Unveiling the role of oxygen defects in facilitating Mg2+ diffusion and charge storage in MgMn2O4 cathodes for aqueous magnesium-ion capacitors

The development of high-performance aqueous magnesium-ion capacitors (AMICs) critically depends on overcoming the inherent challenges of sluggish Mg²⁺ diffusion and limited electronic conductivity in cathode materials. This study presents an effective strategy utilizing oxygen defect engineering in MgMn₂O₄ cathodes to enhance Mg²⁺ storage performance in aqueous electrolytes. Oxygen defect formation induces significant lattice expansion, increasing the crystal plane spacing from 0.22 nm to 0.36 nm, which substantially reduces steric hindrance for bulky hydrated Mg²⁺ ions during intercalation. This structural modification accelerates ion diffusion kinetics and mitigates volumetric changes during cycling, thereby minimizing mechanical stress and enhancing the electrode's structural stability. Density functional theory (DFT) calculations demonstrate that oxygen defects reduce the Mg²⁺ diffusion barrier from 0.97 eV to 0.38 eV, and modify the electronic structure by introducing defect states near the Fermi level, thus improving electronic conductivity and charge transfer efficiency. Furthermore, defect-induced charge redistribution generates energetically favorable adsorption sites with binding energies of −0.44 eV for Mg²⁺ ions. Ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses confirm the structural and chemical stability of the host lattice during Mg²⁺ insertion/extraction, emphasizing the role of oxygen defects in framework stabilization. The optimized oxygen-deficient MgMn₂O₄ cathode demonstrates a remarkable specific capacity of 230.8 mAh g⁻¹ at 0.1 A g⁻¹ and exceptional cycling stability, maintaining 85 % capacity after 3000 cycles. This research provides valuable insights into defect engineering as a versatile approach for advancing aqueous multivalent ion energy storage and establishes a framework for rational cathode design through electronic structure modification.