ABSTRACT Two‐dimensional (2D) van der Waals ferroelectrics have attracted interest for overcoming the scaling limitations of conventional ferroelectric materials owing to their atomic thickness, dangling‐bond‐free interfaces, and natural compatibility with heterostructure integration. Among them, 2D CuInP 2 S 6 (CIPS) is distinct because it simultaneously exhibits room‐temperature ferroelectricity and pronounced ionic conductivity within a single crystalline framework, making it a representative ferroionic material that intrinsically couples polarization order with ionic dynamics. Unlike conventional ferroelectrics, in which polarization reversal is primarily governed by small ionic displacements, the behavior of 2D CIPS is jointly determined by intralayer Cu off‐centering, interlayer Cu relocation, and longer‐range migration through van der Waals gaps. Recent advances further show that this coupled landscape gives rise to multiple polarization states, anomalous field‐driven switching pathways, configurable intralayer/interlayer migration kinetics, and device responses that can be dominated by ferroelectric switching, ionic migration, or their coupling. This review summarizes the crystal structure, Cu‐ion configurational dynamics, temperature and thickness effects, external regulation strategies, synthesis and sample‐quality issues, and device applications of 2D CIPS. Particular emphasis is placed on clarifying the interplay between ferroelectric polarization switching and Cu‐ion migration. This review connects reported observations in 2D CIPS and outlines the critical challenges and opportunities for ferroionic devices, including multistate memory, reconfigurable electronics, and neuromorphic computing.
Li et al. (Sat,) studied this question.