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

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 the formation of high quality solid-electrolyte interface (SEI) layers. All these factors account for its improved electrochemical performance (discharge capacity of 841(25) mAh/g against 758(21) mAh/g of undoped one, after 80 cycles at 100mA/g). In Cu²⁺ -doped samples, Cu²⁺ takes both Fe³⁺ and empty octahedral interstitial sites, forming linear clusters of three neighboring CuO₆ octahedra. Such medium-range phase separation causes electrochemical reduction to metallic Cu before the reduction of Fe³⁺, leading to inactive surface Cu that contributes to poor SEI layers and deteriorates its electrochemical performances. The present work allows a better understanding of how doping affects the crystallographic structures and offers insights into how this strategy can be employed to improve electrochemical performance, in contrast to the ambiguity over material properties associated with the commonly accepted model of simple atomic replacement.