KEYWORDS: Silicon oxycarbides; Spark plasma sintering; Dielectric properties; Precursor derived ceramics; Nanocomposites; Amorphous; Annealing; Anode materials; Pyrolysis; Reversible capacity; Transition metal modified SiOC; Thermal stability; Energy storage
Ultra-light energy storage devices that bestow both high power density and high energy storage along with miniature size, minimal cost, and stable performance are currently the need of the hour for high-speed electronic devices. The dielectric permittivity should be enhanced to achieve high energy storage density in capacitors with decrease in dielectric loss (tanδ). Most contemporary dielectric materials fail to maintain high dielectric permittivity, low dielectric loss, and high breakdown strength simultaneously. Traditionally, combining glass ceramics, such as SiO2, with dielectric materials has been thought to be a possible way to minimize the loss of tanδ and maximize dielectric permittivity, especially for high-temperature applications. However, the high production cost and low breakdown strength still hinder commercial prospects. Interestingly, the free-carbon domains in polymer-derived silicon oxycarbides (SiOC) demonstrated enhanced dielectric permittivity through improved interfacial polarisation. Further, the ever-existent demands in Li-ion batteries for alternative anode materials to graphite to achieve increased capacity and more extended life high-rate capabilities can also be envisaged through SiOC-based polymer-derived ceramics wherein the free carbon network acts as storage centers for Li+ ions.
Silicon oxycarbides are well-known advanced ceramics because of their unique structure composed mainly of a Si–O–C glass phase with a free carbon region. Furthermore, modifying the processing parameters and the composition of the pre-ceramic polymer allows for a considerable variation in the mechanical and functional properties of these ceramics. However, very few authors reported on the dielectric properties of SiOC and metal-modified SiOCs for energy storage devices. Moreover, modifying SiOC with transition metal oxides has resulted in in situ nano-crystallized phases in an amorphous structure, significantly improving the mechanical and functional properties of these ceramics.
This work uses hafnium n tetra butoxide precursor to modify silicon oxycarbide (SiOC) with an easy, and novel method to synthesize Si(Hf)OC nanocomposites. FTIR analysis confirmed that Si-O-Hf linkages formed during cross-linking. As-pyrolyzed powders were sintered using spark plasma sintering technique to produce these nanocomposites. X-ray diffractograms confirmed the in situ crystallization and stabilization of tetragonal HfO2 in the amorphous SiOC matrix. The morphological features were imaged using electron microscopy. The dielectric properties of sintered Si(Hf)OC nanocomposites were investigated by broadband dielectric spectroscopy for the first time at varied temperatures, 25 °C - 250 °C and at a range of frequencies, 10-1 Hz – 106 Hz. At room temperature, the ε՚ value (1 kHz) was found to be 14, which is almost double the value of pure sintered SiOC (ε՚ ~5). The dielectric loss (tanδ) is less than 0.07 (1 kHz) at room temperature, which is significantly less than sintered SiOC (tanδ ~0.5). At frequencies above 10 Hz, this nanocomposite was found to be thermally stable in the measured temperature range. At low-frequency regions, the material exhibits a rise in dielectric permittivity with increasing temperature, most likely due to charge carrier accumulation at the interfaces of t-HfO2, SiO2, and free carbon in the SiOC matrix.
Additionally, the modification of SiOC with titanium tetra butoxide and niobium ethoxide for the first time to produce nanocomposites of Si(Ti)OC and Si(Nb)OC as a hybrid anode material for LIBs was explored. X-ray diffractograms confirmed the amorphous nature of these composites. The elemental composition and bonding properties were investigated using X-ray photoelectron spectroscopy, and electron microscopy was used to image morphological features. In the first cycle, the reversible capacity of pyrolyzed Si(Ti)OC was 520 mAh g-1, which then increased to 736 mAh g-1 for the 1200 °C annealed Si(Ti)OC due to the enhanced free carbon network and TiC conductive phases. Also, the annealed Si(Ti)OC first cycle irreversible capacity was 568 mAh g-1, which was lower than the annealed SiOC irreversible capacity of 695 mAh g-1.
Remarkably, the rate stability of the pyrolyzed Si(Ti)OC performed better than the annealed sample. Localized carbothermal reactions between amorphous Si(Ti)OC and free carbon at annealing temperatures resulted in the loss of structural stability. Furthermore, electrochemical performance of the as-pyrolyzed and annealed Si(Nb)OC nanocomposites was demonstrated by the first cycle charge capacity of 431 mAh g-1 and 256 mAh g-1, respectively, which exceeded the theoretical charge capacity of pure Nb2O5 (200 mAh g-1). The crystallization of NbC/Nb2-xO5-x in an amorphous SiOC matrix of annealed Si(Nb)OC nanocomposite is responsible for the poor rate stability. Nevertheless, amorphous pyrolyzed Si(Nb)OC nanocomposites retained the capacity of 200 mAh g-1 after cycling at various current densities.
This research explored the path to the potential development of Si(TM=Hf, Ti, Nb)OC materials for energy storage applications such as high-energy density dielectric capacitors and high-power negative electrodes for batteries.