Understanding Mg-ion deposition behavior on MgBi alloy in solid-state form

Mg alloys have frequently been studied as anodes for Mg-ion batteries due to their high specific capacity and low electrochemical potential. In the present study, we investigated the interfacial stability of MgBi alloy anodes with solid-state electrolytes. The bubble-like solid electrolyte interface (SEI) was observed between the MgBi alloy anode and Mg(BH₄)₂·1.9NH₃ solid-state electrolyte, leading to the unstable Mg stripping/plating on the MgBi alloy. Theoretical simulations suggest that the bubble-like SEI originates from the different Mg-ion dynamics on the eutectic region (e.g., Mg + Mg₃Bi₂ phases) and the Mg matrix. The addition of MgBr₂·2NH₃ nanoparticles in Mg(BH₄)₂·1.9NH₃ suppresses the formation of a bubble-like SEI through the etching effect of Br- ions. Consequently, interfacial resistance is lowered and the interfacial stability is drastically enhanced, e.g., Mg stripping/plating for over 1,200 cycles at 0.1 mA cm^-2 with a low overpotential around 0.05 V.