We report on a particularly stable 3 V all-solid-state sodium–ion battery built using a closo-borate based electrolyte, namely Na2(B12H12)0.5(B10H10)0.5. Battery performance is enhanced through the creation of an intimate cathode–electrolyte interface resulting in reversible and stable cycling with a capacity of 85 mA h g−1 at C/20 and 80 mA h g−1 at C/5 with more than 90% capacity retention after 20 cycles at C/20 and 85% after 250 cycles at C/5. We also discuss the effect of cycling outside the electrochemical stability window and show that electrolyte decomposition leads to faster though not critical capacity fading. Our results demonstrate that owing to their high stability and conductivity closo-borate based electrolytes could play a significant role in the development of a competitive all-solid-state sodium–ion battery technology.