Cathode Electrolyte Interphase (CEI) Endows Mo 6 S 8 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 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.