Clarification of underneath capacity loss for O3-type Ni, co free layered cathodes at high voltage for sodium ion batteries

Earth abundant O3-type NaFe0.5Mn0.5O2 layered oxide is regarded as one of the most promising cathodes for sodium ion batteries due to its low cost and high energy density. However, its poor structural stability and cycle life strongly impede the practical application. Herein, the dynamic phase evolution as well as charge compensation mechanism of O3-type NaFe0.5Mn0.5O2 cathode during sodiation/desodiation are revealed by a systemic study with operando X-ray diffraction and X-ray absorption spectroscopy, high resolution neutron powder diffraction and neutron pair distribution functions. The layered structure experiences a phase transition of O3 → P3 → OP2 → ramsdellite during the desodiation, and a new O3′ phase is observed at the end of the discharge state (1.5 V). The density functional theory (DFT) calculations and nPDF results suggest that depletion of Na+ ions induces the movement of Fe into Na layer resulting the formation of an inert ramsdellite phase thus causing the loss of capacity and structural integrity. Meanwhile, the operando XAS clarified the voltage regions for active Mn3+/Mn4+ and Fe3+/Fe4+ redox couples. This work points out the universal underneath problem for Fe-based layered oxide cathodes when cycled at high voltage and highlights the importance to suppress Fe migration regarding the design of high energy O3-type cathodes for sodium ion batteries.