Nanoscale Mapping of Cu-Ion Transport in van der Waals Layered CuCrP₂S₆

Ionic conduction of metal thiophosphates (MTPs) is attracting growing attention for promising applications in electrochemical storage and tunable physical properties. Especially, metal-ion migration in copper thiophosphate has been identified as a key factor for the control of their microstructure and phase transition. However, direct evidence for the coupling between Cu-ion motions and the crystal lattice has been elusive at the nanometer scale. Here, the room temperature diffusion kinetics of Cu ions in layered CuCrP₂S₆ (CCPS) is demonstrated. A tip-enhanced electric field based on scanning probe microscopy (SPM) has been used as the driving force for Cu-ion motions through van der Waals gaps. The strong coupling between Cu-ion concentration and crystal lattice and the resulting serial structural transitions have been probed directly by the comprehensive utilization of spatially resolved Raman spectra, cross-section energy dispersion spectrum (EDS), and high-resolution transmission electron microscopy (HR-TEM). This knowledge improves the understanding of the effect of intrinsic Cu-ion migration on the structure transformation in layered van der Waals materials and provides feedback to the nanoscale mechanisms of nanometer devices based on iontronics.