Oxygen-Rich Terminations on Ti3C2 MXene Sheets Enable Superior Stability and Energy Storage

Ti3C2Tx holds great promise as an electrode material for high-capacitance supercapacitors, owing to its large specific surface area, pseudocapacitive behavior, and excellent electrical conductivity. However, the commonly used MILD synthesis method based on HCl/LiF etching introduces electrochemically inert −F terminations, which hinder ion storage and make the surface prone to oxidation. In this work, we report a modified MILD etching protocol using a significantly reduced LiF concentration under hydrothermal conditions at elevated temperatures. The lower LiF concentration and the higher etching temperature promote the formation of −O and minimize −F surface terminations. The resulting Ti3C2 film exhibits far fewer −F terminations than conventional films (Ti3C2–O1.37(OH)0.35F0.36·0.35H2O vs Ti3C2–O0.96(OH)0.39F0.75·0.47H2O). The oxygen-rich Ti3C2 film allows for enhanced proton-coupled electron transfer and reversible redox reactions in H2SO4, thus delivering a gravimetric capacitance of 511 F g–1 at 1 A g–1, 1.61 times higher than its counterpart obtained by conventional MILD method. The film retains 93.6% capacitance at 20 A g–1 and shows no performance degradation over 10,000 cycles. When assembled in asymmetric configuration with activated carbon, the device achieves a maximum energy density of 25.0 Wh kg–1 at 375 W kg–1 and maintains 15.4 Wh kg–1 at a high power density of 15,000 W kg–1. Devices with gel electrolyte also show good performance under practical conditions, including in parallel and series configurations as well as under bending deformation. This study presents a scalable strategy to tailor MXene surface chemistry for high-energy, high-power energy storage applications.