Structural and Electrochemical Characterizations of P2 and New O3-NaxMn1-yFeyO2Phases Prepared by Auto-Combustion Synthesis for Na-Ion Batteries

Using auto-combustion synthesis route followed by a 1000°C heat-treatment, we succeeded to prepare four Nax(Mn,Fe)O2 phases: P2-Na0.67Mn2/3Fe1/3O2, P2-Na0.71Mn1/2Fe1/2O2, and new Na-deficient O3-Na0.82Mn1/3Fe2/3O2 and O3-Na∼0.8Mn1/2Fe1/2O2 phases. We studied their structures by X-ray Diffraction and their electrochemical properties as positive electrode in Na-cells that were reversibly charged and discharged in the 1.5 and 3.8 V range vs. Na+/Na, leading to discharge capacities between 135 and 155 mAh.g−1. The shapes of the cycling curves are discussed together with the stacking of the phases and the redox processes involved in the Na intercalation/deintercalation reaction. In some cases the Fe(IV) state is clearly reached.