Structural and Electrochemical Characterizations of P2 and New O3-Na_xMn_1-yFe_yO₂Phases 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 Na x (Mn,Fe)O 2 phases: P2-Na 0.67 Mn 2/3 Fe 1/3 O 2 , P2-Na 0.71 Mn 1/2 Fe 1/2 O 2 , and new Na-deficient O3-Na 0.82 Mn 1/3 Fe 2/3 O 2 and O3-Na ∼0.8 Mn 1/2 Fe 1/2 O 2 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.