Fast Sodium-Ion Conduction in a Novel Conjuncto-Hydroborate of Na₄B₂₀H₁₈

Hydroborates have attracted extensive attention as a promising class of solid electrolytes for all-solid-state batteries. A conjuncto-hydroborate of Na4B20H18 is successfully developed and exhibits significantly promoted ionic conductivity and electrochemical stability compared to its monomer form of Na2B10H10. Moreover, the anion-mixing method is implemented and the mixed-anion electrolyte of Na4B20H18-3Na2B12H12 achieves an enhanced ionic conductivity exceeding 0.22 mS cm–1 at 25 °C, a remarkable Na-ion transference number up to 0.921 and an electrochemical stability window of 3.45 V vs Na+/Na at room temperature. Meanwhile, Na4B20H18-3Na2B12H12 also has remarkable stability against the Na electrode, and an all-solid-state battery with Na metal anode, TiS2 cathode, and Na4B20H18–3Na2B12H12 electrolyte delivers outstanding capacity retention of 92% after 50 cycles at 0.1C.