Structure Design And Electrochemical Properties Of In-situ Synthesized Flexible Carbon Nanotube-based Composite Film Materials

The discontinuity of solar energy and wind energy in energy supply and the new requirements of flexible electronic devices for endurance and mechanical properties have promoted the rapid development of electrochemical energy storage technologies such as lithium/sodium ion batteries and supercapacitors.One dimensional carbon nanotubes(CNTs)play an important role in the key technology field of energy storage electrode materials due to their outstanding electrical and mechanical properties.However,due to their low capacity,CNTs need to be composited with a variety of high-capacity active materials to build flexible self-supporting CNTs-based composite film electrodes.However,the chemical inertness and hydrophobicity of the CNT surface seriously hinder the effective loading of the active material,which is the main problem to obtain high-performance carbon nanotube matrix composites.Conventional surface modification methods often destroy the original structure of CNT and reduce its electrical and mechanical properties.In this paper,we adopted heterogeneous nucleation mechanism to introduce a variety of active material precursors in the process of in-situ continuous preparation of carbon nanotubes by chemical vapor deposition(CVD),which improved the surface density of composite materials and avoided the damage to the original structure of carbon nanotubes.Three systems were investigated as component materials for the composite including silicon oxide,titanium dioxide,graphene oxide.Take the silicon oxide/carbon nanotube film as an example,the in-situ conversion reaction is designed,and two kinds of derivative material systems,namely silicates and sulfide carbon nanotube composite films,are prepared.All of the above materials inherit the excellent mechanical flexibility and efficient conductivity of the three-dimensional carbon nanotubes network,which lead to high performing battery electrode materials with good rate performance and cycle stability.The research details are as follows:(1)Graphene has a high specific surface area.It is designed to introduce grapheneinto precursor solution to make it combine with CNTs in-situ to modify the electrochemical performance of CNTs,and graphene oxide carbon nanotube composite films(r GO/CNTs)were designed and prepared.In this work,the introduction of graphene can improve the specific surface area of the composite materials by broadening the introduction source,using the random composite of secondary inorganic molecules and CNTs,and at the same time,the introduction of graphene can improve the specific surface area of the composite materials,which is worthy of reference for the field of supercapacitor electrode materials.(2)The in-situ combination of the silicon dioxide and CNTs was completed at the early stage of carbon nanotube growth using tetraethoxysilane as silicon source.The relationship between the three-dimensional conductive network,the uniform silicon oxide coating and the electrochemical performance of the composite was discussed.The results show that SiO_x/CNTs film can bear about 30%of the strain.Compared with the same kind of silicon-based materials,SiO_x/CNTs film has excellent tensile properties and good flexibility.SiO_x/CNTs film is used as anode material of Li-ion battery.After 500 cycles at a current density of 2 A g^-1,the specific capacity of SiO_x/CNTs film remains about 441 m Ah g^-1,demonstrating its good rate performance and cycle stability.(3)Due to the strong bond between the silicon oxygen and CNTs,the silicon oxidecoating layer is too dense,which make the capacity is gradually released.To solve this problem,the titanium dioxide carbon nanotube composite film material(Ti O₂/CNTs)was prepared by one-step method using tetrabutyl titanate as titanium source.Through the weak force between the titanium oxygen group and the CNT,the in-situ combination of the two was completed in the middle growth stage of the CNT.The results show that the composite film inherits the mechanical properties of CNT and highly interconnected three-dimensional conductive network,and can be used as the anode of lithium/sodium ion battery directly.Because of the design of mixed crystal structure,the material has good structural stability.It shows about 330 m Ah g^-1@100 m A g^-1 as anode material of lithium-ion battery.The specific capacity at high current density of 2 A g^-1 shows a steady trend even after 200 cycles.(4)In order to expand the basic system of oxide carbon nanotube composite films,aiming at the problem of high capacity nonoxide secondary material and carbon nanotube composite,the idea of in-situ transformation is proposed.By using the oxide coating of the basic system material,the reaction is designed to transform it into the required high-capacity secondary material to achieve high load,while maintaining the original structure of the carbon nanotube to the maximum extent and maintaining the mechanical flexibility of the film.By in-situ transformation,two kinds of derived system materials,cobalt nickel silicate/CNTs and cobalt nickel sulfur/CNTs composite films were prepared.The electrode materials with excellent electrochemical properties were obtained by optimizing the parameters of solution dynamics and crystal formation.Among them,cobalt nickel silicate/CNTs shows a specific capacity of 453.4 m Ah g^-1at 2 A g^-1 as electrodes for lithium ion batteries and a specific capacity of 502 m Ah g^-1at 0.1 A g^-1 as electrodes for sodium ion batteries,both demonstrating excellent performances among similar materials.The design idea of intermediate template method would inspire the development of other similar CNTs composite materials.