Exploration of Phase Compositions, Crystal Structures, and Electrochemical Properties of NaxFeyMn1-yO2 Sodium Ion Battery Materials

Layered manganese oxide materials are widely used in sodium and lithium ion batteries, but significant discrepancies are encountered in the literature with respect to their electrochemical properties. This could be due to difficulties in establishing the exact phase compositions and crystal structures (typically P2, P3, O2, and O3, their distorted analogues, e.g., P'2, hydrated - PH2, or carbonated - PC2, phases) of a given synthesis product, especially when only crude crystallographic indexing is used without structural analysis. Here we report a benchmark high-resolution synchrotron powder diffraction investigation of a broad composition range of the layered NaxFeyMn1-yO2, cathode materials (x = 0.5, 0.7, and 1.0 and y = 0.3, 0.5, and 0.7) with respect to phase composition, crystal structure, and electrochemical properties. On the basis of multiphase Rietveld refinements, it is shown that crystal structure can be controlled to a certain degree for different x and y. Most synthesis products contain a complex phase mixture, but in a few cases, almost phase pure P2 and O3 type materials can be produced. The P2 phase is observed to be air sensitive, whereas the O3 and P3 structures are not. Clear trends linking electrochemical performance to x and y are observed, where higher x and y result in worse performance. On the other hand, no clear trend is observed linking the type of layered crystal structure to electrochemical performance. Overall, the electrochemical performance of the NaxFeyMn1-yO2 samples seems to be mostly dependent on the initial oxidation state and the transition metal ratio.