Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors

High-power Na-ion batteries have tremendous potential in various large-scale applications. However, conventional charge storage through ion intercalation or double-layer formation cannot satisfy the requirements of such applications owing to the slow kinetics of ion intercalation and the small capacitance of the double layer. The present work demonstrates that the pseudocapacitance of the nanosheet compound MXene Ti2C achieves a higher specific capacity relative to double-layer capacitor electrodes and a higher rate capability relative to ion intercalation electrodes. By utilizing the pseudocapacitance as a negative electrode, the prototype Na-ion full cell consisting of an alluaudite Na2Fe2(SO4)3 positive electrode and an MXene Ti2C negative electrode operates at a relatively high voltage of 2.4V and delivers 90 and 40mAhg-1 at 1.0 and 5.0Ag -1 (based on the weight of the negative electrode), respectively, which are not attainable by conventional electrochemical energy storage systems

Two-Dimensional Vanadium Carbide (MXene) as Positive Electrode for Sodium-Ion Capacitors

Ion capacitors store energy through intercalation of cations into an electrode at a faster rate than in batteries and within a larger potential window. These devices reach a higher energy density compared to electrochemical double layer capacitor. Li-ion capacitors are already produced commercially, but the development of Na-ion capacitors is hindered by lack of materials that would allow fast intercalation of Na-ions. Here we investigated the electrochemical behavior of 2D vanadium carbide, V2C, from the MXene family. We investigated the mechanism of Na intercalation by XRD and achieved capacitance of ∼100 F/g at 0.2 mV/s. We assembled a full cell with hard carbon as negative electrode, a known anode material for Na ion batteries, and achieved capacity of 50 mAh/g with a maximum cell voltage of 3.5 V