Insights into the Redox Chemistry and Structural Evolution of a P2-Type Cathode Material in Sodium-Ion Batteries

Abstract

In view of the tunable composition and higher theoretical capacity, layered transition metal oxide cathode materials (Na_xTMO₂, TM: transition metal) have attained substantial attention. In this work, a P2-type layered metal oxide with the nominal composition Na_0.67Fe_0.20Ni_0.15Mn_0.65O₂ (NFNM) was synthesized via a sol–gel method; electrochemical performance and the operating mechanism of the electrode material in half-cells were investigated. The material delivered an initial discharge capacity of 166 mA h g^–1 where the capacity retention after 50 cycles is 65% when cycled in the voltage range 1.50–4.30 V at a C-rate of C/20. At 1C, the capacity delivered by the material was 110 mA h g^–1 and the capacity retention noted after 80 cycles was 80%. A combination of in operando synchrotron diffraction and X-ray absorption spectroscopy (XAS) elucidates the electrochemical mechanism in a Na/NFNM half-cell. The structural evolution of the electrode material was analyzed using in operando XRD from which the evidence of reversible P2-Z phase transformations was obtained. Investigation of the charge-compensation mechanism and local structure changes in the electrode material during cycling were carried out via the XAS technique which revealed the coupled Fe migration, anionic activity, and phase transformations.