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 Mg2+/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.