Surface ‒OH Termination‐Reduced Ti₃C₂T_x Pseudo‐capacitors with Reinforced Ionic Liquid Accessibility to Achieve High Energy Density

Abstract Highly conductive Ti 3 C 2 T x pseudo‐capacitors hold promise to expand energy density by utilizing ionic liquid (IL) electrolytes (e.g., 1‐ethyl‐3‐methylimidazolium bis trifluoromethane sulfonyl imide, EMITFSI) that possesses broaden voltage window, resulting in various applications involving hybrid vehicle, rail transportation, and power system. However, abundantly random surface ‒OH terminations cannot maintain an extremely stabilized Ti 3 C 2 T x layer spacing framework for efficient ion arrangements, increase ionic diffusion barriers, and decline in energy density. Herein, intensely chemical covalent bonding interactions of ‒SH, ‒COOH, and ‒NH 2 terminated L‐cysteine molecules with ‒OH terminations are proposed to stabilize Ti 3 C 2 T x and expand interlayer structure, allowing sufficient ionic transport and realizing high energy‐density Ti 3 C 2 T x pseudo‐capacitors. It is found that, with cysteine molecules stayed flat to Ti 3 C 2 T x layer, superior hydrophilic surface, and increased interlayer spacing to 1.51 nm, 2.16‐folds higher than Ti 3 C 2 T x electrode, the Ti 3 C 2 T x ‒cysteine electrode delivered high capacitance of 279 F g −1 in EMITFSI/acetonitrile electrolyte. Assembled asymmetric Ti 3 C 2 T x ‒cysteine//activated carbon flexible device exhibited a high voltage of 2.9 V and a high energy density of 72.1 Wh kg −1 at a power density of 874 W kg −1 , which could power various colored smart rollable flexible electronics at bent angle of 90°. Additionally, identical mechanisms are found in Ti 3 C 2 T x ‐amino acid systems, wherein amino acid stayed flat between Ti 3 C 2 T x layers with optimized content, interlayer spacing, and capacitance, no matter amino acid in different charged feature and different sidechain lengths (e.g., glutamine, cysteine, and lysine). The present study provides systematically experimental evidence for improving ion accessibility in IL electrolytes based on an organic‐modified Ti 3 C 2 T x electrode structure.