Recent advances in 2D MXene and solid state electrolyte for energy storage applications: Comprehensive review

•The unique layered structures of MXenes simultaneously enhance electrolyte ion transport.•The essential properties of the electrolytes used in supercapacitors linked to the charge storage process are discussed.•MXene/solid electrolytes exhibit high conductivity.•The assembled supercapacitor exhibits excellent electrochemical properties. A new class of two-dimensional transition metal carbides and nitrides, known as MXenes, was discovered in 2011 by Gogotsi et al., and has since emerged as a bright new star in the energy storage sector. Owing to their unique structures and adjustable surface chemical functional groups, MXenes exhibit a range of remarkable properties. Particular attention has been paid to the utilization of MXenes in electrochemical energy storage, which could be highly beneficial for supercapacitor applications. MXenes are particularly suitable electrode materials for supercapacitors owing to their superior mechanical flexibility, energy density, and electrochemical performance. Despite the fact that MXene layers eventually stack together owing to strong van der Waals forces, similar to other 2D materials, this leads to a significant reduction in the number of electrochemically active sites. The electrochemical performance of MXene layers can be enhanced by effectively preventing self-stacking. Optimizing the MXene structure and doping the composite with other materials are two techniques with considerable effects. This review outlines the latest advances in MXene synthesis, fundamental characteristics, and composite materials, with an emphasis on the most recent electrochemical performance of MXene-based electrodes and devices. This highlights the challenges and potential of MXenes in the burgeoning field of energy storage. Supercapacitors are efficient energy storage devices with quick charge/discharge times, long life cycles, and good temperature performance; however, they have lower energy densities than batteries. Owing to their advantages such as no leakage or separators, user safety, and the ability to construct flexible and curved supercapacitors, solid-state and quasi-solid-state electrolytes have become increasingly popular. Recent advances in quasi-solid-state electrolytes include gel polymer electrolytes, aqueous-based solid-state electrolytes, water-in-salt polymer electrolytes, and hydrophilic conductive polymers.