Asymmetric Rolling-Up Induced Strong Polarization Electric Field for Ultrahigh Areal Electrochemical Capacitance

Electrochemical capacitors are promising candidates for large-scale energy storage devices owing to their high power densities. However, as increasing their energy densities with high-loading thick electrodes, the ion transport kinetics are usually limited during cycling. Here, asymmetric roll-ups of transition metal carbides (MXenes) are fabricated via a modified stiffness-mediated rolling-up strategy. Such asymmetric structure induces an electron concentration gradient along the axis of MXene roll-up, leading to a strong polarization electric field. Under the polarization, ionic transport conductance of the asymmetric MXene roll-up in various ions (e.g., H⁺, Li⁺, Na⁺, NH₄ ⁺, Zn²⁺, Mg²⁺, Al³⁺) is up to ∼10² µS, which is an order of magnitude higher than the constituent building blocks (∼10¹ µS). As a result, the film constructed from asymmetric MXene roll-up exhibits an ultrahigh rate capability of ∼400 F g^-1 at 5000 mV s^-1 and unique thickness-independent capacitive features. Moreover, the areal capacity of the 400-µm-thick MXene roll-up film with a high mass loading of ∼50 mg cm^-2 is up to ∼26 F cm^-2, outperforming the most reported materials.