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, Li 9 AlAs 4 and Li 9 GaAs 4, and compare them with two previously reported phosphide-based superionic conductors, Li 9 AlP 4 and Li 9 GaP 4, 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 Li 9 GaAs 4 reached remarkable levels of 6.5 mS cm –1 and 2.05 × 10 –11 m 2 s –1, respectively, achieving an order of magnitude increase compared with Li 9 GaP 4 . Multinuclear solid-state NMR chemical shifts reveal that As 3– in [ Tr As 4 ] 9– ( Tr = Al, Ga) has a lower negative charge density than P 3– in [ Tr P 4 ] 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.