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 + , NH 4 + , Zn 2+ , Mg 2+ , Al 3+ ) is up to ∼10 2 µS, which is an order of magnitude higher than the constituent building blocks (∼10 1 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.