Two-dimensional MXenes for flexible energy storage devices

With the rapid development of wearable electronics, flexible energy storage devices that can power them are quickly emerging. Among multitudinous energy storage technologies, flexible batteries have gained significant attention, benefiting from high energy density and long cycling life. An ideal flexible battery requires superior electrochemical performance and excellent mechanical deformability. MXenes, 2D transition metal carbides, nitrides, and carbonitrides show substantial encouraging advances due to their unique properties, including excellent mechanical performance, high electrical conductivity, abundant surface chemistries, and convenient processability. Related reports of MXenes in flexible batteries have keenly increased since 2021. However, systematic reviews on this subject are rare. Herein, the latest progresses of MXene-based materials in flexible energy storage devices are comprehensively reviewed. Firstly, the fundamental principles of flexible MXenes, such as types, synthesis methods, and competitive features, are introduced. Subsequently, the design strategies and internal mechanisms of MXene-based materials in flexible metal batteries and metal-ion/oxygen/sulfur/selenium batteries (metal = Li, Na, K, Zn, Mg, Fe, etc.) as electrode, matrix, current collector, interlayer, and binder are comprehensively introduced. At the end of the review, the current trends, limitations, and future outlooks of MXene-based materials in flexible batteries are proposed. The design strategies and internal mechanisms of MXene-based materials in flexible energy storage devices are comprehensively introduced. Besides, the current trends, limitations, and future outlooks are proposed.