Formation of Amorphous H₃Zr₂Si₂PO₁₂ by Electrochemical Substitution of Sodium Ions in Na₃Zr₂Si₂PO₁₂ with Protons

The sodium ions in Na₃Zr₂Si₂PO₁₂ (NASICON) were substituted with protons using an electrochemical alkali-proton substitution (APS) technique at 400 °C under a 5% H₂/95% N₂ atmosphere. The sodium ions in NASICON were successfully substituted with protons to a depth of <400 μm from the anode. Completely protonated NASICON, i.e., H₃Zr₂Si₂PO₁₂, was obtained to a depth <40 μm from the anode, although complete protonation of NASICON cannot be achieved by ion exchange in aqueous acid. H₃Zr₂Si₂PO₁₂ was amorphous, whereas the partially protonated NASICON was crystalline, and its unit cell volume decreased with an increase in the extent of substitution. Amorphous H₃Zr₂Si₂PO₁₂ was prepared by pressure-induced amorphization of the NASICON framework, in which an internal pressure of -3.5 GPa was induced by the substitution of large sodium ions with small protons during APS at 400 °C.