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

• Published in: Electrochimica acta Vol. 394; p. 1
• Main Authors: Siriwardena, Dumindu P, Fernando, Joseph FS, Wang, Tony, Firestein, Konstantin L, Zhang, Chao, Brand, Helen EA, Jones, Michael WM, Kewish, Cameron M, Berntsen, Peter, Jenkins, Tristram, Lewis, Courtney-Elyce M, von Treifeldt, Joel E, Dubal, Deepak P, Golberg, Dmitri V
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier BV 20.10.2021
• Subjects: Alkali metals; Cathodes; Crystal structure; Discharge; Doping; Electrochemical analysis; Electrode materials; Magnesium; Phase transitions; Rechargeable batteries; Redox reactions; Sodium-ion batteries; Titration; Transition metal oxides; X ray absorption
• ISSN: 0013-4686; 1873-3859

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.