Nanoconfinement-induced calcium ion redox charge storage of V2CTx MXene

Rechargeable calcium metal batteries are promising for sustainable stationary energy storage applications but are plagued by the irreversible nature of Ca metal anodes due to the formation of a non-ionically conductive solid electrolyte interphase. Alternatively, calcium-ion batteries require the development of charge storage hosts with the characteristics of reversibility and facile diffusivity of high charge density Ca-ions. We report on a foundational experimental study on vanadium carbide MXene (V2CTx) as a two-dimensional (2D) charge host for the reversible (de)intercalation of calcium-ions. Pseudocapacitance of V2CTx is induced by confining a room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI), which helps in partial charge transfer across the electrode|electrolyte interface. Bode analysis reveals redox charge storage dynamics and corroborates well with a change in the oxidation state of vanadium upon insertion of calcium-ions, confirmed through ex situ X-ray photoelectron spectroscopy. V2CTx shows a reversible specific calciation capacity of 120 mA h g-1 at a current density of 50 mA g-1. This work marks the primary experimental investigation on the feasibility of 2D MXene compositions for Ca-ion storage and provides plenty of opportunities for exploring nano-confined fluids for improved charge storage capacities in multivalent metal-ion electrolytes.