crucible and placed in a tube furnace for two-stage heat treatment in a dry air atmosphere. The first stage involved heating to 300°C at a rate of 2°C / min and holding for 1 h. At this temperature, the lithium source reacted with the niobium-containing precursor to form the Li3NbO4 crystalline phase. Since the crystallization temperature of Li3NbO4 (approximately 280 to 320°C) is lower than that of LiNb... 1-x Ti xThe crystallization temperature of O3 (approximately 420 to 550°C) is used in the first stage of heat treatment, which selectively promotes the transformation of the niobium-containing precursor layer adjacent to the substrate on the surface of the cathode particles into a dense inner transition sublayer of Li3NbO4, while the outer titanium-containing precursor layer remains amorphous. In the second stage, the temperature is increased from 300°C to 500°C at a rate of 3°C / min and held for 3 hours. At this higher temperature, the outer amorphous titanium-containing precursor reacts with the residual niobium-containing precursor and the lithium source to form a stable outer sublayer of LiNb0.6Ti0.4O3 composite oxide crystals on the outer side of the already crystallized Li3NbO4 inner layer. After natural cooling to room temperature, the product is removed, yielding the cathode material with a gradient oxide coating.

[0026] like Figure 1 As shown, characterization using high-angle annular dark-field transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS line scan) revealed a total coating thickness of approximately 12 nm, with an inner transition sublayer of approximately 3 nm and an outer stable sublayer of approximately 9 nm. Selected area electron diffraction (SAED) analysis confirmed that the inner layer is primarily composed of the Li3NbO4 crystalline phase, while the outer layer is composed of the LiNb0.6Ti0.4O3 crystalline phase. Figure 2 As shown in the EDS elemental distribution diagram, the Nb concentration decreases from the cathode substrate towards the outer surface of the coating layer, while the Ti concentration increases from the outer surface, confirming the successful construction of the gradient composition structure. X-ray photoelectron spectroscopy (XPS) further confirmed the presence of Nb. 5+ and Ti 4+ Valence distribution and chemical environment in the coating layer.

[0027] like Figure 3As shown, further phase analysis of the gradient-coated cathode material was performed using X-ray diffraction (XRD). Due to the nanometer-level thickness of the coating layer, no obvious diffraction peaks of the coating layer crystal phase were observed in the conventional XRD pattern. The diffraction peak positions and peak intensity ratios of the hexagonal layered structure of the cathode material were basically consistent with those before coating, with a peak intensity ratio of (003) / (104) of approximately 1.45, indicating that the coating process did not damage the layered structure of the cathode material. Grazing incidence X-ray diffraction (GIXRD) was used to enhance the detection of the surface layer at an incident angle of 0.5°. Weak diffraction signals were observed at 2θ = 23.5° and 30.8°, corresponding to the (111) crystal plane of Li3NbO4 and the (110) crystal plane of LiNb0.6Ti0.4O3, respectively, confirming the coexistence of the two crystal phases in the coating layer. Thermogravimetric-differential scanning calorimetry (TG-DSC) analysis showed that the exothermic peak temperature of the gradient-coated cathode material in the 200-350°C range was delayed by approximately 25°C compared to the uncoated material, and the total heat release was reduced by 18%, indicating that the gradient coating significantly improved the thermal stability of the cathode material. In specific surface area determination, the BET specific surface area of ​​the coated material was 0.58 m². 2 / g, compared to 0.52 m before coating 2 The slight increase in g indicates that the introduction of the nanoscale coating layer creates a micro-rough structure on the particle surface, which helps to increase the effective contact area between the positive electrode and the electrolyte.

[0028] Electron energy loss spectroscopy (EELS) analysis was performed on the gradient-coated cathode material prepared in Example 1. The fine structure of the Nb M4,5 edge in the inner transition sublayer region showed good agreement with the standard Li3NbO4 reference spectrum. Simultaneously, signals from both the Nb M4,5 and Ti L2,3 edges were observed in the outer stable sublayer region, and the fine structure of the Ti L2,3 edge was consistent with the standard perovskite lithium titanate reference spectrum, further confirming the crystal phase assignment of the two sublayers. High-resolution TEM images showed an epitaxial orientation relationship between the inner transition sublayer and the cathode substrate. The (100) crystal plane of Li3NbO4 was parallel to the (003) layered plane of the cathode material. This epitaxial growth characteristic is beneficial to Li... + Rapid ion transport at the cathode / coating interface. The outer stabilizing sublayer exhibits a polycrystalline structure with grain sizes of approximately 3 to 5 nm. Its dense packing without significant porosity forms an effective physical barrier against the diffusion of harmful substances from the electrolyte.

[0029] Example 2

[0030] The difference between this embodiment and Embodiment 1 lies in the types and amounts of niobium-containing and titanium-containing precursors, as well as the adjustment of heat treatment parameters. LiNi0.8Co0.1Mn0.1O2 was used as the cathode core. The niobium-containing precursor was niobium pentaethoxy (calculated based on niobium accounting for 2.5% of the total transition metal content), and the titanium-containing precursor was isopropyl titanate (calculated based on titanium accounting for 1.5% of the total transition metal content). The niobium source deposition temperature was 50°C, with a stirring time of 2 h; the titanium source deposition temperature was 70°C, with a stirring time of 3 h. The vacuum drying temperature was 90°C. Li2CO3 was used as the lithium source, with a molar ratio of cathode material to Li2CO3 of 1:0.05. The two-stage heat treatment parameters were: the first stage was held at 280°C for 1.5 h (heating rate 1°C / min), and the second stage was held at 450°C for 4 h (heating rate 2°C / min), with a pure oxygen atmosphere.

[0031] The total thickness of the resulting coating layer was approximately 18 nm, with an inner transition layer thickness of approximately 5 nm and an outer stabilizing layer thickness of approximately 13 nm. The outer stabilizing layer was composed of LiNb0.5Ti0.5O3, exhibiting a higher Ti content than Example 1, thus contributing to superior chemical stability. TEM-EDS characterization confirmed a continuous gradient in the Nb / Ti ratio from the substrate to the outer surface of the coating layer. The use of niobium pentaethoxy as the niobium source resulted in superior solubility and hydrolysis reactivity in anhydrous ethanol compared to ammonium niobate, leading to more uniform Li3NbO4 grain size (approximately 2 to 3 nm) and higher crystallinity in the inner transition layer. Furthermore, heat treatment under a pure oxygen atmosphere, compared to dry air, helped suppress residual carbon impurities and Li2CO3 residues on the cathode material surface, further purifying the coating layer / substrate interface. This resulted in a slightly higher initial interfacial impedance in Example 2 (38 Ω·cm) due to the thicker coating layer compared to Example 1. 2 vs. 32 Ω·cm 2 However, the impedance growth rate is lower during long cycles (0.28 Ω·cm). 2 / cycle vs. 0.36 Ω·cm 2 The thicker outer stable sublayer ( / cycle) demonstrates the advantage of long-term chemical protection. In 200 cycles, Example 2 showed a capacity retention of 89.5%, slightly better than Example 1's 88.2%, supporting the above conclusion.

[0032] Example 3

[0033] The difference between this embodiment and Embodiment 1 lies in the adjustment of the cathode material composition and the coating layer thickness parameters. The cathode active material core uses LiNi0.7Co0. 15 Mn0. 15 O2 (D) 50=10 μm). The niobium-containing precursor is ammonium niobate (niobium element content 0.5%), and the titanium-containing precursor is tetrabutyl titanate (titanium element content 0.3%). The lithium source is LiOH·H2O, with a molar ratio of 1:0.02. The two-stage heat treatment parameters are: the first stage is held at 310°C for 0.5 h, and the second stage is held at 530°C for 2 h. The total thickness of the resulting coating layer is approximately 6 nm, the inner transition sublayer thickness is approximately 2 nm, the outer stable sublayer thickness is approximately 4 nm, and the outer layer composition is LiNb0.7Ti0.3O3. This embodiment is a scheme near the lower limit of the coating layer thickness. Although the coating layer is thin, it still maintains a complete bilayer gradient structure. The lower precursor dosage and shorter heat treatment time further reduce the preparation cost, which is advantageous in application scenarios where cost-effectiveness is pursued. TEM characterization shows that the 6 nm ultrathin gradient coating layer forms a continuous and dense coverage on the particle surface, with no obvious missing areas at the edges. In electrochemical tests, the ultrathin coating scheme still showed a significant performance improvement compared to the uncoated scheme in Comparative Example 3: the 0.5C discharge specific capacity increased from 135 mAh / g to 190 mAh / g, and the capacity retention rate after 100 cycles increased from 52.8% to 92.1%, proving that even with an extremely thin coating thickness, the gradient structure can still play an effective interface protection role.

[0034] Example 4

[0035] The difference between this embodiment and Embodiment 1 is the use of a cathode material with a higher nickel content. The core of the cathode active material is LiNi0.9Co0. 05 Mn0. 05 O2 (D) 50 =12 μm), this ultra-high nickel cathode material has a higher specific capacity, but the interface stability challenge is more severe. The niobium-containing precursor is niobium pentaethoxy (niobium element content 3%), and the titanium-containing precursor is isopropyl titanate (titanium element content 2%). The lithium source is LiOH·H2O, with a molar ratio of 1:0.06. The two-stage heat treatment parameters are: the first stage is 320°C for 1 h, and the second stage is 550°C for 2 h. The total thickness of the resulting coating layer is about 20 nm, the inner transition sublayer thickness is about 5 nm, the outer stabilizing sublayer thickness is about 15 nm, and the outer layer composition is LiNb0.5Ti0.5O3. The thicker coating layer and the higher Ti content provide enhanced interface protection for the ultra-high nickel cathode. In ultra-high nickel cathode materials, due to the Ni content as high as 90%, Ni during charging... 2+ →Ni 3+ →Ni 4+Oxidation leads to greater lattice volume changes and stronger surface activity, thus placing higher demands on the chemical protection and mechanical toughness of the coating layer. This embodiment addresses these challenges by increasing the total coating thickness to an upper limit of 20 nm and increasing the Ti content in the outer stabilizing sublayer to 50%. Electrochemical test results show that Example 4 achieves a discharge specific capacity of up to 218 mAh / g at 0.1C, which is close to the theoretical utilization limit of ultra-high nickel cathodes, while the 92.5% capacity retention rate after 100 cycles is excellent among ultra-high nickel cathode all-solid-state battery systems.

[0036] Comparative Example 1

[0037] This comparative example uses a traditional single-layer LiNbO3 coating scheme. The same LiNi0.8Co0.1Mn0.1O2 as in Example 1 was used as the positive electrode core, with only a niobium-containing precursor (ammonium niobate, 2.3% niobium, equivalent to the total niobium and titanium content in Example 1) added; no titanium-containing precursor was added. The deposition temperature was 55°C, and stirring was performed for 1.5 h. The lithium source was LiOH·H2O, with a molar ratio of 1:0.035. A single-stage heat treatment was used: heating to 500°C at 3°C / min in dry air and holding for 3 h. The resulting coating layer was a single LiNbO3 crystalline phase, approximately 12 nm thick, with no gradient structure.

[0038] Comparative Example 2

[0039] This comparative example uses a traditional single-layer Li₂ZrO₃ coating scheme. Using the same cathode material as in Example 1 as the core, zirconium acetate (2.3% zirconium element content) was dissolved in anhydrous ethanol and stirred at 65°C for 2 h. The lithium source was LiOH·H₂O, with a molar ratio of 1:0.035. The heat treatment was performed at 500°C for 3 h. The resulting coating layer was a single Li₂ZrO₃ crystalline phase with a thickness of approximately 10 nm.

[0040] Comparative Example 3

[0041] This comparative example uses uncoated bare cathode material. The same batch of LiNi0.8Co0.1Mn0.1O2 cathode material as in Example 1 was used directly without any surface coating treatment.

[0042] Comparative Example 4

[0043] This comparative example employs a uniform Nb-Ti co-deposition scheme (non-gradient structure). Unlike Example 1, niobium-containing and titanium-containing precursors were pre-mixed and simultaneously added to the cathode suspension, and Nb / Ti co-deposition was achieved by stirring at 60°C for 2 h. Subsequent lithiation and heat treatment were the same as in Example 1. The resulting coating layer is a single-component LiNb0.6Ti0.4O3 layer with uniform Nb and Ti distribution, lacking gradient structure characteristics.

[0044] The cathode materials prepared in the above embodiments and comparative examples, and the Li6PS5Cl sulfide solid electrolyte (with an ionic conductivity of approximately 3.5 × 10⁻⁶ at room temperature) were used. -3 Electrochemical performance tests were conducted on all-solid-state batteries assembled in an argon-filled glove box using materials with a positive electrode (S / cm) and a lithium-indium alloy anode. The positive electrode composite layer was prepared by thoroughly mixing and grinding the positive electrode material, Li6PS5Cl electrolyte powder, and conductive carbon in an agate mortar at a mass ratio of 70:27:3. A suitable amount of the positive electrode composite powder, along with the pure Li6PS5Cl electrolyte layer and the lithium-indium alloy anode, was then sequentially cold-pressed in a stainless steel mold under a pressure of 300 MPa to obtain a 10 mm diameter sheet-like all-solid-state battery.

[0045] Charge-discharge tests were conducted at a constant temperature of 25°C, with a voltage range of 2.5 to 4.3 V vs. Li. + / Li (equivalent to approximately 1.88 to 3.68 V vs. Li-In alloys) was tested using a constant current charge-discharge mode. Rate performance was tested after 5 charge-discharge cycles each at 0.1C, 0.2C, 0.5C, 1C, and 2C, followed by recovery to 0.1C. Cycle stability was tested after 100 consecutive charge-discharge cycles at 0.5C. Electrochemical impedance spectroscopy (EIS) was performed at open-circuit potential with a frequency range of 10 Hz. 6 Up to 10 -2 Hz, with a disturbance voltage amplitude of 10 mV.

[0046] like Figure 4 and Figure 5 As shown, the key electrochemical performance data for each embodiment and comparative example are listed below. The gradient oxide-coated cathode material of Example 1 exhibits an initial discharge specific capacity of 210 mAh / g at 0.1C, an initial coulombic efficiency of 89.2%, a discharge specific capacity of 195 mAh / g at 0.5C, a discharge specific capacity of 180 mAh / g at 1C, a discharge specific capacity of 158 mAh / g at 2C, and a capacity retention of 94.5% after 100 cycles at 0.5C. Figure 6 As shown, the initial value of the interface impedance (charge transfer resistance Rct fitted by EIS) is 32 Ω·cm. 2 After 100 cycles, the value is 68 Ω·cm. 2 .

[0047] The coated cathode material of Example 2 exhibited an initial discharge specific capacity of 208 mAh / g at 0.1C, 192 mAh / g at 0.5C, and 176 mAh / g at 1C. After 100 cycles at 0.5C, the capacity retention was 93.8%, and the initial Rct was 38 Ω·cm. 2The coated cathode material of Example 3 exhibited an initial discharge specific capacity of 206 mAh / g at 0.1C and 190 mAh / g at 0.5C. After 100 cycles at 0.5C, the capacity retention was 92.1%, and the initial Rct was 42 Ω·cm. 2 The ultra-high nickel coated cathode material of Example 4 exhibited an initial discharge specific capacity of 218 mAh / g at 0.1C and 198 mAh / g at 0.5C. After 100 cycles at 0.5C, the capacity retention was 92.5%, and the initial Rct was 35 Ω·cm. 2 .

[0048] The conventional LiNbO3 monolayer coated cathode material in Comparative Example 1 exhibits an initial discharge specific capacity of 198 mAh / g at 0.1C and 178 mAh / g at 0.5C. After 100 cycles at 0.5C, the capacity retention is 82.3%, and the initial Rct is 78 Ω·cm. 2 The Li₂ZrO₃ monolayer coated cathode material of Comparative Example 2 exhibited an initial discharge specific capacity of 186 mAh / g at 0.1C and 162 mAh / g at 0.5C. After 100 cycles at 0.5C, the capacity retention was 79.6%, and the initial Rct was 115 Ω·cm. 2 The uncoated bare cathode material of Comparative Example 3 exhibited an initial discharge specific capacity of 165 mAh / g at 0.1C and 135 mAh / g at 0.5C. After 100 cycles at 0.5C, the capacity retention was only 52.8%, and the initial Rct was 220 Ω·cm. 2 The Nb-Ti uniformly mixed coated cathode material of Comparative Example 4 exhibited an initial discharge specific capacity of 202 mAh / g at 0.1C and 184 mAh / g at 0.5C. After 100 cycles at 0.5C, the capacity retention was 87.5%, and the initial Rct was 56 Ω·cm. 2 .

[0049] The above data show that the gradient oxide-coated cathode materials of Examples 1 to 4 of this invention are significantly superior to the comparative examples in terms of discharge specific capacity, cycle stability, and interfacial impedance. Specifically, compared with Comparative Example 1, Example 1 shows an increase in 0.5C discharge specific capacity from 178 mAh / g to 195 mAh / g (an increase of 9.6%), an increase in capacity retention after 100 cycles from 82.3% to 94.5% (an increase of 12.2 percentage points), and an increase in initial interfacial impedance from 78 Ω·cm. 2 Reduced to 32 Ω·cm 2 (Reduced by 59%). Compared with the uncoated solution in Comparative Example 3, the improvement is more significant: discharge specific capacity increased by 44.4%, capacity retention increased by 41.7 percentage points, and interface impedance decreased by 85.5%.

[0050] It is worth noting that the gradient coating schemes of Examples 1 to 4 also showed significant advantages over the uniform mixed coating scheme of Comparative Example 4 (e.g., Example 1 vs. Comparative Example 4: capacity retention 94.5% vs. 87.5%, initial impedance 32 Ω·cm). 2 vs. 56 Ω·cm 2 This fully demonstrates the superiority of gradient component distribution over uniform component distribution in terms of interface protection. The gradient structure allows for higher Li content near the cathode substrate. + The high conductivity of the inner layer of Li3NbO4 effectively reduces charge transfer resistance, while the high stability of LiNb near the electrolyte side... 1-x Ti x The outer layer of O3 effectively blocks the oxidative decomposition of sulfide electrolytes. The two sublayers each perform their respective functions and work together to produce an interface optimization effect far exceeding that of uniform mixing and coating.

[0051] Cyclic voltammetry (CV) tests further revealed the enhancing effect of the gradient coating on electrochemical reversibility. At a scan rate of 0.1 mV / s, the gradient-coated cathode material of Example 1 exhibited a sharp and symmetrical redox peak pair near 3.75 V, with a potential difference ΔEp between the oxidation and reduction peaks of 0.12 V, significantly smaller than that of the LiNbO3 monolayer coating scheme in Comparative Example 1 (ΔEp = 0.18 V) and the uncoated scheme in Comparative Example 3 (ΔEp = 0.35 V). This smaller peak potential difference reflects the effective suppression of electrochemical polarization by the gradient coating, indicating that Li... + The transport kinetics at the cathode / coating / electrolyte three-phase interface were significantly improved. In five consecutive CV scans, the peak current decay rate of Example 1 was only 2.3%, while that of Comparative Example 3 was as high as 15.8%, further confirming the protective effect of the gradient coating layer on the reversibility of the cathode active material structure.

[0052] EIS impedance spectroscopy analysis was performed using an equivalent circuit Rs - (Rct / / CPE1) - Wo, where Rs is the bulk resistance, Rct is the charge transfer resistance, CPE1 is the constant-phase element, and Wo is the Warburg diffusion impedance. The initial Rct value in Example 1 was 32 Ω·cm. 2 After 10 cycles, the temperature dropped to 28 Ω·cm. 2 (Attributable to the interface activation effect), it then slowly increased to 68 Ω·cm after 100 cycles. 2 In contrast, the Rct of Comparative Example 1 increased from an initial 78 Ω·cm. 2 It continued to grow rapidly in the cycle, reaching 115 Ω·cm after 10 cycles. 2 After 100 cycles, it reaches 296 Ω·cm. 2The Rct increase was more dramatic in the uncoated scheme of Comparative Example 3, starting at 220 Ω·cm. 2 It exceeded 500 Ω·cm after only 20 cycles. 2 Linear fitting was performed on the curve of Rct changing with the number of cycles. The slope of the impedance increase in Example 1 was 0.36 Ω·cm. 2 / cycle, Comparative Example 1 is 2.18 Ω·cm 2 / cycle, Comparative Example 3 is 14.5 Ω·cm 2 / cycle. The gradient coating layer reduces the impedance growth rate by more than an order of magnitude, which strongly supports the long-term cycle stability of the all-solid-state battery.

[0053] The results of potentiostatic intermittent titration (GITT) showed that the apparent Li of the gradient-coated cathode material in Example 1 was [missing information]. + The diffusion coefficient is 2.8 × 10⁻⁶ throughout the entire charge-discharge range. -11 Up to 5.6×10 -10 cm 2 / s, compared to the LiNbO3 monolayer coating scheme of Comparative Example 1 (1.5×10⁻⁶). -11 Up to 3.2×10 -10 cm 2 The improvement was approximately 80% in speed (s), compared to the uncoated solution in Comparative Example 3 (4.2 × 10⁻⁶). -12 Up to 1.1×10 -10 cm 2 / s) increased by an order of magnitude. Li + The significant increase in diffusion coefficient indicates that the gradient coating not only reduces interfacial impedance but also improves Li₂ properties by reducing the space charge layer thickness and suppressing the formation of interfacial byproduct layers. + Kinetics of cross-interface migration. This kinetic advantage is even more pronounced at high rates (2C): Example 1 has a discharge specific capacity of 158 mAh / g at 2C, equivalent to 75.2% of the 0.1C capacity, while Comparative Example 1 has only 110 mAh / g at 2C (55.6% of the 0.1C capacity), and Comparative Example 3 has only 68 mAh / g at 2C (41.2% of the 0.1C capacity).

[0054] To gain a deeper understanding of the interface protection mechanism of the gradient oxide coating, a systematic interface characterization analysis was performed on the all-solid-state batteries of Example 1 and Comparative Examples 1 to 3 before and after cycling. XPS depth profiling results showed that after 10 cycles, the uncoated cathode material of Comparative Example 3 exhibited a large number of P 2p and S 2p signals on its cathode surface, indicating that P and S elements in the sulfide electrolyte had significantly diffused towards the cathode side, forming a thick layer of byproducts containing polysulfides, phosphates, and sulfates at the interface. Simultaneously, the Ni 2p spectrum on the cathode side showed a significant shift towards lower binding energy after cycling, indicating that Ni... 2+ / Ni 3+ Imbalance and surface rock salt phase transition. These results visually demonstrate the severe chemical degradation process at the cathode / sulfide electrolyte interface without a protective coating.

[0055] XPS analysis of the LiNbO3 monolayer coated cathode material in Comparative Example 1 after cycling showed that the diffusion of P and S elements towards the cathode side was significantly reduced but not completely eliminated, and trace amounts of phosphorus and sulfur compounds were still detected on the outer surface of the coating layer. This indicates that although the single LiNbO3 coating layer can partially block interfacial side reactions, it still exhibits certain chemical reactivity with the sulfide electrolyte. During long-term cycling, the outer surface of the coating layer gradually degrades, forming a thin layer of sulfur-containing byproducts, leading to a slow increase in interfacial impedance. EIS analysis confirmed that the interfacial impedance of Comparative Example 1 increased to 3.8 times its initial value after 100 cycles (from 78 to 296 Ω·cm). 2 Example 1 only increased to 2.1 times the initial value (from 32 to 68 Ω·cm), while Example 1 only increased to 2.1 times the initial value (from 32 to 68 Ω·cm). 2 ).

[0056] XPS depth profiling of the gradient-coated cathode material in Example 1 after 100 cycles showed that the elemental distribution gradients of Nb and Ti within the coating layer remained essentially unchanged, the P and S signals on the surface of the outer stabilizer layer were extremely weak, and the Ni2p spectrum of the cathode substrate showed no significant changes. These results demonstrate that the gradient oxide coating layer maintains excellent structural stability and chemical inertness during long-term cycling. The Ti in the outer stabilizer layer... 4+ By enhancing the covalent nature of the metal-oxygen bond and increasing the lattice energy, the reactive sulfur in sulfide electrolytes is effectively resisted. 2- Chemical erosion; the high ionic conductivity of Li3NbO4 in the inner transition layer ensures the chemical erosion of Li; + Rapid migration at the cathode / coating interface is unaffected by coating thickness.

[0057] Based on density functional theory (DFT) calculations, the Li3NbO4 / LiNi0.8Co0.1Mn0.1O2 interface exhibits Li +The migration barrier is 0.28 eV, significantly lower than the 0.42 eV at the LiNbO3 / LiNi0.8Co0.1Mn0.1O2 interface, explaining the physical mechanism by which the Li3NbO4 phase in the inner transition sublayer effectively reduces the interfacial impedance on the cathode side. On the other hand, the calculated reaction energy (ΔE_rxn) at the LiNb0.6Ti0.4O3 and Li6PS5Cl interface is -0.03 eV / atom, much lower than the -0.12 eV / atom at the LiNbO3 and Li6PS5Cl interface. This indicates that the Ti doping in the outer stabilizing sublayer reduces the thermodynamic driving force of the interfacial reaction between the cathode coating layer and the sulfide electrolyte by 75%, which is the fundamental reason why the gradient coating scheme can effectively suppress interfacial degradation during long-term cycling.

[0058] From the perspective of space charge layer suppression, although traditional monolayer LiNbO3 coating alleviates the Li-electrolyte space charge layer suppression to some extent, + Despite the chemical potential difference, a chemical potential step interface still exists between LiNbO3 and the high-nickel cathode, and between LiNbO3 and the sulfide electrolyte. This means the space charge layer is not completely eliminated but transferred to new interfaces. The gradient coating layer of this invention, through a continuous and gradual change in composition from the cathode substrate to the outer surface, transfers the LiNbO3 that was originally concentrated at a single interface. + The chemical potential difference is dispersed across the entire thickness of the coating layer, resulting in a significant reduction in the local electric field strength. Simulations using the Poisson-Boltzmann equation show that the maximum space charge layer thickness in the gradient coating layer is only 0.8 nm, far smaller than the 3.2 nm of the LiNbO3 monolayer coating scheme and the 8.5 nm of the uncoated scheme. This significant reduction in space charge layer thickness directly corresponds to a substantial decrease in interfacial impedance and a marked improvement in ion transport kinetics observed in experiments.

[0059] In summary, the excellent interface protection effect of the gradient oxide coating layer of this invention stems from a triple synergistic mechanism: firstly, the transition sublayer within Li3NbO4 provides low impedance Li + The transmission channel ensures the charge transfer dynamics on the positive electrode side; secondly, LiNb 1-x Ti x The outer stable sublayer of O3 provides high chemical stability, preventing electrolyte oxidation and decomposition, as well as element interdiffusion; thirdly, the continuous gradient composition distribution eliminates chemical potential steps, compressing the space charge layer thickness to the sub-nanometer level. The combined effect of these three mechanisms enables the all-solid-state battery to maintain stable low interfacial impedance and high capacity output during long-term cycling.

[0060] like Figure 7As shown, to further verify the structural stability and interface evolution behavior of the gradient coating layer, focused ion beam scanning electron microscopy (FIB-SEM) cross-sectional analysis was performed on the all-solid-state batteries of Example 1 and Comparative Example 1 after 100 cycles. The cross-sectional image after cycling of Example 1 shows that the gradient coating layer still completely covers the surface of the cathode particles after cycling, without obvious cracking, peeling or uneven thickness. The interfaces between the coating layer and the cathode substrate, as well as between the coating layer and the electrolyte, are tightly adhered, and no obvious gaps or by-product layers are observed. EDS surface scanning results show that the interdiffusion of elements between the cathode particles and the Li6PS5Cl electrolyte is effectively confined within the thickness range of the gradient coating layer (approximately 12 nm). The diffusion fronts of P and S elements stop at the surface of the outer stable sublayer, and transition metal elements such as Ni, Co, and Mn do not diffuse detectably towards the electrolyte side.

[0061] In contrast, after 100 cycles, the LiNbO3 monolayer coated cathode material in Comparative Example 1 showed cracking and localized detachment of the coating layer on the surface of some cathode particles in FIB-SEM cross-sectional images, especially at particle edges and secondary grain boundaries. In the detached areas, the cathode substrate directly contacted the electrolyte, forming a gray byproduct reaction layer approximately 20 to 50 nm thick. EDS analysis confirmed that this reaction layer was rich in P, S, Ni, and other elements, representing products of the cathode / electrolyte interface chemical reaction. In Comparative Example 3, the uncoated cathode material showed a continuous gray reaction layer of 100 to 200 nm thick on the surface of the cathode particles after only 50 cycles. This reaction layer severely hindered the LiNbO3 electrochemical reaction. + The interface transmission causes rapid degradation of battery performance.

[0062] Time-of-flight secondary ion mass spectrometry (ToF-SIMS) provides highly sensitive analytical information for interfacial chemical reactions. ToF-SIMS analysis of the cathode composite layer after cycling in Example 1 revealed trace amounts of PO3 on the outer surface of the gradient coating layer. - and SO3 - The fragment ion signal (signal intensity only about 1 / 8 of that in Comparative Example 1) indicates that the outer stabilizer layer suppressed the interfacial oxidation reaction to an extremely low degree. (LiF) - and Li2O - Fragment ion signals exhibit a continuous distribution within the coating layer, consistent with the gradient composition structure. Notably, ToF-SIMS detected NbO2 in the transition region between the inner transition sublayer and the outer stable sublayer of the gradient coating layer. - and TiO2 - The cross-gradual characteristics of the signal directly confirm that there is no abrupt interface between the two sublayers but a smooth transition. This component continuity is the structural basis for achieving a smooth distribution of the chemical potential gradient.

[0063] From the perspective of the structural evolution of cathode materials, micro-area Raman spectroscopy and synchrotron X-ray absorption near-edge structure (XANES) tests were performed on the cycled cathode particles. After 100 cycles, the edge-front peak position and main absorption edge shape of the NiK edge XANES spectrum of the gradient-coated cathode material in Example 1 were basically consistent with those before cycling, indicating that the coordination environment and valence state distribution of Ni in the cathode matrix did not change significantly, and the layered structure remained intact. Micro-area Raman spectroscopy detected clear E_g and Al_g vibrational modes on the surface region of the cathode particles, corresponding to a typical layered cathode structure. No characteristic Raman shifts representing rock salt or spinel phases were observed, further confirming the effective protection of the gradient coating layer for the surface structural stability of the cathode material.

[0064] In contrast, the uncoated cathode material in Comparative Example 3, after 50 cycles, exhibited a significant edge-front peak enhancement and a shift of the main absorption edge towards lower energies in its Ni K-edge XANES spectrum, indicating an irreversible structural transformation of the cathode surface layer from a layered phase to a rock-salt phase. 2+ / Ni 3+ The proportion increased significantly. Micro-area Raman spectroscopy detected a newly formed region located at approximately 550 cm⁻¹ on the cathode surface. -1 The broad peak at this location is attributed to a characteristic vibrational mode of the rock salt phase NiO structure, with the surface rock salt phase layer thickness estimated to be approximately 3 to 8 nm. The LiNbO3 monolayer coated cathode material in Comparative Example 1 also exhibited a weak but identifiable rock salt phase Raman signal after 100 cycles, indicating that the monolayer coating's protective effect on the cathode surface structure is not durable. These comparative results, from the perspective of the evolution of the cathode material's bulk structure, verify the superior effectiveness of the gradient oxide coating layer in suppressing the degradation of the cathode surface structure.

[0065] To further verify the universality of gradient oxide-coated cathode materials in different all-solid-state battery systems, Li3PS4 and Li 10 GeP2S 12 Two sulfide solid electrolytes were assembled with the gradient-coated cathode material prepared in Example 1 to form an all-solid-state battery for testing.

[0066] When using Li3PS4 electrolyte (room temperature ionic conductivity approximately 2 × 10⁻⁶), -4 At a rate of S / cm, the initial discharge specific capacity was 198 mAh / g at 0.1C and 178 mAh / g at 0.5C, with a capacity retention of 90.3% after 50 cycles. Although the bulk conductivity of Li3PS4 is lower than that of Li6PS5Cl, the gradient coating effectively suppressed the side reactions at the cathode / Li3PS4 interface, resulting in acceptable cycle performance. When using Li... 10 GeP2S 12Electrolyte (room temperature ionic conductivity approximately 1.2 × 10⁻⁶) -2 At a rate of S / cm, the initial discharge specific capacity is 215 mAh / g at 0.1C and 200 mAh / g at 0.5C, with a capacity retention of 93.1% after 100 cycles. Li 10 GeP2S 12 The ultra-high ionic conductivity fully leverages the capacity advantage of the gradient-coated cathode material, and the all-solid-state battery exhibits electrochemical performance close to that of a liquid battery.

[0067] Furthermore, the coated cathode material from Example 1 was assembled with a Li6PS5Cl electrolyte into a 10-layer stacked pouch-type all-solid-state battery (capacity approximately 0.5 Ah), and charge-discharge tests were conducted at 25°C. This pouch battery exhibited an initial discharge specific capacity of 192 mAh / g at 0.2C rate, and retained 91.2% of its capacity after 50 cycles, demonstrating the transferability of the gradient oxide coated cathode material from laboratory coin cells to engineered pouch batteries. Under high-temperature testing conditions at 55°C, the pouch battery achieved an initial discharge specific capacity of 205 mAh / g at 0.5C rate, and retained 88.5% of its capacity after 30 cycles, indicating that the gradient coating layer effectively suppresses interfacial side reactions even at higher operating temperatures. Under low-temperature testing conditions (-10°C), the pouch battery still released a discharge specific capacity of 135 mAh / g at 0.1C rate, reaching 70% of its room-temperature capacity, demonstrating the good adaptability of the gradient coating layer to a wide temperature range.

[0068] To evaluate the long-cycle performance potential of the gradient-coated cathode material, the battery prepared in Example 1 was subjected to 300 cycles at 0.3C. The test results showed that the capacity retention rate was 94.5% after 100 cycles, 88.2% after 200 cycles, and 82.6% after 300 cycles, with an average capacity decay rate of approximately 0.058% per cycle. In contrast, the LiNbO3 monolayer coating scheme in Comparative Example 1 only retained 58.3% of its capacity after 300 cycles under the same conditions, with an average capacity decay rate of approximately 0.139%, which is 2.4 times that of the gradient coating scheme. The uncoated scheme in Comparative Example 3 had its capacity decayed to less than 50% of its initial capacity after approximately 150 cycles, rendering it unable to continue operating effectively. The long-cycle data further validates the outstanding advantages of the gradient oxide coating layer in maintaining long-term interface stability.

[0069] To address the pressure requirements of all-solid-state batteries in practical applications, comparative tests were conducted on the battery prepared in Example 1 under different applied pressure conditions. At an applied pressure of 5 MPa, the 0.5C discharge specific capacity was 195 mAh / g; at 50 MPa, it increased to 198 mAh / g; and at 150 MPa, it slightly decreased to 193 mAh / g. After 100 cycles under the three pressure conditions, the capacity retention rates were 92.8%, 94.5%, and 91.6%, respectively. The results indicate that the gradient-coated cathode material exhibits stable electrochemical performance and good mechanical adaptability across a wide pressure range of 5 to 150 MPa. The polycrystalline dense structure of the outer stabilizing sublayer in the gradient coating layer endows the coating layer with appropriate mechanical flexibility, allowing it to undergo elastic deformation during cycling along with the volume expansion and contraction of the cathode particles without cracking or detaching, maintaining complete coverage of the cathode particle surface.

[0070] The stepwise solution deposition and two-stage heat treatment process employed in this invention has promising prospects for industrial scale-up. Both niobium-containing and titanium-containing precursors are commercially available chemical raw materials, and only industrial-grade anhydrous ethanol is used as the solvent, ensuring controllable raw material costs. The stepwise solution deposition process can be completed in a conventional stirred reactor, resulting in low equipment investment and operational complexity. The two-stage heat treatment can be carried out continuously in an industrial-grade rotary kiln or pusher kiln, enabling batch preparation of gradient structures through precise temperature program control. In pilot-scale verification, the batch-to-batch consistency of products processed from 5 kg of cathode powder was good, with coating thickness deviation controlled within ±2 nm. The reproducibility of the gradient component distribution was verified by EDS characterization to meet the requirements for industrial production.

[0071] From a cost-effectiveness perspective, based on the process parameters of Example 1, approximately 9.3 g of ammonium niobate and 5.35 g of tetrabutyl titanate are required per kilogram of cathode material. Calculated at current market prices, the cost of coating raw materials increases by approximately RMB 15 to 20 per kilogram of cathode material, representing only about 8% to 10% of the unit price of high-nickel cathode material. However, the overall economic benefits of improved electrochemical performance (specific capacity increased by 9.6%, cycle life increased by over 50%) far outweigh the increase in coating cost. The ethanol solvent in the stepwise solution deposition process can be recovered and recycled through distillation, with a recovery rate exceeding 90%, further reducing production costs and environmental impact. The total time for the two-stage heat treatment is approximately 5 to 6 hours (including heating and holding), significantly reducing energy consumption compared to traditional high-temperature sintering processes, as the first-stage heat treatment temperature is only around 300°C, far lower than the 700 to 900°C sintering temperature required for conventional oxide coating. These characteristics indicate that the technical solution of this invention is perfectly suitable for technology transfer from laboratory research and development to industrial mass production. The core competitiveness of the preparation process of this invention lies in the organic combination of the low cost advantage of traditional wet coating process with the high performance advantage of gradient functional structure. It does not require the introduction of expensive vacuum equipment such as atomic layer deposition or magnetron sputtering. The controllable preparation of nanoscale gradient oxide coating layer can be achieved on ordinary chemical equipment by simply designing the order of precursor addition and the heat treatment temperature program. This opens up a feasible path for the large-scale low-cost production of cathode materials for all-solid-state batteries.

[0072] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A gradient oxide-coated cathode material, comprising a cathode active material core and a gradient oxide coating layer covering the surface of the core, characterized in that: The core of the positive electrode active material is a high-nickel ternary layered positive electrode material LiNi. a Co b Mn c O2, where 0.7≤a≤0.9, 0.05≤b≤0.15, 0.05≤c≤0.15, a+b+c=1; the gradient oxide coating layer consists of an inner transition sublayer near the core and an outer stable sublayer located outside the inner transition sublayer; the thickness of the inner transition sublayer is 2 to 5 nm, mainly composed of Li3NbO4 crystal phase, and the content of the Li3NbO4 crystal phase is not less than 80% of the total mass of the inner transition sublayer; the thickness of the outer stable sublayer is 3 to 15 nm, mainly composed of LiNb 1-x Ti x The structure is composed of O3 crystal phase, wherein 0.2≤x≤0.5; the total thickness of the gradient oxide coating layer is 5 to 20 nm.

2. The gradient oxide coated cathode material according to claim 1, characterized in that, The high-nickel ternary layered cathode material is LiNi0.8Co0.1Mn0.1O2, with a primary particle size of 200 to 500 nm and a secondary particle size D. 50 The size is 8 to 15 μm.

3. The gradient oxide coated cathode material according to claim 1, characterized in that, The Li3NbO4 crystal phase in the inner transition sublayer contains Li + Ionic conductivity not less than 1×10 -6 S / cm, LiNb in the outer stable sublayer 1-x Ti x The electrochemical oxidation potential of the O3 crystal phase is not less than 4.5 V vs. Li. + / Li.

4. The gradient oxide coated cathode material according to claim 1, characterized in that, The total molar amount of niobium and titanium in the gradient oxide coating layer accounts for 1% to 5% of the total molar amount of transition metals in the core of the positive electrode active material, and the molar ratio of niobium to titanium is 1:0.3 to 1:

1.

5. The gradient oxide coated cathode material according to claim 1, characterized in that, When the gradient oxide-coated cathode material is assembled with the Li6PS5Cl sulfide solid electrolyte into an all-solid-state battery, the interfacial impedance between the cathode and the electrolyte is no higher than 50 Ω·cm. 2 The initial discharge specific capacity at 0.5C rate is no less than 188 mAh / g, and the capacity retention rate after 100 cycles is no less than 92%.

6. A method for preparing the gradient oxide-coated cathode material according to any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1: Disperse the high-nickel ternary layered cathode material powder in anhydrous ethanol to form a suspension with a concentration of 50 to 100 g / L. Add a niobium-containing precursor under an inert atmosphere and stir at 50 to 60°C for 1 to 2 hours to allow the niobium-containing precursor to be uniformly adsorbed onto the surface of the cathode particles. The niobium-containing precursor is selected from one or two of ammonium niobate and niobium pentaethoxy. Step 2: Add a titanium-containing precursor to the suspension obtained in Step 1 and continue stirring at 60 to 70°C for 2 to 3 hours. h, the titanium-containing precursor is selected from one or two of tetrabutyl titanate and isopropyl titanate; Step 3, the suspension obtained in Step 2 is centrifuged and then vacuum dried at 70 to 90°C to obtain precursor-coated powder; Step 4, the precursor-coated powder and lithium source are uniformly mixed at a molar ratio of cathode material to lithium source of 1:0.02 to 1:0.06, wherein the lithium source is selected from one or two of lithium hydroxide and lithium carbonate; Step 5, the mixture is subjected to a two-stage heat treatment in an oxygen-containing atmosphere, wherein the first stage is held at 280 to 320°C for 0.5 to 1.5 h to form the inner transition sublayer, and the second stage is heated to 450 to 550°C and held for 2 to 4 h to complete the crystallization of the outer stable sublayer.

7. The preparation method according to claim 6, characterized in that, The amount of niobium-containing precursor in step one is calculated as 0.5% to 3% of the total molar amount of niobium to the transition metals in the cathode material, and the amount of titanium-containing precursor in step two is calculated as 0.3% to 2% of the total molar amount of titanium to the transition metals in the cathode material.

8. The preparation method according to claim 6, characterized in that, The oxygen-containing atmosphere mentioned in step five is dry air or pure oxygen atmosphere, the first stage heating rate is 1 to 3°C / min, and the second stage heating rate is 2 to 5°C / min.

9. The preparation method according to claim 6, characterized in that, The inert atmosphere mentioned in step one is argon or nitrogen, and the stirring speed is 200 to 500 r / min.

10. An all-solid-state battery, comprising a positive electrode, a sulfide solid electrolyte, and a negative electrode, characterized in that, The positive electrode contains a gradient oxide-coated positive electrode material as described in any one of claims 1 to 5, and the sulfide solid electrolyte is selected from Li6PS5Cl, Li3PS4, and Li 10 GeP2S 12 One or more of the following, wherein the negative electrode is a lithium metal negative electrode or a lithium alloy negative electrode.

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