Understanding the Electrochemical Reaction Mechanism of the Co/Ni Free Layered Cathode Material P2–Na$_{2/3}$Mn$_{7/12}$Fe$_{1/3}$Ti$_{1/12}$O$_{2}$ for Sodium-Ion Batteries

Iron- and manganese-based layered electrodes for sodium-ion batteries have attracted renewed interest due to their low cost and environmental friendliness. However, phase changes at high voltage and the Jahn–Teller effect lead to a short cycle life and poor rate capability. Herein, we describe the optimization of the structure of a Co/Ni free Na2/3Mn1/2Fe1/2O2 cathode via partial substitution of Fe by Mn and Ti and explore the redox activity of P2-type Mn/Fe-based layered cathodes. The obtained P2–Na2/3Mn7/12Fe1/3Ti1/12O2 (NMFTO) exhibits a solid solution mechanism during the complete desodiation/resodiation process and delivers an initial discharge capacity of 170 mA h g–1 at a 0.1 C rate and a capacity retention of 80% after 50 cycles. The main focus is to understand the electrochemical mechanism of P2–Na2/3Mn7/12Fe1/3Ti1/12O2 by exploring the redox processes of transition metal cations and oxygen anions upon cycling. In situ synchrotron radiation diffraction reveals a single-phase reaction of NMFTO during cycling, which is beneficial to improving cycle stability. In situ X-ray absorption spectroscopy (XAS), in situ 57Fe Mössbauer spectroscopy, and ex situ 23Na nuclear magnetic resonance spectroscopy are used to elucidate the changes in the crystallographic/electronic structure during desodiation/resodiation. ... mehrEx situ soft XAS reveals the participation of oxygen anions in the electrochemical reactions.