Fast Lithium Ion Conduction in Arsenides Li₉AlAs₄ and Li₉GaAs₄

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 As3– in [TrAs4]9– (Tr = Al, Ga) has a lower negative charge density than P3– in [TrP4]9–, which leads to a smaller Coulomb force between Li+ and As3– than between Li+ and P3–. 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.