2D transition metal carbides, nitrides, and carbonitrides, also known as MXenes, are versatile materials due to their adjustable structure and rich surface chemistry. The physical and chemical diversity has recognized MXenes as a potential 2D material with a wide spectrum of application domains. Since the discovery of MXenes in 2011, a wide variety of synthetic routes has been proposed with advancement toward large‐scale preparing methods for MXene nanosheets and derivative products. Herein, the critical synthesis aspects and the operating conditions that influence the physical and chemical characteristics of MXenes are discussed in detail. The emerging etching methods including HF etching methods, in situ HF‐forming etching methods, electrochemical etching methods, alkali etching methods, and molten salt etching methods, as well as delamination strategies are discussed. Considering the future developments and practical applications, the large‐scale synthesis routes and the antioxidation strategies of MXenes are also summarized. In summary, a generalized overview of MXenes synthesis protocols with an outlook for the current challenges and promising technologies for large‐scale preparation and stable storage is provided.

MXenes with metallic conductivity, high pseudo-capacitance and 2D structure, are promising flexible electrode materials for supercapacitors, but suffer from the restacking issue, which hinders ion accessibility and causes sluggish ion kinetics. Herein, a simple in situ ice template strategy is proposed to fabricate free-standing, flexible 3D porous Ti 3 C 2 T x /carbon nanotubes (CNTs) film (3D-PMCF) by freeze-drying Ti 3 C 2 T x-based hydro-films without any postprocessing. During the freeze-drying process, small ice grains are in situ transformed from the residual water molecules in the Ti 3 C 2 T x interlayer and then act as a self-sacrifice template to construct a 3D porous network. CNTs introduced in the hydro-film increase the amount of interlayer water and the resultant porosity. The 3D structure of Ti 3 C 2 T x significantly increases the exposed surface active sites and accelerates the ion transport, meanwhile maintaining good flexibility. Consequently, the flexible 3D-PMCF film delivers a capacitance of 375 F g −1 at 5 mV s −1 and retains 251.2 F g −1 at 1000 mV s −1 with excellent cycling stability, much superior to the conventional densely stacked Ti 3 C 2 T x film. Being assembled into a symmetric supercapacitor, an energy density of 9.2 Wh kg −1 is realized. This work demonstrates a simple and efficient route for constructing high-performance and flexible 3D MXene film electrodes for supercapacitors.

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2D MXenes have emerged as promising supercapacitor electrode materials due to their metallic conductivity, pseudo‐capacitive mechanism, and high density. However, layer‐restacking is a bottleneck that restrains their ionic kinetics and active site exposure. Herein, a carbon dots‐intercalated strategy is proposed to fabricate flexible MXene film electrodes with both large ion‐accessible active surfaces and high density through gelation of calcium alginate (CA) within the MXene nanosheets followed by carbonization. The formation of CA hydrogel within the MXene nanosheets accompanied by evaporative drying endow the MXene/CA film with high density. In the carbonization process, the CA‐derived carbon dots can intercalate into the MXene nanosheets, increasing the interlayer spacing and promoting the electrolytic diffusion inside the MXene film. Consequently, the carbon dots‐intercalated MXene films exhibit high volumetric capacitance (1244.6 F cm−3 at 1 A g−1), superior rate capability (662.5 F cm−3 at 1000 A g−1), and excellent cycling stability (93.5% capacitance retention after 30 000 cycles) in 3 m H2SO4. Additionally, an all‐solid‐state symmetric supercapacitor based on the carbon dots‐intercalated MXene film achieves a high volumetric energy density of 27.2 Wh L−1. This study provides a simple yet efficient strategy to construct high‐volumetric performance MXene film electrodes for advanced supercapacitors.

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