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 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.

Direct Cation–Cation Interactions Induced by Mg Dopants for Electron–Gas Behavior in α-Fe2O3

“…The doping also induces the shift of O 1s signals toward lower binding energy from 529.9(1) to 529.6(1) eV (Figure 3f). Such shifts were not observed in Fe 2 O 3 with other divalent dopants, 12 and, thus, it is likely that the specific structure 34 The T M determined from the temperature-dependent magnetization under a field of 500 Oe is 249.5( 5) and 232.6(5) K under ZFC and FC, respectively (Figures 4a and S8). The antiferromagnetic coupling is significantly decreased by Mg doping, as characterized by the weak Morin transition and lower T M in Mg4 (Figure 4b,c), which is reduced to 197.6(5) and 142.7(5) K under ZFC and FC, respectively.…”

“…Despite the subtle structural change, the variation of the electronic configuration is suggested in some doped Fe 2 O 3 , generally resulting from the electron transfer or crystal-field splitting caused by dopants. − According to density functional theory (DFT) calculations, the reduced Fe moments can be a result of Fe 3+ reduction to Fe 2+ by electron transfer from high-valent dopants (e.g., Ti 4+ and Sn 4+ ), though no Fe 2+ was observed experimentally in these doped materials . Furthermore, neutron diffraction experiments have shown that most of these doped Fe 2 O 3 still retain the antiferromagnetic ground state identical to that of Fe 2 O 3 .…”

Recovery of High Purity Chlorine by Cu‐Doped Fe2O3 in Nitrogen Containing Carbon Matrix: A Bifunctional Electrocatalyst for HCl Electrolysis

“…Moreover, the peak at 950 eV and satellite peak at 956 eV corresponds to Cu 2p 1/2. In the end, XP spectra of Fe 2p was deconvoluted and found that it has two characteristic peaks at 712.3 eV and 724.4 eV for 2p 3/2 and 2p 1/2 with a peak separation of 12.1 eV which validate that catalyst contains iron as Fe 2+ [38,46] . More interestingly, the peak at 714.3 eV evidenced the presence of Fe 3+ as well (Figure 3f) which is in well agreement with the reported literature.…”

All‐In‐One Stainless‐Steel Mesh Oxide Composites Anode for Flexible Li‐Ion Battery

“…The strong peak at 0.63 V attributes to the formation of SEI layers which disappear in the next cycle and the peak at 0.75 V may be the attributed to the Li + insertion of MOx on the surface, whereas the broad peak is associated with the formation of a cubic Li–Fe–O phase because of Li insertion. [ 23 ] The CV curves of SSM‐MOx‐900 ( Figure a) change significantly: 1) The sharp reduction peak occurs between 0.01 and 0.3 V derived from the insertion of the formed hybrid MnO, SiO 2 , and Cr 2 O 3 ; 2) The reduction peaks of Fe 2 O 3 occur at 0.92 V; 3) The relative area of the CV curves of SSM‐MOx‐900 is much larger than that of SSM, indicating that the capacity density of SSM‐MOx‐900 is much larger than that of SSM. 4) The relative area of SSM‐MOx‐900 from the 1st cycle to the 2nd cycle is much smaller, reflecting the lower consumption of Li + during SEI formation.…”