Inorganic Nanomaterials For Li-S Battery And Investigation By Core-Level X-ray Spectroscopies

Due to the unique structure and excellent physical and chemical properties, inorganic nanomaterials have potential applications in energy conversion and storage. In recent decades, various inorganic nanomaterials have been widely used in Li-ion batteries, supercapacitors and fuel cells applications. As Li-S battery could break through thecapacity limitation of traditional Li-ion battery cathode materials, Li-S battery has been extensively investigated in recent years. The development of Li-S battery performance by inorganic nanomaterials, including metal oxides, carbon nanomaterials and their composites, is a promising strategy for Li-S battery application.In this thesis, various inorganic nanomaterials, including titanium oxide, titanium oxide-carbon nanocomposite and carbon nanomaterialswere used to improve the performance of Li-S batteries. Furthermoe, the electronic structures of these inorganic nanomaterials were investigated by synchrotron based X-ray spectroscopiesand the relationship between electronic structure and performance of these nanomaterials has been established. Detailedare briefed as follows.(1) We synthesized various TiOx nanomaterials, including TiO2 and sub-stoichiometrictitanium oxide by hydrogen reduction method at high temperature. The morphology of TiOxwill change from nanoparticle to nanosheet when temperature is higher than 900 oC. With the temperature up to 900 oC, the anatase structure will transform into magneli phase Tin O2n-1. X-ray spectroscopies, including X-ray absorption spectroscopy(XAS), X-ray emission spectroscopy(XES), clearly illustrated the evolution of electronic structure at different annealing temperature. The sodium impurity in TiOx nanomaterials was also clearly identified. The sodium doped tianium oxide(Na0.23TiO2) was obtained at 800 oC and the Na0.23TiO2-S nanocomposite could help develop the long cycling performance of Li-S battery.(2) TiOx@C nanocomposites were synthesized by annealing of titanium glycolateat high temperature. The morphology and structure of nanocomposite could be tuned by annealing temperature, atmosphere, reaction time, and precursor. The growth mechanism of TiOx@C nanocomposite was well studied: carbon reduction and hydrogen reduction. On the basis, we further synthesized the Ti2O3-TiC@C nanocomposite(TCNW). We also developedan on-site chemical adsorption strategy of metal oxide toward superior stability of sulfur electrode by using TCNW for sulfur loading. The S@TCNW composite exhibited a superior cycling stability and high Coulombic efficiency during the charge/discharge process.(3) We synthesized S@N-doped graphene nanocomposite(S@NG) for cathode of Li-S battery by a solution method. S@NG could dramatically enhance the cycling performance of Li-S battery. The S@NG cathode could work over 1000 cycles. For comprehensive understanding of the degradation and chemical absorption mechanism of sulfur electrodes in Li/S batteries, the electronic and chemical structure evolution of the S@NG cathode were investigated by X-ray absorption spectroscopy(XAS). The N doped graphene have strong adsorption for polysulfide. The experimental resultswill help us to recognize the major issuesin theperformance development of Li/S@NG battery and then obtain thebetter performance of the battery.(4) The N/P doping effect on the electronic structure of carbon nanocages(CNCs) have been studied by various core-level X-ray spectroscopies, including XAS, X-ray photoelectron spectroscopy(XPS), X-ray emission spectroscopy(XES) and resonant XES. The doping configurations for N/P dopants are identified from the spectra. There are three major doping configurations for nitrogen doping but only one doping configuration for phosphorus doping. The nitrogen doping reveals the complex coexistence of graphite-like, pyridine-like and pyrrole-like configurations that are proved by density functional theory(DFT) simulations, while the phosphorus doping presents only the “graphite-like” configuration. The different configuration profiles result in less atomic structure ordering of N-CNCs than that of P-CNCs. XAS spectra obtained from both surface and bulk sensitivedetection suggest different chemical environments between the interior and shell for all types of nanocages.(5) We have stabilized the iron oxide nanoparticles(NPs) of various sizes on layered carbon materials(Fe-oxide/C) by self-assemble and annealing at high temperature. From the characterization of XAS, XES, scanning transmission X-ray microscopy(STXM) and X-ray magnetic circular dichroismspectroscopy(XMCD), a strong interfacial interaction in theFe-oxide/C hybrids has been observed between the small ironoxide NPs(~2 nm) and layered carbon in contrast to the weakinteraction in the large iron oxide NPs(~5 nm). In addition, the Fe L-edge XMCD spectra show that the large iron oxide NPs aremainly γ-Fe2O3 with a strong ferromagnetic property, whereas the small iron oxide NPs with strong interfacial interaction aremainly α-Fe2O3 or amorphous Fe2O3 with a nonmagnetic property. The results strongly suggest that the strong interfacial interactionplays a key role for the catalytic performance, and the experimental findings may provide guidance toward rational design of highperformancecatalysts.