Probing the effect of Mg doping on triclinic Na2Mn3O7 transition metal oxide as cathode material for sodium-ion batteries

Triclinic Na2Mn3O7 has been identified as a promising material for high-capacity sodium-ion batteries. However, the knowledge on the effect of doping of metal ions and structural transformations of Na2Mn3O7 during dis(charge) is limited. Integration of alkali metal-ions, specially Mg2+ can enhance the electrochemical properties in transition metal oxides. Herein, a series of Mg2+ doped triclinic Na2Mn3O7 cathode materials was explored for the first time. Electrochemical analysis revealed that Mg2+ improves specific capacities, and rate capabilities. Ex situ X-ray diffraction (XRD) and Galvanostatic charge discharge cycling (GCD) showed that the triclinic phase reversibly converts into two monoclinic phases at high Na+ insertion levels. Na+ extraction at high potentials is supported by another biphasic region which converts to a major triclinic phase at the end of the charge. GCD, cyclic voltammetry (CV) and ex situ X-ray absorption spectroscopy (XAS) documented that the capacity mainly evolved through a Mn4+/3+ redox couple and a reversible O2-/n− redox reaction. CV and Galvanostatic intermittent titration techniques (GITT) showed that Mg2+ reduces the Na+-vacancy ordering and improves the Na+ diffusion. The 2 mol.% Mg-doped material exhibited a high specific capacity of 143 mAh/g after 30 cycles and a rate capability of 93 mAh/g (at 500 mA/g). GCD analysis demonstrated that O2-/n− redox is remarkably stable up to at least 90 cycles. Full cells made using the 0.5 mol.% Mg-doped material displayed a promising discharge specific capacity of 80 mAh/g. The effects of cation doping into the complex crystal structures, phase transformations during Na+ de(intercalation) and the importance of O2-/n− redox for achieving high capacities were uncovered. The findings of this work will guide the design of novel cathode materials for sodium-ion batteries. ©2021 Elsevier Ltd.

Keywords

Sodium ions, Sodium, Manganese, Manganese oxides, Electric batteries, Cathodes, Anions

Citation

Siriwardena, D. P., Fernando, J. F., Wang, T., Firestein, K. L., Zhang, C., Brand, H. E. A., Jones, W. M. W., Kewish, C. M., Berntsen, P., Jenkins, T., Lewis, C.-E. M., von Triefeldt, J. E., Dubal, D. P. & Golberg, D. V. (2021). Probing the effect of Mg doping on triclinic Na2Mn3O7 transition metal oxide as cathode material for sodium-ion batteries. Electrochimica Acta, 394, 139139. doi:10.1016/j.electacta.2021.139139