NSRRC Activity Report 2023

064 NSRRC ACTIVITY REPORT 2023 by sluggish redox kinetics, as seen for Na–Se and Li–S batteries. Other batteries that utilize organic electrolytes show superior performance; however, these may exhibit safety issues, as seen for the Li ion battery. Several studies have been undertaken to resolve the disadvantages of these different battery types. Single-atom catalysts and complex metal oxides have been applied as materials for electrodes to develop new types of batteries with excellent safety or to improve their performance, such as charging capacity and charge/discharge efficiency. However, these materials usually have a complex composition and do not possess structures with long-range ordering, making it difficult to observe how these electrodes work using traditional techniques, such as X-ray diffraction or transmission electron microscope analyses. XAFS spectroscopy is a powerful technique to provide information about the environment surrounding the absorbing atoms in these materials, whether amorphous materials or single-atom catalysts. Here we present several studies undertaken by various accomplished research teams. To solve the issue of sluggish redox kinetics, a single-atom catalyst was added to the cathode. The teams of Bao-Lian Su (Wuhan University of Technology, China) and Liang-Ching Hsu (NSRRC) confirmed using XAFS that no metallic Co existed in the Co–N 4 C 2 catalyst for the Na/Se battery ( Fig. 1 ). 1 The experimental and density functional theory (DFT) calculations revealed that the Co–N 4 C 2 electrocatalyst bidirectionally catalyzes the two-step conversion of solid Se/Na 2 Se and inhibits the phase transition of amorphous Fig. 1 : (a) XANES and (b) FT-EXAFS in R space for Co–N 4 C 2 and reference samples, including CoO, CoPc, and Co-foil. (c) Co K-edge EXAFS fitting of Co-N 4 C 2 in R space. (d) XANES and (e) FT-EXAFS in R space for Ni–NC single anion catalysts and reference samples including NiO and Ni- foil. (f) The fitting curve of the proposed Ni–NC single anion catalysts. [Reproduced from Refs. 1 and 2] Se, enabling higher utilization of active Se species. For the Li/S battery, the teams of Aiwen Lei (Wuhan University, China) successfully synthesized a Ni–NC single atom catalyst, which has vacancy defects, to promote Li–S battery conversion kinetics. According to XAS ( Fig. 1 ), 2 DFT, and other analyses, single Ni atoms are stably embedded in the N-doped carbon lattice. The Ni is coordinated with N atoms to form Ni–N 3 –V centers, which exhibit large binding energies for polysulfides, rapid charge transfer, and improved immobilization of the S species around active sites to enable constant conversion. To observe the charge/discharge process, ex-situ XAS was measured at different potential states. For this, the teams of Watchareeya Kaveevivitchai (National Cheng Kung University) collaborated with Jeng-Lung Chen (NSRRC) and successfully elucidated the change in the transition metal redox center of NaMnFe–PB during charge/discharge in Zn ion batteries (ZIBs) through ex-situ XANES measurements as shown in Fig. 2 . 3 The results of XANES present 1.70 and 2.0 V charge plateaus originating from the redox of Fe and Mn, respectively. Furthermore, the pre-edge peaks for both Fe and Mn shift back to the original position of the pristine compound at the 0.3 V discharge plateau, suggesting that the redox reaction in ZIBs is reversible. Anode free lithium metal batteries (AF LMBs) have attracted the attention of the teams of Yu Qiao (Xiamen University, China) and Yan-Gu Lin (NSRRC) and ex-situ XAS was also utilized during the charge/discharge of these materials. From the ex-situ XAS, as shown in