Engineering Solution-Processed Non-Crystalline Solid Electrolytes for Li Metal Batteries

Non-crystalline Li-ion solid electrolytes (SEs), such solid-state battery operation over thousands of cycles in thin film form. However, they are typically produced by expensive and low throughput vacuum deposition, limiting their wide application and study. Here, we report non-crystalline SEs of composition Li-Al- P-O (LAPO) with ionic conductivities > 10-7 S cm(-1) at room temperature made by spin coating from aqueous solutions and subsequent annealing in air. Homogenous, dense, flat layers can be synthesized with submicrometer thickness at temperatures as low as 230 degrees C. Control of the composition is shown to significantly affect the ionic conductivity, with increased Li and decreased P content being optimal, while higher annealing temperatures result in decreased ionic conductivity. Activation energy analysis reveals a Li-ion hopping barrier of approximate to 0.4 eV. Additionally, these SEs exhibit low room temperature electronic conductivity (< 10-11 S cm(-1)) and a moderate Young's modulus of approximate to 54 GPa, which may be beneficial in preventing Li dendrite formation. In contact with Li metal, LAPO is found to form a stable but high impedance passivation layer comprised of Al metal, Li-P, and Li-O species. These findings should be of value when engineering non-crystalline SEs for Li-metal batteries with high energy and power densities.