Electrochemical Performance Of MXene （Ti₃C₂） Composites And Derivative For Sodium（potassium）ion Batteries

Lithium-ion batteries（LIBs）have been widely used as power sources of portable electronic devices in normal life.Recent years,the popularization of electric vehicles have leaded to the demands of LIBs are keeping arise.However,the further development and application of LIBs is largely hindered by the rare mineral resources of lithium.Based on this concern,researchers concentrate their attention on the new-form ion batteries,especially speaking,the sodium-ion batteries（SIBs）and potassium-ion batteries（PIBs）.SIBs and PIBs have similar construction and electrochemical energy storage mechanism with LIBs,which accelerate the development of SIBs and PIBs.MXenes has attracted great attention in the field of energy storage due to its highly adjustable metal components and surface functional groups,especially its excellent conductivity,unique two-dimensional structure,adjustable layer spacing and low ionic diffusion barrier.Studies have shown that Ti₃C₂T_x MXene has excellent stability as an electrode material for sodium ion batteries,but has a low capacity.Based on this background,the purpose of this paper is to explore the preparation of MXene-based composites/derivative and to study their electrochemical properties as electrode materials of SIBs/PIBs.The main research contents and results of this paper are as follows:1.In order to solve the problem of the low capacity of pure Ti₃C₂T_x and poor cyclic stability of transition metal oxide when they are used as anode materials of SIBs,CoNiO₂/Ti₃C₂T_x composite were synthesized by a two-step method.First,Co-Ni double hydroxide was loaded on the multi-layered Ti₃C₂T_x which was used as matrix by hydrothermal method.And then the CoNiO₂/Ti₃C₂T_x composite was obtained by annealing the precursor.When the CoNiO₂/Ti₃C₂T_x composite was used as anode material in SIBs,it displays a high capacity of 413 mA h g^-1 at a current density of 0.1 A g^-1,which is a significant improvement than the pure Ti₃C₂T_x,and the cyclic stability is also improved compared with the pure CoNiO₂.The double hydroxide loading onto the Ti₃C₂T_x matrix can prevent the agglomeration of it during the annealing process,which is favorable for the contact between the CoNiO₂/Ti₃C₂T_x composite and electrolyte,improve the reactivity and ensure a good cyclic performance.Meanwhile,MXene could improve the electronic conductivity of the composite since it served as an electronic conductive matrix,which ensure the good rate performance of the composite.All in all,combining MXene with transition metal oxide is the way to obtain a kind of SIBs anode material with high reversible capacity,steady cyclic performance and high rate capability.2.In order to solve the safety issue of traditional liquid SIBs and suppress the dissolution of polysulfide of transition metal sulfide,CoS₂/CNTs/TiO_x N_y composite was synthesized by a two-step method,and the PFSA-Na membrane was also fabricated.Based on CoS₂/CNTs/TiO_xN_y composite and PFSA-Na membrane,solid-state sodium-ion batteries（SSIBs）are assembled,and the electrochemical reaction process of CoS₂/CNTs/TiO_xN_y composite in SSIBs was explored.First of all,the multi-layered Ti₃C₂T_x was used as matrix and the CoS₂/CNTs/TiO_x N_y composite were synthesized by high temperature pyrolysis and vulcanization method.Then,the PFSA-Na membranes was fabricated by ionic-exchange method and solution coating process.The as-assembled SSIBs displays a capacity of 234 mA h g^-1 at 1 A g^-1 and good rate performance at room temperature.In-situ Raman and ex-situ XPS analysis suggests that the CoS₂ of the composite could form the intermediate polythionate complex during the reaction process which could suppress the dissolution of polysulfide and ensure a steady cyclic performance.Ti₃C₂T_x matrix and in-situ formed CNTs improve the conductivity of the composite together,which result in a good rate capability.The as-synthesized CoS₂/CNTs/TiO_xN_y composite has promising electrochemical performance and the as-assembled SSIBs based on CoS₂/CNTs/TiO_xN_y composite and PFSA-Na membrane is a new choice for safety energy storage.3.In order to use the MXene derivative as anode material for PIBs and achieve a promising electrochemical performance,the carbon encapsulated TiO_xN_y was synthesized by a two-step method based on Ti₃C₂T_x flake.First of all,Ti₃C₂T_x flake was combined with melamine through electrostatic adsorption,and then the carbon encapsulated TiO_xN_y was obtained by high-temperature pyrolysis.When it used as anode material for PIBs,good rate capability and excellent cyclic performance was obtained,the sample could deliver 150 mA h g^-1 at 0.2 A g^-1 after 1250 cycles.And after rate test,the structure of electrode material is maintained.By particle size statistical analyzing,it can be found that the size of TiO_x N_y/C is concentrated at about 20-40 nm,which is conducive to increasing the specific surface area,the number of active sites as well as shortening the length of ion diffusion path.In addition,the carbon encapsulated structure is also conducive to maintaining the structure stability of the material during the electrochemical cycling process.The isothermal nitrogen adsorption and desorption curves shows that the material has a slit pore type and a rich mesoporous distribution,which are conducive to the infiltration of electrolyte and improve the performance.In this work,the synthesis method of TiO_xN_y/C is simple and the electrochemical performance is promising.Therefore,the TiO_xN_y/C provided in this work is a kind of anode material for PIBs with promising further.In this paper,CoNiO₂/Ti₃C₂T_x composite with higher reversible capacity and steady cyclic performance was synthesized and used as anode material for SIBs.And CoS₂/CNTs/TiO_xN_y composite was synthesized and used in SSIBs,which demonstrates a good electrochemical performance,and the CoS₂ could form the intermediate polythionate complex to suppress the dissolution of polysulfide during the reaction process.Furthermore,the carbon encapsulated Ti₃C₂T_x derivative TiO_x N_y was explored as anode material for PIBs and promising electrochemical performances were obtained.All the materials synthesized in this paper demonstrate good electrochemical performance which provide some new choice as for SIBs or PIBs anode materials.Meanwhile,the usage of MXene in the field of energy storage is expanded and some guidance is given for the future study.