Katsuya Eguchi

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.

Lithium Species in Electrochemically Lithiated and Delithiated Silicon Oxycarbides

The work described herein deals with efforts to make a persuasive correlation between structural characteristics and electrochemical lithium storage for a silicon oxycarbide prepared from poly(methylhydrogensiloxane) and divinylbenzene. Structural characterization reveals that the silicon oxycarbide includes excess free carbon in an amorphous network. The reversibility of lithiation and delithiation in the silicon oxycarbide reaches 74% between 0.005 and 3 V relative to lithium at the first cycle but falls to only ca. 30% between 0.4 and 3 V. We found two resonances at 0 and 2.4 ppm in the (7)Li magic angle spinning nuclear magnetic resonance spectrum of the silicon oxycarbide lithiated to 0.4 V, whose contributions are 67 and 33%, respectively, and thus are in good agreement with the reversibility observed between 0.4 and 3 V. The fully lithiated silicon oxycarbide shows a single resonance at ca. 3-9 ppm, which tends to broaden at lower temperatures to -120 °C, whereas the fully delithiated silicon oxycarbide has a single resonance at 0 ppm. These results indicate that both reversible and irreversible lithium species have ionic natures. The Li K edge in electron energy loss spectroscopy does not show clearly any identified near-edge fine structures in the inner part of the silicon oxycarbide after delithiation. Near the surface, on the other hand, LiF and oxygen- and phosphorus-containing compounds were found to be the major constituents of a solid electrolyte interface (SEI) layer. Over repeated lithiation and delithiation, the SEI layer appears to become thick, which should in part trigger capacity fading.

Vinyl ether-modified poly(hydrogen silsesquioxanes) as dielectric materials

Vinyl ether-modified poly(hydrogen silsesquioxanes) or PHSQ were prepared via a platinum-catalyzed hydrosilylation reaction of PHSQ with an alkyl vinyl ether (VE) in toluene. The product formed in a near quantitative yield and its composition was characterized by multinuclear magnetic resonance spectroscopy. Multi-detector size exclusion chromatography revealed that relative to the PHSQ starting material, the PHSQ–VEs increased in molecular weight and radius of gyration, and the relationship between intrinsic viscosity and molecular weight suggested a branched structure. Thermal analyses indicated a cure onset around 100 °C; an onset of thermal decomposition at ca. 230 °C; and mass loss completed by 550 °C. Evolved gas analysis from thermogravimetric experiments revealed the initial elimination of the ethylene linkage, followed by cleavage of the carbon–carbon bonds. The materials prepared by pyrolysis at 425 °C were porous. Nitrogen porosimetry measured an increase in microporosity—from 0.187 to 0.295 cm3 g−1 (<5 nm)—when the VE content was increased from 10 to 50 wt%. The PHSQ–VEs were spin-coated onto silicon wafers and cured either at 400, 425, or 450 °C. The dielectric constant of the spin-coated films ranged from 2.3 to 3.0, and the modulus was between 2.2 and 12.9 GPa depending on material composition.

SYNTHESIS AND DONOR-π-ACCEPTOR PROPERTIES OF POLYFLUORENE DERIVATIVES CONTAINING A PHENAZASILINE MOIETY AND AN ELECTRON ACCEPTOR

Two polyfluorene derivatives containing a phenazasiline moiety and an electron acceptor were synthesized by the Suzuki coupling reaction. Molecular weight analysis showed that the two polymers had approximately 14 and 9 fluorene units as well as functional groups. Since these polymers had donor-p-acceptor systems, their absorption edges were red-shifted compared to that of polyfluorene. Cyclic voltammetry experiments indicated that the polymers showed amphoteric redox behavior. Bulk heterojunction solar cells were fabricated using films made of the polymers and a fullerene derivative, and their photovoltaic properties were investigated. Currently, extensive research is being carried out on conjugated polymers, with special focus on the development of electronic devices such as bulk heterojunction (BHJ) solar cells. Conjugated polymers with donor-p-acceptor systems have attracted considerable attention as candidate materials for BHJ solar cells. Their HOMO and LUMO energy levels can be controlled by appropriate choice of the donor and acceptor groups, and thus, semiconducting polymers with the desired band structures can be obtained. On the other hand, polyfluorenes containing alkyl chains are useful as organic-soluble polymer backbones, and several polyfluorene derivatives with donor-p-acceptor systems have been synthesized and investigated for use in BHJ solar cells. Furthermore, some phenazasiline polymers exhibiting p-type semiconducting behavior have been reported. Therefore, in this study, we synthesized polyfluorene derivatives containing a phenazasiline moiety and an electron acceptor such as 2,1,3-benzothiadiazole or anthraquinone via Suzuki coupling (Scheme 1). Herein, we report the synthesis and spectroscopic and electrochemical properties of polyfluorene derivatives (1) and (2), as well as the use of these polymers in the fabrication of BHJ solar cells.