Improving the electrochemical performance of α-MoO3 electrode using aluminium trifluoromethanesulfonate water-in-salt electrolyte

Orthorhombic molybdenum trioxide (α-MoO3) electrode material experiences severe capacity fading and poor cycling stability in aqueous electrolytes. We investigated the charge-storage performance of α-MoO3 electrode in aluminium trifluoromethanesulfonate (Al(OTf)3)-based salt-in-water electrolyte (SiWE) and water-in-salt electrolyte (WiSE). It was found that α-MoO3 electrode exhibits significantly different cycling stabilities in both electrolytes with capacity retentions of 8% using the former and 87% using the latter. This is because α-MoO3 electrode maintains its crystal structure upon cycling in WiSE, but experiences substantial structural collapses and partial dissolution upon cycling in SiWE. This behaviour was inferred from both operando electrogravimetry and ex situ analyses. Research results suggest that the predominant charge-storage mechanism in α-MoO3 electrode using WiSE is the intercalation of protons produced from electrolyte hydrolysis with some contribution from surface pseudocapacitance enabled by Al3+ ions. A two-volt full cell fabricated from α-MoO3 electrode as anode and copper hexacyanoferrate (CuHCF) electrode as cathode using WiSE delivers volumetric and gravimetric energies of 10.4 Wh/L and 26.5 Wh/kg, respectively, with 78% capacity retention after 2500 cycles. This study provides an insightful understanding of the electrochemical performance of α-MoO3 electrode in Al(OTf)3-based electrolytes.