Size Effect of MgO on the Ionic Conduction Properties of a LiBH₄·1/2NH₃-MgO Nanocomposite

A solid-state electrolyte (SSE) is the core component for fabricating solid-state batteries competitive with the currently commercial Li-ion batteries. In the present study, a LiBH₄·1/2NH₃-MgO nanocomposite has been developed as a fast Li-ion conductor. The conductive properties depend strongly on the size of MgO nanopowders. By adding MgO nanoparticles, the first-order transition at 55 °C observed in the crystalline LiBH₄·1/2NH₃ is suppressed due to the conversion of LiBH₄·1/2NH₃ into the amorphous state. When the size of MgO decreases from 163.6 to 13.9 nm, the MgO amount required for the phase-transition suppression of LiBH₄·1/2NH₃ decreases linearly from 92 to 75 wt %, accompanied by a significant enhancement of ionic conductivity. The optimized nanocomposite with 75 wt % MgO of size 13.9 nm exhibits a pronouncedly high conductivity of 4.0 × 10^-3 S cm^-1 at room temperature, which is 20 times higher than that of the crystalline LiBH₄·1/2NH₃. Furthermore, a smaller size MgO contributes to a higher electrochemical stability window (ESW) owing to the stronger interfacial interaction via B-O bonds, i.e., an ESW of 4.0 V is achieved with the addition of 75 wt % MgO of size 13.9 nm.