Electrolyte Engineering with Carboranes for Next-Generation Mg Batteries

To realize an energy storage transition beyond Li-ion competitive technologies, earth-abundant elements, such as Mg, are needed. Carborane anions are particularly well-suited to realizing magnesium-ion batteries (MIBs), as their inert and weakly coordinating properties beget excellent electrolyte performance. However, utilizing these materials in actual electrochemical cells has been hampered by the reliance on the Mg²⁺ salts of the commercially available [HCB₁₁H₁₁]^- anion, which is not soluble in more weakly binding solvents apart from the higher glymes. Herein, we demonstrate it is possible to iteratively engineer the [HCB₁₁H₁₁]^- anion surface synthetically to address previous solubility issues and yield a highly conductive (up to 7.33 mS cm^-1) and electrochemically stable (up to +4.2 V vs Mg^2+/0) magnesium electrolyte that surpasses the state of the art. This novel non-nucleophilic electrolyte exhibits highly dissociative behavior regardless of concentration and is tolerant of prolonged periods of cycling in symmetric cells at high current densities (up to 2.0 mA cm^-2, 400 h). The hydrocarbon functionalized carborane electrolyte presented here demonstrates >96% Coulombic efficiency when paired with a Mo₆S₈ cathode. This approach realizes a needed candidate to discover next-generation cathode materials that can enable the design of practical and commercially viable Mg batteries.