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

Abstract P2‐type Na 2/3 Fe 2/3 Mn 1/3 O 2 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 Na 2/3 Fe 2/3 Mn 1/3 O 2 is successfully obtained by a two‐step calcination method. A voltage plateau of 2.5 V, which is attributed to the oxidation of Mn 3+ to Mn 4+ , 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 Mn 3+ is not contained in this sample. Charge/discharge mechanisms of nearly stoichiometric Na 2/3 Fe 2/3 Mn 1/3 O 2 are also examined by X‐ray absorption spectroscopy and X‐ray diffractometry, and it is found that reversible redox reaction of Fe 3+ /Fe 4+ and Mn 3+ /Mn 4+ proceeds on electrochemical cycles. Moreover, stacking faults associated with metal layer glide on charge are more pronounced for P2 Na 2/3 Fe 2/3 Mn 1/3 O 2 in comparison to the P2‐type Mn‐rich system, Na 2/3 Fe 1/2 Mn 1/2 O 2 . From these results, factors affecting electrode performance of sodium insertion materials are discussed.