Reversible Intercalation of Mg²⁺ in V₂O₅ at Elevated Temperatures Leads to Enhanced Electrochemical Performances

Barriers to lithium ion battery development, such as the limited energy density of Li-ion batteries and the scarcity of Li, have led to great interest in exploring multi-valent ionic batteries containing earth abundant intercalation elements, such as magnesium or calcium. Reversible intercalation of Mg²⁺ ions is a crucial part in constructing a stable battery, which, when combined with high capacity, charts a promising pathway towards high performance and future designs of Mg-ion batteries. High levels of reversible Mg²⁺ intercalation have been reported in α-V₂O₅ by cycling assembled cells at an elevated temperature of 110°C, the benefit of which is retained at room temperature.<br/><br/>Experiments were carried out with assembled cells consisted of pressed V₂O₅ powders as the cathode, ionic liquid MgTFSI₂-PY14TFSI as the electrolyte, and activated carbon cloths or Mg foil as the anode. After cycling at room temperature and 110°C, the samples were characterized structurally with electron microscopy and X-ray absorption spectroscopy. The ex-situ experiments demonstrate that α-V₂O₅ can intercalate at least one mole of Mg²⁺ reversibly at 110°C, as opposed to &lt;0.6 mole at 25°C. The capacity of the α-V₂O₅ is increased 20-fold, from 16 mAh g^-1 at 25°C to 295 mAh g^-1 at 110°C, matching the performance of Li-ion batteries. This increase in electrochemical performance is accompanied by structural changes; the morphology of the α-V₂O₅ powders drastically change from platelets to delaminated layers. Particles with delaminated morphologies show the change in Mg content, which echoes with the change in capacity, calculated to be 1 mol Mg²⁺ per mole α-V₂O₅. These findings pave the road toward reversible Mg-ion batteries with high degrees of Mg²⁺ intercalation and high potential, and current efforts in developing in-situ characterization functionalities will lead to imperative capabilities.<br/><br/>Current research is ongoing to explore in-situ characterization techniques with scanning transmission electron microscope, combined with capabilities of simultaneous heating and electrochemical cycling. If realized, this can open up more possibilities to deepen the understanding of structural and chemical developments during the intercalation process.