Gregg Alan Zank

Pyrolyzed Polysiloxanes for Use as Anode Materials in Lithium‐Ion Batteries

More than sixty siloxane polymers containing various organofunctional siloxane units were synthesized. The synthesized siloxane polymers were pyrolyzed in inert gas at 1000°C. Chemical analysis showed that the products of pyrolysis were distributed over a well-defined region in the Si-C-O Gibbs phase diagram. The electrochemical and structural properties of these materials were measured using coin-type test cells and x-ray powder diffraction, respectively. The most interesting materials are found near the line in the Si-C-O Gibbs triangle connecting carbon to SiO 1.3 . Materials with the largest reversible specific capacity for lithium (about 900 mAh/g) are on this line and were at about 43% carbon, 32% oxygen, and 25% silicon (atomic percent). Materials which were almost pure carbon showed diffraction patterns characteristic of disordered carbons. Along the line from carbon to SiO 1.3 the sample structure can be described as a mixture of single or small groups of graphene sheets mixed with regions of Si-C-O amorphous glass. The amount and composition of the glass changed according to the overall sample composition. Moving from carbon to SiO 1.3 , the reversible capacity first rises from about 340 mAh/g for pure carbon, to a maximum of 900 mAh/g near 50% carbon, and then falls to near zero mAh/g at 0% carbon. This suggests that the amorphous glass can reversibly react with lithium, provided the carbon is present to provide a path for electrons and Li ions. However, the hysteresis in the voltage profile (difference between charge and discharge voltages) and the irreversible capacity increase almost linearly along this line. There is a clear correlation between both the irreversible capacity and hysteresis in these materials with their oxygen content. Along the line connecting carbon to silicon, the reversible capacity rises from 340 mAh/g for pure carbon to about 600 mAh/g for samples with about 15 atomic percent Si. It then decreases to near zero as the composition nears SiC. Along the C-SiC line, the irreversible capacities remain below about 200 mAh/g. We are quite convinced that optimized silicon-containing carbons can be good alternatives to pure carbons as anode materials in lithium-ion batteries

Pyrolysed silicon-containing polymers as high capacity anodes for lithium-ion batteries

We describe the characteristics of materials prepared by the pyrolysis of over 50 different silicon-containing polymers, including polysilanes, polysiloxanes, and pitch silane blends. We investigate the electrochemical behaviour and structural properties of these materials as a function of their stoichiometry. Based on our findings we propose a structural phase diagram which illustrates possible structures of these materials. Our results suggest that the electrochemical behaviour of these materials, as might be expected, varies with stoichiometry and structure. We recommend stoichiometric ranges to be avoided for lithium-ion battery applications.

Second‐generation polymeric precursors for BN and SiNCB ceramic materials

Our recent work directed at the design, synthesis, characterization and applications of new types of polyborazylene and polyborosilazane polymers is reviewed with a focus on the use of these polymers as processable precursors to BN and SiNCB composites. A design strategy based on the controlled functionalization of preformed polymers with pendant groups of suitable compositions and crosslinking properties has been employed to yield second-generation dipentylamine‐polyborazylene (DPA) and pinacolborane‐hydridopolysilazane (PIN‐HPZ) polymers, which, unlike the parent polyborazylene (PB) and the borazine‐hydridopolysilazane (B‐HPZ) polymers, are stable as melts and can be easily melt-spun into polymer fibers. Subsequent pyrolyses of these polymer fibers then provide excellent routes to BN and SiNCB ceramic fibers.# 1998 John Wiley & Sons, Ltd.

Insights into the oxidation chemistry of SiOC ceramics derived from silsesquioxanes

We have undertaken a systematic study of the oxidation chemistry for a range of SiOC ceramics derived from silsesquioxane polymeric precursors. This study examines the oxidation for 500 hours at 600, 800, 1000 and 1200°C for four SiOC powders. The material changes upon oxidation were characterized qualitatively by color change and optical microscopy and quantitatively by weight and composition change. In this study we employ a very easy method that uses the weight change upon oxidation and a carbon analysis after oxidation to arrive at the composition of the oxidized SiOC. Combined these qualitative and quantitative techniques have shown that on oxidation at 800 and 600°C the SiOC composition is more rapidly changed to that of silica than oxidation over the same time frame at 1000 or 1200°C. The data indicates that this difference is due to the relative rates of oxidation of the excess carbon versus the Si—C bonds in the SiOC. At lower temperatures initially the carbon oxidation predominates which leads to higher porosity throughout the material and an increase in the surface area with eventually ‘complete’ oxidation to silica. At higher temperatures the Si—C bond oxidation rate is comparable to the rate of oxidation of carbon. This allows a silica-like surface to build up on the SiOC, which slows all subsequent reactions due to the necessity to diffuse O2 in and COx out of the bulk. Under these oxidation conditions materials that originally contain high amounts of excess carbon are more quickly oxidized to silica than those that contain minimal amounts of excess carbon, as confirmed by elemental analysis and optical microscopy. Regardless of the time or temperature of the oxidation conditions no materials were found to be completely stable to oxidation. SiOC materials with low levels of excess carbon showed the best resistance to change upon oxidation.

Fine-diameter polycrystalline SiC fibers

Abstract Various organosilicon polymers have been converted to small-diameter, polycrystalline silicon carbide fibers by melt-spinning, cross-linking and pyrolyzing to high-temperature in argon. Several wt% boron was doped into the fibers before pyrolysis. Use of polycarbosilane precursor gave 8–10 μm diameter fibers having up to 2.6 GPa tensile strength, 450 GPa elastic modulus, 3.1–3.2 g/cm3 density. The microstructure consists of >95 wt% β-SiC crystallites of 30–40 nm average crystallite size. Stoichiometric fibers or fibers having excess carbon content have been prepared. Fiber has been thermally aged under inert conditions at 1800°C for 12 h with minimal strength and microstructural change. Stoichiometric fiber maintains higher strength after oxidative aging at 1370°C. Current processing efforts are aimed at preparing the fiber in continuous tow form.

Stoichiometry control of SiOC ceramics by siloxane polymer functionality

The guidelines, or empirical rules, previously described in the literature to estimate ceramic compositions from preceramic polymer compositions have been refined and quantified. Thermogravimetric and residual gas analyses of the pyrolysis of organosilsesquioxane polymers have identified the organic degradation products at various temperatures and indicated that essentially all of the silicon and oxygen atoms of these highly branched polymers are retained in the 1200 °C ceramic residue. Series of organosilsesquioxanes with systematically varied amounts of organosilsesquioxane and endcapping components were synthesized, cured, and pyrolyzed to 1200 °C under an inert atmosphere. Multiple linear regression analysis was used to quantify the relationships between the amount of carbon retained in the ceramic residues and the mole fractions of the various organic components of the preceramic polymer, allowing for retention of the silicon and oxygen of the silsesquioxane. Specifically, a phenylsilsesquioxane fragment contributes an average of 3.94 carbons to the resulting ceramic material, vinylsilsesquioxane, 1.52 carbons, methylsilsesquioxane, 0.59 carbons and a vinyldimethylsilyl endcapping group, 2.75 carbons. The utility of the model was shown by employing this information to predict a select set of candidate precursors to an SiOC with a carbon content near a desired 18 wt.% level. One of the candidate precursors (MeSiO1.5)0.84(Me2ViSiO0.5)0.16 (predicted to afford an SiOC at 18.1 wt.% carbon) was then prepared, cured, pyrolyzed and analyzed to test the accuracy of the model. The 1200 °C ceramic was found to have 18.4 wt.% carbon, indicating good agreement between the actual and predicted values.

Silicon Oxycarbide Glasses Derived from Polymer Precursors

This paper describes a new predictive model to estimate compositions of ceramic materials from the structure of the preceramic polymer. The polymers are prepared by sol-gel methods and have M and T silicone functionality. The ceramic predictive model works well for highly branched silsesquioxanes, which are some of the most common ceramic precursors. This report describes the control of ceramic SiOC compositions which in turn make the materials useful as: (1) matrices for ceramic matrix composites and; (2) anodes for rechargeable lithium ion batteries.

New Polymer Precursors To SiNCB Materials

The first borazine/silazane backbone copolymers derived from the parent borazine, B{sub 3}N{sub 3}H{sub 6}, have been obtained by the thermal condensation of borazine with either of two silazanes, tris(trimethylsilylamino)silane (TTS) or 1,1,3,3,5,5-hexamethylcyclotrisilazane (HCT), to yield copolymers of typical composition (B{sub 3}N{sub 3}H{sub 4}){sub 1.00}(N){sub 1.17}(SiMe{sub 3}){sub 1.16}(SiH){sub 0.34} and (B{sub 3}N{sub 3}H{sub 4}){sub 1.00}(N){sub 1.67}(SiMe{sub 2}){sub 1.49}(H){sub 1.5}, respectively. Despite their similar compositions, upon pyrolysis the TTS copolymers yield B{sub 1.0}N{sub 1.0}Si{sub <0.2} ceramics, while the ceramics derived from the HCT copolymers showed greater retention of silicon and carbon with typical compositions of B{sub 1.0}N{sub 1.5}Si{sub 0.4}C{sub 0.2}. The XRD spectra show the materials are amorphous to 1,400 C, but show crystalline phases of {beta}-Si{sub 3}N{sub 4}, {beta}-SiC and Si at 1,800 C, with no diffraction from any boron-containing species. The DRIFT spectra of the ceramics, however, indicate the presence of boron nitride.

Preceramic Polymer — Derived Silicon Oxycarbides

Silicon-based polymeric precursors to ceramics have been reviewed a number of times. Many of these reviews stand as excellent references to the technology being developed at the time of the writing. For general reviews of preceramic polymer chemistry the following references provide a good foundation for the reader [1], [2], [3]. There are also reviews that focus on a specific ceramic composition or ceramic shapes such as fibres [4], [5], [6]. Other reviews emphasise the precursor polymers and their chemistry [7], [8], [9], [10]. While these previous reviews can be crudely categorised as focussing on oxide, silica; or non-oxide silicon carbide or silicon nitride or silicon carbonitrides; this present review concerns itself with only the mixed system of silicon oxycarbides. One of the reasons for the large numbers of reviews covering organosilicon preceramic polymers is that this area has seen a lot of research activity since the pioneering work by Yajima [11], [12]. While this work of Yajima involved the synthesis of polycarbosilane and its conversion to silicon carbide ceramics, the main commercial product that is made today from this polycarbosilane is Nicalon™ fibre a silicon oxycarbide ceramic [13] [14]. This review will cover studies that have examined the conversion of organofunctional silicon sol-gels and silicone resins as precursors to silicon oxycarbide ceramics as well as the properties of these ceramic materials.

Polysilacyclobutasilazanes: pre-ceramic polymers for the preparation of sintered silicon carbide monoliths

A family of pre-ceramic polymers based upon silacyclobutasilazanes was prepared. Upon heating to 200–250 °C the polymers crosslink to intractable resins through a ring-opening polymerization of the silacyclobutyl group. In an inert atmosphere the polymers convert to Si-C-N-O chars upon pyrolysis to 1200 °C. At higher temperatures (> 1400 °C) the Si-C-N-O chars loose nitrogen and carbon monoxide to give stable chars containing only silicon carbide and carbon. The polymers were used as binders in press- and-sinter and transfer-moulding applications to give silicon carbide monoliths with sintered densities above 3.13 g cm−3 and four-point flexural strengths above 70 kpsi (483 MPa).