Materials Compatibility in Rechargeable Aluminum Batteries: Chemical and Electrochemical Properties between Vanadium Pentoxide and Chloroaluminate Ionic Liquids

To demonstrate the importance of electrode/electrolyte stability in rechargeable aluminum (Al) batteries, we investigate the chemical compatibility between vanadium pentoxide (V2O5), a proposed positive electrode material for Al batteries, and the common chloroaluminate ionic liquid electrolytes. We reveal that V2O5 reacts with both the Lewis acidic (Al2Cl7–) and the Lewis neutral species (AlCl4–) within the electrolyte. The reaction products are identified using a combination of electrochemical analyses, Raman spectroscopy, liquid-state and solid-state nuclear magnetic resonance (NMR) spectroscopy, and density functional theory (DFT) calculations. The results establish that V2O5 chemically reacts with Al2Cl7– to form vanadium oxychloride (VOCl3) and amorphous aluminum oxide. V2O5 also chemically reacts with AlCl4– to produce dioxovanadium chloride (VO2Cl) and a new species of metavanadate anion coordinated with aluminum chloride (AlCl3VO3–). These products furthermore exhibit electrochemical redox activity between V5+ and V2+ oxidation states. Our results have significant implications when interpreting the electrochemical properties and mechanisms of rechargeable Al–V2O5 batteries.