Proton-Driven Intercalation and Ion Substitution Utilizing Solid-State Electrochemical Reaction

The development of an unconventional synthesis method has a large potential to drastically advance materials science. In this research, a new synthesis method based on a solid-state electrochemical reaction was demonstrated, which can be made available for intercalation and ion substitution. It was referred to as proton-driven ion introduction (PDII). The protons generated by the electrolytic dissociation of hydrogen drive other monovalent cations along a high electric field in the solid state. Utilizing this mechanism, Li⁺, Na⁺, K⁺, Cu⁺, and Ag⁺ were intercalated into a layered TaS₂ single crystal while maintaining high crystallinity. This liquid-free process of ion introduction allows the application of high voltage around several kilovolts to the sample. Such a high electric field strongly accelerates ion substitution. Actually, compared to conventional solid-state reaction, PDII introduced 15 times the amount of K into Na super ionic conductor (NASICON)-structured Na_3-xK_xV₂(PO₄)₃. The obtained materials exhibited a thermodynamically metastable phase, which has not been reported so far. This concept and idea for ion introduction is expected to form new functional compounds and/or phases.