An ultrahigh closo-borate concentration solid-state electrolyte is presented that is a combined salt of an alkali metal or alkali earth metal closo-borate and conductivity enhancing SISE. The combined salt allows significantly higher conductivities in the solid state than the included alkali metal or alkali earth metal closo-borate. The combined salt can be prepared by mechanical combination or combination in solution. The salts can be used in solid-state electrochemical devices.