Understanding the Electrochemical Reaction Mechanism of the Co/Ni Free Layered Cathode Material P2–Na<sub>2/3</sub>Mn<sub>7/12</sub>Fe<sub>1/3</sub>Ti<sub>1/12</sub>O<sub>2</sub> for Sodium-Ion Batteries
Jiali Peng, Angelina Sarapulova, Qiang Fu, Hang Li, Hao Liu, Oleksandr Dolotko, Thomas Bergfeldt, Karin Kleiner, Bixian Ying, Yi Wu, Volodymyr Baran, Edmund Welter, Peter Nagel, S. Schuppler, Michael Merz, Michael Knapp, Helmut Ehrenberg, Sylvio Indris
Abstract
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.