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A central goal in current battery research is to increase the safety and energy density of Li-ion batteries. Electrolytes nowadays typically consist of lithium salts dissolved in organic solvents. Solid electrolytes could facilitate safer batteries with higher capacities, as they are compatible with Li metal anodes, prevent Li dendrite formation and sulphur shuttling, and eliminate risks associated with flammable organic solvents. Less than 10 years ago, LiBH4 was proposed as a solid state electrolyte. It showed a high ionic conductivity, but only at elevated temperatures. Since then strategies have been developed to extend the high ionic conductivity of LiBH4 down to room temperature, and other light metal hydrides have been explored as solid electrolytes [1]. Using LiBH4 as an example we will discuss how the properties of solid electrolytes can be modified by forming nanocomposites with metal oxides, leading to an enhancement of the room temperature ionic conductivity of more than three orders of magnitude[2]. DSC measurements combined with solid state NMR allow to identify how the nanoconfinement and presence of interfaces modify the phase stability and the Li+ mobility [3]. Systematic studies show how the ionic conductivity can be optimized by tuning the nanostructure and interfaces in these nanocomposites. Finally, promising results have been obtained in using these materials as solid-state electrolytes in next generation all-solid state lithium-sulphur batteries. [4] [1] de Jongh et al. J. Appl. Phys. A (2016), 122:251. [2] Blanchard et al., Adv. Funct. Mater. 25 (2015), 182. [3] Verkuijlen et al., J. Phys. Chem. C 116 (2012) 22169. [4] Blanchard et al, J. Electrochem. Soc. (2016).