Realization of Highly Stable O-Redox Cathodes via Real-Time Analysis on the Solid-State Synthesis for High-Energy-Density Batteries

Layered sodium manganese oxides (NMO) have been proposed as promising cathode materials for large-scale batteries owing to their cost-effectiveness [1]. However, their low capacities have hindered the increase in energy density, necessitating the development of high-capacity NMO cathodes. One approach to increasing the theoretical capacities of NMO cathodes is by utilizing the oxygen (O)-redox reaction, which is enabled by the electrochemically-active lattice oxygens through the introduction of substitutes or vacancies in the transition metal (TM) layer [1]. The stability of the O-redox reaction depends on the in-plane distribution of substitutes or vacancies in the TM layer, influencing the distance between the participating O atoms and the energy barrier for TM migration [1-2]. However, the realization of in-plane structure during synthesis and its correlation with the O-redox stability, has not been studied.<br/>In this context, we aim to explore the structural changes of NMO during solid-state synthesis and assess the effect of synthetic parameters on its in-plane structure and O-redox stability. To monitor the changing structure during synthesis, we employ time-resolved X-ray diffraction (TR-XRD) analysis throughout the synthesis process. In addition, electrochemical tests are conducted to evaluate the O-redox stability. By analyzing these results and conducting further structural characterization (e.g., high-resolution electron microscopy (HR-TEM) and X-ray absorption spectroscopy (XAS)), the correlation between synthetic parameter and O-redox stability is suggested.<br/><br/>[1] C. Wu et al., <i>Adv. Mater.</i> 34, 2106171 <b>(2022)</b><br/>[2] P. G. Bruce et al., <i>Nature</i> 577, 502–508 <b>(2020)</b>