Anion Effects on Cathode Electrochemical Activity in Rechargeable Magnesium Batteries: A Case Study of V₂O₅

One of the holy grails in research and development focused on rechargeable magnesium batteries is development of "conventional" electrolyte solutions that are compatible with both anode and cathode and support highly reversible magnesium electrochemical activity. In the last couple of years, MgTFSI₂, a "simple" salt, attracted considerable attention owing to its high solubility in a range of relevant solvents and apparent compatibility with magnesium anode and cathode materials. Nonetheless, questions were raised regarding the validity of the chemical and electrochemical inertness attributed to the TFSI anion, in particular when electrochemistry of magnesium is in the spotlight. Here, we demonstrate the impact of the TFSI anion on the intercalation kinetics of Mg ions into V₂O₅. The importance of this work is the fact that V₂O₅ is considered to be the most attractive high-voltage/high-capacity cathode for secondary Mg batteries, while MgTFSI₂ is considered a very important electrolyte for the same systems.