Cathode Electrolyte Interphase (CEI) Endows Mo6S8 with Fast Interfacial Magnesium‐Ion Transfer Kinetics

Abstract Magnesium (Mg) metal secondary batteries have attracted much attention for their high safety and high energy density characteristics. However, the significant issues of the cathode/electrolyte interphase (CEI) in Mg batteries are still being ignored. In this work, a significant CEI layer on the typical Mo 6 S 8 cathode surface has been unprecedentedly constructed through the oxidation of the chloride‐free magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip) 4 ] 2 ) salt under a proper charge cut‐off voltage condition. The CEI has been identified to contain B x O y effective species originating from the oxidation of [B(hfip) 4 ] − anion. It is confirmed that the B x O y species is beneficial to the desolvation of solvated Mg 2+ , speeding up the interfacial Mg 2+ transfer kinetics, thereby improving the Mg 2+ ‐storage capability of Mo 6 S 8 host. The firstly reported CEI in Mg batteries will give deeper insights into the interface issues in multivalent electrochemical systems.