Local and Global Structures in the Phase Evolution of P2-Nax_xFey_yMn1–y_{1–y}O2_2 Electrodes for Na-Ion Batteries

The limited availability of raw materials for the production of Li-ion batteries creates a strong incentive to develop Na-ion batteries due to the higher abundance of sodium raw materials. Layered transition-metal oxides are among the most promising electrode materials for Na-ion batteries due to their high capacities. Unfortunately, they still suffer from poor capacity retention. Furthermore, for the Na-ion battery technology to be truly sustainable, the Na-ion electrodes must be free of scarce elements like Ni, Co, and Li (often used as stabilizing dopants). This study investigates the sustainable materials P2-Na$_x$Fe$_y$Mn$_{1–y}$O$_2$ (y = 0.33 and 0.5) to correlate the structural evolution during Na-ion extraction and insertion (i.e., battery charge and discharge) to the Fe:Mn ratio. Using operando powder diffraction, we map the complete structural evolution during deep charge and discharge. Through a combination of Rietveld refinement and pair distribution function analysis, structural models for the distorted and disordered phases at deep discharge and charge are deduced at a global (average) and local scale. By combining the overview of the full structural evolution with the details from the structural analysis, insight into the effects of the Fe:Mn ratio and the origin of phase transitions are elucidated