Understanding the Electrochemical Reaction Mechanism of the Co/Ni Free Layered Cathode Material P2–Na_2/3Mn_7/12Fe_1/3Ti_1/12O₂ for Sodium-Ion Batteries

High Resolution Image Download MS PowerPoint Slide 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 Na 2/3 Mn 1/2 Fe 1/2 O 2 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–Na 2/3 Mn 7/12 Fe 1/3 Ti 1/12 O 2 (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–Na 2/3 Mn 7/12 Fe 1/3 Ti 1/12 O 2 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 57 Fe Mössbauer spectroscopy, and ex situ 23 Na nuclear magnetic resonance spectroscopy are used to elucidate the changes in the crystallographic/electronic structure during desodiation/resodiation. Ex situ soft XAS reveals the participation of oxygen anions in the electrochemical reactions.