Influence of Synthesis Conditions on Electrochemical Properties of P2-Type Na2/3Fe2/3Mn1/3O2 for Rechargeable Na Batteries

P2-type Na2/3Fe2/3Mn1/3O2 samples are synthesized by solid-state calcination with different conditions and the influence of synthesis conditions on the structural and electrochemical properties is systematically studied. By controlling the synthesis conditions, a fraction of O3-type impurity phase is effectively decreased, and nearly stoichiometric P2 Na2/3Fe2/3Mn1/3O2 is successfully obtained by a two-step calcination method, A voltage plateau of 2.5 V, which is attributed to the oxidation of Mn3+ to Mn4+, appears in initial charge profiles for quenched and one-step calcined samples. In contrast, for the initial charge process, the voltage plateau of 2.5 V is not observed in the sample prepared in the two-step calcination process, suggesting that Mn3+ is not contained in this sample. Charge/discharge mechanisms of nearly stoichiometric Na2/3Fe2/3Mn1/3O2 are also examined by X-ray absorption spectroscopy and X-ray diffractometry, and it is found that reversible redox reaction of Fe3+/Fe4+ and Mn3+/Mn4+ proceeds on electrochemical cycles. Moreover, stacking faults associated with metal layer glide on charge are more pronounced for P2 Na2/3Fe2/3Mn1/3O2 in comparison to the P2-type Mn-rich system, Na2/3Fe1/2Mn1/2O2. From these results, factors affecting electrode performance of sodium insertion materials are discussed.