Stable All‐Solid‐State Na Batteries Enabled by In Situ Formed Na─B─H─F Electrolyte.

All‐solid‐state Na‐ion batteries are considered to be one of the most promising candidates for large‐scale applications at low‐cost. Hydride electrolytes have been pursued as a research hotspot more recently, owing to their high Na‐ion conductivities, while there is an urgent need to address their electrochemical stabilities in order to meet the requirements for applications in Na batteries. Herein, a novel and universal strategy is proposed to improve the electrochemical stabilities of hydride electrolytes, which takes advantage of the in situ reactions between hydrides and NaHF2. As a representative example, Na2B12H12 can react with NaHF2 to form NaF nanoparticles that are uniformly embedded in the Na2B12H11F matrix, which exhibits superior electrode compatibilities without apparent reduction in conductivity. The symmetrical Na cell thus derived shows a long‐term cycling and the quasi‐symmetrical Na3V2(PO4)3 cell shows high Coulombic efficiency. They give rise to a stable cycling of the Na||Na3V2(PO4)3 all‐solid‐state batteries with a capacity retention of 87.7% after 100 cycles, which can be attributed to the stabilization of electrolyte/electrode interfaces by the F‐enriched interphases. Beyond Na batteries, the present study sheds timely new light on the development of hydride electrolytes for other long‐waited applications. [ABSTRACT FROM AUTHOR]