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

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.