A low-expansion high-toughness composite oxide solid electrolyte and a method for preparing the same

Abstract

The application discloses a preparation method of a low-expansion high-toughness composite oxide solid electrolyte, which comprises the following steps: S1: compounding a solid electrolyte precursor, a grain boundary toughening agent and a stress buffer into a composite powder; S2: mixing the composite powder with a negative thermal expansion filler to prepare a plurality of mixed powders with a gradient change in the content of the negative thermal expansion filler; S3: preparing a green body from the plurality of mixed powders, and making the negative thermal expansion filler form a concentration gradient in the green body along the thickness direction; and S4: sintering the green body, making the grain boundary toughening agent form a toughening phase at the grain boundary in the sintering process, and finally obtaining the low-expansion high-toughness composite oxide solid electrolyte. The preparation method can make the oxide solid electrolyte have low thermal expansion, high fracture toughness, high density and self-healing characteristics, and can meet the strict requirements of long-term durability and reliability of electrolytes in a wide temperature range, long service life and high-power solid-state batteries.

Description

Technical Field

[0001] This invention relates to the technical field of electrolytes, and more particularly to a low-expansion, high-toughness composite oxide solid electrolyte and its preparation method. Background Technology

[0002] Oxide solid electrolytes are considered core materials for all-solid-state batteries due to their high ionic conductivity and wide electrochemical window. However, their large-scale application still faces challenges such as thermal expansion mismatch, high grain boundary brittleness, and large residual stress after sintering. During sintering cooling and temperature fluctuations, the difference in thermal expansion between the electrolyte, electrodes, and encapsulation metals leads to interfacial stress concentration, easily causing warping, microcracks, and delamination. Simultaneously, the low toughness of ceramic grain boundaries allows cracks to propagate rapidly under mechanical impact or long-term electrochemical cycling, resulting in short-circuit failure. Furthermore, high-temperature sintering is often accompanied by lithium volatilization and insufficient density, causing grain boundary impedance to continuously increase with cycling time, exacerbating battery polarization. Existing homogenization modification methods struggle to simultaneously achieve low expansion, high toughness, and high ionic conductivity across a wide temperature range, limiting the long-term reliability and high power output of solid-state batteries. Summary of the Invention

[0003] In view of the shortcomings of the prior art, the present invention provides a method for preparing a low-expansion, high-toughness composite oxide solid electrolyte to solve the technical problems of battery failure caused by high thermal expansion coefficient, high grain boundary brittleness, sintering internal stress concentration, poor thermal shock resistance, and easy cracking and interface peeling during long-term cycling of traditional oxide solid electrolytes.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a low-expansion, high-toughness composite oxide solid electrolyte, the method comprising the following steps: S1: A composite powder is prepared by compounding a solid electrolyte precursor, a grain boundary toughening agent, and a stress buffer. Step S1, through the introduction of the grain boundary toughening agent, aims to form a toughening phase at the grain boundaries during subsequent sintering. This addresses the problem of crack initiation and propagation in traditional oxide ceramic electrolytes due to high grain boundary brittleness, and is expected to significantly improve the material's fracture toughness and resistance to mechanical impact. The addition of the stress buffer, particularly transition metal oxide nanoparticles that undergo phase transition and volume change within the range of room temperature to 200°C, aims to utilize their phase transition volume effect to absorb and buffer localized stress concentrations caused by temperature fluctuations, lithium dendrite growth, or electrochemical cycling during the material's preparation or use. This inhibits microcrack initiation and improves the material's structural integrity.

[0005] S2: The composite powder obtained in step S1 is mixed with a negative thermal expansion filler to prepare various mixed powders with a gradient in the content of the negative thermal expansion filler. Step S2 prepares a series of mixed powders with gradually changing compositions by mixing fillers with negative thermal expansion coefficients (i.e., volume shrinkage upon heating) (such as ZrW2O8) with the base composite powder in different contents. This design aims to use negative thermal expansion fillers to offset or reduce the thermal stress generated by positive thermal expansion in the solid electrolyte matrix during sintering and cooling processes and subsequent temperature changes. The gradient design further solves the problem of large internal stress at the interface caused by abrupt changes in the coefficient of thermal expansion in homogeneous composite materials. By continuously varying the filler content, a smooth transition in the coefficient of thermal expansion within the material can be achieved, thereby more effectively dispersing and reducing thermal mismatch stress, preventing interlayer peeling or cracking, and improving the dimensional stability and interfacial bonding strength of the electrolyte sheet under thermal cycling.

[0006] S3: The various mixed powders obtained in step S2 are prepared into a green body, and the negative thermal expansion filler forms a concentration gradient along the thickness direction in the green body. Step S3 aims to accurately transfer the gradient composition design from the powder stage to the formed green body, achieving structural and functional integration. By employing forming processes such as casting and stacking, mixed powders with different negative thermal expansion filler contents are stacked in the designed gradient order and formed into a green body. This results in a continuous composition gradient in the thickness direction of the final green body, thereby achieving a gradient distribution of thermal expansion performance on a macroscopic scale. This structural design makes the thermal stress distribution generated inside the electrolyte more gradual when subjected to temperature changes, avoiding stress concentration at specific interfaces and greatly reducing the risk of delamination or fracture caused by thermal stress. At the same time, the gradient structure also provides the material with better stress matching capabilities, enabling it to better adapt to the thermal expansion differences when combined with electrode materials or other battery components, improving the interface stability and cycle life of the full battery.

[0007] S4: The green body obtained in step S3 is sintered, causing the grain boundary toughening agent to form a toughening phase at the grain boundaries during sintering, ultimately yielding the low-expansion, high-toughness composite oxide solid electrolyte. In step S4, under a specific sintering regime (e.g., inert atmosphere, 1100–1200°C), the green body undergoes densification, and the solid electrolyte precursor forms the main crystalline phase. During this process, the grain boundary toughening agent (e.g., a combination of lanthanum borate, alkali metal borate, and 3Y-PSZ) forms a second phase (toughening phase) with high toughness or phase transformation toughening effect in situ at the grain boundaries through liquid-phase sintering or solid-phase reaction. These toughening phases can effectively hinder crack propagation through mechanisms such as crack deflection, bridging, and phase transformation stress field shielding, thereby significantly improving the fracture toughness and mechanical strength of the material. Simultaneously, the sintering process also solidifies the negative thermal expansion filler gradient structure constructed in step S3, ensuring good bonding with the matrix. Ultimately, through the integration of sintering processes, multiple functions such as gradient thermal stress management, grain boundary toughening, and stress buffering are integrated into a dense electrolyte body, synergistically achieving the comprehensive performance of low thermal expansion, high mechanical toughness, high ionic conductivity, and good structural stability of the material, thus meeting the requirements of solid-state batteries for long-term electrolyte durability and reliability.

[0008] As a preferred technical solution, the solid electrolyte precursor is lithium lanthanum zirconium oxide (LLZO) or other oxide solid electrolyte precursors with a garnet-type structure.

[0009] As a preferred embodiment, the grain boundary toughening agent is composed of lanthanum borate, alkali metal borate, and 3Y-PSZ (3 mol% Y2O3 partially stabilized ZrO2). As a further preferred embodiment, the mass ratio of lanthanum borate, alkali metal borate, and 3Y-PSZ is (0.5-2):(0.5-2):1. In this application, lanthanum borate and alkali metal borate first melt to form a low-viscosity liquid phase, promoting grain rearrangement and densification. Simultaneously, a boron-rich glass film is generated in situ at the grain boundaries, which both inhibits lithium volatilization and provides continuous lithium-ion transport channels, significantly reducing grain boundary resistance. 3Y-PSZ uniformly nucleates at the grain boundaries in the liquid environment and retains the metastable tetragonal phase. During crack propagation, stress-induced phase transformation occurs, accompanied by volume expansion, which creates a shielding effect on the crack tip. Through crack bridging and deflection mechanisms, the fracture toughness is greatly improved. The boron-rich glass and tetragonal zirconium oxide together constitute a self-healing composite grain boundary. Under high temperature or local stress, the glass can flow to fill microcracks, while the tetragonal phase continuously provides phase transformation toughening, thereby achieving the simultaneous integration of high density, high toughness, and long-life self-healing function.

[0010] As a preferred technical solution, the alkali metal borate is at least one of lithium metaborate, lithium tetraborate, sodium metaborate, sodium tetraborate, potassium metaborate, and potassium tetraborate.

[0011] As a preferred technical solution, the stress buffer is a transition metal oxide nanoparticle that undergoes a first-order phase transition and is accompanied by a volume change in the range of room temperature to 200°C.

[0012] As a preferred technical solution, the negative thermal expansion filler is ZrW2O8 powder, the content of which continuously varies from 3 to 5 wt% on one side to 0 wt% on the other side in the thickness direction.

[0013] As a preferred technical solution, the mixed powders prepared in step S2 have a variation step of less than or equal to 1 wt% in the content of negative thermal expansion filler, and contain at least 3 different content levels.

[0014] As a preferred technical solution, in step S3, the mixed powders with different negative thermal expansion filler contents are formed into a green body with a continuous concentration gradient by means of casting and stacking.

[0015] As a preferred technical solution, in step S4, the sintering is carried out in an inert atmosphere, the sintering temperature is 1100-1200℃, and the holding time is 2-10 hours.

[0016] Another aspect of the present invention is to provide a low-expansion, high-toughness composite oxide solid electrolyte, wherein the solid electrolyte is prepared by the preparation method of the low-expansion, high-toughness composite oxide solid electrolyte described above.

[0017] The beneficial effects of this invention are: This invention discloses a method for preparing a low-expansion, high-toughness composite oxide solid electrolyte. By innovatively introducing a triple synergistic architecture of "grain boundary toughening, stress buffering, and negative expansion gradient" at the powder stage, it not only achieves instantaneous wetting and densification of the grain boundary glass phase during sintering but also enables continuous adjustment of the coefficient of thermal expansion in the thickness direction and self-balancing of internal stress. This design not only enhances the electrolyte's resistance to thermal shock and mechanical bending but also maintains the high ionic conductivity and chemical stability of the garnet matrix, helping to suppress lithium dendrite penetration, reduce interfacial contact resistance, and extend the cycle life of all-solid-state batteries. Test results show that the composite oxide solid electrolyte prepared using this method, in a full cell matched with a lithium metal anode, maintains a capacity retention rate of over 92% after 500 cycles at 0.5C, demonstrating excellent long-term cycle stability and interfacial durability.

[0018] In summary, the low-expansion, high-toughness composite oxide solid electrolyte of the present invention not only possesses excellent low thermal expansion, high fracture toughness, high density, and self-healing properties, but also significantly improves the thermal / mechanical compatibility with existing positive and negative electrode materials and encapsulation metals, thus meeting the long-term durability and reliability requirements of high-energy-density solid-state batteries under wide temperature range, long cycle, and high-power conditions. Detailed Implementation

[0019] The following description is intended to disclose the invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art.

[0020] Example 1 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this embodiment includes the following steps: S1: Lithium lanthanum zirconium oxide (LLZO) powder with a particle size of 0.5–2 μm was used as the precursor for the solid electrolyte. Subsequently, a grain boundary toughening agent and a stress buffer were compounded with the precursor. The grain boundary toughening agent was prepared by weighing and mixing lanthanum borate, lithium metaborate, and 3Y-PSZ in a mass ratio of 1:1:1. The stress buffer was selected from vanadium oxide (VO2) nanoparticles that undergo a first-order phase transition with accompanying volume change in the temperature range of room temperature to 200°C, with an average particle size of 20–40 nm, and was added at 2% of the mass of the LLZO powder. All raw materials were placed in a ball mill jar and ball-milled at 300 r / min for 6 hours using anhydrous ethanol as the medium to ensure thorough mixing. The mixture was then dried at 80°C for 12 hours and passed through a 200-mesh sieve to obtain the composite powder. The LLZO powder was synthesized in a solid phase from La2O3, Li2CO3, and ZrO2 in a molar ratio of 6.5:3.3:2, and obtained by pre-calcination at 900℃ for 6 hours, crushing, and passing through a 400-mesh sieve.

[0021] S2: Take the composite powder obtained in step S1 and perform gradient mixing with negative thermal expansion filler ZrW2O8 powder (particle size 0.5-1μm). The ZrW2O8 content is set to continuously change from 5wt% on one side to 0wt% on the other side along the thickness direction, with a change step of 1wt%, to prepare six mixed powders with different ZrW2O8 contents: 0wt%, 1wt%, 2wt%, 3wt%, 4wt%, and 5wt%. For each content, weigh the corresponding mass of composite powder and ZrW2O8 powder, place them in a ball mill jar, and ball mill at 200r / min for 4 hours using anhydrous ethanol as the medium. After uniform mixing, dry and sieve to obtain six mixed powders with gradient changes in the content of negative thermal expansion filler. The ZrW2O8 is obtained by solid-phase reaction of ZrO2 and WO3 at a 1:2 molar ratio at 1200℃ / 2h followed by ice-water quenching.

[0022] S3: A casting and lamination process was used to prepare a preform with a continuous concentration gradient from the above six mixed powders. First, each mixed powder was mixed with a binder (polyvinyl butyral), a plasticizer (dibutyl phthalate), and a dispersant (castor oil) at a mass ratio of 92:5:2:1 in an organic solvent (ethanol to ethyl acetate volume ratio 2:1), and ball-milled to form a uniform slurry. Then, the slurries with different ZrW2O8 contents were sequentially cast into films approximately 50 μm thick using a casting machine, and then laminated layer by layer in order of ZrW2O8 content from 5 wt% to 0 wt%, applying a pressure of 5 MPa during lamination to ensure tight bonding between layers. After lamination, the film was dried at 60°C for 24 hours, and then subjected to cold isostatic pressing at 200 MPa for 5 minutes to finally obtain a dense preform with a continuous concentration gradient along the thickness direction.

[0023] S4: The green body obtained in step S3 is placed in an alumina crucible and then placed in a tube sintering furnace. First, the temperature is raised to 600℃ at a rate of 2℃ / min and held for 2 hours to completely remove organic matter. Then, under a nitrogen inert atmosphere, the temperature is raised to 1150℃ at a rate of 5℃ / min and held at this temperature for 6 hours for sintering. During sintering, the grain boundary toughening agents (lanthanum borate and lithium metaborate) melt to form a liquid phase, promoting grain rearrangement and densification, and generating a boron-rich glassy phase in situ at the grain boundaries. 3Y-PSZ is uniformly distributed at the grain boundaries and retains a metastable tetragonal phase structure. The ZrW2O8 filler maintains its negative thermal expansion characteristics in the matrix. After sintering, the material is cooled to room temperature at a rate of 3℃ / min, ultimately obtaining a dense composite oxide solid electrolyte sheet with low expansion, high toughness, and self-healing properties.

[0024] Test results show that the composite oxide solid electrolyte prepared in this embodiment has an ionic conductivity of 3.8 × 10⁻⁶ at room temperature. -4 The strength is S / cm, the fracture toughness is 2.1 MPa·m^(1 / 2), and the coefficient of thermal expansion is 8.5 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 95.2%, the electrolyte surface was smooth and crack-free, and there was no delamination in the thickness direction.

[0025] Example 2 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this embodiment includes the following steps: S1: Lithium lanthanum zirconium oxide (LLZO) powder with a particle size of 0.5–2 μm was selected as the solid electrolyte precursor. Subsequently, a grain boundary toughening agent and a stress buffer were compounded with the precursor. The grain boundary toughening agent was prepared by weighing and mixing lanthanum borate, sodium tetraborate, and 3Y-PSZ at a mass ratio of 0.5:1.5:1. The stress buffer was selected as vanadium oxide (VO2) nanoparticles that undergo a first-order phase transition with accompanying volume change in the temperature range of room temperature to 200℃, with an average particle size of 20–40 nm, and added at 1.5% of the LLZO powder mass. All raw materials were placed in a planetary ball mill and ball-milled at 400 r / min for 8 hours using isopropanol as the medium to ensure thorough mixing. Afterward, the mixture was vacuum-dried at 75℃ for 10 hours and passed through a 300-mesh sieve to obtain the composite powder.

[0026] S2: Take the composite powder obtained in step S1 and perform gradient mixing with negative thermal expansion filler ZrW2O8 powder (particle size 0.8-1.2μm). Set the ZrW2O8 content to continuously change from 4.8wt% on one side to 0wt% on the other side along the thickness direction, with a change step size of 0.8wt%, to prepare seven mixed powders with different ZrW2O8 contents: 0wt%, 0.8wt%, 1.6wt%, 2.4wt%, 3.2wt%, 4.0wt%, and 4.8wt%. For each content, weigh the corresponding mass of composite powder and ZrW2O8 powder, place them in a mixer, and dry mix for 2 hours to ensure uniform dispersion, obtaining seven mixed powders with gradient changes in the content of negative thermal expansion filler.

[0027] S3: A casting and lamination process was used to prepare a preform with a continuous concentration gradient from the above seven mixed powders. First, each mixed powder was mixed with a binder (polyvinyl butyral), a plasticizer (dibutyl phthalate), and a dispersant (castor oil) at a mass ratio of 90:6:3:1 in an organic solvent (ethanol to ethyl acetate volume ratio 2:1), and stirred at high speed to form a homogeneous slurry. Then, using a casting machine, the slurries with different ZrW2O8 contents were sequentially cast into films approximately 40 μm thick, and then laminated layer by layer in order of ZrW2O8 content from 5 wt% to 0 wt%, applying a pressure of 3 MPa during lamination to ensure tight bonding between layers. After lamination, the films were dried at 50°C for 20 hours, followed by cold isostatic pressing at 150 MPa for 8 minutes, finally obtaining a dense preform with a continuous concentration gradient along the thickness direction.

[0028] S4: The green body obtained in step S3 is placed on a zirconia sintering plate and placed in a high-temperature atmosphere sintering furnace. First, the temperature is raised to 500℃ at a rate of 3℃ / min and held for 1.5 hours to completely remove organic matter. Then, under a nitrogen inert atmosphere, the temperature is raised to 1180℃ at a rate of 4℃ / min and held at this temperature for 4 hours for sintering. During sintering, the grain boundary toughening agents (lanthanum borate and sodium tetraborate) melt to form a liquid phase, promoting grain rearrangement and densification, and generating a boron-rich glassy phase in situ at the grain boundaries. 3Y-PSZ is uniformly distributed at the grain boundaries and retains a metastable tetragonal phase structure. The ZrW2O8 filler maintains its negative thermal expansion characteristics in the matrix. After sintering, the substrate is cooled to room temperature at a rate of 2℃ / min, finally obtaining a dense composite oxide solid electrolyte sheet with low expansion, high toughness, and self-healing properties.

[0029] Test results show that the composite oxide solid electrolyte prepared in this embodiment has an ionic conductivity of 3.5 × 10⁻⁶ at room temperature. -4 The strength is S / cm, the fracture toughness is 2.0 MPa·m^(1 / 2), and the coefficient of thermal expansion is 8.8 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 94.0%, the electrolyte surface was smooth and crack-free, and there was no delamination in the thickness direction.

[0030] Example 3 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this embodiment includes the following steps: S1: Lithium lanthanum zirconium oxide (LLZO) powder with a particle size of 0.5–2 μm was selected as the solid electrolyte precursor. Subsequently, a grain boundary toughening agent and a stress buffer were compounded with the precursor. The grain boundary toughening agent was prepared by weighing and mixing lanthanum borate, potassium metaborate, and 3Y-PSZ at a mass ratio of 2:0.5:1. The stress buffer was selected from vanadium oxide (VO2) nanoparticles that undergo a first-order phase transition with volume change in the temperature range of room temperature to 200℃, with an average particle size of 20–40 nm, and was added at 2.5% of the LLZO powder mass. All raw materials were placed in a vibratory mill and treated with deionized water at 350 r / min for 5 hours to ensure thorough mixing. Afterward, the mixture was dried at 85℃ for 15 hours and passed through a 250-mesh sieve to obtain the composite powder.

[0031] S2: Take the composite powder obtained in step S1 and perform gradient mixing with negative thermal expansion filler ZrW2O8 powder (particle size 0.3-0.8μm). The ZrW2O8 content is set to continuously change from 5wt% on one side to 0wt% on the other side along the thickness direction, with a change step size of 0.5wt%. A total of 11 mixed powders with different ZrW2O8 contents are prepared: 0wt%, 0.5wt%, 1.0wt%, 1.5wt%, 2.0wt%, 2.5wt%, 3.0wt%, 3.5wt%, 4.0wt%, 4.5wt%, and 5wt%. For each content, weigh the corresponding mass of composite powder and ZrW2O8 powder, place them in a drum mixer, and dry mix at 50r / min for 3 hours to ensure uniform dispersion, obtaining 11 mixed powders with gradient changes in the content of negative thermal expansion filler.

[0032] S3: The above 11 mixed powders were prepared into a preform with a continuous concentration gradient using a casting and lamination process. First, each mixed powder was mixed with a binder (polyvinyl butyral), a plasticizer (dibutyl phthalate), and a dispersant (castor oil) at a mass ratio of 94:4:1.5:0.5 in an organic solvent (ethanol to ethyl acetate volume ratio 2:1), and then ultrasonically dispersed and mechanically stirred to form a homogeneous slurry. Subsequently, the slurries with different ZrW2O8 contents were sequentially cast into films approximately 30 μm thick using a casting machine, and then laminated layer by layer in order of ZrW2O8 content from 5 wt% to 0 wt%, applying a pressure of 8 MPa during lamination to ensure tight bonding between layers. After lamination, the films were dried at 70°C for 18 hours, and then subjected to cold isostatic pressing at 250 MPa for 3 minutes to finally obtain a dense preform with a continuous concentration gradient along the thickness direction.

[0033] S4: The blank obtained in step S3 is placed in a magnesium oxide crucible and then placed in a box-type atmosphere furnace. First, the temperature is raised to 550℃ at a rate of 1.5℃ / min and held for 3 hours to completely remove organic matter. Then, under a nitrogen inert atmosphere, the temperature is raised to 1120℃ at a rate of 3℃ / min and held at this temperature for 8 hours for sintering. During sintering, the grain boundary toughening agents (lanthanum borate and potassium metaborate) melt to form a liquid phase, promoting grain rearrangement and densification, and generating a boron-rich glassy phase in situ at the grain boundaries. 3Y-PSZ is uniformly distributed at the grain boundaries and retains a metastable tetragonal phase structure. The ZrW2O8 filler maintains its negative thermal expansion characteristics in the matrix. After sintering, the blank is cooled to room temperature at a rate of 5℃ / min, finally obtaining a dense composite oxide solid electrolyte sheet with low expansion, high toughness, and self-healing properties.

[0034] Test results show that the composite oxide solid electrolyte prepared in this embodiment has an ionic conductivity of 3.2 × 10⁻⁶ at room temperature. -4The strength is S / cm, the fracture toughness is 2.3 MPa·m^(1 / 2), and the coefficient of thermal expansion is 8.3 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 93.5%, the electrolyte surface was smooth and crack-free, and there was no delamination in the thickness direction.

[0035] Example 4 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this embodiment is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this embodiment, lithium tetraborate is used instead of lithium metaborate in Example 1 for the alkali metal borate.

[0036] Test results show that the composite oxide solid electrolyte prepared in this embodiment has an ionic conductivity of 3.6 × 10⁻⁶ at room temperature. -4 The strength is S / cm, the fracture toughness is 2.0 MPa·m^(1 / 2), and the coefficient of thermal expansion is 8.6 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 94.8%, the electrolyte surface was smooth and crack-free, and there was no delamination in the thickness direction.

[0037] Example 5 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this embodiment is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this embodiment, sodium metaborate is used instead of lithium metaborate in Example 1 for the alkali metal borate.

[0038] Test results show that the composite oxide solid electrolyte prepared in this embodiment has an ionic conductivity of 3.4 × 10⁻⁶ at room temperature. -4 The strength is S / cm, the fracture toughness is 1.9 MPa·m^(1 / 2), and the coefficient of thermal expansion is 8.9 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 93.2%, the electrolyte surface was smooth and crack-free, and there was no delamination in the thickness direction.

[0039] Example 6 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this embodiment is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this embodiment, potassium tetraborate is used instead of lithium metaborate in Example 1.

[0040] Test results show that the composite oxide solid electrolyte prepared in this embodiment has an ionic conductivity of 3.7 × 10⁻⁶ at room temperature.-4 The strength is S / cm, the fracture toughness is 2.0 MPa·m^(1 / 2), and the coefficient of thermal expansion is 8.7 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 94.5%, the electrolyte surface was smooth and crack-free, and there was no delamination in the thickness direction.

[0041] Example 7 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this embodiment is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this embodiment, the gradient change step size of the negative thermal expansion filler is adjusted to 0.5wt% (corresponding to a total of 11 different content grades of mixed powder), replacing the 1wt% step size (6 grades) in Example 1. Other raw material types, dosages and process parameters remain unchanged.

[0042] Test results show that the composite oxide solid electrolyte prepared in this embodiment has an ionic conductivity of 3.5 × 10⁻⁶ at room temperature. -4 The strength is S / cm, the fracture toughness is 2.2 MPa·m^(1 / 2), and the coefficient of thermal expansion is 8.4 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 95.0%, the electrolyte surface was smooth and crack-free, and there was no delamination in the thickness direction.

[0043] Example 8 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this embodiment is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this embodiment, the content gradient of the negative thermal expansion filler in the thickness direction changes continuously from 3wt% on one side to 0wt% on the other side, replacing 5wt%→0wt% in Example 1. The gradient change step size is still 1wt%, corresponding to a total of 4 different content levels of mixed powder. The other raw material types, dosages and process parameters remain unchanged.

[0044] Test results show that the composite oxide solid electrolyte prepared in this embodiment has an ionic conductivity of 3.9 × 10⁻⁶ at room temperature. -4 The strength is S / cm, the fracture toughness is 1.8 MPa·m^(1 / 2), and the coefficient of thermal expansion is 9.2 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 92.8%, the electrolyte surface was smooth and crack-free, and there was no delamination in the thickness direction.

[0045] Comparative Example 1 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this comparative example is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this comparative example, no grain boundary toughening agent is added. Only solid electrolyte precursor (LLZO), stress buffer and negative thermal expansion filler are used for composite and gradient molding. The remaining process conditions are completely consistent with those in Example 1.

[0046] Test results show that, in this comparative example without the addition of grain boundary toughening agent, the ionic conductivity of the electrolyte at room temperature is 3.1 × 10⁻⁶. -4 The strength is S / cm, the fracture toughness is 1.6 MPa·m^(1 / 2), and the coefficient of thermal expansion is 10.5 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at a 0.5C rate, the capacity retention was only 78.5%, obvious intergranular cracks appeared on the electrolyte surface, and localized spalling occurred.

[0047] Comparative Example 2 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this comparative example is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this comparative example, no stress buffer is added. Only solid electrolyte precursor (LLZO), grain boundary toughening agent and negative thermal expansion filler are used for composite and gradient molding. The remaining process conditions are completely consistent with those in Example 1.

[0048] Test results show that, in this comparative example without the addition of stress buffer, the ionic conductivity of the electrolyte at room temperature is 3.5 × 10⁻⁶. -4 The strength is S / cm, the fracture toughness is 1.5 MPa·m^(1 / 2), and the coefficient of thermal expansion is 11.0 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 82.3%, and dendrite puncture marks and microcrack networks were visible on the electrolyte surface.

[0049] Comparative Example 3 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this comparative example is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this comparative example, no negative thermal expansion filler is added and no gradient design is performed. The solid electrolyte precursor (LLZO), grain boundary toughening agent and stress buffer are simply mixed uniformly and directly molded into a homogeneous preform. The remaining process conditions are completely consistent with those in Example 1.

[0050] Test results show that, in this comparative example, without the addition of negative thermal expansion fillers and without a designed gradient structure, the ionic conductivity of the electrolyte at room temperature is 3.1 × 10⁻⁶. -4The strength is S / cm, the fracture toughness is 1.4 MPa·m^(1 / 2), and the coefficient of thermal expansion is 12.9 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 80.1%, the electrolyte showed significant warping and deformation, and severe interfacial delamination.

[0051] Comparative Example 4 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this comparative example is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this comparative example, the negative thermal expansion filler is uniformly distributed, that is, the ZrW2O8 content is kept constant (2.5wt%) in the whole green body and does not form a gradient structure. The other raw materials and process conditions are completely consistent with those in Example 1.

[0052] Test results show that when 2.5 wt% ZrW₂O₈ is uniformly distributed in this comparative example, the ionic conductivity of the electrolyte at room temperature is 3.4 × 10⁻⁶. -4 The strength is S / cm, the fracture toughness is 1.8 MPa·m^(1 / 2), and the coefficient of thermal expansion is 10.5 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 85.7%, stress concentration zones appeared in the thickness direction of the electrolyte, and cracks were present at the edges.

[0053] Comparative Example 5 The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte in this comparative example is basically the same as that in Example 1 in terms of raw material composition and preparation steps. The difference is that in the preparation method of this comparative example, only lanthanum borate is used as the grain boundary toughening agent, and no alkali metal borate and 3Y-PSZ are added. The other raw materials and process conditions are completely consistent with those in Example 1.

[0054] Test results show that, in this comparative example, when only lanthanum borate is used as a grain boundary toughening agent, the ionic conductivity of the electrolyte at room temperature is 2.6 × 10⁻⁶. -4 The strength is S / cm, the fracture toughness is 1.5 MPa·m^(1 / 2), and the coefficient of thermal expansion is 10.2 × 10⁻⁶. -6 K -1 (Room temperature to 200°C). After 500 cycles at 0.5C, the capacity retention was 83.6%, and pores and an inhomogeneous glassy phase were visible at the grain boundaries on the electrolyte surface.

[0055] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for preparing a low-expansion, high-toughness composite oxide solid electrolyte, characterized in that, The preparation method includes the following steps: S1: Solid electrolyte precursor, grain boundary toughening agent and stress buffer are compounded into composite powder; S2: The composite powder obtained in step S1 is mixed with the negative thermal expansion filler to prepare a variety of mixed powders in which the content of the negative thermal expansion filler varies in a gradient. S3: Prepare a green body from the various mixed powders obtained in step S2, and make the negative thermal expansion filler form a concentration gradient along the thickness direction in the green body; S4: The blank obtained in step S3 is sintered so that the grain boundary toughening agent forms a toughening phase at the grain boundary during the sintering process, and finally the low expansion and high toughness composite oxide solid electrolyte is obtained.

2. The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in claim 1, characterized in that, The solid electrolyte precursor is lithium lanthanum zirconium oxide or other oxide solid electrolyte precursors with a garnet-type structure.

3. The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in claim 1, characterized in that, The grain boundary toughening agent is composed of lanthanum borate, alkali metal borate and 3Y-PSZ.

4. The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in claim 3, characterized in that, The alkali metal borate is at least one of lithium metaborate, lithium tetraborate, sodium metaborate, sodium tetraborate, potassium metaborate, and potassium tetraborate.

5. The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in claim 1, characterized in that, The stress buffer is a transition metal oxide nanoparticle that undergoes a first-order phase transition and is accompanied by a volume change in the range of room temperature to 200°C.

6. The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in claim 1, characterized in that, The negative thermal expansion filler is ZrW2O8 powder, the content of which varies continuously from 3 to 5 wt% on one side to 0 wt% on the other side in the thickness direction.

7. The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in claim 1, characterized in that, The various mixed powders prepared in step S2 have a variation step of less than or equal to 1 wt% in the content of negative thermal expansion filler, and contain at least 3 different content levels.

8. The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in claim 1, characterized in that, In step S3, the mixed powders with different negative thermal expansion filler contents are formed into a green body with a continuous concentration gradient by casting and stacking.

9. The preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in claim 1, characterized in that, In step S4, the sintering is carried out in an inert atmosphere, the sintering temperature is 1100-1200℃, and the holding time is 2-10 hours.

10. A low-expansion, high-toughness composite oxide solid electrolyte, characterized in that, The solid electrolyte is prepared using the preparation method of the low-expansion, high-toughness composite oxide solid electrolyte as described in any one of claims 1 to 9.