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 PO 4 3–, 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 Na 2 Ti 3 O 7 anode is predicted to be much more stable against a broad range of solid electrolytes. We identify several promising binary oxides, Sc 2 O 3, SiO 2, TiO 2, ZrO 2, and HfO 2, that are similarly or more chemically compatible with most electrodes and solid electrolytes than the commonly used Al 2 O 3 is. Finally, we show that ab initio molecular dynamics simulations of the NaCoO 2 /Na 3 PS 4 interface model predict that the formation of SO 4 2– -containing compounds and Na 3 P is kinetically favored over the formation of PO 4 3– -containing compounds, in contrast to the predictions of the thermodynamic models. 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.