Charge-clustering induced fast ion conduction in 2LiX-GaF₃: A strategy for electrolyte design

2LiX-GaF₃ (X = Cl, Br, I) electrolytes offer favorable features for solid-state batteries: mechanical pliability and high conductivities. However, understanding the origin of fast ion transport in 2LiX-GaF₃ has been challenging. The ionic conductivity order of 2LiCl-GaF₃ (3.20 mS/cm) > 2LiBr-GaF₃ (0.84 mS/cm) > 2LiI-GaF₃ (0.03 mS/cm) contradicts binary LiCl (10^-12 S/cm) < LiBr (10^-10 S/cm) < LiI (10^-7 S/cm). Using multinuclear ⁷Li, ⁷¹Ga, ¹⁹F solid-state nuclear magnetic resonance and density functional theory simulations, we found that Ga(F,X) polyanions boost Li⁺-ion transport by weakening Li⁺-X^- interactions via charge clustering. In 2LiBr-GaF₃ and 2LiI-GaF₃, Ga-X coordination is reduced with decreased F participation, compared to 2LiCl-GaF₃. These insights will inform electrolyte design based on charge clustering, applicable to various ion conductors. This strategy could prove effective for producing highly conductive multivalent cation conductors such as Ca²⁺ and Mg²⁺, as charge clustering of carboxylates in proteins is found to decrease their binding to Ca²⁺ and Mg²⁺.