Borohydride Ammoniate Solid Electrolyte Design for All‐Solid‐State Mg Batteries

Searching for novel solid electrolytes is of great importance and challenge for all‐solid‐state Mg batteries. In this work, we develop an amorphous Mg borohydride ammoniate, Mg(BH 4 ) 2 ·2NH 3 , as a solid Mg electrolyte that prepared by a NH 3 redistribution between 3D framework‐γ‐Mg(BH 4 ) 2 and Mg(BH 4 ) 2 ·6NH 3 . Amorphous Mg(BH 4 ) 2 ·2NH 3 exhibits a high Mg‐ion conductivity of 5 × 10 −4 S cm −1 at 75 °C, which is attributed to the fast migration of abundant Mg vacancies according to the theoretical calculations. Moreover, amorphous Mg(BH 4 ) 2 ·2NH 3 shows an apparent electrochemical stability window of 0–1.4 V with the help of in‐situ formed interphases, which can prevent further side reactions without hindering the Mg‐ion transfer. Based on the above superiorities, amorphous Mg(BH 4 ) 2 ·2NH 3 enables the stable cycling of all‐solid‐state Mg cells, as the critical current density reaches 3.2 mA cm −2 for Mg symmetrical cells and the reversible specific capacity reaches 141 mAh g −1 with a coulombic efficiency of 91.7% (first cycle) for Mg||TiS 2 cells.