Direct Investigation of Mg Intercalation into the Orthorhombic V₂O₅ Cathode Using Atomic-Resolution Transmission Electron Microscopy

Batteries based on Mg metal anode can promise much higher specific volumetric capacity and energy density compared to Li-ion systems and are, at the same time, safer and more cost-effective. While previous experimental reports have claimed reversible Mg intercalation into beyond Chevrel phase cathodes, they provide limited evidence of true Mg intercalation other than electrochemical data. Transmission electron microscopy techniques provide unique capabilities to directly image Mg intercalation and quantify the redox reaction within the cathode material. Here, we present a systematic study of Mg insertion into orthorhombic V₂O₅, combining aberration-corrected scanning transmission electron microscopy (STEM) imaging, electron energy-loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDX) analysis. We compare the results from an electrochemically cycled V₂O₅ cathode in a prospective full cell with Mg metal anode with a chemically synthesized MgV₂O₅ sample. Results suggest that the electrochemically cycled orthorhombic V₂O₅ cathode shows a local formation of the theoretically predicted ϵ-Mg_0.5V₂O₅ phase; however, the intercalation levels of Mg are lower than predicted. This phase is different from the chemically synthesized sample, which is found to represent the δ-MgV₂O₅ phase.