Insights into P2-Type Layered Positive Electrodes for Sodium Batteries: From Long- to Short-Range Order.

P2-type Fe- and Mn-based layered sodium transition metal oxides are promising positive electrodes for sodium batteries due to their high energy density and low costs of the constituting transition metals. However, the poor structural reversibility and fast capacity decay have prevented their breakthrough so far. Herein, the real-time dynamic phase transitions and capacity fading mechanism of P2 Na0.67Fe0.5Mn0.5O2 positive electrode are revealed by operando X-ray diffraction, operando/ex situ X-ray absorption spectroscopy, neutron powder diffraction and neutron pair distribution functions. Upon the desodiation process, a layered OP4 phase with long-range order is found as an intermediate state. With further deep desodiation, the formation of a Na-depleted ramsdellite phase with a short coherent length of 30 Å is observed for the first time. However, the latter transition from OP4 to ramsdellite is considered to be irreversible due to the breakdown of the layered structural characteristics resulting in poor cycling performance in a variety of Fe-based layered sodium transition metal oxides. This work suggests that stabilizing the crystal structure by substitution or chemical modification can be a favorable strategy to avoid the degradation of positive electrodes and thus to improve the cycling performance.