A Layer-Structured High Entropy Oxide with Highly Reversible Fe3+/Fe4+ Redox as Advanced Cathode for Sodium Ion Batteries

Abstract

A single-phase layer-structured high entropy oxide O3-Na(MgCu)1/12(NiCoFeMnTi)1/6O2 (HEO-MgCuNi)  has been synthesized and investigated as cathode in sodium ion battery. The as-synthesized high entropy oxide delivers a discharge capacity of 146.6 mAh g-1 at 10 mA g-1, and can retain 83.2% of capacity after 700 cycles at 100 mA g-1 between 2.0 and 4.1 V vs. Na+/Na. A reversible O3-P3 phase-transition takes place during the charge/discharge process. In addition, the electronic and local structures of the electrochemical active centres (Ni, Cu, Co, Fe) are enormously reversible. Particularly, highly reversible Fe3+/Fe4+ redox was revealed by X-ray absorption fine structure (XAFS). Meanwhile, Mn and Ti stabilize the layered structure without alternation in their oxidation states. The highly reversible crystal structure, electronic structure and local structure synergistically enable HEO-MgCuNi a long cycle life.

Keywords: High entropy oxide, layer-structured, cathode material, highly reversible Fe3+/Fe4+ redox, sodium ion batteries