Cathode Electrolyte Interphase (CEI) Endows Mo S with Fast Interfacial Magnesium-Ion Transfer Kinetics.

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 S cathode surface has been unprecedentedly constructed through the oxidation of the chloride-free magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip) ] ) salt under a proper charge cut-off voltage condition. The CEI has been identified to contain B O effective species originating from the oxidation of [B(hfip) ] anion. It is confirmed that the B O species is beneficial to the desolvation of solvated Mg , speeding up the interfacial Mg transfer kinetics, thereby improving the Mg -storage capability of Mo S host. The firstly reported CEI in Mg batteries will give deeper insights into the interface issues in multivalent electrochemical systems.