Synthesis of O3-Na0.8Fe0.6Mn0.3O2 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 Na_0.8Fe_0.4Mn_0.5O_2, Na_0.8Fe_0.5Mn_0.4O_2, Na_0.8Fe_0.6Mn_0.3O_2, Na_0.8Fe_0.6Mn_0.4O_2, Na_0.8Fe_0.7Mn_0.4O_2, Na_0.9Fe_0.6Mn_0.3O₂ in order to attain a high performing cathode. Based on this transition metal stoichiometry, an interesting sodium magnate combination of Na_0.8Fe_0.6Mn_0.3O_2, 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 Na_0.8Fe_0.6Mn_0.4O₂, Na_0.9Fe_0.6Mn_0.3O₂ 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 Na_0.8Fe_0.6Mn_0.3O₂ does not show detectable impure phase while Na_0.8Fe_0.6Mn_0.4O₂ and Na_0.9Fe_0.6Mn_0.3O₂ 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 °.