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

2LiX-GaF (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-GaF has been challenging. The ionic conductivity order of 2LiCl-GaF (3.20 mS/cm) > 2LiBr-GaF (0.84 mS/cm) > 2LiI-GaF (0.03 mS/cm) contradicts binary LiCl (10 S/cm) < LiBr (10 S/cm) < LiI (10 S/cm). Using multinuclear Li, Ga, F 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-GaF and 2LiI-GaF, Ga-X coordination is reduced with decreased F participation, compared to 2LiCl-GaF. 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 Ca and Mg, as charge clustering of carboxylates in proteins is found to decrease their binding to Ca and Mg.