First-Principles Insights into Lithium-Rich Ternary Phosphide Superionic Conductors: Solid Electrolytes or Active Electrodes

Lithium-rich ternary phosphides are recently found to possess high ionic conductivity and are proposed as promising solid electrolytes (SEs) for solid-state batteries. While lithium ions can facilely transport within these materials, their electrochemical and interfacial stability in complex battery setups remain largely uncharacterized. We study the phase stability and electrochemical stability of phosphide-type SEs via first-principles calculations and thermodynamic analysis. Our results indicate that these SEs have intrinsic electrochemical stability windows narrower than 0.5 V. The SEs exhibit low anodic limits of about 1 V vs Li/Li+ due to the oxidation of the electrolytes to form various P binary compounds, indicating the poor electrochemical stability in contact with the cathode. In particular, we find that the thermodynamic driving force of such electrochemical decomposition is critically dependent on the new phases formed at the interfaces. Therefore, these phosphides might not be suitable as electrolytes. Despite the electrochemical instability, further calculations of Li diffusion kinetics show that the Li conduction is highly efficient through face-sharing octahedral and tetrahedral sites with low energy barriers, in spite of the possible variation of the local environments. In addition, an analysis of the terminal decomposition products shows impressive Li storage capacity as high as 2547 mAh.g(-1) based on the conversion mechanism, indicating they are capable as high-rate and energy-dense anode materials for battery applications.