Magnesocene-Based Electrolytes: A New Class of Electrolytes for Magnesium Batteries

Abstract Unlike ferrocene, bis(η 5 ‐cyclopentadienyl)magnesium (magnesocene, MgCp 2 ) is slightly dissociated in solvents, such as ethers, resulting in electrolyte solutions with low conductivity. MgCp 2 /tetrahydrofuran solutions make possible reversible magnesium plating and stripping with low over‐potentials for many cycles. The Mg deposits appear with a cauliflower‐like morphology. IR and NMR spectroscopy confirm that the electrolyte is stable and not decomposed during prolonged cycling. The anodic stability limit is in the range of 1.5 V (at platinum) and 1.8 V versus Mg/Mg 2+ (at stainless steel), which may be sufficient for low‐voltage cathode materials. MgCp 2 is a first example of a completely new class of halide‐free electrolytes, which may open up a new research direction for future magnesium metal and magnesium‐ion batteries.