Free Carbon Content Research Articles

Effects of carbon on the synthesis and densification of tantalum carbide powder

Tantalum carbide (TaC) powder was synthesised at 1500 ºC by sol-gel and carbothermal reduction processes using tantalum pentachloride (TaCl5) and phenolic resin as the starting materials. The effects of the C/Ta ratios in the Ta-containing precursor on the reaction yield, microstructure, chemical composition, and sinterability of the powders were investigated. The results showed that a high C/Ta ratio was favourable for the formation of TaC powder. With an increase in the C/Ta ratio, the oxygen content of the powder decreased, whereas the free carbon content increased. Consolidated TaC ceramics with high relative density (> 97 %) were obtained at 1900 ºC for 5 min under 80 MPa after sintering the powder synthesised at C/Ta ratios of 4.00 and above. However, the hardness and fracture toughness of the TaC ceramics were slightly reduced when the C/Ta ratio exceeded 4.00, owing to weak interface bonding caused by excessive free carbon in the powder. It was found that sintering TaC powders prepared at a C/Ta ratio of 4.00 produced dense TaC ceramics, with a Vickers hardness and fracture toughness of 16.54 GPa and 3.72 GPa∙m1/2, respectively.

Modulation of Free Carbon Structures in Polysiloxane-Derived Ceramics for Anode Materials in Lithium-Ion Batteries.

Polymer-derived silicon oxycarbide (SiOC) ceramics have garnered significant attention as novel silicon-based anode materials. However, the low conductivity of SiOC ceramics is a limiting factor, reducing both their rate capability and cycling stability. Therefore, controlling the free carbon content and its degree of graphitization within SiOC is crucial for determining battery performance. In this study, we regulated the free carbon content using divinylbenzene (DVB) and controlled the graphitization of free carbon with the transition metal iron (Fe). Through a simple pyrolysis process, we synthesized SiOC ceramic materials (CF) and investigated the impact of Fe-induced changes in the carbon phase and the amorphous SiOC phase on the comprehensive electrochemical performance. The results demonstrated that increasing the DVB content in the SiOC precursor enhanced the free carbon content, while the addition of Fe promoted the graphitization of free carbon and induced the formation of carbon nanotubes (CNTs). The electrochemical performance results showed that the CF electrode material exhibited a high reversible capacity of approximately 1154.05 mAh g-1 at a low current density of 100 mA g-1 and maintained good rate capability and cycling stability after 1000 cycles at a high current density of 2000 mA g-1.

Evolution of high temperature oxidation properties of SiCf/SiCN composites with BN interphase

In this study, SiC/SiCN composites with BN interphase were prepared using chemical vapor infiltration (CVI) and polymer infiltration and pyrolysis (PIP) methods. The evolution and related mechanisms of mechanical and microwave absorption properties of the composites after oxidation at 800 °C, 1000 °C, 1200 °C, and 1400 °C were investigated. The results showed that with the elevating of oxidation temperature, the flexural strength of SiC/SiCN composites gradually decreased, with a retention rate of 56.7 % after oxidation at 1400 °C. The main reasons for the deterioration of mechanical properties were the changes in the crystal structure of SiC fibers under high-temperature conditions and the formation of brittle SiO2 phases on the surface and interior of the SiCN matrix. With the increase in oxidation temperature, the microwave absorption performance of the composites showed an enhancing trend. The main reasons were that after oxidation treatment, the reduction of free carbon content and the formation of SiO2 film layers from the oxidation of SiC and SiCN led to a decrease in the complex permittivity of the composites, resulting in better impedance matching and dielectric loss capabilities. The contributions of BN interphase to stress dispersion, interface polarization, and oxidation protection significantly enhance the mechanical and electromagnetic absorption properties of the composites. Therefore, SiC/SiCN composites are high-temperature structural microwave absorbing materials with great application prospects.

Structural, spectroscopic and improved photoelectrochemical properties of TiO2/C composites with anatase/brookite matrix, prepared by air-thermolysis of microspherical titanium glycolate

X-ray powder diffraction, Raman spectroscopy, HRTEM, electron diffraction and differential thermal analysis methods were used to study structural and spectroscopic properties and thermal stability of the products obtained by air-thermolysis of Ti-glycolate in the temperature range 300–450°С. TiO2/C composites were obtained, which have anatase/brookite matrix with a variable degree of crystallinity and contain different contents of brookite and free polymeric-like carbon with COO− groups. It was found that the residual content of carbonaceous component and the intensity of exothermal decomposition of Ti-glycolate have an effect on the formation of brookite-rich TiO2 matrix. Correlations between the phase composition and the free carbon content suggest that during crystallization brookite/carbon nanoparticles are more stable than anatase/carbon nanoparticles. PEC tests demonstrated that the composite photoanode, which has the greatest degree of crystallinity, the optimal carbon content and is most enriched with brookite, exhibits the greatest incident photon-to-current conversion efficiency (IPCE), and its IPCE peak value is twice as great as the IPCE peak value for Degussa P25. The increased IPCE value is associated with the preferable formation of carbon/brookite/anatase heterostructures. The carbon component has an optical gap of approx. 1.1 eV, and at the optimal contents it ensures efficient photogeneration and transfer of electrons into the conduction band of brookite and then into the conduction band of anatase. HRTEM study demonstrated the formation of such heterostructure with anatase/brookite heterojunction.

Preparation of ultrafine WC-Co powder via fluidized bed

In this paper, the effects of reaction parameters on the deep-reduction and carbonization process of WO2-Co to WC-Co were studied. The results indicate that oxygen loss rate of WO2 is positively correlated with temperature and methane partial pressure. The partial pressure of methane has no significant effect on the formation rate of WC. The carbon content and particle size of the product increase with the increase of CH4 partial pressure. By synergistically regulating the reaction temperature to 950 °C, methane partial pressure to 1.25%, and reaction time to 60 min, ultrafine WC-Co powder without η phase can be obtained. The particle size of the composite powder is 128 nm, with total carbon content of 6.16%, free carbon content of 0.4%, and residual oxygen content of 0.05%, respectively. The growth rate relationship of tungsten carbide is as follows:

Tailoring crystal structure of high-entropy carbides in Si-based ceramic nanocomposites through precursor engineering

High-entropy carbides with tunable crystallization and growth have been demonstrated using single-source precursor derived ceramic route. The in situ nanocrystallization of high-entropy carbide phases, (Ta0.2W0.2V0.2Mo0.2Nb0.2)SiδC and (Ta0.167W0.167V0.167Mo0.167Nb0.167Si0.167)C in amorphous Si-based ceramic matrices was achieved by using polysiloxanes and polycarbosilanes as polymer precursors respectively. The results exemplify a prominent role of the architectures of the polymeric precursors in controlling the structural features of these ceramics at various length scales. In particular, it was observed that high-entropy carbides with rock salt and zinc blende crystal structures were formed when polysiloxanes and polycarbosilanes with different backbone structure were used as polymeric precursors respectively. This is attributed to the thermodynamics of nucleation of the carbidic phases in these nanocomposites. Furthermore, the precursor architecture that dictates free carbon content, influenced nanostructural features and porosity in the material. Therefore, engineering such compositionally complex phases is feasible by selecting suitable polymeric precursors.

Microstructure-electrical conductivity relationship of Si(Ni)BCN ceramics

Wireless passive sensors based on surface acoustic wave (SAW) provide a good solution for the measurement demand under high temperature and harsh conditions. SiBCN ceramics have excellent mechanical properties at high temperature and semiconductor behavior, making them potential for electrode of SAW sensors. To achieve the target conductivity, a series of SiBCN and SiNiBCN coatings are prepared by polymer derived ceramics (PDC) method and the influence of microstructure on the conductivity of the coatings is discussed. The results show that the conductivity of the SiBCN coatings is affected by both the content of free carbon and the degree of graphitization. The introduction of Ni leads to the formation of Ni2Si conductive microcrystals at lower temperatures and catalyzes the graphitization of the carbon phase, so that the SiNiBCN coatings show lower electrical resistivity. The amount of the metal conductive phase and the graphitization degree of the carbon phase increase with the pyrolysis temperature, resulting in decrease of the SiNiBCN coatings resistivity.

Capacity degradation of lithium-ion cell: The role of free carbon black content in the slurry and drying induced cracks in LiFePO4 electrode

Conductive additives, such as carbon black (CB), are incorporated into LiFePO4 (LFP) electrodes to address their low electronic conductivity. Within the electrode, a portion of CB remains as free carbon while the remaining amount attaches itself to the LFP particles. We have attempted to quantify the free carbon content in an LFP/CB/binder electrode composition via rheology studies. A low carbon content leads to an increased CB-binder domain formation thereby reducing electronic conduction. An increase in the carbon content is found to induce cracks and form islands within the electrode at the time of solvent evaporation and first cycling. Here we demonstrate the impact of free carbon content on the changes in the percentage of electronically well-connected and ill-connected active materials and its effect on the capacity in the nominal voltage zone during cycling studies. The electrode with 2.06 vol% CB exhibited a higher nominal zone capacity compared to electrodes with higher and lower CB content throughout 200 cycles of charge and discharge at 1C. Finally, the influence of premixing time, which affects the percentage of free CB, on the cyclic stability for a specific volume percentage of CB is investigated.

(Digital Presentation) Electrochemical Conversion of CO2 into Tungsten Carbides in Molten Salts

Huge amounts of anthropogenic emissions of the greenhouse gas carbon dioxide into the Earth's atmosphere are one of the key factors causing global warming. To mitigate the consequences of the severe climate changes caused by this phenomenon, over the last two decades great efforts of researchers have been directed towards the development of sustainable, environmentally friendly, carbon neutral and, if possible, not very expensive (in terms of used energy and inexpensive consumables) technologies for capture, conversion and storage (CCS) of CO2.Electrochemical conversion of CO2 using molten salts can rightfully be classified as CCS technology. In this case, carbon dioxide from various sources of its generation (fossil fuel power plants, industrial enterprises with a high carbon footprint) can be captured by molten salt (as a result of its physical dissolution, or chemical absorption by molten salt), and then electrochemically be converted into high value-added carbon-containing compounds: (a) carbon monoxide [1]; (b) carbon allotropes of various structures and modifications [2]; (c) refractory metal carbides [3], and various composites based on them. The reaction path and composition of the cathode products will depend on the electrolysis conditions. Elemental carbon synthesis precursor can be – carbon dioxide, directly dissolved in the molten salt mixture (direct reduction of CO2), as well as the carbonate anion, formed as a result of carbon dioxide interaction with oxide ions which are presented in the electrolyte bath (indirect reduction of CO2).This work presents the result of research concerning the electrochemical synthesis of the powders of tungsten carbides (WC and W2C) in chloride melt NaCl-KCl (1:1) under carbon dioxide pressure at the temperature range 700 – 800 оС. Refractory metal precursors are its oxy-anions (WО3; W2O7 2-; Меn x[WO4]nx-2; WO3F3 3- where Me – Na; K; Li; Mg; Ca; n – valance of metal Me). The formation of the new forms of tungsten electrochemical active particles in electrolyte is realized by the changing (control) of acidity of the melt. Carbon source is CO2, which was introduced into the electrolyzer under the excessive pressure of 0.1 – 1.7 MPa. The creation of excessive gas pressure is necessary condition for the increasing of the rate of electrolytic process (current densities) throw the rise of CO2 solubility in chloride melt. The general scheme of the high-temperature synthesis of tungsten carbides by the method of Molten Salt Carbon Electrochemical Transformation (MS-CCT) is presented in Fig. 1.The electrochemical investigations of partial and joint electroreduction of tungsten carbide precursors were carried out by the method of cyclic voltammetry. The areas of potentials and current densities, where the joint electrochemical discharge of tungsten carbide precursors (a narrow range of deposition potentials) occurs up to refractory metal and carbon takes place were found.Electrolytical synthesis of nano-sized (10 – 30 nm) powders of tungsten carbides (WC, W2C) and composites WC-C (up to 5 wt % of free carbon); W2C-WC; WC-C-Pt was carried out from the melts of different chemical composition; and the characterization of obtained products was fulfilled by the methods of chemical analysis, X-ray diffraction, DTG, BET adsorption, scanning and transmission electron microscopy.Synthesized composite materials based on tungsten carbides of various compositions were investigated as a cathode material in the reaction of electrolytic splitting of water for hydrogen production in a sulfuric acid solution [4]. The obtained results showed that the best activity has a composite of tungsten monocarbide WC with a content of free carbon up to 5 wt.%. The hydrogen onset potential for this electrode is -0.02 V, the overvoltage of hydrogen release at ik = 10 mA/cm2 is -110 mV, the exchange current is 7.0×10-4 A/cm2, the Tafel slope – -85 mV/dec.The presence of free carbon on the surface of tungsten carbides electrode improves its catalytic activity, increasing the area of the active surface. The catalytic activity of electrodes made of tungsten monocarbide increases with the introduction of platinum (up to 10 wt %) into the composite.References Kaplan V, Wachtel E, Gartsman K et al (2010) Conversion of CO2 to CO by electrolysis of molten lithium carbonate. J Electrochem Soc 157:B552–B556.Novoselova I.A., Kushkhov Kh.B., Malyshev V.V., Shapoval V.I. (2001) Theoretical foundations and implementation of high-temperature electrochemical synthesis of tungsten carbides in ionic melts. Theor. Found. Chem. Eng. 35:175–187.Novoselova I.A., Kuleshov S.V., Volkov S.V. et al (2016) Electrochemical synthesis, morphological and structural characteristics of carbon nanomaterials produced in molten salts. Electrochim Acta 211:343–355.Novoselova I, Kuleshov S, Fedoryshena E et al (2018) Electrochemical synthesis of tungsten carbide in molten salts, its properties and applications. ECS Trans 86:81–94. Figure 1

ELECTROLYTIC CATALYSTS BASED ON TUNGSTEN AND CARBON COMPOUNDS FOR THE HYDROGEN EVOLUTION REACTION

The hydrogen evolution reaction (HER) is one of the most promising methods of obtaining high-purity hydrogen. However, the high cost and limited resources of materials with low cathodic hydrogen evolution overvoltage values, such as platinum group metals, are the main obstacles to the use HER for obtaining hydrogen on an industrial scale. Therefore, it is necessary to develop new alternative materials and methods of their production. One of the promising materials are catalysts based on refractory metals, in particular tungsten carbides. Metal tungsten can also be used for these purposes. In our opinion, high-temperature electrochemical synthesis (HTES) in molten salts can be a promising method of obtaining materials with properties that meet the requirements for effective catalysts, namely: ultra-dispersity, high specific surface area, mesoporosity and defective structure, high chemical and electrochemical stability. Therefore, the purpose of this work is to evaluate the electrocatalytic activity of a group of materials for HER, which are obtained by HTES in melts. Four samples of electrolytic materials were chosen for the study: tungsten, carbon, tungsten mono- and semi-carbides (WC and W2С). All samples were characterized in detail using X-ray diffraction (phase composition), SEM (morphology), Raman spectroscopy (structure of carbon phases), DTG (free carbon content).
 Based on the analysis of the obtained data, it was established that all samples can be used as catalysts: crystallites have a nanometer size and a large number of structural defects; morpho­logy provides increased surface area; tungsten carbide particles are covered with a layer of free carbon, which prevents oxidation of carbide to WO3, which has a lower catalytic acti­vity; carbon particles are nanosized (20–30 nm) and contain a large number of structural defects; tungsten carbide-based samples contain free carbon, which increases the specific surface area, but does not cause clogging of pores.
 Polarization measurements were carried out at room temperature at a polarization rate of 5 mV/s in a standard three-electrode cell with an Ag|AgCl reference electrode. 1N H2SO4 was used as a base solution, which was bubbled with high-purity argon. Onset potentials for all samples are -0.05 – -0.25 V (in order WC/C – W2C/WC/C – C – W). The overvoltage and Tafel slope were calculated and WC/C composite was shown to have the lowest values of -0.2 V and -75 mV, respectively.
 Electrolytic composite of tungsten carbide/carbon have demonstrated the best characteristics, so we plan to continue the development of synthesis method of carbide compounds, which will allow us to reveal even greater potential of carbide catalysts and pave the way for their wide application in catalytic processes.

Effect of free carbon content changed by divinylbenzene on the conductivity of SiCN ceramic

Effect of free carbon content changed by divinylbenzene on the conductivity of SiCN ceramic

Electrochemical performance of SiCN embedded carbon (SiCN–C) fiber mat electrodes for lithium-ion battery: Electrospinning polysilazane in air or protective atmosphere

Here polysilazane/polyacrylonitrile fiber mats were manufactured by electrospinning in air or protective atmosphere. After pyrolysis, SiCN embedded carbon (SiCN–C) was tested as lithium-ion battery (LIB) electrode. Fibers electrospun in nitrogen are found to be thinner due to the slower solidification of the flying jet. XPS analysis confirmed that fibers electrospun in air incorporated a great oxygen content during shaping. The highest charge capacity of 773 mA h g−1 at 50 mA g−1 was recovered for SiCN–C electrospun in air. Oxygen increased the capacity due to its high character to attract Li+ ions, but SiCN–C electrospun in air suffered voltage hysteresis. Contrariwise, the SiCN–C electrospun in nitrogen demonstrated stable cycling with a charge capacity of 299 mA h g−1 and 98% recovery of the initial capacity due to enhanced free carbon content. These findings make electrospinning very promising for the highly controlled production of freestanding SiCN fiber mat electrodes.

Study on the preparation mechanism of novel WC-Cr2(C, N)-Co nanocomposites

In this work, the WC-Cr2(C, N)-Co nanocomposites were synthesized by using the one-pot carbothermal reduction nitridation hydrothermal precursor method. The preparation mechanisms, including phase and morphology evolutions, were investigated by differential thermal analysis (TG-DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and thermodynamic analysis. The results showed that phase evolution, with temperature increasing, follows the sequences of (amorphous phase, WO2.72) → (CoWO4, WO2.72, WO2, CrWO3) → (WO2, CoWO4, Co6W6C, CrWO3) → (W2C, Co3W3C, WC) → (WC, W2C, Co3W3C) → (WC, Co, Co3W3C trace amount). Through comparing different heat treatment conditions, the powder particle size was more easily affected by temperature than holding time. The higher temperatures or longer time holding resulted in the aggregation of powder particles. Finally, WC-Cr2(C, N)-Co nanopowders with about 60 nm were obtained at 1050 °C for 1 h under the N2 flow of 1 L/min with the oxygen content of 0.22 wt% and the free carbon content of 0.19 wt%. The TEM elements mapping revealed that Cr distributes uniformly with WC grains, Co and WC grains are combined well. The surface of WC-Cr2(C, N)-Co determined by XPS is mainly composed of W, Co, Cr, C, N, and O. The spark plasma sintering (SPS) sintered WC-Cr2(C, N)-Co composite shows a nanocrystalline microstructure with the average grain size ∼170 nm, and the excellent comprehensive mechanical properties with the Vickers hardness 1730 Kg·mm−2 and fracture toughness 11.5 MPa·m1/2, respectively.

Effect of an annealing treatment on the microstructure and EMW-absorbing properties of SiCw/Si3N4 ceramics fabricated by 3D printing

SiC whisker (SiCw)/Si3N4 ceramics were prepared via selective laser sintering (SLS) and precursor infiltration pyrolysis (PIP) to achieve good electromagnetic wave (EMW) absorption performance. The as-pyrolyzed composite ceramics were annealed at 1200–1600 °C in a N2 atmosphere. The microstructural variation, phase evolution, and EMW-absorbing properties of the SiCw/Si3N4 ceramics were subsequently studied. SiC whiskers were generated in the composite ceramics during pyrolysis, and their content increased as a function of the annealing temperature. An increase in the SiCw content of the annealed samples improved the reflection loss of the composite ceramics. The absorption properties of the SiCw/Si3N4 ceramics first degraded and then improved with increasing annealing temperature. Among all the samples tested, the composite ceramics annealed at 1600 °C showed the optimal EMW absorption performance in the range of 2–18 GHz due to the lower content of free carbon and higher content of SiCw. The ceramics annealed at 1600 °C exhibited a superior absorption loss of −14.1 dB and an effective absorption bandwidth of 2.16 GHz. These results demonstrate that the EMW-absorbing performance of the SiCw/Si3N4 ceramics was significantly improved after annealing at 1600 °C. Thus, SiCw/Si3N4 composite parts with complex shapes were successfully manufactured by combining the SLS and PIP processes.

Production of graphitic carbons from plant-based SiC/C nanocomposites for Li-ion batteries

Barley husks are a byproduct of grain processing produced in large volumes globally. They contain a large amount of nanostructured silica (nSiO2) which can be converted via magnesiothermic reduction into carbon-rich nanostructured silicon carbide composite (nSiC/C). The nSiC/C can be further utilized as a precursor for the production of graphitic carbons (GCs) under atmospheric pressure at 2400 °C, typically utilized as active material in Li-ion battery (LIB) anodes. However, the current synthesis of graphite utilizes fossil-based precursors and requires temperatures up to 3000 °C. Therefore, we propose the sustainable and renewable plant-based GCs as an alternative anode material for LIBs. The synthesized GCs demonstrated a fully crystalline 3D carbon structure with stacking faults between graphene layers. Particle morphology, carbon structure, and elemental composition of the GCs were not influenced by the amount of free carbon in the precursors but the yield (11–41 wt%) of the GCs increased with increasing content of free carbon (1–26 wt%) in the SiC/C composite. The GC material had a specific capacity of 294 mAh g−1 as an active anode material in LIBs, demonstrated significantly better capacity retention at high current rates (1 and 2 C) compared with commercial graphite, and withheld 80% of the specific capacity over 175 cycles at 1 C. Synthesis of GCs from barley husks may provide an advantageous route of valorization of this underutilized sidestream.

Synthesis and characterization of mesoporous TiC nanopowder/nanowhisker with low residual carbon processed by sol–gel method

In the present research, Titanium carbide nanopowders and nanowhiskers were synthesized by the sol-gel process using phenolic resin and titanium alkoxide as precursors. The synthesis process was performed in a quaternary alkoxide-catalyst-dispersant-water system. The effect of synthesis temperature and soaking time on the free carbon content and the formation and morphology of TiC nanopowders was investigated. It was revealed that titanium carbide began to nucleate at a temperature of approximately 1200 °C. This process ended at 1400 °C. Surface studies demonstrated that the pores on the surface of the synthesized nanopowders were in the meso range and had a specific surface area of 150 m2/g. Microstructural images indicated that over time, TiC nanowhiskers heterogeneously nucleated from the particle surface and grew in a soaking time of 3 h. The free carbon content detected in the synthesized product under a soaking time of 3 h was reported to be less than 1%.

Synthesis and Sintering of Tungsten and Titanium Carbide: A Parametric Study

The three primary steps in the production of tungsten carbide WC and titanium carbide TiC powders are the preparation of the green mixture, carbidization by furnace annealing, and ball milling of the annealed products. This work performed a comprehensive parametric investigation of these three steps. The impact of several factors was examined including the carbon precursor, the mass and diameter of the milling bodies (balls), the milling time and speed, the temperature and length of the annealing process, the height of the powder in the furnace boats, and the rate at which the furnace boats move. Regression models for every stage of the process were verified by 10-fold validation and used to optimize the synthesis sequence, resulting in high-quality WC and TiC with a grain size below 2 microns and a content of free carbon below 0.1%. Additionally, solid solution (W,Ti)C was fabricated by mechanochemical synthesis from the elemental mixtures; however, further modification of this technique is necessary because of the observed relatively high concentration of residual free carbon (0.2–0.8%) and contamination by Fe.

Mechanical Properties of a Stainless Steel after Annealing in Uranium Carbide

The aim of this study was to investigate the interaction of carbide nuclear fuels with steel that is being used as cladding material for nuclear reactors. The specimens prepared from steel EN 1.4988 were consecutively annealed in three uranium carbide (UC) powders, having different carbon contents, at 600 °C for 1000 h. Both Ar and Na were used as bonding elements. The increase in the carbon content of the carburized specimens was determined and evaluated according to the bound and free carbon contents in the UC powders. The migration of free and bound carbon atoms into steel via self-diffusion and over Fe3C formation is interpreted as carburizing. Microhardness measurements and stress-strain tests were used to determine the mechanical properties of crude and carburized steel specimens. Maximum hardness at the contact surface and depth of the carburized zone were determined from the microhardness profiles and discussed depending on the bonding elements and carbon content in the specimens. These variables have a significant impact on the elongation percent, 0.2% yield stress, and tensile stress.

SiOC Phase Control and Carbon Nanoribbon Growth by Introducing Oxygen at Atom Level for Lithium-Ion Batteries.

Poor intrinsic conductivity and the presence of irreversible lithiation phase affect the electrochemical performance of silicon oxycarbide anode materials. Even though it can be improved by increasing free carbon content or composition, scarification of reversible capacity and initial Coulombic efficiency (ICE) remain as challenge. Here, polycarbosilane (PCS) with alternating distribution of silicon and carbon atoms is employed as precursor of SiOC ceramics. Air oxidation cross-linking is used to regulate the content of oxygen and carbon elements in PCS at atom level, so as to explore a solution to improve the intrinsic conductivity and reversible lithium phase content of SiOC ceramics. This strategy provides extremely excellent rate capability, areal/volumetric capacity, and ICE. This is also the first concept for feasible precursor structure design to control the SiOC glass phase and regulate the growth of C nanoribbon that can improve the intrinsic conductivity and reversible capacity of SiOC ceramic anode materials.

Impact of blending with polystyrene on the microstructural and electrochemical properties of SiOC ceramic

AbstractIn this work, we present the electrochemical behavior and microstructural analysis of silicon oxycarbide (SiOC) ceramics influenced by an addition of polystyrene (PS). Polymer‐derived ceramics were obtained by pyrolysis (1000°C, Ar atmosphere) of different polysiloxanes prepared by sol–gel synthesis. This method is very effective to obtain desired composition of final ceramic. Two alkoxysilanes phenylthriethoxysilane and diphenyldimethoxysilane were used as precursors. Before pyrolysis polysiloxanes were mixed with PS using toluene as a solvent. Blending with PS affects the microstructure and free carbon content in the final ceramic material. Free carbon phase has been confirmed to be a major lithium storage host. Nevertheless, we demonstrate here that capacity does not increase linearly with increasing carbon content. We show that the amount of SiO4 units in the SiOC microstructure increases the initial capacity but decreases the cycling stability and rate capability of the material. Furthermore, the microstructure of the free carbon influences the electrochemical performance of the ceramic: More ordered graphitic clusters favor better rate capability performance.