Next-generation all-solid-state batteries promise higher energy and power densities, and improved operational safety, compared to the state-of-the-art lithium-ion batteries. As their key component, several material classes of the ion-conducting solid electrolytes are currently under investigation. Among them, hydroborates draw attention as an emerging class, combining e.g. high Li+ and Na+ conductivity, compatibility with alkali metal anodes, low gravimetric density, low toxicity, and facile all-solid-state cell fabrication by cold pressing. To further improve energy density and interface compatibility, this thesis studies the electrochemical and thermal stability of hydroborate solid electrolytes and their cathode compatibility for high-voltage (>4 V) all-solid-state batteries. By introducing a self-passivating functionality into a solid electrolyte, it demonstrates the first 4 V hydroborate-based all-solid-state battery at room temperature with an excellent long-term capacity and energy retention. This work records the highest average discharge cell voltage and cathode-based specific energy among all reported all-solid-state sodium batteries so far.