Electrochemical Insertion of Magnesium into Hydrated Vanadium Bronzes

The Mg²⁺insertion into layered vanadium bronzes, MeV₃O₈(H₂O)_y (Me = Li, Na, K, Ca_0.5, and Mg_0.5), has been studied with regard to their use as positive electrodes of magnesium ion transfer batteries. In acetonitrile-based electrolytes maximum specific charges of ~200 Ah/kg were measured but the charge decreases rapidly with the increasing cycle number. A salt melt (liquid at room temperature) based on MgCl₂, AlCl₃, and l-ethyl-3-methylimidazolium chloride, offers an interesting alternative to common aprotic electrolytes. It is possible to insert Mg²⁺electrochemically from the salt melt into the bronzes. The behavior of all bronzes is similar in this electrolyte with the exception of the first few cycles. Steady state is reached after about five cycles, and a reversible insertion and expulsion of Mg²⁺is observed for all bronzes. Specific charges of up to 150 Ah/kg for Mg²⁺ insertion were measured in the first cycle, and >80 Ah/kg can be utilized during 60 deep cycles. Variations in the content of bound lattice water in the bronzes are responsible for a difference in the electrochemical properties of the same starting material dried at different temperatures. The presence of this water seems to be essential. Unfortunately, the lattice water is removed during cycling.