Modulation Of Group ⅥA And ⅦA End-Base MXenes Materials And Their Lithium Storage Properties

Lithium-ion batteries have been widely used in everyday life as a portable and efficient technology for energy storage and conversion.However,the current batteries made up of commercial graphite negative electrodes have poor multiplier performance and long cycle stability,and are no longer able to meet the demand for fast charging and long battery life.Transition metal carbide or nitride（MXenes）materials with two-dimensional sheet structure are characterized by large specific surface area,high electrical conductivity and abundant surface groups,which can be used as electrode active materials or electrode additives to help improve the multiplication performance and cycling stability of batteries with good application prospects.However,there are currently two problems with MXenes:on the engineering side,the traditional HF preparation method is risky and polluted,and cannot be prepared on a large scale.In terms of performance,the complex and difficult-to-regulate functional groups on the surface of MXenes greatly restrict their electrochemical performance.Therefore,this paper is dedicated to develop a safe and efficient route for the preparation of MXenes and can precisely regulate the functional groups on the surface of MXenes,including seven elements of group VIA（O,S,Se）and group VIIA（F,Cl,Br,I）,and systematically investigate their effects on the physical properties and electrochemical lithium storage dynamics of MXenes materials.Study of the properties of theⅦA functional groups:HF or Li F+HCl etchants were first utilised;then Lewis acid molten salts were selected for etching,and the method can successfully obtained Ti₃C₂-F,Ti₃C₂-Cl,Ti₃C₂-Br,and Ti₃C₂-I.The synthesis of Ti₃C₂-F involves prolonged exposure to highly concentrated corrosive acid solutions,the molten salt method not only improves experimental safety,but also shortens the reaction time.The lithium storage properties of MXenes modified with different Group VIIA reactive groups were investigated.With Ti₃C₂-F,Ti₃C₂-Cl,Ti₃C₂-Br,and Ti₃C₂-I at a current density of 1.0A·g^-1,the discharge specific capacities are 66,116,223,and 156 m Ah·g^-1;and Ti₃C₂-Br exhibits a better lithium storage multiplicity.Among these,Ti₃C₂-I has a large pseudocapacity contribution,with a capacity fraction of 85%at 5.0 m V·s^-1.Studies have shown that molten salt process is environmentally friendly and safe,offering improved cycling performance and product kinetics compared to conventional acid etching.Br-terminated base has the best performance and the I-terminated base has the most capacitive lithium storage behaviour.Study of the properties of VIA functional groups:the halogen-based Ti₃C₂T_x with excellent performance was selected and the oxidative replacement of surface groups was carried out by high temperature calcination.The results showed that the halogen content in the products was reduced,the Ti-O,Ti-S and Ti-Se bonds were generated,and Ti₃C₂-O,Ti₃C₂-S and Ti₃C₂-Se materials were successfully synthesized.The addition of VIA group elements change the physicochemical properties of MXenes and maintains the excellent layered structure of the 2D material.At a current density of 1.0 A·g^-1,the electrochemical test results showed that the discharge specific capacities of Ti₃C₂-O,Ti₃C₂-S,and Ti₃C₂-Se are 376,260,and 231 m Ah·g^-1,respectively,with good structural stability;among these,Ti₃C₂-O exhibits good multiplicative performance,achieving a specific capacity of 114m Ah·g^-1 even at high currents of 4.0 A·g^-1;the impedance gradually increases and the Warburg coefficientσdecreases.The results show that Ti₃C₂-O has the highest lithium storage specific capacity,the best cycling stability and fast lithium storage power.Finally,the electrochemical properties of MXenes modified with seven different elements of VIA and VIIA were systematically compared.Lower operated voltage for Group VIIA elements modified materials.In contrast,Ti₃C₂T_x materials modified with Group VIA elements have higher specific capacitance and Coulomb efficiency.The results of the paper show that the functional groups on the surface of the material are important for the lithium storage performance,the thermal stability and electrochemical kinetics of MXenes can be effectively improved by precise modulation of the functional groups on the surface.The findings of this paper will help to facilitate the practicalisation of MXenes materials and the comparison of the advantages and disadvantages of different end groups will be a guide to the design of 2D materials.