Phase Transition Induced Enhanced Performance of Sodium-Rich Na1.2Mn0.8O2–yFy (y = 0–0.5) Cathodes

Abstract

Increasing capacity and cycle stability is key to successfully commercializing sodium battery technologies. Currently, cation-rich cathodes are well suited for this trend owing to superior overall performance through fluorine substitution in the cation. However, the exact effects and synergy due to fluorine substitution as anions are still unknown. Understanding such synergistic effects can significantly facilitate increasing the capacity, leading to maximum performance at optimum conditions. In this work, the systematic addition of F into sodium-layered oxide and its corresponding changes in the crystal structure or phase formation are extensively studied. More specifically, the effect of fluorine substitution in the anion site on Na diffusion and the Na–Tm polyhedral are extensively studied using X-ray diffraction and subsequent Rietveld analysis. In addition, the effects of F on transition metals (i.e., Mn) and its valence states are also analyzed using X-ray absorption spectroscopy. Na1.2Mn0.8O1.5F0.5 not only showed superior capacity of approximately 172 mAh g–1 and capacity retention but also remarkable cycling stability for up to 300 cycles at higher current densities. Improvements in the performance due to distortion induced by F substitution are also presented. This study provides insight into the transition of cathode material properties from F doping (y = 0–0.2) to F substitution (y ≥ 0.3) and the associated structural changes and capacity improvements.