Operando X-ray Reflectivity Reveals the Dynamical Response of Ti3C2 MXene Film Structure during Electrochemical Cycling

Abstract

The dynamical structural response of MXenes during electrochemical ion intercalation is critical for their functionality in fast chemical actuation and high-power electrical energy storage but has yet to be observed. Here, the dynamically evolving layer spacing of a Ti3C2Tx MXene film was observed using time-resolved operando X-ray reflectivity during cyclic voltammetry for applied potentials between +0.3 and −0.7 V (vs Ag/AgCl) at sweep rates 1 ≤ v ≤ 500 mV/s. The pseudocapacitive electrochemical MXene response in the aqueous 0.1 M Li2SO4 electrolyte, including both capacitive and redox characteristics, is characterized by kinetically limited lithium intercalation and layer contraction. Two types of dynamical responses were observed: slow and fast lattice contraction regimes versus applied potential are correlated with the capacitive and redox features, respectively, with structural relaxation rates that scale as ∼v1/2 and ∼v, revealing two distinct dynamical structural responses during electrochemical ion intercalation.