Nanoconfinement of an ammine magnesium borohydride composite electrolyte in a mesoporous silica scaffold

Abstract

Improvements of magnesium ionic conducting materials are important for the development of novel magnesium solid-state batteries. Here we investigate the effect of nanoconfinement of one of the most conductive magnesium electrolytes − the (Mg(BH4)2·NH3)x(Mg(BH4)2·2NH3)1−x composite. The synthesis of nanoconfined Mg(BH4)2·1.47NH3 in a mesoporous silica scaffold (SBA-15, pore size 5.8 nm) is achieved through melt infiltration. Various degrees of pore filling are investigated, ranging from 100 to 300 %. Solid-state 11B nuclear magnetic resonance analysis confirms the successful stabilization of the highly dynamic eutectic molten state of Mg(BH4)2·1.47NH3 through nanoconfinement. The confined sample exhibits notable thermal stability, maintaining integrity up to approximately 100 °C, and the eutectic molten composite does not recrystallize within five months after synthesis, upon storing at room temperature. Notably, among the confined samples, 200 % pore filling displays promising Mg2+ ionic conductivity within the range of 9.1 × 10−6 to 2.7 × 10−4 S cm−1 and a low activation energy of 0.69 eV at temperatures from 32 to 80 °C. Furthermore, the compound was found to have a low electronic conductivity of 7.93 × 10−11 S cm−1 at 70 °C resulting in an ionic transport number close to unity.