Low Melting Alkali-Based Molten Salt Electrolytes for Lithium Metal Batteries

Liquid electrolytes for lithium metal batteries (LMBs) raise safety concerns because the highly volatile and flammable organic solvents hinder their practical deployment. In contrast, solid-state electrolytes promise improved safety but suffer from poor electrode/electrolyte interfaces. In this work, we develop nonvolatile and nonflammable low melting alkali-based molten salts as electrolytes for LMBs. A ternary mixture of bis(fluorosulfonyl)imide salts shows a eutectic temperature at 45 ^oC and exhibits higher current densities and improved cycling compared to nonvolatile ionic liquids, polymer composites, and inorganic solid-state conductors. The molten salt electrolyte exhibits excellent compatibility with both Li metal anodes (Coulombic efficiency ~99.8% and fast activation within five cycles) and high-voltage cathodes (~6 V stability). Li||NMC batteries retain 80% capacity after 100 cycles with an average Coulombic efficiency of 99.98%. Our low-melting molten salts provide a new class of electrolytes for the development of high-energy, long-cycling Li metal batteries.