Free Carbon Phase Research Articles

Effect of KOH etching on the structure and electrochemical performance of SiOC anodes for lithium-ion batteries

The SiOC ceramic was prepared with vinyltriethoxysilane as the sol−gel precursor, and followed by etching with KOH. The effect of KOH etching on the morphology, structure and electrochemical properties of SiOC anodes was investigated. The amorphous SiOC ceramics with different free carbon content is obtained by KOH etching. The reaction of oxygen-enriched units with KOH and the activation of free carbon resulted in the formation of micro and mesopores. The galvanostatic charging/discharging and rate capability suggest that the electrochemical performance of SiOC ceramics is improved by KOH etching. As compared with SiOC sample, KOH-SiOC-5 (KOH/SiOC weight ratio of 5:1) with a high BET surface area of 249.2m2g−1 deliveres a discharge capacity of 607mAhg−1 at the current density of 36mAg−1. Nano-sized pores, mixed SiOxC4-x structures and the free carbon phase both work together for the excellent electrochemical performance. The contribution of carbon to reversible capacity is higher than that of SiOxC4-x phases and surface area. However, the specific surface area, pore volume and free carbon content are decreased by excessive etching with KOH, which resulted in the decrease of electrochemical properties.

Models Proposed to Explain the Electrical Conductivity of Polymer‐Derived Silicon Carbide Fibers

Two types of silicon carbide fibers (SiCf) were prepared employing different pyrolysis techniques. The relationship between the microstructure and the electrical resistivity of the fibers was investigated. The results indicated that the carbon layer present on the fiber surface acted as the main conductive phase in the SiCf obtained by direct pyrolysis, whereas a free carbon phase determined the conductivity of the SiCf prepared by the preheated pyrolysis method. A core‐shell model and a general effective media (GEM) theory were proposed to explain the conductivity of different types of SiCf. Quantitative analysis based on these models indicated an electrical resistivity of ~10−2 Ω·cm for the carbon layer on the surface of SiCf obtained by direct pyrolysis. The electrical resistivity and the percolation threshold of the free carbon in SiCf prepared by the preheated pyrolysis method were 10−1 Ω·cm and 11.3% respectively.

Impact of the electrical conductivity on the lithium capacity of polymer-derived silicon oxycarbide (SiOC) ceramics

Within this work, we systematically investigate the influence of the amount of free carbon phase on the electrical conductivity and the lithium storage capability of silicon oxycarbide (SiOC) materials. A series of SiOC ceramics comprised of free carbon content ranging from 8 to 54wt.% is examined. The ability to reversibly host lithium ions is correlated with the elemental composition and the electrical conductivity. The materials with low carbon content up to ∼20wt.% reveal conductivities lower than 310−5Sm−1 and initially high capacities of more than 1000mAhg−1, which fade rapidly with continuous cycling. For the samples with higher amounts of free carbon phase, higher electrical conductivities ranging from 0.07 to 2.2Sm−1 are measured. In comparison to the carbon-poor materials, the carbon-rich samples demonstrate enhanced accessibility of Li-ion storing sites with capacities of more than 500mAhg−1. Moreover, a linear dependence between a stable capacity after 60 cycles and free carbon content in the ceramic is found.

Sol–gel derived borosilicate glasses and thin film coatings on SiC substrates: Boron loss and carbon retention due to processing and heat treatment

Synthesis of freestanding borosilicate glass and borosilicate thin films (<1μm thick) applied on silicon carbide substrates was carried out by a sol–gel method for the purpose of determining boria effects on silicon carbide oxidation. Extensive characterization of boron loss, compositional reproducibility, and film quality was carried out through every stage in the gel to glass conversion. Boron loss in the gel synthesis and during heat treatment ranged from 30 to 68mol% and increased with solvent loading. Hydrolyzed products were evident in glasses heat treated at lower temperatures, but were not present as crystalline phases in the amorphous glass network. Reproducibility of boron concentration in the glass was relatively unaffected by heat treatment conditions, but formation of the borosilicate bond was enhanced in certain gas environments. Carbon retention was characterized for total concentration. Carbon bonding both within the glass network and as a free carbon phase was evident by spectroscopic analysis.

First-Principle Calculations on Structure, Thermodynamics and Electrochemistry of Si-O-C Electrodes

Siliconoxycarbides (SiOC) have emerged as promissing electrode material for Li-ion batteries, showing capacities of up to 800 mAh/g and reasonable cycling stability. SiOCs are amorphous networks that are typically obtained by pyrolysis of various precursors. Depending on the choice of the precursor and the pyrolysis temperature, SiOCs with different chemical compositions and thus different morphologies, including an additional free carbon phase, can be obtained. Moreover, Si or Sn nanoparticles can be embedded into SiOC networks, either by mixing or by modification of the precursor. In this way, a further increase of Li capacities has been demonstrated. Despite numerous experimental investigations our understanding of atomic structure and electrochemical behaviour of SiOCs is still limited. In this contribution, density functional theory (DFT) calculations are used to model lithiation of SiOC from a an atomistic and electronic perspective. We first address the challenge of designing realistic structure models. For pristine SiOC, the lack of the existence of a crystalline phase prevents straightforward structure-generation using a cook-and-quench approach. In recent literature, SiOC models that were obtained by replacing O by C in amorphous SiO2 networks have been presented. However, such models suffer from the fact that O is only 2-fold coordinated in SiO2, while C prefers a 4-fold coordination in the vicinity of Si. Therefore, the carbon in such SiOC models is expected to be more reactive than in reality. We present a strategy that results in model structures that consist of Si and C that are mainly 4-fold coordinated while O is 2-fold coordinated. In addition, a free carbon phase is included in our models. After structural optimization and thermodynamic characterization, we study Li insertion. In particular we present information about Li storage sites, energetic evolution (including voltage profiles), structural evolution (including specific volume changes) and elastic properties. We compare our results to experiment and previous modelling studies.

Si(O,N)C Ceramics of Controlled Porosity for Storage of Lithium Ions

Porous carbons have been investigated as high capacity electrodes for lithium storage for more than 10 years [ 1 ]. Realization of active materials with high surface to volume ratio leads to intensification of the presence of active sites for lithium storage. Although high active electrode leads to significant losses during SEI formation, it is recompensed by a considerable increase of the specific capacity. Moreover, the high surface area implies a high contact area with the electrolyte, hence leading to high lithium-ion flux across the electrode/electrolyte interface. Moreover, controlled porosity leads to an improved lithium diffusion due to the reduction of its path length, hence batteries with enhanced power capability and higher electron transfer rates[ 2 ]. Within this work we present a novel approach to prepare porous, polymer-derived Si(O,N)C ceramic for lithium storage. Through controlled crosslinking/pyrolysis process, materials with significantly higher surface area compared to standard polymer derived ceramic (PDC) route are synthesised. Free carbon phase formed after pyrolysis of the polymeric samples is characterized by means of Raman spectroscopy. The characteristic carbon vibrations appear in all spectra [ 3 ]. Unusually high I(D)/I(G) ratio ranging from 1.8 to 2 is calculated, whereas fitting[ 4 ] reveals the presence of strong D3 band at 1540 cm-1, originating from the presence of amorphous carbon. In parallel, the intensity of G band attributed to an ideal graphitic lattice vibration mode with E2g symmetry remains low. In general, the increase of pyrolysis temperature leads to increase of ordering in free carbon phase. Elemental analysis reveals similar composition with average free carbon content of ~45 wt.%. Galvanostatic cycling with potential limitation show a significant improvement in capacity and rate capability (Figure1) for porous samples compared to dense sample. Namely discharge capacity increases from about 160 mAh·g-1 (Poly1_1100) to 320 mAh·g-1 for samples pyrolysed at 1100 °C (Poly2_1100). The sample exposed to 900 °C (Poly2_900) reveals very high capacities around 520 mAh·g-1 after 100 cycles (Figure1, insert). Furthermore, the porous materials demonstrate high and stable capacities when subjected to higher currents, namely 180 mAh·g-1 is recovered with the rate of 5C (1860 mA·g-1). In order to understand the differences in the ion storage mechanism in dense and porous materials solid state NMR study is in progress. Acknowledgments: We gratefully acknowledge the financial support of the German Research Foundation (DFG) SPP1473/JP8. Authors thank to Christina Schitco for fruitful discussions. Literature: 1. Zhou, H.; Zhu, S.; Hibino, M.; Honma, I.; Ichihara, M. Advanced Materials 2003, 15, 2107–2111. 2. Roy, P.; Srivastava, S. K. J. Mater. Chem. A 2015, 3, 2454–2484. 3. Ferrari, A. C. Solid State Commun. 2007, 143, 47–57. 4. Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Poschl, U. Carbon 2005, 43, 1731–1742. Figure 1. Recovered capacity of Poly2_1100 at different C-rates ranging from C/2 up to 5C. Insert: Extended cycling curves for Poly1_1100, Poly2_1100, Poly2_900 at current of 72 mA·g-1. Figure 1

Controlled atmosphere pyrolysis of polyureasilazane for tailored volume fraction Si3N4/SiC nanocomposites powders

A controlled atmosphere pyrolysis process is used to develop processing protocols for controlled volume fraction Si3N4/SiC composites from a polyureasilazane. Various processing protocols are explored such as gas composition (Argon, NH3 and a 10%NH3/Argon mix) and a gas switching process where gas is switched from NH3 to Argon at specified temperatures and specified times to quench the reaction. Results showed an accurate control of the volume fraction of SiC in 0–30 volume% range in a Si3N4 matrix. FTIR spectroscopic results confirmed that a substitution reaction occurs between NH3 and the polyureasilazane. Raman spectroscopy shows a free carbon phase present up until 1850°C and carbon present in specimens that do not show observable XRD peaks of SiC.

Free carbon phase in SiOC glasses derived from ladder-like silsesquioxanes

The main objective of this paper is to attempt to determine the structure of free carbon phase in SiOC glasses derived from ladder-like silsesquioxane precursors. The samples were fired in argon flowing atmosphere, in a wide temperature range (200–800° C). Conducted XRD, Raman, MIR spectroscopy research allowed to determine the structure of obtained materials. Combined XPS and Raman studies enabled to establishing the point of formation of free carbon in silicon oxycarbide matrix. Two most popular concepts were confronted with Raman and XPS results of free carbon phase in studied materials. This allowed to conclude that for temperatures below 800 °C carbon is present in a form of graphite domains inside glassy matrix.

Preparation and Characterization of Axial Gradient Silicon Carbide Fibers with Sinusoidal Electrical Resistivity

Two types of silicon carbide fibers with sinusoidal electrical resistivity were prepared by using different pyrolysis technology. The relationship between the microstructure and the electrical resistivity of these fibers was investigated and compared. The results indicated that carbon layer was the main conductive phase in the SiC fibers obtained by means of one step pyrolysis, whereas a free carbon phase governed the conductivity of the SiC fibers prepared through two step pyrolysis mode.

Electric conductivity and microstructure evolution of polymer-derived SiAlCO ceramics

The electric conductivity and microstructure of polymer-derived SiAlCO ceramic were studied. The conductivity increased drastically with increasing pyrolysis temperature and exhibited a typical Arrhenius dependence on pyrolysis temperature with the activation energy of ~7.15eV. The microstructure was analyzed by XRD, Raman spectroscopy and XPS. Unlike previous reported PDCs, the increase in the conductivity of the SiAlCO cannot be explained by the structural changes of the free carbon phase. It is speculated that the increased conductivity is possibly due to the redistribution of C–O bonds within the free carbon phase.

Experimental investigation and thermodynamic assessment of the C–Co–Fe–Ni–W system

The liquid phase formation temperatures of WC–(Co,Fe,Ni) alloys with free carbon and M6C phase, respectively, were determined by means of Differential Scanning Calorimetry (DSC) analysis. Based on the experimental results, a thermodynamic modelling is conducted on the C–Co–Fe–Ni–W quinary system using the CALPHAD (CALculation of PHAse Diagram) technique. Temperature-composition sections and projections concerning the sintering areas of the WC–(Co,Fe,Ni) hardmetals are calculated to verify the rationality of the present modelling. Comprehensive comparisons between the experimental and calculated results show that all the experiments can be well reproduced by the present modelling.

Improving the oxidation resistance of carbon fibers using silicon oxycarbide coatings

Silicon oxycarbide (SiOC) ceramic was coated on carbon fibers using a vinyl group-modified silicon alkoxide as the sol precursor. The coatings were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and thermogravimetry. The mechanical properties of the carbon fibers were investigated by single fiber tensile tests. Results showed that the amorphous SiOC coating was composed of SiCxO4-x and a free carbon phase. The oxidation resistance of the carbon fibers was improved by the SiOC coatings. However, debonding of the coatings and surface cracks led to a reduction of tensile strength and an increase of Weibull modulus. The coating with a thickness of 200 nm increased the onset oxidation temperature by about 150 °C, but decreased the tensile strength from 3.18 to 2.32 GPa.

Highly Porous SiOC Bulk Ceramics with Water Vapor Assisted Pyrolysis

Micro/mesoporous SiOC bulk ceramics with the highest surface area and the narrowest pore size distribution were prepared by water‐assisted pyrolysis of polysiloxane in argon atmosphere at controlled temperatures (1100°C–1400°C) followed by etching in hydrofluoric acid (HF) solution. Their pyrolysis behaviors, phase compositions, and microstructures were investigated by DSC, FTIR, XRD, and BET. The Si–O–Si bonds, SiO2‐rich clusters, and SiO2 nanocrystals in the pyrolyzed products act as pore‐forming species and could be etched away by HF. Water injection time and pyrolysis temperature have important effects on phase compositions and microstructures of the porous SiOC bulk ceramics, which have a maximum‐specific surface area of 2391.60 m2/g and an average pore size of 2.87 nm. The porous SiOC ceramics consist of free carbon phase, silicon carbide, and silicon oxycarbide.

Silicon Oxycarbide Ceramics As Anode Materials for Li-Ion Batteries: Modification of the Structure and Composition to Enhance the Li Storage Properties

Li-ion battery technology is one of the promising energy storage solution for the future. Studies based on electrode materials are one of the key step to improve the energy storage performance of these systems. Recent studies shows that polymer derived silicon oxycarbide (SiOC) ceramics can be considered as a potential anode material. SiOC ceramics are generally amorphous in nature and usually consists of a network of disordered free carbon segregated in amorphous SiOC matrix. The complex nanostructure plays a crucial role in determining their functional properties. Carbon rich SiOCs (>20wt% free carbon) is found to be the best candidate for Li storage. The enhanced Li ion storage capacity of SiOC ceramics is generally explained in connection with the disordered free carbon phase which stores lithium in interlayer spaces along with at the edges of the carbon clusters and mixed SiOC phase stoichiometry supporting the stability of the system. The main disadvantage of this material is the first cycle irreversible capacity loss which arises mainly due to the presence of oxygen, defects, micro pores etc. By synthesizing SiOC compositions with different stoichiometry it has been shown that carbon-rich SiOC ceramics with ~50wt% of free carbon and remaining amorphous SiOC matrix is the best candidate for reversible lithium storage demonstrating higher reversible capacity of 600 mAh g-1with a good cycling stability. Within our work SiOC ceramics have been synthesized from polysiloxanes using different cross-linking and heat treatment approach. In a typical preparation a linear polysiloxane is cross-linked with divinylbenzene (DVB) via hydrosilylation reaction in presence of a Pt catalyst. The phenyl groups present in divinylbenzene act as the major carbon source in the final ceramics. Correlating the mixed SiOC stoichiometry composition with initial charging capacity points out a linear trend in capacity increase with amount of mixed bond networks. Mixed SiOC compositions has been characterized using 29Si MASNMR studies. It has been shown that the composition changes with pyrolysis temperature. With increase in pyrolysis temperature there is a phase separation in to SiO4 and SiC4 phases with the consumption of mixed SixOyCz units. Electrochemical studies reveal that 1000 °C is the optimal temperature of pyrolysis to have maximum SiOC capacity with good cycling stability. A specific capacity of ~600 mAh g-1 is recorded at a rate of C/20 (18 mA g-1) with 60 % efficiency. Pyrolysis atmosphere also plays a role in determining the final properties. Using a hydrogen containing atmosphere (Ar/H2 with 5% H2) for pyrolysis helps to increase the specific capacity up to an extent by removing carbon dangling bonds which acts as a trap for irreversible Li storage. This has been well supported by EPR investigations showing reduced defect concentration for H2 pyrolysed samples. A capacity increase of around 150 mAh g-1 is observed for the H2 pyrolysed samples. Recently our research focus turned to porous SiOC ceramic as a perspective anode for high rate Li storage applications. The porous nature of the anode material can enhance the fast charging/discharging behavior by reducing the diffusion path of Li ions due to electrolyte penetration and also the porous space can accommodate the volume changes associated with large Li intake. Aerogel approach is followed to produce SiOC ceramics with controlled porosity. The crosslinking is carried out under highly diluted conditions using acetone as solvent (80 vol% dilution). Wet gels after cross-linking are then aged in respective solvent followed by drying under supercritical conditions using liquid carbon dioxide followed by pyrolysis at 1000 °C under controlled argon atmosphere. Final SiOC ceramic aerogels have ~ 40 wt % of free carbon distributed within remaining SiOC matrix. The BET surface area for aerogel sample equals 180 m2/g. Electrochemical characterization of aerogels reveals a high specific capacity of more than 600 mAh g-1 at a charging rate of C (360 mA g-1) along with a good cycling stability compared to ~300 mAh g-1 recorded for dense SiOC at the same rate and ~400 mAh g-1 for H2 pyrolysed counterparts. Electrochemical studies with different SiOC compositions reveal the excellent Li host properties of these materials. Capacities are enhanced by tailoring the structure and composition of final ceramic. Incorporating Li active hetero elements such as metallic Si, Sn etc. in aerogels may further improve the Li storage performance, i.e. porous matrix can be used to compensate the volume expansion associated with the alloying systems. Figure 1

Preparation of Micro‐/Mesoporous SiOC Bulk Ceramics

Micro‐/mesoporous SiOC bulk ceramics with high surface area and bimodal pore size distribution were prepared by pyrolysis of polysiloxane in argon atmosphere at 1100°C–1400°C followed by etching in hydrofluoric acid solution. Their thermal behaviors, phase compositions, and microstructures at different nano‐SiO2 filler contents and pyrolysis temperatures were investigated by XRD, SEM, DSC, and BET. The SiO2 fillers and SiO2‐rich clusters in the SiOC matrix act as pore‐forming sites and can be etched away by HF. At the same time, the SiO2 filler promotes SiOC phase separation during the pyrolysis. The filler content and pyrolysis temperature have important effects on phase compositions and microstructures of porous SiOC ceramics. The resulting porous SiOC bulk ceramic has a maximum specific surface area of 822.7 m2/g and an average pore size of 2.61 nm, and consists of free carbon, silicon carbide, and silicon oxycarbide phases.

Frequency-dependent conductive behavior of polymer-derived amorphous silicon carbonitride

The AC conductive behavior of a polymer-derived amorphous silicon carbonitride ceramic was systemically studied. The conductivity exhibited a frequency-dependent switch: at low frequencies, the conductivity is constant and independent of frequency; while at high frequencies, the conductivity increases with frequency, showing a strong relaxation process. Both the frequency-independent conductivity and the characteristic frequency for the relaxation follow the Arrhenius relation with respect to the annealing temperature and follow a band-tail hopping process with respect to the testing temperature. XPS analysis revealed that a sp3–sp2 transition took place in the free-carbon phase of the material with increasing annealing temperature. The activation energy of the transition is similar to those for the Arrhenius relations. The following conductive mechanisms were proposed to account for the observed behaviors: the frequency-independent conductivity in the low frequency region is dominated by a long-distance transport of charge carriers via matrix-free carbon path, enhanced by an electric-field concentration effect; while the frequency-dependent conductivity in the high frequency region is dominated by a interfacial polarization process governed by charge carrier relaxation within the free-carbon phase.

New Insights into Understanding Irreversible and Reversible Lithium Storage within SiOC and SiCN Ceramics.

Within this work we define structural properties of the silicon carbonitride (SiCN) and silicon oxycarbide (SiOC) ceramics which determine the reversible and irreversible lithium storage capacities, long cycling stability and define the major differences in the lithium storage in SiCN and SiOC. For both ceramics, we correlate the first cycle lithiation or delithiation capacity and cycling stability with the amount of SiCN/SiOC matrix or free carbon phase, respectively. The first cycle lithiation and delithiation capacities of SiOC materials do not depend on the amount of free carbon, while for SiCN the capacity increases with the amount of carbon to reach a threshold value at ~50% of carbon phase. Replacing oxygen with nitrogen renders the mixed bond Si-tetrahedra unable to sequester lithium. Lithium is more attracted by oxygen in the SiOC network due to the more ionic character of Si-O bonds. This brings about very high initial lithiation capacities, even at low carbon content. If oxygen is replaced by nitrogen, the ceramic network becomes less attractive for lithium ions due to the more covalent character of Si-N bonds and lower electron density on the nitrogen atom. This explains the significant difference in electrochemical behavior which is observed for carbon-poor SiCN and SiOC materials.

The influence of the pyrolysis temperature on the electrochemical behavior of carbon-rich SiCN polymer-derived ceramics as anode materials in lithium-ion batteries

Within this study we report on the impact of the pyrolysis temperature on the structural and electrochemical properties of the poly(phenylvinylsilylcarbodiimide) derived silicon carbonitride (SiCN) ceramic. Materials pyrolysed at 800 °C and 1300 °C, SiCN 800 and SiCN 1300, are found amorphous. Raman spectroscopy measurements indicate the increase in ordering of the free carbon phase with increasing pyrolysis temperature which leads to lower capacity recovered by SiCN 1300. Significant hysteresis is found for materials pyrolysed at 800 °C during electrochemical lithium insertion/extraction. This feature is attributed to much higher hydrogen content in SiCN 800 sample. An aging of SiCN 800 reflected by a change of elemental composition upon contact to air and a strong film formation are attenuated at a higher pyrolysis temperature. Single particle microelectrode investigation on SiCN 800 and SiCN 1300 clarify different electrochemical behavior of the materials. Much lower charge transfer resistance of SiCN 1300 in comparison to SiCN 800 explains better high currents electrochemical performance. Lithium ions diffusion coefficient Dmin ranges from 3.2 10−9 cm2s−1 to 6.4 10–11 cm2s−1 and is independent on the potential.

Structural evolutions in polymer-derived carbon-rich amorphous silicon carbide.

The detailed structural evolutions in polycarbosilane-derived carbon-rich amorphous SiC were investigated semiquantitatively by combining experimental and analytical methods. It is revealed that the material is comprised of a Si-containing matrix phase and a free-carbon phase. The matrix phase is amorphous, comprised of SiC4 tetrahedra, SiCxOx-4 tetrahedra, and Si-C-C-Si/Si-C-H defects. With increasing pyrolysis temperature, the amorphous matrix becomes more ordered, accompanied by a transition from SiC2O2 to SiCO3. The transition was completed at 1250 °C, where the matrix phase started to crystallize by forming a small amount of β-SiC. The free-carbon phase was comprised of carbon nanoclusters and C-dangling bonds. Increasing pyrolysis temperature led to the transition of the free carbon from amorphous carbon to nanocrystalline graphite. The size of the carbon clusters decreased first and then increased, while the C-dangling bond content decreased continuously. The growth of carbon clusters was attributed to Ostwald ripening and described using a two-dimensional grain growth model. The calculated activation energy suggested that the decrease in C-dangling bonds is directly related to the lateral growth of the carbon clusters.

Formation of Al2O3‐HfO2 Eutectic EBC Film on Silicon Carbide Substrate

The formation mechanism of Al2O3‐HfO2 eutectic structure, the preparation method, and the formation mechanism of the eutectic EBC layer on the silicon carbide substrate are summarized. Al2O3‐HfO2 eutectic EBC film is prepared by optical zone melting method on the silicon carbide substrate. At high temperature, a small amount of silicon carbide decomposed into silicon and carbon. The components of Al2O3 and HfO2 in molten phase also react with the free carbon. The Al2O3 phase reacts with free carbon and vapor species of AlO phase is formed. The composition of the molten phase becomes HfO2 rich from the eutectic composition. HfO2 phase also reacts with the free carbon and HfC phase is formed on the silicon carbide substrate; then a high density intermediate layer is formed. The adhesion between the intermediate layer and the substrate is excellent by an anchor effect. When the solidification process finished before all of HfO2 phase is reduced to HfC phase, HfC‐HfO2 functionally graded layer is formed on the silicon carbide substrate and the Al2O3‐HfO2 eutectic structure grows from the top of the intermediate layer.