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

Highlights: • A series of novel Mg-substituted triclinic Na₂Mn_3-xMg_xO₇ cathode materials was introduced. • Mg-substitution improved specific capacity and rate capability of the cathodes. • Ex situ methods (X-ray diffraction and X-ray absorption spectroscopy) demonstrated that the capacity evolved through reversible Mn^3+/4+ and O^2-/n- redox reactions. • Capacity evolution was assisted through a reversible triclinic to monoclinic structural transition. • Full cell performance was analyzed. -- Abstract: 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 Mn^4+/3+ redox couple and a reversible O^2-/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 O^2-/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 O^2-/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.