Solid-state batteries combining an alkali metal anode and a high-voltage cathode have the potential to double the energy density of current-generation rechargeable batteries. We recently demonstrated the integration of hydroborate solid electrolytes with a 4 V class cathode through in-situ formation of a passivating interface layer. Combined with their high ionic conductivity > 1 mS/cm at room temperature, low gravimetric density 1.2 g/cm3, low toxicity, high thermal and chemical stability, stability vs lithium and sodium metal, soft mechanical properties enabling cold pressing, compatibility with solution infiltration, and potential for low cost, hydroborate electrolytes represent a promising option for a competitive next-generation solid-state battery technology.