Chemical Compatibility of Solid-State Electrolytes with Hydroflux Cathode-Coating Process (Supporting Information)

We apply the hydroflux method as a potential cathode-coating process for solid-state batteries. The garnet-type Li-ion conductor Li_6.4La₃Zr_1.4Ta_0.6O₁₂ reacts with the precursor material of LiCoO₂. Water molecules in the molten alkaline hydroxide initiate Li⁺/H⁺ ion exchange and dissolution of the Zr⁴⁺ species. The NASICON-type lithium-ion conductor LATP also reacts with molten hydroxide owing to the formation of soluble species of Al(OH)₄⁻ under high pH conditions. Perovskite-type Li_0.33La_0.55TiO₃ is stable under the hydroflux condition because titanium and lanthanum do not form soluble species in alkaline solution. The chemical compatibility of the solid-state electrolyte is mostly estimated using Pourbaix diagram of each element in the system. The solid-state electrolyte containing only insoluble species in an alkaline solution is preferable for the hydroflux cathode-coating process.