4 V room-temperature all-solid-state sodium battery enabled by a passivating cathode/hydroborate solid electrolyte interface

Designing solid electrolytes for all-solid-state-batteries that can withstand the extreme electrochemical conditions in contact with an alkali metal anode and a high-voltage cathode is challenging, especially when the battery is cycled beyond 4 V. Here we demonstrate that a hydroborate solid electrolyte Na-4(CB11H12)(2)(B12H12), built from two types of cage-like anions with different oxidative stability, can effectively passivate the interface to a 4 V-class cathode and prevent impedance growth during cycling. We show that B12H12 anions decompose below 4.2 V vs. Na+/Na to form a passivating interphase layer, while CB11H12 anions remain intact, providing sufficient ionic conductivity across the layer. Our interface engineering strategy enables the first demonstration of a 4 V-class hydroborate-based all-solid-state battery combining a sodium metal anode and a cobalt-free Na-3(VOPO4)(2)F cathode without any artificial protective coating. When cycled to 4.15 V vs. Na+/Na, the cells feature a discharge capacity of 104 mA h g(-1) at C/10 and 99 mA h g(-1) at C/5, and an excellent capacity and energy retention of 78% and 76%, respectively, after 800 cycles at C/5 at