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 (V 2 O 5 ), a proposed positive electrode material for Al batteries, and the common chloroaluminate ionic liquid electrolytes. We reveal that V 2 O 5 reacts with both the Lewis acidic (Al 2 Cl 7 – ) and the Lewis neutral species (AlCl 4 – ) 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 V 2 O 5 chemically reacts with Al 2 Cl 7 – to form vanadium oxychloride (VOCl 3 ) and amorphous aluminum oxide. V 2 O 5 also chemically reacts with AlCl 4 – to produce dioxovanadium chloride (VO 2 Cl) and a new species of metavanadate anion coordinated with aluminum chloride (AlCl 3 VO 3 – ). These products furthermore exhibit electrochemical redox activity between V 5+ and V 2+ oxidation states. Our results have significant implications when interpreting the electrochemical properties and mechanisms of rechargeable Al–V 2 O 5 batteries.