Anion and Cation (Co-)intercalation Behavior from an Inorganic Molten Salt for Dual-Ion Batteries

Abstract Long-duration energy storage is needed to accelerate decarbonization. Lithium-ion batteries are currently state-of-the-art but require costly transition metal oxide cathodes, which can limit their scalability. Dual-graphite batteries are a promising chemistry featuring a graphite anode and graphite cathode, where cation and anion intercalation occur at the former and latter electrodes, respectively. Conventional small-molecule-based electrolytes face safety issues due to their flammability and volatility, and they limit the energy density of dual-graphite batteries due to the presence of species other than the active ions. Molten salt electrolytes are thus a promising alternative to conventional dual-graphite battery electrolytes due to their intrinsic safety and energy density. Here, we report the investigation of a low-melting, single-cation imide-based binary molten salt electrolyte to enable “Carbon-and-Salt” batteries. We find that both anions in the Li(FSI)x(FTFSI)1–x electrolytes that were studied reversibly intercalate into graphite cathodes, but that FTFSI– preferentially intercalates and is present in greater compositions in anion-graphite intercalation compounds compared to the bulk electrolytes. Furthermore, we find that lithium co-intercalates with the anions, suggesting the inability of anions to fully de-solvate from tightly-bound aggregate networks. The systematic investigation of anion and cation (co-)intercalation behavior from imide-based molten salts is important to guide the design of next-generation solvent-free and transition-metal-free “Carbon-and-Salt” batteries.