Triggering Fast Lithium Ion Conduction in LiPS₄I

All-solid-state batteries are receiving keen interest as emerging alternatives to conventional liquid electrolyte batteries owing to potential benefits that include the possibility of higher energy densities, enablement of anode-less designs, a wider range of temperature operation, and improvement in battery safety. The discovery and development of new solid-state electrolytes is a critical factor. One sought-after material is Li₄PS₄I that has long been predicted to exhibit high ionic conductivity, while experiment has disappointingly proven the contrary. Here, we address this long-standing issue and show that inducing Li sublattice disorder is key. We demonstrate the remarkable effects of aliovalent substitution in Li_4+xP_1–xSi_xS₄I (x = 0.12 and 0.30) using a combination of single-crystal X-ray and powder neutron diffraction; Raman and impedance spectroscopy; ab initio molecular dynamics simulations; and the bond valence site energy approach. With increasing Si⁴⁺ and thus Li⁺ content in Li_4+xP_1–xSi_xS₄I, configurational disorder of the Li sublattice is induced, leading to isotropic 3D-fast Li ion conductivity of 1.46 mS·cm^–1 at room temperature for the Li_4.3P_0.7Si_0.3S₄ composition and a low activation energy of 0.32 eV. The unit cell volume is half that of the parent phase, Li₄PS₄I, which exhibits a fully ordered Li substructure that explains the latter’s poor conductivity and high activation energy (0.046 mS·cm^–1 and 0.44 eV, respectively). The deliberate creation of Li sublattice disorder (via Li ion “stuffing”) is a pivotal strategy toward the development of new, fast ion conductors.