Structural Regulation And Electrochemical Performance Of Two-Dimensional Titanium Carbide Based Electrode Material

The wide use of electrochemical energy storage（EES）devices can effectively alleviate the consumption rate of non-renewable energy,and the most critical part of EES is the development of high-performance and long-life electrode materials.2D Ti₃C₂T_x shows a promising prospect in EES because of its superior conductivity,hydrophilicity and solution processability.However,the active sites（-O groups）of Ti₃C₂T_x are not enough.The hydrogen bonding and van der Waals force between terminations will leading to the re-stacking,hinder the ion transport of electrolyte,reduce the surface area and available active sites.In addition,Ti₃C₂T_x is easy to collapsed and become unstable due to the volume change during charge-discharge process.In this work,the high-performance Ti₃C₂T_x-based electrodes were prepared through the interface and structure regulation.The relationship between the structure,composition and properties of the material was systematically studied,the preparation conditions of the electrode material were optimized,and the mechanism of enhancing the electrochemical properties of Ti₃C₂T_xwas analyzed.Ti₃C₂T_x films were heat treated at low temperature,the influence of annealing temperature on the structure and properties of Ti₃C₂T_x films was firstly studied.The-OH groups of Ti₃C₂T_x film can transformed into-O groups when annealing at 200℃under Ar gas,which increases the number of electrochemical active sites and avoids oxidation.Moreover,the removal of H atom and/or O atom in-OH group can increase the interlayer spacing of Ti₃C₂T_x,which is favor of intercalation/deintercalation of electrolyte ion.Combined enlarged interlayer spacing and more-O groups,the 200-Ti₃C₂T_x film electrode has superior capacitance of 416 F/g and energy density of 29.2 Wh/kg at 1 A/g,which could withstand 5000 cycles with 89%capacitance retention under 10 A/g.Starting from the negative electric properties of Ti₃C₂T_x terminations,Ti₃C₂T_x hydrogel was formed by introduction of cation.The introduced cation could not only closely connect the 2D Ti₃C₂T_x nanosheets and maintain 3D network to avoid re-stacking,but also be intercalated into the Ti₃C₂T_x nanosheets interlayer and expand interlayer spacing by“pillar”effect.The capacitance and energy density of H-Ti₃C₂T_x hydrogel can reach428 F/g and 29.1 Wh/kg at 1 A/g.The specific capacitance of H-Ti₃C₂T_x hydrogel can realize 206 F/g when scan rate up to 1 V/s,demonstrating their excellent rate-capability.Moreover,the cycling performance could achieve 91.9%after 5000 cycles under 10 A/g.Cellulose/Ti₃C₂T_x films were prepared by the hydrogen bonding between-OH groups of cellulose and the terminations of Ti₃C₂T_x nanosheets,followed by the vacuum-assisted filtration.Cellulose could not only buffer the volume change of Ti₃C₂T_x during charging and discharging,and effectively improve the cycle stability.In addition,high strength cellulose could act as the reinforcement of mechanical strength,helping to improve the tensile strength and strain.The electrochemical properties of MFC@Ti₃C₂T_x composite films have been effectively improved on the basis of enhanced mechanical properties:when the 10%MFC,the capacitance is 452 F/g when the tensile strength is increased by44%.The energy density of 10%MFC is 30.8 Wh/kg,which could withstand 10000cycles with 95.2%capacitance retention under 10 A/g.This is because the coating structure could expose more active surfaces,form a shorter ion transport path,and promote the mass transfer and occurrence of electrochemical process.