Haitao Fang

Electronic structure of TiO2 nanotube arrays from X-ray absorption near edge structure studies

We report an X-ray absorption near edge structure (XANES) investigation of several TiO2nanotube arrays, including the as-prepared nanotube arrays from electrochemical anodic oxidation of Ti foil (as-prepared ATNTA), as-prepared nanotube arrays after annealing at 580 °C (annealed ATNTA) and annealed ATNTA after electrochemical intercalation with Li (Li-intercalated ATNTA). XANES at the O K-edge and Ti L3,2 and K edges shows distinctly different spectral features for the as-prepared and the annealed ATNTA, characteristic of amorphous and anatase structures, respectively. Intercalation of Li into annealed ATNTA induces a surprising, yet spectroscopically unmistakable, anatase to rutile transition. XANES at the Li K-edge clearly shows ionic features of Li in ATNTA. The charge relocation from Ti 3d to O 2p at the conduction band in TiO2 was also observed when Li ions were intercalated into annealed ATNTA albeit no noticeable reduction of Ti4+ to Ti 3+ was observed. The O K-edge shows a distinctly enhanced feature in the multiple scattering regime, indicating a close to linear O–Li–O arrangement in Li-intercalated ATNTA. These results show bonding changes between Ti and O resulting from the interaction of Li ions in the TiO2 lattices. Such bonding variation has also been supported by X-ray excited optical luminescence (XEOL), which suggests Li+-defect interactions. The implications of these results are discussed.

Structural variation and water adsorption of a SnO2 coated carbon nanotube: a nanoscale chemical imaging study

The local chemistry and electronic structure variations of an individual carbon nanotube coated with tin oxide nanoparticles have been studied by nanoscale chemical imaging using Scanning Transmission X-ray Microscopy (STXM). Specifically, STXM images at the O and C K-edges with a spatial resolution of 30 nm have been obtained and in turn used to extract X-ray absorption near edge structures (XANES) at both edges from regions of different morphology, i.e. core/shell and side by side. A comparison of the spectra at these regions reveals their structural and bonding variation. In situSTXM with humidity control was then used to image water adsorption on the nanocomposite and its effect on the electronic structure of the nanocomposite, which is crucial for its applications.

Mechanism for improving the cycle performance of LiNi0.5Mn1.5O4 by RuO2 surface modification and increasing discharge cut-off potentials

The cycle performance of LiNi0.5Mn1.5O4 (LNMO) is improved greatly by a surface modification with discrete RuO2 particles in combination with setting discharge cut-off potentials to 4.5 V. A specific capacity value as high as 113.8 mA h g−1 after 1000 cycles is attained. In order to not only clarify the mechanism for the improvement by the RuO2 particles deposited, but also to elucidate which chemical properties of surface-modifying materials play a critical role in the evolution of the solid electrolyte interphase (SEI) layer with cycling, the electrochemical impedance spectra and X-ray photoelectron spectra (XPS) of a bare LNMO electrode, discrete RuO2 particle modified-LNMO electrode and discrete Al2O3 particle modified-LNMO electrode were compared. The XPS spectra of the SEI layers of these electrodes cycled between 4.5 and 5.2 V indicate that a stable SEI layer on the RuO2-modified LNMO electrode has the characteristics of higher content of LiF and longer poly-ethylenecarbonate chains with extremely low content of –CF2O– groups. The crucial point for the formation of the stable SEI layer is the sustainable consumption of the F radicals generated by the electrochemical decomposition of LiPF6 at high potentials. We conclude that the hydrolysable property of ruthenium fluorides, intermediate products obtained during the formation of the SEI layer on the RuO2-modified LNMO electrode, and the high catalytic activity of RuO2 itself for electrochemical oxygen evolution guarantee the sustainable consumption of the F radicals. Al2O3 lacks these special chemical properties to attain the sustainable consumption, thus leading to no improvement by the modification with discrete Al2O3 particles.

Chelate-induced formation of Li2MnSiO4 nanorods as a high capacity cathode material for Li-ion batteries

Li2MnSiO4 with a theoretical capacity of 333 mA h g−1 is considered as a potential high capacity cathode material for lithium-ion batteries. However, it suffers from impure phases, low electronic conductivity, and poor cycle performance, which hinder its application in electric vehicles (EVs) and hybrid electric vehicles (HEVs). In this work, a chelating agent-assisted hydrothermal method was proposed to synthesize pure phase Li2MnSiO4 nanorods. Taking advantage of the strong chelating effect of ethylenediamine tetraacetic acid tetrasodium salt (EDTA-4Na), the reaction kinetics was substantially improved by changing the Mn source from Mn(OH)2 precipitate to soluble Mn-containing chelates, which simultaneously controlled the purity and nanoscale architecture of Li2MnSiO4. After coating with amorphous carbon, Li2MnSiO4@C with a 9% carbon coating exhibited a discharge capacity of 275 mA h g−1 in the initial cycle with a current density of 8 mA g−1 (0.05C, 1C = 166 mA g−1), and a better cycle property with a capacity retention of 115 mA h g−1 after 50 cycles was obtained with a higher carbon coating (19%). The good electrochemical properties may be attributed to synergetic effects of the high phase purity and well-dispersed one-dimensional morphology of Li2MnSiO4.

Synthesis of self-stacked CuFe2O4–Fe2O3 porous nanosheets as a high performance Li-ion battery anode

Self-stacked CuFe2O4–Fe2O3 porous nanosheets were prepared via a facile polyol-mediated route followed by calcination. Because of its highly porous structures and good electrical and ion conductivity of the well-dispersed CuFe2O4 phase in the matrix, the hybrid material exhibits high specific capacity of 910 mA h g−1 at 0.5 C after 200 cycles, superior capacity retention (0.02% capacity loss per cycle) and good rate capability (417 mA h g−1 at 4 C) as a promising anode material for Li-ion batteries.