Design And Synthesis Of Carbon Nanotube-based Ni_xS_y,MoS₂,and TiO₂ Nanocomposites As Advanced Anodes For Lithium Ion Batteries

The cathode material is an important part of the lithium ion batteries?LIBs?.Transition metal compounds are considered to be the most promising electrode materials for LIBs due to their advantages of high specific capacity,abundant resources,low price and environmental friendliness.Among them,Ni,Mo and Ti have attracted a great deal of attention.Because of the low electrical conductivity and obvious volume effect of the sulfur compounds of Ni and Mo,the cyclic stability and multiplier performance of the materials are not ideal.Due to its low theoretical capacity and conductivity,TiO₂ can not be used as the cathode material of lithium ion battery alone with out composite with other materials or modified.Based on above factors,this paper designed structures of carbon nanotube loaded with Ni_xS_y thio-nickel compound composite material,MoS₂ nanosheets-wrapped carbon nanotube-in-nanotube composite material,TiO₂nanocrystalline-assembledmesoporoussandwichlayerembeddedincarbon nanotube-in-nanotube composite material,explored and studied the properties of the above materials as the cathode of LIBs.The main contents of this paper are as follows:?1?homogeneous coated Ni_xS_y@CNTs composite was prepared by chemical bath deposition combined with high temperature calcination and hydrothermal method.Ni_xS_y surrounded the carbon nanotubes evenly in the form of thionicol nanoparticles with micropores between them.In Ni_xS_y@CNTs composite,the center CNTs provide support of the whole structure,as well as a conductive substrate to improve the conductivity of the composite,the mesoporous Ni_xS_y nanoparticles covered on CNT are tightly coupled with it,the nanopores not only reduce the volume effect of the material,but also provide space for fully infiltrate of the electrolyte.After electrochemical test,the first charge/discharge capacity of the material was 1007 mAh/g and1151 mAh/g under a current density of 0.1 C,respectively.The coulombic efficiency of the first cycle was 87.4%.After 100 constant current charge-discharge cycles,the charge-discharge capacity is 762 mAh/g and 756 mAh/g.In the multiplier test,when the current density turn back to 0.1 C,the capacity restored to 94.4%under the initial current density of 0.1 C.The material has shown good reversibility and multiplier performance,also shows great potential in the cathode material of li-ion battery.?2?Via chemical bath deposition combined with etching and hydrothermal methods,hollow carbon nanotube-in-tubes?CTTs?were produced,and we got MoS₂@CTTs composite with MoS₂nanosheets coverd on CTTs.Expectations of this design is that growth of extending outward MoS₂ nanosheets can shorten the diffusion distance of Li⁺,the CTTs can provide a buffer for inflationing of MoS₂,as well as a conductive substrate,and the hollow part of the CTTs also can be used as the electrolyte storage space to improve the activity of the electrochemical reaction.In the electrochemical test,the first charging and discharging capacity of the material reached1042/1057 mAh/g and the coulombic efficiency reached 98.6%under a current density of 0.2 C.After 175 cycles,the capacity was 856 mAh/g and tend to be stable.While the capacity retention rate of the composite was as high as 90.3%after the current density restored to 0.1 C in the multiplier test.This material exhibits excellent long cycle stability and multiplier performance,which is attributed to the design of the structure and the synergistic effect of various factors,maks the MoS₂@CTTs composite show excellent electrochemical performance.?3?TiO₂@CTTs composite with sandwich structure was successfully synthesized through chemical bath deposition combined with etching and calcining.In this composite,the ultrafine size of TiO₂ nanocrystals provided more active sites for the electrochemical reaction,increased the electrochemical reaction activity,shortened the Li⁺diffusion distance and improved the electrochemical reactivity of the material.In addition,the presence of inner and outer carbon layers ensures the structural stability of TiO₂ nanocrystals and prevented fragmentation or collapse.The outer carbon nanotubes provided both protective support and mutual promotion with the inner carbon nanotubes to improve the electrical conductivity of TiO₂.At a current density of 0.2 C,the discharge capacity of the composite was 242 mAh/g for the first cycle.After200 cycles,the discharge capacity of the composite was still 191 mAh/g.In the multiplier test,the specific capacity of 115 mAh/g is available under the current density of 5 C.When the current is restored to 0.1 C,the discharge capacity restored to 207 mAh/g.The composite shows excellent cyclic stability and multiplier performance,and the structure shows great application prospect.