Triggering Fast Lithium Ion Conduction in Li₄PS₄I

Abstract

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 Li4+x P1–x Si x S4I (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 Li4+x P1–x Si x S4I, 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 Li4.3P0.7Si0.3S4 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.