Achieving reversible Mg chemistry by tunning electric double layer structure with highly-fluorinated asymmetric magnesium salt

Abstract

The interfacial instability of the Mg metal in chlorine-free magnesium electrolytes severely hinders the practical applications of rechargeable magnesium batteries (RMBs). Conventionally, the reversibility and longevity of Mg metal have been primarily associated with the bulk Mg²⁺ solvation structure in organic electrolytes. However, the electric double layer (EDL) is the real microscopic region where all electrochemical reactions occur. With the region, both electronic and ionic charges undergo periodic rearrangement with surface charge fluctuation during electrochemical processes. Consequently, the structure and properties of EDL critically influence the Mg electroplating process, which has been severely overlooked for a long time. Here, for the first time, we propose a novel strategy of tuning the anion structure in Mg salt to rearrange EDL, thereby simultaneously regulating the uniformity of electroplated Mg morphology and building an inorganic-rich, high-quality solid electrolyte interphase (SEI) layer, achieving a highly reversible Mg anode. We designed a novel highly fluorinated asymmetric Mg salt, magnesium perfluoro(2-ethoxyethane)sulfonate (Mg(PES)₂), which endows the Mg metal anode with superior Coulombic efficiency (CE) of over 99.2%, ultra-low overpotential of 20 mV and remarkable cyclability of over 4500 h. This work introduces a new strategy for multivalent cation anodes, emphasizing interfacial chemistry regulation over traditional bulk solvation structure design. We believe this insightful approach will open up new frontiers in the design for advanced multivalent electrolyte development.