Synthesis of O3-Na<sub>0.8</sub>Fe<sub>0.6</sub>Mn<sub>0.3</sub>O<sub>2</sub> Positive Electrode and Its Application to Sodium Ion Batteries

Abstract

Herein, we report precise variation of Fe and Mn constituent in the sodium magnate layered cathodes with the compositions of Na0.8Fe0.4Mn0.5O2, Na0.8Fe0.5Mn0.4O2, Na0.8Fe0.6Mn0.3O2, Na0.8Fe0.6Mn0.4O2, Na0.8Fe0.7Mn0.4O2, Na0.9Fe0.6Mn0.3O2 in order to attain a high performing cathode. Based on this transition metal stoichiometry, an interesting sodium magnate combination of Na0.8Fe0.6Mn0.3O2, with O3-type crystal phase, possess R3m space group along with the superior electrochemical behavior is obtained. On charge-discharge capacities in the range of 2.0-3.8 V at 0.1 C, it shows the comparatively higher performance of the first and the second charge capacity of 115 and 180 mAhg-1 and discharge capacity of 184 and 181 mAhg-1, respectively. The best sample was then compared with the closely related Na0.8Fe0.6Mn0.4O2, Na0.9Fe0.6Mn0.3O2 combination in terms of valence ratio and influence of excess sodium for the structure robustness, stability along with purity. The best sample with the composition Na0.8Fe0.6Mn0.3O2 does not show detectable impure phase while Na0.8Fe0.6Mn0.4O2 and Na0.9Fe0.6Mn0.3O2 shows a tendency of P-type (Cmca space group) behavior with 30.8% and 32.8%, respectively. The enhancement of iron constituent increases not only the performances but also the stabilization of sodium vacancy ordering and substitution of Mn with a substantial reduction of Jahn-Teller distortion, mounting biphasic characteristics and high peak intensity of 41.5 °.