Synthesis And Electrochemical Properties Of Ti-based Compound Nanosturctures

Rechargeable lithium ion batteries with multiple properties,such as high power density,high energy density and high safety,are excellent power sources for portable electronic devices,electric vehicles and stationary energy storage systems.Ti-based compound nanostructures exhibit excellent cycling reversibility owing to their unique zero-strain insertion crystal structure.Their high working potential ensures the high security for lithium ion batteries application.However,their intrinsic electrical conductivity(ca.10-13 S cm-1)and lithium ion diffusion coefficient(ca.10-9-10-13 cm2 s-1)are relatively low,which severely prohibit their application for achieving high rate performance.In our work,the hydrogenated mesoporous TiO2 microspheres,hierarchical hollow microspheres composed ofN-doped carbon coated Li4Ti5O12 nanosheets and flower-like hydrogenated TiO2(B)nanostructures are successful synthesized to improve their slow lithium ion diffusion coefficient and electronic conductivity.Electrochemical performances demonstrate that the above attractive Ti-based compound nanostructures exhibit enhanced specific capacity,excellent rate performance and superior cycling stability.The main results are as follows:(1)Hydrogenated mesoporous TIO2 microspheres(H-TiO2):Hydrogenated mesoporous TIO2 microspheres have been synthesized by a combined sol-gel and hydrothermal approach followed by a facile hydrogenation route.The as-synthesized hydrogenated mesoporous TiO2 microspheres show clear disordered mesopores throughout the whole microsphere with the polycrystalline nature of anatase TiO2 and a specific area of 95.9 m2 g-1,The primary crystallite sizes of the H-TiO2 microspheres are estimated to be 8.2 ± 0.5 nm.The introduction of oxygen vacancies by hydrogenation process reduces the band gaps of the TiO2 microspheres from 3.21 to 2.42 eV,which substantially improve the charge carrier density and the electronic conductivity of TiO2.The hydrogenated mesoporous TiO2 microspheres show enhanced specific capacity(291 mAh g-1 at 0.5 C),excellent rate performance(141 mAh g-1 at 20 C)and superior cycling stability(97.4%capacity retention after 100 cycles at 20 C).The results indicate that the hydrogenation of TiO2 can improve both the electronic conductivity and the lithium ion diffusion kinetics,which is critical for the excellent rate capability of the hydrogenated mesoporous TiO2 microspheres.(2)Hierarchical hollow microspheres composed of N-doped carbon coated Li4Ti5O12 nanosheets(NC-LTO):Hierarchical hollow microspheres composed of NOLTO have been synthesized on a large scale by a facile low-temperature solution-based approach combined with high temperature calcination.N-=containing carbon derived from the self-carbonization of hexadecylamine is low content(7.24%)and amorphous.The primary Li4Ti5012 nanosheets show zigzag morphology with a typical thickness of about 5.0 nm,which are uniformly coated with N-doped carbon layers.There are different chemical states of N,such as Ti-N,C-N and Ti-O-N species.The distribution of C and N elements is consistent with that of Ti and O,which indicates that C and N are distributed uniformly on/in the NC-LTO hollow microspheres.When evaluated for lithium storage capacity,the NC-LTO hollow microspheres display enhanced electrochemical energy storage performances compared to the pristine hollow microspheres composed of Li4Ti5O12 nanosheets(A.LTO),including high capacity(181.4 mAh g-1 at 0.5 C),excellent rate capability(140.8 mAh g-1 at 20 C),and good cyclic stability(92.8%capacity loss after 100 cycles at 20 C).The introduction of N element into the carbon coatings can lead to the production of C-vacancies,which is beneficial for lithium ion diffusion.N-doping can stabilize the coating layers and reduce the chemical activity of surface Ti,resulting in better reversibility of the electrodes.(3)Flower-like hydrogenated TiO2(B)nanostructures[H-TiO2(B)]:Flower-like hydrogenated TiO2(B)nanostructures have been synthesized via a facile solvothermal approach combined with hydrogenation treatment.The obtained H-TiO2(B)nanostructures show uniform and hierarchical flower-like morphology with a diameter of 124 ± 5 nm,which are further constructed by primary nanosheets with a thickness of 10±1.2 nm.The Ti3+ species and/or oxygen vacancies are well introduced into the structures of TiO2(B)after hydrogen reduction,resulting in an enhancement in the OTs3?content(14.7%)and the OH content(4.4%).The increased charge carrier density and the OH groups are attributed to the enhanced electronic conductivity(up to 2.79 ×10-3 S cm-1)and the modified surface electrochemical activity,respectively.When evaluated for lithium storage capacity,the H-TiO2(B)nanostructures exhibit enhanced electrochemical energy storage performances compared to the pristine TiO2(B)nanostructures[A-TiO2(B)],including high capacity(292.3 mA h g-1 at 0.5 C),excellent rate capability(179.6 mA h g-1 at 10C),and good cyclic stability(98.4%capacity retention after 200 cycles at10 C).The reasons for these improvements are explored in terms of the increased electronic conductivity and the facilitation of lithium ion transport arising from the introduction of oxygen vacancies and the unique flower-like morphologies.