All-solid-state batteries promise to simultaneously yield higher energy and power density as well as improved safety as compared to state-of-the-art lithium-ion technologies based on organic liquid electrolytes. A competitive all-solid-state battery requires a solid-state electrolyte with high ionic conductivity near room temperature and high thermal and electrochemical stability. Meeting these requirements simultaneously represents a major challenge. Closo-borates represent a promising, yet underexplored, class of solid electrolytes. Specifically, their high-ionic conductivity, wide electrochemical stability window, and advantageous processing properties make them serious candidates to address the major challenges faced by all-solid-state batteries. This thesis presents the development and characterization of a new solid electrolyte within this family, namely Na4(B12H12)(B10H10). It demonstrates that this material possesses the appropriate combination of properties to develop a 3 V all-solid-state battery and discusses aspects related to the assembly and successful cycling of such a device.