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 PO₄^3-, 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 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 SO₄^2--containing compounds and Na₃P is kinetically favored over the formation of PO₄^3--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. © 2017 American Chemical Society.