Anionic Effects on Lithium‐Ion Transport in Highly Concentrated Lithium Salt/Propylene Carbonate Solutions

Highly concentrated electrolytes (HCEs) exhibit unique ion‐transport properties that fundamentally differ from those of conventional electrolytes; however, the role of anion species in governing Li + transport remains unknown. Herein, Li + ‐transport properties in lithium salt/propylene carbonate (LiX/PC) mixtures were systematically investigated by varying the basicity of the Lewis base anion: PF 6 − , N(SO 2 F) 2 − , N(SO 2 CF 3 ) 2 − , ClO 4 − , BF 4 − , and SO 3 CF 3 − (TfO − ). Ionic conductivity, viscosity, self‐diffusion coefficients, and Li + transference numbers under anion‐blocking conditions were evaluated and correlated with molecular‐scale structures obtained from molecular dynamics simulations. Weak Lewis‐base anions exhibited high ionic conductivity and coupled Li + ‐solvent diffusion at high salt concentrations. Conversely, strong Lewis‐base anions promoted ion‐pair and aggregate formation, resulting in structural diffusion of Li + and high transference numbers. Notably, Li + transference numbers increased with anion Lewis basicity and concentration, attaining 0.83 for LiTfO/PC = 1/2.5, while conductivity decreased, revealing an intrinsic tradeoff between these transport descriptors. Therefore, anion Lewis basicity critically governs ion association, correlated motion, and Li + ‐transport mechanisms in HCEs.