The demand for a new generation of flexible, portable, and high‐capacity power sources increases rapidly with the development of advanced wearable electronic devices. Here we report a simple process for large‐scale fabrication of self‐standing composite film electrodes composed of NiCo2O4@carbon nanotube (CNT) for supercapacitors. Among all composite electrodes prepared, the one fired in air displays the best electrochemical behavior, achieving a specific capacitance of 1,590 F g−1 at 0.5 A g−1 while maintaining excellent stability. The NiCo2O4@CNT/CNT film electrodes are fabricated via stacking NiCo2O4@CNT and CNT alternately through vacuum filtration. Lightweight, flexible, and self‐standing film electrodes (≈24.3 µm thick) exhibit high volumetric capacitance of 873 F cm−3 (with an areal mass of 2.5 mg cm−2) at 0.5 A g−1. An all‐solid‐state asymmetric supercapacitor consists of a composite film electrode and a treated carbon cloth electrode has not only high energy density (≈27.6 Wh kg−1) at 0.55 kW kg−1 (including the weight of the two electrodes) but also excellent cycling stability (retaining ≈95% of the initial capacitance after 5000 cycles), demonstrating the potential for practical application in wearable devices.

While pseudocapacitive electrodes have potential to store more energy than electrical double-layer capacitive electrodes, their rate capability is often limited by the sluggish kinetics of the Faradaic reactions or poor electronic and ionic conductivity. Unlike most transition-metal oxides, MoO 2 is a very promising material for fast energy storage, attributed to its unusually high electronic and ionic conductivity; the one-dimensional tunnel is ideally suited for fast ionic transport. Here we report our findings in preparation and characterization of ultrathin MoO 2 sheets with oriented tunnels as a pseudocapacitive electrode for fast charge storage/release. A composite electrode consisting of MoO 2 and 5 wt % GO demonstrates a capacity of 1097 C g −1 at 2 mV s −1 and 390 C g −1 at 1000 mV s −1 while maintaining ∼80% of the initial capacity after 10,000 cycles at 50 mV s −1 , due to minimal change in structural features of the MoO 2 during charge/ discharge, except a small volume change (∼14%), as revealed from operando Raman spectroscopy, X-ray analyses, and density functional theory calculations. Further, the volume change during cycling is highly reversible, implying high structural stability and long cycling life.

Recently, water-in-salt electrolytes have been widely reported because of their ability in broadening the potential window of aqueous based energy storage devices.