Fast Lithium\nIon Conduction in Arsenides Li<sub>9</sub>AlAs<sub>4</sub> and Li<sub>9</sub>GaAs<sub>4</sub>

Understanding the structure–property relationship\nin ionic\nconductors is crucial for developing more efficient solid-state electrolytes\nand improving cell architecture. Here, we present two new arsenide-based\nfast ion conductors, Li<sub>9</sub>AlAs<sub>4</sub> and Li<sub>9</sub>GaAs<sub>4</sub>, and compare them with two previously reported phosphide-based\nsuperionic conductors, Li<sub>9</sub>AlP<sub>4</sub> and Li<sub>9</sub>GaP<sub>4</sub>, to study the substitution effect of element P by\nAs. It is established that anion substitution dramatically enhances\nlithium-ion mobility and conductivity. In particular, the ionic conductivity\nand diffusion coefficient at room temperature of Li<sub>9</sub>GaAs<sub>4</sub> reached remarkable levels of 6.5 mS cm<sup>–1</sup> and 2.05 × 10<sup>–11</sup> m<sup>2</sup> s<sup>–1</sup>, respectively, achieving an order of magnitude increase compared\nwith Li<sub>9</sub>GaP<sub>4</sub>. Multinuclear solid-state NMR chemical\nshifts reveal that As<sup>3–</sup> in [<i>Tr</i>As<sub>4</sub>]<sup>9–</sup> (<i>Tr</i> = Al, Ga) has a\nlower negative charge density than P<sup>3–</sup> in [<i>Tr</i>P<sub>4</sub>]<sup>9–</sup>, which leads to a smaller\nCoulomb force between Li<sup>+</sup> and As<sup>3–</sup> than\nbetween Li<sup>+</sup> and P<sup>3–</sup>. This weakened Coulomb\nforce on lithium ions, caused by As substitution, together with an\nenlarged lattice volume, lowers the activation barrier and promotes\nLi ion conductivity.