Preparation And Electrochemical Performance Of CNTs@TiO₂ Composites As Anode Materials For Lithium-ion Batteries

Anatase titanium dioxide(TiO₂)has been widely studied as one of the most prominent anode materials for lithium-ion batteries(LIBs),because of its low cost,chemical and physical stability,long cycling life,and favorable theoretical capacity.However,low intrinsic electronic conductivity and poor lithium-ion diffusion coefficient of anatase TiO₂ restrict its electrochemical performance,especially at high charging-discharging rate.In order to improve the rate performance of anatase TiO₂,multi-walled carbon nanotubes(CNTs)@TiO₂ composites with different contents of CNTs were prepared by employing solvothermal process using ethanol-glycerol mixture as solvent and subsequent calcination.The role of glycerol during solvothermal process and the effect of calcination temperature on electrochemical performance of composites were discussed.On this basis,F127 was added to further improve the electrochemical performance of the CNTs@TiO₂ composites.The as-prepared CNTs@TiO₂ composites were characterized by scanning electron microscopy,high-resolution transmission electron microscopy and X-ray diffraction.The main results are as follows:(1)The results indicate glycerol is one of the solvothermal reactants and also plays an important role in the coating of TiO₂ particles on the surface of CNTs because of its high viscosity.Moreover,with the increase of CNTs content in the composites,the specific surface area of the composites increases gradually,and the TiO₂ coating on the surface of CNTs is more uniform.Notably,CNTs@TiO₂ composites with 42 wt.%CNTs have a specific surface area of 265.7 m²·g^-1,and exhibit excellent high-rate performance and cyclic stability.The specific capacities of the composites are 248,228,225,212,200,194 and 191 m Ah/g at 1C,2C,5C,10C,20C,30C and 40C,respectively.Even at50C and 60C,the specific capacities can still reach 187 and 184 m Ah/g.After 1000cycles at 10C,90.1%of the reversible capacity is retained.The excellent performance can be ascribed to the electronic and ionic rapid conductive network formed by CNTs in composites and the synergistic contribution from pseudo-capacitance.(2)With the increase of calcination temperature,the crystallinity of anatase TiO₂ in the CNTs@TiO₂ composites increases significantly.The growth of TiO₂ grains leads to the increase of particle size,the agglomeration between TiO₂ particles,and the decline of the specific surface areas of the composites.Thus the pseudo-capacitance dominated by lithium storage on the surface reduces.While the electrochemical performance of TiO₂ with larger particles is poor.Therefore,the rate performance and cycle performance of the CNTs@TiO₂composites decrease greatly with the increase of calcination temperature,and the composites calcined at 350?have the best electrochemical performance.(3)With the increase of F127 content,the CNTs@TiO₂composites show improved electrochemical performance.The CNTs@TiO₂composites with 0.06 g F127 have the best rate performance and cycle performance.The specific capacities at 1C,2C,5C,10C,20C,30C,40C,50C and 60C are 225,211,186,166,151,144,137,132 and 130m Ah/g,respectively.After 1000 cycles at 10C,92.2%of the reversible capacity is retained.When the addition amount of F127 is further increased to 0.08 g,the electrochemical performance of the composites is not significantly improved,indicating that the role of F127 in the system is limited.The enhanced electrochemical performance of the CNTs@TiO₂composites may be due to the encapsulation and assembly between the micelles formed by the dissolution of F127 in solvent and the solvothermal precursor TiGly.