Non-crystalline Li+-ion solid electrolytes (SEs), such as lithium phosphorous oxynitride, can uniquely enable high-rate 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 a non- crystalline SE of composition Li-Al-P-O (LAPO) with an ionic conductivity >10-7 S cm-1 at room temperature by spin coating from aqueous solutions and subsequent annealing in air. Homogenous, dense, flat layers can be synthesised with sub- micron thickness at temperatures as low as 230 °C. Control of the composition is shown to significantly affect the ionic con- ductivity, 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 0.42(1) eV. Additionally, these SEs exhibit low room temperature electronic conductivity (<10-11 S cm-1) and moderate Young’s modulus of ≈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.