Multifunctional Effect of Fe Substitution in Na Layered Cathode Materials for Enhanced Storage Stability

Abstract

Developing stable cathode materials that are resistantto storagedegradation is essential for practical development and industrialprocessing of Na-ion batteries as many sodium layered oxide materialsare susceptible to hygroscopicity and instability upon exposure toambient air. Among the various layered compounds, Fe-substituted O3-typeNa(Ni1/2Mn1/2)(1-x )Fe x O2 materials haveemerged as a promising option for high-performance and low-cost cathodes.While previous reports have noted the decent air-storage stabilityof these materials, the role and origin of Fe substitution in improvingstorage stability remain unclear. In this study, we investigate theair-resistant effect of Fe substitution in O3-Na(Ni1/2Mn1/2)(1-x )Fe x O2 cathode materials by performing systematic surfaceand structural characterizations. We find that the improved storagestability can be attributed to the multifunctional effect of Fe substitution,which forms a surface protective layer containing an Fe-incorporatedspinel phase and decreases the thermodynamical driving force for bulkchemical sodium extraction. With these mechanisms, Fe-containingcathodes can suppress the cascades of cathode degradation processesand better retain the electrochemical performance after air storage.