Preparations And Characterizations Of Hierarchically Heterostructured Transition Metal Compounds/Carbon Nanofibers As Ultrathin Film Anodes For Lithium Batteries

Rechargeable lithium ion batteries (LIBs) and especially all-solid flexible LIBs have been the most ubiquitous power source for portable electronic devices, medical implants and system-on-a-chip, owing to their unique functionalities including high energy density, high power density, long lifespan, low weight, high area and shape versatility. However, the electrochemical properties of commercial graphite anodes still lie behind the stringent requirements for next generation high-performance LIBs. In this regard, it is still a formidable challenge to design and fabricate suitable anode materials with high energy and power density.In this paper, we employed sol-gel, electrospinning, hydrothermal (or solvothermal) treatment and chemical activation to prepare hierarchically heterostructured transition metal compounds/carbon nanofibers, which were directly used free-standing anode candidates for LIBs without any binders and conductive additives. The relationship among structures, compositions and electrochemical properties was systematically investigated to realize the tunable preparation and synergistic lithium storage effect of nanocomposite anodes. The aim of this work was to prepare a series of novel anode materials with controllable structure, stable cycling performance and high rate capability.First, nano-particulate TiO2/PCNFs were facilely fabricated by electrospinning Ti(OC4H9)4/PAN/PMMA nanofibers containing different amounts of PMMA, followed by subsequent hot-stretching/stabilization and carbonization treatments. The crystal phase and grain size of TiO2were little affected by the PMMA porogen, thus TiO2showed a typical anatase phase structure and an average grain size of about10nm. However, the PMMA-derived pore structure can improve cycling performance and especially rate capability of TiO2/PCNFs by promoting Li+diffusion and transfer. The TiO2/PCNFs (PAN:PMMA=3:1) had the best rate capability, exhibiting a reversible capacity of about445mAh g"1at25mA g-1as well as190mAh g-1at800mA g-1from0.001to3V.Second, the optimal0D nanostructured TiO2/PCNFs (PAN: PMMA=3:1) was selected as a raw material to further improve its lithium storage performance. A series of intriguing1D nanostructured TiO2/MPCNFs were fabricated by successive hydrothermal, ion-exchange and dehydration treatments of0D TiO2/PCNFs. The microstructure and distribution of1D TiO2were controlled by changing hydrothermal temperature, time and solution composition. Anatase TiO2nanotubes could be transformed to TiO2-B nanowires by increasing hydrothermal temperature from150℃to190℃and adding ethanol as a co-solvent, which should be attributed to further crystallization, oriented growth and accumulation of amorphous and layered Na2Ti3O7. Such the1D TiO2/MPCNFs showed a superior rate capability to the0D TiO2/PCNFs, presenting a reversible capacity of about140mAh g-1at2000mA g-1from0.001to3V.Third, various hierarchically micro-/nano-structured TiO2/MPCNFs were reproductively prepared using hydrothermal/solvothermal method. The geometrical structures of TiO2were tailored by adopting different ratios of water to ethylene glycol and alkalis. Nano-particulate, micro-particulate and flower-like TiO2/MPCNFs were respectively prepared by using ethylene glycol, water and ethylene glycol/water (3:7, volume) as solvents and urea as a solute. Moreover, nanowire-like, nanosheet-like and flower-like TiO2/MPCNFs were also respectively prepared by using NaOH, LiOH and urea as solutes and ethylene glycol/water (3:7, volume) as a solvent. The morphological diversification of TiO2/MPCNFs can be elucidated by interdiffusion-dissolution-nucleation-assembly growth mechanism. As a virtue of its appropriate crystallization for Li+insertion and unique spatial configuration for Li+diffusion, the flower-like TiO2/MPCNFs anode demonstrated remarkable cycling reversibility and rate capability, delivering a reversible capacity of about105mA h g-1at2000mA g-1from1to3V.At last, a novel Ti3O5/TiP2O7/MPCNFs hierarchical nanocomposite was meticulously designed and facilely prepared by phosphoric acid pre-impregnation activation treatment of Ti(OC4H9)4/PAN hybrid nanofibers. As a consequence of the favourable synergistic effect between Ti3O5/TiP2O7nanoparticles and MPCNFs, the topographically "nanoparticles-micropores-nanofibers" triplex-roughened exhibited superior rate capability.