A Descriptor and Tool for Guiding Composition Design toward Developing Single- or Mixed/Hybrid-Phase “Layered” Transition Metal Oxide Cathode Materials for Na-Ion Batteries

Abstract The structure type and phase assemblage of “layered” Na-transition metal (TM) oxides dictate their performance as cathode materials, resulting in an impetus toward developing biphasic or hybrid Na-TM-oxides. However, a fundamental understanding as to what drives the formation of such biphasic structures, instead of a single phase, remains unestablished. This also means that a coherent strategy to rationally design hybrid Na-TM-oxides and tune the relative contents of the constituent phases is missing. Against this backdrop, we report a simple strategy and an associated descriptor, that is based on the overall covalency of cation−oxygen bonds in the TMO2-“slab”, viz., the average charge-to-radius ratio of the cation combination in the TM-layer. Tuning the above parameter gives access to compositions leading to either phase-pure or biphase/hybrid O3/P2-type structures with varied relative contents of the constituent phases (at least in qualitative terms) for a fixed Na-content. The usage of this simple descriptor and the associated phase map has been successfully demonstrated here by developing a series of phase-pure and hybrid P2/O3-structured Na-TM-oxides based on two Na-contents using the same synthesis protocol. This confirms the suitability of this rational design strategy across various compositions, which can be adapted for widespread development of biphasic/hybrid “layered” TM-oxides for usage as cathode materials for Na-ion batteries and beyond.