Ether-Functionalized Asymmetric Lithium Imide Salts: Molecular Design Principles for Low-Melting Molten Salt Electrolytes

Abstract

Solvent-free molten lithium salts have attracted growing interest as intrinsically safe electrolytes with high lithium-ion transference numbers; however, their practical application has been limited by high melting temperatures and insufficient transport. Here, we systematically investigate a series of asymmetric lithium imide salts bearing alkyl- and ether-functionalized anions to elucidate molecular design principles governing melting behavior, supercooling stability, and ion transport in molten salt electrolytes. Incorporation of short ether segments proved critical for melting-point depression, with anions containing two ether oxygen atoms undergoing genuine solid-liquid transitions around 100 °C, whereas related salts with restricted conformational freedom exhibit only solid-solid transitions or remain solid. These diether motifs provide an optimal balance between enhanced conformational entropy and Li+ coordination strength. Molecular dynamics simulations show that Li+ ions preferentially coordinate to the imide and sulfonyl moieties, while terminal ether groups remain barely coordinated, leading to pronounced Li+-anion clustering and nanoscale ionic/non-ionic segregation in the molten state. Consistent with this picture, the molten ether-functionalized lithium salts exhibit nearly identical Li+ and anion diffusion coefficients, near-unity lithium-ion transference numbers, and prolonged supercooled lifetimes. Electrochemical measurements revealed that interfacial charge-transfer kinetics are governed primarily by anion chemistry rather than bulk transport properties, highlighting the importance of interfacial design in molten salt electrolytes. Overall, this study clarifies how controlled ether functionalization modulates melting, coordination structure, and ion transport in molten lithium salts, and provides molecular-level design guidelines for next-generation solvent-free electrolytes for lithium batteries.