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: PF₆ ^-, N(SO₂F)₂ ^-, N(SO₂CF₃)₂ ^-, ClO₄ ^-, BF₄ ^-, and SO₃CF₃ ^- (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.