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 Na3B24H23 (6.7 V vs Na+/Na). Its mixed-anion composite Na3B24H23-5Na2B12H12 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 Na3B24H23-5Na2B12H12 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 degrees C and maintain 0.7 mA cm-2 at 25 degrees C. Finally, Na3B24H23-5Na2B12H12 exhibits good compatibility with 4 V-class cathodes and the high-voltage all-solid-state sodium batteries, Na[Ni1/3Fe1/3Mn1/3] O2/Na3B24H23-5Na2B12H12/Na, delivers outstanding capacity retention of 84 % after 100 cycles at 0.1C at 60 degrees C.