Prior to lithiation, Nb18W8O69 is off-white (light yellow-green) with an optical bandgap of 2.84 eV (Supplementary Figure ). Upon lithium insertion, the color becomes dark blue. Experimental and theoretical work on related cs phases suggests that lithiation reduces d 0 Nb 5+ and W 6+ , 1 ndoping the structure into the conduction band and causing an increase in electronic conductivity, which motivates detailed future work to understand subtle differences between compositionally and structurally similar cs compounds.