Beyond Half‐Cell Success: Cathode‐Electrolyte Reactivity Driving Magnesium Battery Full‐Cell Degradation at Elevated Temperature

Rechargeable magnesium battery (RMB) is gaining attention as a promising alternative to lithium-ion batteries, offering advantages such as low cost and high theoretical capacity of magnesium metal anodes. Yet, realizing stable, high-voltage RMB full cells remains a considerable challenge. In this study, a full-cell configuration is explored combining a vanadium oxide (VO₂) cathode with a weakly coordinating anion-based electrolyte. While encouraging performance is observed in half-cell setups, translating it into full-cell operation proves complex, particularly at elevated temperatures. At 60 °C, the initial discharge capacity of 77 mAh g^-1 decreases notably to 28 mAh g^-1 in the second cycle, whereas performance at 30 °C remains more stable ≈25 mAh g^-1. Three-electrode measurement suggests increasing overpotentials at the Mg anode as a key factor in the capacity degradation. Further analysis points to issues such as uneven Mg plating/stripping, surface pitting, and minor vanadium dissolution, contributing to impedance growth and cross-over effects. These are linked to cathode-electrolyte side reactions, particularly under high-voltage. Overall, the results emphasize the importance of developing stable interphases to enhance the long-term performance of RMB full cells, especially at elevated temperatures.