Preparation And Li/Na Storage Performance Of Max-based And Mxene-based Composites

With the increasing environmental crisis,it is urgent to develop new,green,and efficient energy storage and conversion devices.In recent years,MXenes,a new group of two-dimensional laryered materials,have attracted wide research in the field of energy storage and conversion because of their high conductivity,hydrophilicity,and rich surface physical and chemical properties.MAX phases,as their precursors,are also attracted researchers’interests because of their metal like conductivity,high stability and special compositions.In this project,Ti-based MAX phase and MXene are used as the research objects by means of modification to prepare anodes with superior performance.The formation mechanism and energy storage mechanism of materials are described in detail.The detailed research contents are as follows:Si C/Ti C composite and Si C/Ti C@Si O_x/Ti O₂ porous core-shell composite were successfully prepared by ball milling and subsequent heat treatment Ti₃Si C₂ MAX.After ball milling,the micron three-dimensional layered Ti₃Si C₂ decomposed into Si C/Ti C nanopartical,and the specific capacity increases 1.6 times after 200 cycles at 0.1 A g^-1.Under the condition of heat treatment,the Si C/Ti C can be gradual oxidized from outside to inside to form Si C/Ti C@Si O_x/Ti O₂ porous core-shell composite.With the increase of heat treatment temperature and holding time,the oxidation degree gradually deepens,hence the thickness of the shell gradually becomes larger.In this process,CO₂ was volatilized and then holes were formed on the surface.This specifical composition and structure significantly improves the lithium storage performance of the Si C/Ti C@Si O_x/Ti O₂,it delivers a reversible specific capacity of 363.3 m Ah g^-1 at 0.1 A g^-1 after 200 cycles,which is almost 2.6 times higher than that of Si C/Ti C,and 4.1 times higher than that of Ti₃Si C₂.Ni²⁺ions were adsorbed on d-MXene by electrostatic attraction,then nucleated and grown into Ni（OH）₂ under hydrothermal conditions,hence Ni（OH）₂/d-MXene composite was successfully prepared.Ni（OH）₂ nanosheets uniformly grown on d-MXene can not only effectively prevent d-MXene self-stacking and increase the reaction active sites,but also shorten the ions diffusion distance and accelerate the ions transport.And d-MXene can not only effectively prevent the agglomeration of Ni（OH）₂ nanosheets and avoid the volume expansion of Ni（OH）₂ during charge and discharge process,but improve the conductivity.This synergistic effect makes the composite exhibit excellent lithium storage performance,it delivers a reversible specific capacity of 732.6 m Ah g^-1 after 200 cycles at 0.1 A g^-1.To further improve lithium storage performance of Ni（OH）₂/d-MXene,Fe³⁺was introduced,and the Ni Fe-LDH/d-MXene composite was successfully prepared.Ni Fe-LDH nanosheets grown vertically and uniformly on d-MXene can effectively prevent d-MXene flakes self-stacking,and d-MXene with high conductivity can effectively alleviate the volume changes of Ni Fe-LDH during cycles.The synergism makes Ni Fe-LDH/d-MXene composite have excellent lithium storage performance,at 0.1 A g^-1 for 200 cycles,it delivers capacities of 898.9 m Ah g^-1,almost 1.2 times of Ni（OH）₂/d-MXene.After assembled with activated carbon into lithium ion capacitor,it also shows better performance,and its energy density is 168 Wh kg^-1 at 47 W kg^-1,and power density is 1158 W kg^-1 at 53 Wh kg^-1.Further improve the preparation method of LDH/d-MXene composite,a new method was developed,in this method,ZIF-67 was used as a self-sacrificial template,and it was hydrolyzed into Co-LDH nanosheets during washing in aqueous solution.Similarly,the Co-LDH/d-MXene composite was prepared by washing ZIF-67/d-MXene in aqueous solution.In this process,ZIF-67 cubes grown on d-MXene nanoflakes hydrolyzed and then formed the ultra-thin Co-LDH nanosheets vertically grown on d-MXene nanosheets.Compared with hydrothermal method,this method can alleviate the agglomeration of LDH nanosheets and improve the specific surface area.As lithium ion battery anode,Co-LDH/d-MXene delives a reversible capacity of 854.9 m Ah g^-1 at 0.1 A g^-1.The good lithium storage performance of Co-LDH/d-MXene composite is related to high specific surface,multivalent Co,and high conductivity.Ni₃S₂/d-MXene was prepared by in-situ vulcanization of Ni（OH）₂/d-MXene.Ni₃S₂nanoparticles grown on d-MXene nanosheets effectively prevent d-MXene self-stacking,increase the layer spacing of d-MXene,and provide rich active sites.Besides,d-MXene nanosheets as conductive frameworks not only improve the conductivity,but effectively prevent the volume expansion of Ni₃S₂ nanoparticles during charge and discharge process.The synergistic effect makes Ni₃S₂/d-MXene shows a capacitive and diffusive dual-model energy storage mechanism,exhibiting better sodium storage performance,it delivers a reversible capacity of 193.8 m Ah g^-1 at 1 A g^-1 after 800 cycles and without obvious capacity decay.