Probing Solid–Solid Interfacial Reactions in All-Solid-State Sodium-Ion Batteries with First-Principles Calculations

We present an exposition of first-principles approaches to elucidating interfacial reactions in all-solid-state sodium-ion batteries. We will demonstrate how thermodynamic approximations based on assumptions of fast alkali diffusion and multispecies equilibrium can be used to effectively screen combinations of Na-ion electrodes, solid electrolytes, and buffer oxides for electrochemical and chemical compatibility. We find that exchange reactions, especially between simple oxides and thiophosphate groups to form PO43–, are the main cause of large driving forces for cathode/solid electrolyte interfacial reactions. A high reactivity with large volume changes is also predicted at the Na anode/solid electrolyte interface, while the Na2Ti3O7 anode is predicted to be much more stable against a broad range of solid electrolytes. We identify several promising binary oxides, Sc2O3, SiO2, TiO2, ZrO2, and HfO2, that are similarly or more chemically compatible with most electrodes and solid electrolytes than the commonly used Al2O3 is. Finally, we show that ab initio molecular dynamics simulations of the NaCoO2/Na3PS4 interface model predict that the formation of SO42–-containing compounds and Na3P is kinetically favored over the formation of PO43–-containing compounds, in contrast to the predictions of the thermodynamic models. As a result, this work provides useful insights into materials selection strategies for enabling stable electrode/solid electrolyte interfaces, a critical bottleneck in designing all-solid-state sodium-ion batteries, and outlines several testable predictions for future experimental validation.