Electron donor enabling Mn-Fe based layer oxide cathode with durable sodium ion storage

Abstract

Enhancing the specific capacity of P2-type layered oxide cathodes via elevating the upper operation voltage would inevitably deteriorate electrochemical properties owing to the irreversible anionic redox reaction at high voltage. In this work, the strategy of the electron donor was utilized to address this issue. Remarkably, the earth-abundant P2-layered cathode Na_2/3Al_1/6Fe_1/6Mn_2/3O₂ with the presence of K₂S renders superior rate capability (187.4 and 79.5 mA h g⁻¹ at 20 and 1000 mA g⁻¹) and cycling stability (a capacity retention of 85.6% over 300 cycles at 1000 mA g⁻¹) within the voltage region of 2–4.4 V Na⁺/Na. Furthermore, excellent electrochemical performance is also demonstrated in the full cell. Detailed structural analysis of as-proposed composite cathode illustrates that even at 4.4 V irreversible phase transition can be avoided as well as a cell volume variation of only 0.88%, which are attributed to the enhanced performance compared with the control group. Meanwhile, further investigation of charge compensation reveals the crucial role of sulfur ions in actively control of reversible redox reaction of oxygen species in the lattice structure. This work inspires a new strategy to enhance the structural stability of layered sodium ion cathode materials at high voltages.