A novel high-voltage solid electrolyte of Na3B24H23 for 4 V all-solid-state sodium battery

Designing high-voltage solid electrolyte that can be compatible with high-voltage cathode and alkali metal anode is challenging and urgently required for the development of all-solid-state batteries with high energy density. Herein, the electrochemical stability window of hydroborate electrolyte beyond 6 V vs Na + /Na has been realized for the first time by a novel conjuncto -hydroborate of Na 3 B 24 H 23 (6.7 V vs Na + /Na). Its mixed-anion composite Na 3 B 24 H 23 -5Na 2 B 12 H 12 features high conductivity (1.42 mS cm −1 ), high Na-ion transference number (0.97) and wide electrochemical window (5.8 V vs Na + /Na). The sodium symmetrical battery using Na 3 B 24 H 23 -5Na 2 B 12 H 12 electrolyte can stably cycle without short circuit for 100 h at 0.2 mA cm −2 and the critical current density can reach 1.0 mA cm −2 at 60 °C and maintain 0.7 mA cm −2 at 25 °C. Finally, Na 3 B 24 H 23 -5Na 2 B 12 H 12 exhibits good compatibility with 4 V-class cathodes and the high-voltage all-solid-state sodium batteries, Na[ Ni 1/3 Fe 1/3 Mn 1/3 ]O 2 /Na 3 B 24 H 23 -5Na 2 B 12 H 12 /Na, delivers outstanding capacity retention of 84 % after 100 cycles at 0.1C at 60 °C.