Papers by Lokesh S S Vendra
Amorphous silicon oxycarbides are known to be an effective anode material for lithium-ion batteri... more Amorphous silicon oxycarbides are known to be an effective anode material for lithium-ion batteries. Despite their exceptional properties and high charge capacities, however, their practical uses are limited by their significant first-cycle loss, considerable hysteresis, and low cyclic ability. Comparatively, SiOC/metal oxide materials have demonstrated increased rate capability and cyclic stability. This study utilized a liquid precursor-derived ceramic method to modify SiOC with titanium (IV) butoxide precursor to synthesize SiOC/TiO x C y. X-ray diffractograms confirmed the amorphous nature of SiOC/TiO x C y. The elemental composition and bonding properties were investigated using X-ray photoelectron spectroscopy, and electron microscopy was used to explore morphological features. In the first cycle, the reversible capacity of pyrolyzed SiOC/TiO x C y was 520 mAh g −1 , which then increased to 736 mAh g −1 for the 1200 • C annealed SiOC/TiO x C y due to the increased free carbon network and TiC conductive phases. The irreversible capacity of the first cycle was 568 mAh g −1 , which was lower than the annealed SiOC irreversible capacity of 695 mAh g −1. Interestingly, the rate stability of the pyrolyzed SiOC/TiO x C y performed more stability than the annealed sample. Localized carbothermal reactions between amorphous SiOC/TiO x C y and free carbon at annealing temperatures resulted in loss of structure stability.
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This work represents a first attempt to synthesize Si(Nb)OC ceramic composites through the polyme... more This work represents a first attempt to synthesize Si(Nb)OC ceramic composites through the polymer pyrolysis or the precursor-derived ceramics (PDC) route for use as a hybrid anode material for lithiumion batteries (LIB). Electron microscopy, X-ray diffraction, and various spectroscopy techniques were used to examine the micro/nano structural features and phase evolution during cross-linking, pyrolysis, and annealing stages. During the polymer-to-ceramic transformation process, in situ formation of carbon (so-called "free carbon"), and crystallization of t-NbO 2 , NbC phases in the amorphous Si(Nb)OC ceramic matrix are identified. The first-cycle reversible capacities of 431 mA h g −1 and 256 mA h g −1 for the as-pyrolyzed and annealed Si(Nb)OC electrodes, respectively, exceeded the theoretical Li capacity of niobium pentaoxide or m-Nb 2 O 5 (at approximately 220 mA h g −1). With an average reversible capacity of 200 mA h g −1 and close to 100% cycling efficiency, as-pyrolyzed Si(Nb)OC demonstrates good rate capability. X-ray amorphous SiOC with uniformly distributed nanosized Nb 2 O 5 and graphitic carbon structure likely provides stability during repeated Li + cycling and the formation of a stable secondary electrolyte interphase (SEI) layer, leading to high efficiency.
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The precursor route enables in situ crystallization of metastable t-HfO 2 in amorphous SiOC. • SP... more The precursor route enables in situ crystallization of metastable t-HfO 2 in amorphous SiOC. • SPS-sintered Si(Hf)OC nanocomposites exemplify high permittivity and low loss values than SiOC at room temperature. • Cole-Cole relaxation due to the interfacial polarization between t-HfO2 and SiOC is the dominant relaxation mechanism. • Si(Hf)OC composites demonstrated thermally stable and frequency-independent dielectric properties.
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Hafnium dioxide (HfO 2) attracts attention because of its high dielectric constant, wide band gap... more Hafnium dioxide (HfO 2) attracts attention because of its high dielectric constant, wide band gap, and also since it can be quite effortlessly grown on the surface of a silicon substrate. In this work, dense ceramics of hafnium oxide in the monoclinic phase was synthesised from a liquid precursor of hafnium tri-methyl-tetra-oxide (Hf [O(CH 3) 3 ] 4) and sintered by conventional techniques. The electrical properties of the sample were studied by broadband dielectric spectroscopy over a wide range of frequencies (10 − 1-10 6 Hz) and temperature range (300-670 K). The temperature and frequency dependencies of the dielectric response exemplify high stability of the material in the investigated temperature range. The dielectric constant of conventionally sintered ceramics at room temperature is ~13. The frequency dispersion of ceramics was revealed and analysed over the entire investigated temperature range. It is shown that the main contribution to the response is made by lowfrequency charge polarisation. DC conductivity of HfO 2 has a thermally activated character with an activation energy of 1.2 eV in the temperature range below 600 K and 0.9 eV above 600 K. The conductivity is determined by oxygen vacancies.
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Papers by Lokesh S S Vendra