The Quest for Stable Sodium-Ion Batteries in Molten Salt Electrolyte

Sodium-ion batteries (SIBs) offer a cost-effective and sustainable solution for grid storage and short-range electric vehicles. However, their advancement is hampered by safety concerns from flammable organic electrolytes and performance limitations of solid-state alternatives. Molten salt electrolytes (MSEs), specifically a blend of sodium and potassium bis(fluorosulfonyl)imide (NaKFSA), offer a promising, nonflammable, and thermally stable solution compatible with existing manufacturing processes. Despite this potential, MSE compatibility with advanced SIB electrodes remains understudied. This work evaluates the NaKFSA MSE paired with high-performance cathodes (Na3V2(PO4)3, Na2.4Fe1.8(SO4)3, Na4Fe3(PO4)2P2O7) and anodes (hard carbon, Li4Ti5O12, micro-Sb). In half-cells, all cathodes compete with conventional ester/ether electrolyte performance. Among anodes, micro-Sb exhibited exceptional Na+ storage, while Li4Ti5O12 cycled stably (with initial irreversibility), and hard carbon showed limited capacity. Crucially, a full cell integrating the Na2.4Fe1.8(SO4)3 cathode, micro-Sb anode, and NaKFSA electrolyte demonstrated practical viability. This study underscores electrode–electrolyte compatibility as key to developing inherently safe, high-performance molten salt SIBs, bridging fundamental research and application.