Fast Lithium Ion Conduction in Arsenides Li9AlAs4 and Li9GaAs4

Abstract

Understanding the structure–property relationship in ionic conductors is crucial for developing more efficient solid-state electrolytes and improving cell architecture. Here, we present two new arsenide-based fast ion conductors, Li₉AlAs₄ and Li₉GaAs₄, and compare them with two previously reported phosphide-based superionic conductors, Li₉AlP₄ and Li₉GaP₄, to study the substitution effect of element P by As. It is established that anion substitution dramatically enhances lithium-ion mobility and conductivity. In particular, the ionic conductivity and diffusion coefficient at room temperature of Li₉GaAs₄ reached remarkable levels of 6.5 mS cm^–1 and 2.05 × 10^–11 m² s^–1, respectively, achieving an order of magnitude increase compared with Li₉GaP₄. Multinuclear solid-state NMR chemical shifts reveal that As^3– in [TrAs₄]^9– (Tr = Al, Ga) has a lower negative charge density than P^3– in [TrP₄]^9–, which leads to a smaller Coulomb force between Li⁺ and As^3– than between Li⁺ and P^3–. This weakened Coulomb force on lithium ions, caused by As substitution, together with an enlarged lattice volume, lowers the activation barrier and promotes Li ion conductivity.