Fiber-shaped supercapacitors with improved specific capacitance and high rate capability are a promising candidate as power supply for smart textiles. However, the synergistic interaction between conductive filaments and active nanomaterials remains a crucial challenge, especially when hydrothermal or electrochemical deposition is used to produce a core (fiber)-shell (active materials) fibrous structure. On the other hand, although 2D pseudocapacitive materials, e.g., Ti C T (MXene), have demonstrated high volumetric capacitance, high electrical conductivity, and hydrophilic characteristics, MXene-based electrodes normally suffer from poor rate capability owing to the sheet restacking especially when the loading level is high and solid-state gel is used as electrolyte. Herein, by hosting MXene nanosheets (Ti C T ) in the corridor of a scrolled carbon nanotube (CNT) scaffold, a MXene/CNT fiber with helical structure is successfully fabricated. These features offer open spaces for rapid ion diffusion and guarantee fast electron transport. The solid-state supercapacitor based on such hybrid fibers with gel electrolyte coating exhibits a volumetric capacitance of 22.7 F cm at 0.1 A cm with capacitance retention of 84% at current density of 1.0 A cm (19.1 F cm ), improved volumetric energy density of 2.55 mWh cm at the power density of 45.9 mW cm , and excellent mechanical robustness.

As an essential member of 2D materials, MXene (e.g., Ti3C2Tx) is highly preferred for energy storage owing to a high surface‐to‐volume ratio, shortened ion diffusion pathway, superior electronic conductivity, and neglectable volume change, which are beneficial for electrochemical kinetics. However, the low theoretical capacitance and restacking issues of MXene severely limit its practical application in lithium‐ion batteries (LIBs). Herein, a facile and controllable method is developed to engineer 2D nanosheets of negatively charged MXene and positively charged layered double hydroxides derived from ZIF‐67 polyhedrons into 3D hollow frameworks via electrostatic self‐assembling. After thermal annealing, transition metal oxides (TMOs)@MXene (CoO/Co2Mo3O8@MXene) hollow frameworks are obtained and used as anode materials for LIBs. CoO/Co2Mo3O8 nanosheets prevent MXene from aggregation and contribute remarkable lithium storage capacity, while MXene nanosheets provide a 3D conductive network and mechanical robustness to facilitate rapid charge transfer at the interface, and accommodate the volume expansion of the internal CoO/Co2Mo3O8. Such hollow frameworks present a high reversible capacity of 947.4 mAh g−1 at 0.1 A g−1, an impressive rate behavior with 435.8 mAh g−1 retained at 5 A g−1, and good stability over 1200 cycles (545 mAh g−1 at 2 A g−1).

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The increasing demands of portable, flexible, and wearable electronics have greatly stimulated the development of miniaturized supercapacitors (MSCs) with features of high flexibility, low weight, long lifespan, and safety. The techniques which can effectively incorporate functional nanomaterials into flexible electrodes are extremely important for future wide‐spread applications of MSCs, and thus are identified as the research focus in the field. Herein, this review focuses on recent advances in the fabrication of flexible electrodes and their applications in MSCs. In particular, the principles of the newly developed fiber spinning techniques and their utilization in designing microstructures and resulting electrochemical properties of fibrous electrodes are first described. Second, a few novel approaches to prepare planar MSCs and their special advantages are introduced, and third, the state‐of‐the‐art research in 3D stereo MSCs based on 3D printing and morphing technologies are discussed. In the end, the prospects and key challenges based on the considerations of performance improvement, device design, and system integration are outlined.