(Invited) Ion Conduction and Interface Stability of Sulfide Based Solid State Electrolytes – an Atomistic Perspective

Abstract

All-solid-state batteries are promising in terms of safety as well as high energy density compared to the conventional organic liquid-based lithium batteries. However, the characteristic low ionic conductivity of solid-state electrolytes is a major challenge towards commercialization of solid-state sodium ion batteries. Sulfide-based electrolytes especially in amorphous form have been reported as promising solid electrolytes owing to their relatively high ionic conductivity at room temperature. However, a fundamental understanding of the local structure of these amorphous electrolytes and its subsequent impact on the ion transport could be instrumental in establishing guidelines for designing novel solid electrolytes. In the present work, we utilize first principles and classical atomistic simulations to characterize the local structure and investigate the ion transport in amorphous sulfide electrolytes. We selected sodium thiosilicate [xNa2S – (1-x) SiS2] and sodium thiophosphate [xNa2S – (1-x) P2S5] based electrolytes as a model system wherein we characterized the local structure, ion conduction mechanism and ultimately calculated the ionic conductivity of these electrolytes. We utilized experimental X-ray and neutron scattering data for model validation. Our theoretical calculations provide fundamental insights into ion conduction mechanisms as well as correlate ionic conductivity with electrolyte structure and composition.Along with ionic conductivity, interfacial stability is extremely important factor influencing the overall performance of solid-state batteries. Interfacial instability with sulfide electrolytes is detrimental to Li-ion transport, leading to poor cycling performance. Inherent thermodynamic instability of sulfide-based solid electrolytes drives the chemical reaction across the interface leading to formation of undesired secondary phases. The secondary phases are formed dynamically during the cycling and therefore a careful investigation into the dynamics at the electrolyte-cathode interfaces is crucial. To generate fundamental understanding of the interface stability, we utilized ab initio molecular dynamics (AIMD) simulations to carefully investigate the dynamics of secondary phase formation across the Li3PS4 | LiCoO2 interfaces. High-resolution microscopy and spectroscopy studies were used to complement the the first principles simulations methods and unravel the interphases formed under different cycling conditions. These calculations provide crucial insights into formation of secondary phases across the interface, which could be leveraged to evaluate possible avenues to inhibit formation of such undesirable secondary phases.