Enhanced Sodium Ion Storage Behavior of P2-Type Na_2/3Fe_1/2Mn_1/2O₂ Synthesized via a Chelating Agent Assisted Route

On the basis of resource abundance and low cost, high capacity layered P2-type Na_2/3Fe_1/2Mn_1/2O₂ material is regarded as a potential cathode material for sodium-ion batteries but suffers from its unstable structure during cycling. In this work, P2-type Na_2/3Fe_1/2Mn_1/2O₂ layered materials were synthesized by a chelating agent assisted sol-gel method with NH₃·H₂O. With the addition of NH₃·H₂O and the control of the synthesis conditions, highly active material with a more stable structure and better electrochemical performance was obtained. Furthermore, the influences of structure changes during different voltage ranges (1.5-4.0 V and 1.5-4.3 V vs Na⁺/Na) on the Na⁺ storage behaviors were also evaluated and compared. It is confirmed that, when being charged to 4.2 V, an OP4-type phase emerges, which can reduce the damage by the gilding of the MeO₂ layers but leads to an unstable crystal structure. For long-term cycling, it is preferred to cut off at 4.0 V rather than at 4.3 V. For the optimized P2-type Na_2/3Fe_1/2Mn_1/2O₂ calcined at 900 °C, a discharge capacity of 92 mAh/g remains after 40 cycles in the voltage range of 1.5-4.0 V, and the Coulombic efficiency remains 100%.