NSRRC Activity Report 2022

Energy Science 059 lithiation process. Figure 3(b) shows a schematic of the cation migration mechanism in the HLC and LLC cathodes. Since the HLC cathode has more Li 2 MnO 3 phases than the LLC cathode, cation migration is performed easily and the vacancies are clustered in the TM layer. Thus, the lattice oxygen with different coordination environments can cause shifts in the oxygen reduction potential and the occurrence of voltage fading during the lithiation process. There is a lower fraction of the Li 2 MnO 3 phase in the LLC cathode, which limits the cation migration and suppresses the voltage fading. (Reported by Heng-Liang Wu, National Taiwan University) This report features the work of Heng-Liang Wu, Maw-Kuen Wu and their collaborators published in Adv. Funct. Mater. 32 , 2112394 (2022). TPS 44A Quick-scanning X-ray Absorption Spectroscopy TLS 01C2 X-ray Powder Diffraction TLS 20A1 XAS • XANES, EXAFS, XAFS • Materials Science, Physics, Energy Storage Devices, Battery Materials, Chemistry, Condensed-matter Physics, Functional Materials Reference 1. H.-L. Yu, K. B. Ibrahim, P.-W. Chi, Y.-H. Su, W.-T. Chen, S.-C. Tseng, M.-T. Tang, C.-L. Chen, H.-Y. Tang, C.-W. Pao, K.-H. Chen, M.-K. Wu, H.-L. Wu, Adv. Funct. Mater. 32 , 2112394 (2022). Charge–Discharge Mechanism of High-Entropy Co-Free Spinel Oxide in Li-ion Batteries Li + storage mechanism of high-entropy Co-free spinel oxide was firstly evaluated using operando quick- scanning X-ray absorption spectroscopy. T ransition-metal high-entropy oxides (HEO) that integrate five (or more) elements into a single phase are considered as potential anodes for Li-ion batteries (LIB). Entropy-derived phase stabilization enhances the electrode reversibility and cyclic stability. Moreover, the HEO lattice is distorted because of the distinct atomic sizes of the constituents, which cause a lattice residual stress that can alter material properties. An as-synthesized cobalt-free spinel (CrMnFeNiCu) 3 O 4 LIB anode is studied in this work. A homogeneous distribution of the constituent elements is verified by energy-dispersive X-ray spectroscopy mapping data and the K-edge k 3 -weighted extended X-ray absorption fine structure (EXAFS) and Fourier- transform magnitude spectra, as shown in Figs. 1(a) and 1(b) (see next page). The galvanostatic charge–discharge profiles of the HEO anode exhibit a reversible specific capacity of 750 mAh g − 1 at a rate of 50 mA g − 1 . At a high rate of 2000 mA g − 1 , a good specific capacity of 340 mAh g − 1 is still achieved, corresponding to a satisfactory capacity retention of 45% compared to the value measured at 50 mA g − 1 ( Fig. 1(c) ). The multiple electroactive centers result in a complex Li + storage mechanism. The charge storage mechanism of HEOs appears to vary with their crystal structures and chemical composition and thus it is still disputed in the literature. The detailed HEO redox mechanism needs to be further examined to facilitate the design of electrode materials. Recently, Jeng-Kuei Chuang (National Yang Ming Chiao Tung University) and Chih-Wen Pao (NSRRC) explored the charge storage mechanism of a cobalt-free spinel (CrMnFeNiCu) 3 O 4 HEO in detail by using operando quick-scanning X-ray absorption spectroscopy (XAS) of TPS 44A . XAS is sensitive to the local electronic structures of the absorbing atoms, making it suitable for investigation of the redox behavior of the HEO constituent elements. The XAS spectra are recorded in the transmission mode at a monochromator oscillation frequency of 2 Hz ( i.e. , two complete spectra are recorded per second). A total of 240 spectra are integrated to achieve a good signal-to-noise ratio and a 2-min time resolution. This setup makes it feasible to investigate the detailed changes in the material and short-lived intermediate species formed during lithiation/delithiation. The valence/ coordination state variations, multiple transition steps, redox sequence, reversibility, and redox overpotentials of Cr, Mn, Fe, Ni, and Cu species in the HEO electrode are examined, as illustrated in Fig. 2 (see next page). In pristine HEO, the cations are present as Mn 2+/3+ , Cu 2+ , Cr 3+ , Fe 2+/3+ , and Ni 2+ , which are distributed at both the tetrahedral and octahedral sites. Upon the first lithiation ( i.e. , charging), the Mn 2+/3+ → Mn 2+ and Cu 2+ → Cu 0 reactions contribute to the capacity in the potential sloping region. Then, the (Mn 2+ and Mn 2+/3+ ) → Mn 0 , Cr 3+ → Cr 2+ , Fe 2+/3+ → Fe 2+ , Fe 2+ → Fe 0 , and