High capacity, low cost and long cycle life cathode materials are very much desired to develop high performance sodium-ion batteries (SIBs), which are considered as a possible solution for grid level electrochemical energy storage. Sodium manganese oxides with different compositions and crystal structures are a class of intercalation cathode materials that have high promise because of their high initial capacity and low cost, owing to the use of earth abundant metal, Mn. However, many sodium manganese oxides compounds suffer from fast capacity decay primarily attributed to the irreversible phase transitions in the electrochemical cycling processes. In this work, we report our investigation on a group of Mn-based cathodes with using an effective strategy of mitigating and delaying the phase transitions by compositional design. We report the design, synthesis, electrochemical testing and cycling mechanism study on the compounds with a general formula of Na_x(Li_yMn_1-y)O₂ with using lithium as the doping element. Many of this group of materials demonstrated very high initial capacity of >200 mAh/g and good capacity retentions. In-operando X-ray diffraction (XRD), Rietveld refinement and ⁷Li solid state NMR characterizations are conducted to elucidate the cycling mechanisms of the materials. The stability of the Li-doped structure, the effects of lithium doping and the fate of lithium in extended cycling are also discussed.

Figure 1. The voltage profiles (a) and in operando XRD patterns (b) of the lithium doped sodium manganese oxide cathode.