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

Abstract 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 NaHF 2 . As a representative example, Na 2 B 12 H 12 can react with NaHF 2 to form NaF nanoparticles that are uniformly embedded in the Na 2 B 12 H 11 F 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 Na 3 V 2 (PO 4 ) 3 cell shows high Coulombic efficiency. They give rise to a stable cycling of the Na||Na 3 V 2 (PO 4 ) 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.