Operando soft X-ray emission spectroscopy of the Fe₂O₃ anode to observe the conversion reaction

Drastic electronic-structure changes in an Fe₂O₃ thin film anode for a Li-ion battery during discharge (lithiation) and charge (delithiation) processes were observed using operando Fe 2p soft X-ray emission spectroscopy (XES). The conversion reaction forming metallic iron due to the lithiation reaction was confirmed by operando XES in combination with the analysis using full-multiplet calculation. The valence of Fe at the open-circuit voltage (OCV) before the second cycle was not Fe³⁺, but Fe²⁺ with a weak p-d hybridization, suggesting a considerable irreversibility upon the first discharge-charge cycle and a weakened Fe-O bond after the first cycle. Moreover, we revealed that the Fe 3d electronic-structure change during the second cycle was to some extent reversible as Fe²⁺ (2.7 V vs. Li/Li⁺: open circuit voltage) → Fe⁰ (0.1 V vs. Li/Li⁺: discharged) → Fe^(2+δ)+ (3.0 V vs. Li/Li⁺: charged). This operando Fe 2p XES in combination with the full-multiplet calculation provides detailed information for redox chemistry during a discharge-charge operation that cannot be obtained by other methods such as crystal-structure and morphology analyses. XES is thus very powerful for investigating the origin and limitation of the lithiation function of anodes involving conversion reactions.