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(BH4)(2)center dot 2NH(3), as a solid Mg electrolyte that prepared by a NH3 redistribution between 3D framework-gamma-Mg(BH4)(2) and Mg(BH4)(2)center dot;6NH(3). Amorphous Mg(BH4)(2)center dot 2NH(3) exhibits a high Mg-ion conductivity of 5 x 10(-4) S cm(-1) at 75 ?, which is attributed to the fast migration of abundant Mg vacancies according to the theoretical calculations. Moreover, amorphous Mg(BH4)(2)center dot 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(BH4)(2)center dot 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||TiS2 cells.