Interface Engineering Endowing High Reversibility of SPAN Cathode and Mg Metal Anode for Mg‐SPAN Batteries

Abstract Magnesium–sulfur (Mg–S) batteries have garnered considerable attention due to their high energy density, dendrite free, and low cost. However, the sluggish interfacial kinetics of the Mg anode and the polysulfide shuttling effect of the sulfur cathode severely impede their practical implementation. Herein, a novel chlorine‐free ether‐based electrolyte is developed by incorporating 3‐methoxypropylamine (MOPA) co‐solvent and Mg 2+ /Li + dual salts, which simultaneously endow the high reversibility of the Mg anode and sulphurised polyacrylonitrile (SPAN) cathode via interface engineering. Experimental and theoretical analyses prove the formation of organic–inorganic solid‐electrolyte interphase (SEI) on Mg anode and Mg 3 N 2 ‐rich cathode‐electrolyte interphase (CEI) on SPAN cathode, which effectively suppresses interfacial side reactions and enhances Mg 2+ migration kinetics. Consequently, the Mg‐SPAN full cell exhibits exceptional cycling stability, rate capability, and high Coulombic efficiency (CE), that is, a reversible capacity of 941.42 mAh g s −1 with 99.38% CE after 300 cycles at 0.1C (25 °C) and 530 mAh g s −1 with 99.92% CE after 180 cycles at 0.5C (50 °C), while maintaining excellent rate capability. This work highlights the pivotal roles of solvation structures and electrode‐electrolyte interfaces to cell performance, providing fundamental insights for the development of advanced SPAN‐based batteries.