Researchers at Tokyo Metropolitan University have developed a room temperature process for making flexible ceramic electrolyte sheets for lithium metal battery designs.
The team, led by Professor Kiyoshi Kanamura, combined a garnet-type ceramic, a polymer binder, and an ionic liquid, producing a quasi-solid-state sheet electrolyte. The synthesis is carried out at room temperature, requiring significantly less energy than existing high-temperature processes that operate at over 1000°C. The sheet works over a wide range of temperatures, particulary 60°C and 30°C, making it a promising electrolyte for lithium battery pack in electric vehicles.
The flexible ceramic is of particular interest for solid state battery designs. A garnet-type ceramic material, Li7La3Zr2O12, better known as LLZO, is now widely regarded as a promising solid-state electrolyte material for its high ionic conductivity and compatibility with Li metal. However, producing high-density LLZO electrolytes requires sintering temperatures as high as 1200 °C. This is both energy inefficient and time-consuming, making large-scale production of LLZO electrolytes difficult. In addition, the poor physical contact between brittle LLZO electrolytes and the electrode materials usually results in high interfacial resistance, greatly limiting their application in all-solid-state Li-metal batteries.
The flexible sheet is created from spreading a LLZO ceramic slurry onto a thin polymer substrate. After drying in a vacuum oven, the 75-micron thick sheet electrolyte was soaked in an ionic liquid (IL) to improve its ionic conductivity. ILs are salts which are liquid at room temperature, known to be highly conductive while being almost non-flammable and non-volatile. Inside the sheets, the IL successfully filled the microscopic gaps in the structure and bridged the LLZO particles, forming an efficient pathway for Li-ions. They also effectively reduced interfacial resistance at the cathode.
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The researchers also found that Li-ions diffused through both the IL and the LLZO particles in the structure, highlighting the role played by both. The team say that the mechanical robustness and operability of the flexible composite sheet at a wide range of temperatures makes it a promising electrolyte for lithium metal battery designs, and the simplicity of the process could speed up commercialisation of the technology.
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