Metal hydroborates: From hydrogen stores to solid electrolytes

The last twenty years of an intense research on metal hydroborates as solid hydrogen stores and solid electrolytes are reviewed. It is shown that from the most promising application in hydrogen storage due to their high gravimetric and volumetric capacities, the focus has moved to solid electrolytes due to high cation mobility in disordered structures with rotating or tumbling anions-hydroborate clusters. Various strategies of overcoming the strong covalent bonding of hydrogen in hydroborates for hydrogen storage and disordering their structures at room temperature for solid electrolytes are discussed. The important role of crystal chemistry and crystallography knowledge in material design can be read in the cited literature.