Manganese-based layered oxide materials are promising positive electrode for sodium ion batteries. It has been shown that the partial substitution of Mn with Mg can smooth the voltage curve and improve the cycling stability.¹ Mg substitution can also induce the activation of the redox reaction of oxide ions.² In this study, the Na_xMg_x/2Mn_1-x/2O₂ (1.0 ≥ x ≥ 1/3) series of layered oxides were synthesized and characterized in Na cells.

Experimental

Stoichiometric amounts of MgO, MnO and Na₂CO₃ were combined by ball milling, then heated at 900 ºC in air to obtain samples of
Na_xMg_x/2Mn_1-x/2O₂.  Electrodes were made with active material, PVDF binder, carbon black at a ratio of 8:1:1 and dried under vacuum at 120 ℃ overnight. 1M NaPF₆ in a solution of EC, DEC, FEC (volume ratio 3:6:1) was used as electrolyte and Na foil was used as counter/reference electrode. X-ray diffraction (XRD) patterns were measured with a Rigaku Ultima IV X-ray diffractometer equipped with a Cu anode X-ray tube and a diffracted beam monochromator.

Results and discussion

Figure 1 shows the XRD patterns of Na_xMg_x/2Mn_1-x/2O₂ (1.0 ≥ x ≥ 1/3). All the samples were found to be P2-dominant, even when x approachs 1. The sample with x = 2/3 has the highest P2 phase purity. The further x deviates from 2/3, the more impurities are present in the XRD patterns.

Figure 2 shows the voltage curve of Na_2/3Mg_1/3Mn_2/3O₂. Na_2/3Mg_1/3Mn_2/3O₂ was found to have a reversible capacity of ~250 mAh/g (corresponding to a theoretical energy density of approximately 520 Wh/kg) with 80% capacity retention after 20 cycles. The voltage curve exhibits a plateau at > 4 V during the first charge, and a slope between 1.5 V and 3.5 V during discharge. The loss of capacity of the 4 V plateau during cycling indicates irreversible structure changes upon first charging, possibly due to oxygen loss.  After about 10 cycles, the 4 V plateau becomes stabilized and the material cycles reversibly.

The voltage curve of Na_2/3Mg_1/3Mn_2/3O₂ is consistent with the voltage curve of Li₂MnO₃, wherein all the Mn is also in the 4+ oxidation state. This result is the first report of the reversible sodium extraction from layered manganese oxide materials where all the Mn exist as Mn⁴⁺.

References

1. J. Billaud et al., Energy Environ. Sci., 7, 1387-1391 (2014)

2. N. Yabuuchi et al., J. Mater. Chem. A, 2, 16851-16855 (2014)