Self-supporting carbon-rich SiOC ceramic electrodes for lithium-ion batteries and aqueous supercapacitors

C60 Fullerene-Reinforced Silicon Oxycarbide Composite Fiber Mats: Performance as Li-Ion Battery Electrodes

Precursor-derived silicon oxycarbide (SiOC) has emerged as a potential high-capacity anode material for rechargeable Li-ion batteries. The polymer processing and pyrolysis route, a hallmark of polymer-derived ceramics, allows chemical interfacing with a variety of nanoprecursors and nanofiller phases to produce composites with low-dimensional structures such as fibers and coatings not readily attained in traditional sintered ceramics. Here, buckminsterfullerene or C60 was introduced as a filler phase in a hybrid precursor of 1,3,5,7-tetramethyl-1,3,5,7-tetravinyl-cyclotetrasiloxane (TTCS) along with polyvinylpyrrolidone or PVP as a spinning agent to fabricate electrospun fiber mats, which upon a high-heat treatment transformed to a C60-reinforced SiOC ceramic composite. Tested as the self-supporting working electrode in a Li-ion half-cell, C60-reinforced fiber mats show a much-improved reversible capacity (825 mA h g-1), nearly 100% Coulombic efficiency, and superior rate capability with low-capacity decay at high currents (only 25.5% decay at 800 mA g-1) compared to neat C60 and neat carbonized fiber electrodes.

Silicon Oxycarbide Coatings Consisting of Defined Bottom-Up-Grown Nanostructures

Silicon oxycarbide (SiOC) materials have arisen in the past few decades as a promising new class of glasses and glass-ceramics thanks to their advantageous chemical and thermal properties. Many applications, such as ion storage, sensing, filtering, or catalysis, require materials or coatings with high surface area and might benefit from the high thermal stability of SiOC. This work reports the first facile bottom-up approach to textured high surface area SiOC coatings obtained via direct pyrolysis of polysiloxane structures of well-defined shapes, such as nanofilaments or microrods. This work further investigates the thermal behavior of these structures by means of FT-IR, SEM, and EDX up to 1400 °C. The rods shrink in volume by ≈30% while their aspect ratio remains unaffected by pyrolysis until at least 1100 °C. The nano-sized filaments show signs of viscous flow already at a comparably low temperature of 900 °C which is very probably due to the nano-size effect. This might open a way to experimentally study the size-effect on the glass transition temperature of oxide glasses, an experimentally unexplored but very relevant topic. These structures have great potential, for example, as ion storage materials and supports in high temperature catalysis and CO2 conversion.

New insights into the electrochemical performance of precursor derived Si(Nb)OC composites as anode materials for batteries

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 lithium-ion 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-NbO2, 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-Nb2O5 (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 Nb2O5 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.

WS2 Nanosheet Loaded Silicon-Oxycarbide Electrode for Sodium and Potassium Batteries

Transition metal dichalcogenides (TMDs) such as the WS₂ have been widely studied as potential electrode materials for lithium-ion batteries (LIB) owing to TMDs' layered morphology and reversible conversion reaction with the alkali metals between 0 to 2 V (v/s Li/Li⁺) potentials. However, works involving TMD materials as electrodes for sodium- (NIBs) and potassium-ion batteries (KIBs) are relatively few, mainly due to poor electrode performance arising from significant volume changes and pulverization by the larger size alkali-metal ions. Here, we show that Na⁺ and K⁺ cyclability in WS₂ TMD is improved by introducing WS₂ nanosheets in a chemically and mechanically robust matrix comprising precursor-derived ceramic (PDC) silicon oxycarbide (SiOC) material. The WS₂/SiOC composite in fibermat morphology was achieved via electrospinning followed by thermolysis of a polymer solution consisting of a polysiloxane (precursor to SiOC) dispersed with exfoliated WS₂ nanosheets. The composite electrode was successfully tested in Na-ion and K-ion half-cells as a working electrode, which rendered the first cycle charge capacity of 474.88 mAh g^-1 and 218.91 mAh g^-1, respectively. The synergistic effect of the composite electrode leads to higher capacity and improved coulombic efficiency compared to the neat WS₂ and neat SiOC materials in these cells.

Enhanced Li-Ion Rate Capability and Stable Efficiency Enabled by MoSe2 Nanosheets in Polymer-Derived Silicon Oxycarbide Fiber Electrodes

Transition metal dichalcogenides (TMDs) such as MoSe2 have continued to generate interest in the engineering community because of their unique layered morphology-the strong in-plane chemical bonding between transition metal atoms sandwiched between two chalcogen atoms and the weak physical attraction between adjacent TMD layers provides them with not only chemical versatility but also a range of electronic, optical, and chemical properties that can be unlocked upon exfoliation into individual TMD layers. Such a layered morphology is particularly suitable for ion intercalation as well as for conversion chemistry with alkali metal ions for electrochemical energy storage applications. Nonetheless, host of issues including fast capacity decay arising due to volume changes and from TMD's degradation reaction with electrolyte at low discharge potentials have restricted use in commercial batteries. One approach to overcome barriers associated with TMDs' chemical stability functionalization of TMD surfaces by chemically robust precursor-derived ceramics or PDC materials, such as silicon oxycarbide (SiOC). SiOC-functionalized TMDs have shown to curb capacity degradation in TMD and improve long term cycling as Li-ion battery (LIBs) electrodes. Herein, we report synthesis of such a composite in which MoSe2 nanosheets are in SiOC matrix in a self-standing fiber mat configuration. This was achieved via electrospinning of TMD nanosheets suspended in pre-ceramic polymer followed by high temperature pyrolysis. Morphology and chemical composition of synthesized material was established by use of electron microscopy and spectroscopic technique. When tested as LIB electrode, the SiOC/MoSe2 fiber mats showed improved cycling stability over neat MoSe2 and neat SiOC electrodes. The freestanding composite electrode delivered a high charge capacity of 586 mAh g-1electrode with an initial coulombic efficiency of 58%. The composite electrode also showed good cycling stability over SiOC fiber mat electrode for over 100 cycles.