Study of Structural Transitions, Jahn-Teller Distortion, Cyclability, and Specific Energy in P2-type Na_xMO₂ Na-ion Battery Cathodes

Recent years have seen an interest in exploring the viability of Na-ion based battery cathode materials. Here, we report relationships between crystal structure and properties in the Na_xMO₂ (M=transition metal) family of layered metal-oxide battery cathodes. Using detailed Rietveld fitting, we characterize structural transitions that occur as a result of cycling. We further report the effect of these transitions on battery cyclability, and the effect of transition metal substitution in crystal structure and distortion in Na_2/3Mn_xFe_1-xO₂ (NMFO). Using specially designed transfer chambers, we have studied ex situ x-ray diffraction at different potentials in the charge-discharge cycle to examine the nature of the structural transitions both in the metal oxide planes and in the layering structure. We report specific energy, capacity, and cyclability of cells when cycling over several voltage ranges for different cathodes and find that substituents for Mn perform well when charged no higher than 4.0 V (vs. Na). Conversely, charging to 4.3 V degrades crystallinity.