Probing Mg Intercalation in the Tetragonal Tungsten Bronze Framework V₄Nb₁₈O₅₅

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Commercial lithium-ion batteries (Li-ion) are nearing their energy storage limits.
• Magnesium batteries (Mg) offer potential for higher energy densities than Li-ion.
• Development of high-voltage oxide electrode materials is crucial for advanced Mg batteries, but faces challenges with Mg2+ insertion and diffusion kinetics.

Purpose of the Study:

• To investigate the electrochemical insertion of Mg2+ into the tetragonal tungsten bronze V4Nb18O55.
• To evaluate V4Nb18O55 as a potential intercalation electrode material for Mg batteries.

Main Methods:

• Electrochemical magnesiation and de-magnesiation experiments.
• Hybrid-exchange density functional theory (DFT) calculations to analyze Mg diffusion pathways.
• Powder X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and energy-dispersive X-ray spectroscopy (EDX) for structural and compositional analysis.

Main Results:

• Conclusive evidence of reversible Mg2+ electrochemical insertion into V4Nb18O55.
• Achieved a maximum reversible electrochemical capacity of 75 mA h g-1 (Mg4V4Nb18O55).
• Observed a significant voltage hysteresis (1.12 V) during charge/discharge, reducible to 0.5 V by limiting magnesiation to Mg2V4Nb18O55.
• DFT calculations indicated limited accessible Mg sites and kinetic barriers for diffusion.
• Reversible intercalation involved concurrent V and Nb redox activity and structural changes.

Conclusions:

• Tetragonal tungsten bronzes, exemplified by V4Nb18O55, show potential as intercalation electrode materials for Mg batteries.
• Understanding and mitigating voltage hysteresis is key for optimizing performance.
• Further research into Mg diffusion kinetics within oxide frameworks is warranted.