Remote synergy versus facet reconstruction in electrochemically driven interface evolution of Fe3C/Fe3O4/C and Cu2O/Cu/C anodes for potassium storage

The electrochemical evolution of heterointerfaces in conversion-type anodes has long been poorly understood. Whether engineered interfaces survive cycling, and if not, how separated phases maintain function, remains unclear. Using two model systems Fe 3 C/Fe 3 O 4 /C (inert conductive phase) and Cu 2 O/Cu/C (oxidizable conductive phase), we uncover two diametrically opposite evolution pathways that are dictated solely by the oxidation behavior of the conductive phase. In the inert Fe 3 C system, the initial core-shell interface completely dissociates. Yet the porous carbon network alone maintains electronic connectivity, creating a remote synergy mode in which the carbon network sustains battery operation by compensating for lost heterointerface contact, without requiring direct interfacial contact between the active phases. In the oxidizable Cu system, we directly visualize a phenomenon. Cu 2 O undergoes an electrochemically driven facet reconstruction from the (111) to the (200) orientation, which progressively lowers the overpotential for the Cu → Cu 2 O reaction and makes the electrode self-optimizing with cycling. Post-mortem electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy provide direct visual evidence for both pathways. Kinetic analysis shows mixed diffusion capacitive control, with the CCC system reaching a 46.5% capacitive contribution at 1.0 mV s −1 . Full cells with K-PBA cathodes achieve 75.0% capacity retention after 1000 cycles at 0.5 A g −1 , demonstrating practical viability. This work challenges the long-held assumption that interfacial retention is necessary for electrochemistry. Instead, it establishes a universal design principle for durable conversion-type anodes. Ensuring electron-transport pathway integrity, whether through direct contact or indirect network connectivity, is the key. The discovery of electrochemically driven facet reconstruction also opens new avenues for controlling surface crystallography under operating conditions.