NSRRC Activity Report 2022

060 NSRRC ACTIVITY REPORT 2022 Fig. 1 : (a) Energy-dispersive X-ray spectroscopy elemental mapping data of (CrMnFeNiCu) 3 O 4 powder. (b) K-edge k 3 -weighted EXAFS spectra and Fourier-transform magnitude spectra of the constituent elements of HEO. (c) Charge–discharge curves of the (CrMnFeNiCu) 3 O 4 HEO electrode measured at various rates. [Reproduced from Ref. 1] Fig. 2 : Use of operando quick-scanning XAS to examine valence/coordination state variations, transition steps, redox sequence, reversibility, and redox overpotential of multiple electroactive centers in the (CrMnFeNiCu) 3 O 4 HEO electrode. [Reproduced from Ref. 1] Ni 2+ → Ni 0 transitions lead to a charging potential plateau at ~0.5 V. Finally, the Cr 2+ → Cr 0 conversion is responsible for the lithiation capacity from ~900 to 1200 mAh g −1 . Upon delithiation, the Cu conversion is hardly reversible and thus provides little capacity. Ni can be slightly re-oxidized, but a high overpotential of up to ~1.67 V is required. The oxidation of Mn occurs first during discharge. The metallic Mn signal decreases and the Mn 2+ and Mn 2+/3+ signals increase upon delithiation. The Cr and Fe oxidation reactions start at a discharge capacity of ~275 mAh g −1 (from ~1.13 V). At full delithiation, besides metallic Cr and Fe, the species Cr 3+ , Fe 2+ , and Fe 2+/3+ are observed. Figure 3 summarizes the transition behaviors of all the constituent elements in the HEO electrode during the first charge–discharge cycle. With an understanding of the detailed redox