Establishing a Stable Anode-Electrolyte Interface in Mg Batteries by Electrolyte Additive

Simple magnesium salts with high electrochemical and chemical stability and adequate ionic conductivity represent a new-generation electrolyte for magnesium (Mg) batteries. Similar to other Mg electrolytes, the simple-salt electrolyte also suffers from high charge-transfer resistance on the Mg surface due to the adsorbed species in the solution. In the current study, we built a model Mg cell system with the Mg[B(hfip)₄]₂/DME electrolyte and Chevrel phase Mo₆S₈ cathode, to demonstrate the effect of such anode-electrolyte interfacial properties on the full-cell performance. It was found that the cell required additional activation cycles to achieve its maximal capacity. The activation process is mainly attributed to the conditioning of the anode-electrolyte interface, which could be boosted by introducing an additive amount of Mg(BH₄)₂ to the Mg[B(hfip)₄]₂/DME electrolyte. Electrochemical and spectroscopic analyses revealed that the Mg(BH₄)₂ additive helps to remove the native oxide layer and promotes the formation of a solid electrolyte interphase layer on Mg. As a result, the full cell with the additive-containing electrolyte delivered a stable capacity from the second cycle onward. Further battery tests showed a reversible cycling for 600 cycles and an excellent rate capability, indicating good compatibility of the Mg(BH₄)₂ additive. The current study not only provides fundamental insights into the interfacial phenomena in Mg batteries but also highlights the facile tunability of the simple-salt Mg electrolytes.