Controlling the Valence State of Cu Dopant in α-Fe2O3 Anodes: Effects on Crystal Structure and the Conversion Reactions with Alkali Ions

Abstract

Doping is one of the most important ways to tailor the performance of energy materials. However, the crystal structure of doped materials is usually oversimplified as a simple substitution of dopants. Here, we characterized the doped α-Fe2O3 with different Cu cations using synchrotron X-ray diffraction, X-ray absorption, and X-ray photoelectron spectroscopy, and electrochemically evaluated it as an anode in lithium batteries. The results suggest that doping is not the simple replacement of Fe3+ sites by Cu2+ or Cu+ but induces a complex local structure change, which may be a characteristic of this class of materials. In Cu+-doped samples, Cu+ not only replaces the Fe3+ site and distorts the FeO6 octahedra, but also gives rise to oxygen vacancies in CuO6 octahedra in the bulk structure and peroxides at the surface, leading to uniform precipitation of Cu as a conductive and buffering agent. These CuO6 octahedra also facilitate homogeneous reactions (electrochemical reduction of Cu+ and Fe3+ together) and th...