Fluorine substitution enabled superior performance of Na_xMn_2-xO_1.5F_0.5 (x = 1.05–1.3) type Na-rich cathode

Among the various sodium cathodes, the potential of Na-rich layered oxides is yet to be fully utilized. Unlike their Li counterparts, they are least explored and are at least a generation behind in development. Addressing the same, herein, Na-rich Na_xMn_2-xO_1.5F_0.5 (x = 1.05–1.3) type cathodes were synthesized successfully and analyzed as potential electrodes for Na-ion battery applications. Oxygen loss in Na-based transition metal oxides is a common issue, and it is effectively addressed by fluorine substitution. In contrast to exploring a particular stoichiometry as in other Na-deficient layered cathodes, herein, Na-content was gradually increased from 1.05 to 1.3. The cathodes were synthesized using a conventional solid-state approach and quenched to achieve high crystallinity. Compounds with different sodium stoichiometry were electrochemically tested in a half-cell configuration. Among these compounds, the Na_1.2Mn_0.8O_1.5F_0.5 electrode exhibited very high capacities of 178 and 122 mAhg⁻¹ at current densities of 10 and 1000 mA g⁻¹, respectively. The Na-rich Na_1.2Mn_0.8O_1.5F_0.5 cathode was systematically analyzed to understand the mechanism underlying its superior performance using various structural and electrochemical analyses. Furthermore, to demonstrate its practicality, the Na-rich Na_1.2Mn_0.8O_1.5F_0.5 cathode was coupled with a hard carbon and Na-In alloy anode in a full-cell assembly.