Kinetic surface control for improved magnesium-electrolyte interfaces for magnesium ion batteries

Magnesium (Mg) metal is a compelling battery anode material for Mg ion batteries because of its high volumetric energy density, good operational safety and abundance. The metal promises extremely high energy density and much safer compared to Li metal due to less dendrite growth. However, most conventional battery electrolytes react with Mg to form blocking layers. Much attention has been dedicated to synthesizing electrolytes that are thermodynamically stable with respect to Mg metal. Here we report a new approach to improving the (electro) chemical compatibility of the Mg-electrolyte interface by exerting kinetic control instead. A chemically inert magnesium fluoride (MgF2) layer is formed through controlled reaction of Mg surface with hydrofluoric acid. The tailored surface layer improves the voltage stability, Coulombic efficiency, and cycling performance of full-cell Mg ion batteries compared to cells using bare Mg metal as an anode. Remarkably, unlike almost all other Mg surface films, the MgF2 passivating layer not only suppresses side reactions with the electrolyte but also allows for Mg2+ transport between the electrode and electrolyte. This result reinforces the importance of controlling Mg surface chemistry for the successful development of high-energy magnesium ion batteries.