Van Der Waals Ferroionic CuInP2S6: Emergent Properties and Device Application.

Low-dimensional van der Waals (vdW) ferroelectrics are promising for next-generation low-power, non-volatile electronics and brain-inspired computing. Among them, CuInP2S6 (CIPS) has emerged as one of the most intensively explored systems. Distinct from conventional ferroelectrics, CIPS features a strong "ferroionic" coupling between ferroelectric order and long-range Cu+ migration, unlocking unique properties such as multiple polarization states, negative capacitance, and richly tunable conductance states. To date, however, a comprehensive review centered on this ferroionic coupling remains lacking. This review aims to fill that gap by systematically elucidating the ferroionic coupling mechanism, summarizing its manipulation through chemical composition engineering and external fields, and clarifying the dynamic conductive responses and related mechanism. This review further surveys the high-performance CIPS-based nanoelectronic devices enabled by unique properties and concludes with an outlook on future challenges and research directions.