A layer-structured high entropy oxide with highly reversible Fe3+/Fe4+ redox as advanced cathode material for sodium ion batteries

Reversible redox center is essential for long-life electrode materials. Iron is an earth abundant element, in lithium-ion batteries, highly reversible Fe²⁺/Fe³⁺ redox in LiFePO₄ has play important role as redox center, Fe³⁺/Fe⁴⁺ redox in lithium layer-structured oxides display poor electrochemical performance. In sodium ion batteries, Fe³⁺/Fe⁴⁺ redox in sodium layer-structured oxides are active, while the cycle performance of Fe-contained sodium layer-structured oxide cathode need to be further improved. Herein, A pure-phase layer-structured high entropy oxide O3-Na(MgCu)_1/12(NiCoFeMnTi)_1/6O₂ is synthesized and investigated as cathode for sodium ion battery. A reversible phase-transition takes place during the charge/discharge process. Particularly, highly reversible Fe³⁺/Fe⁴⁺ redox is revealed by X-ray absorption fine structure (XAFS). 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. In this work, Fe K-edge of Fe³⁺/Fe⁴⁺ redox displays rigid shift, HEO-MgCuNi could be a platform to investigate the fundamental property of Fe³⁺/Fe⁴⁺ redox.