William R. Even

Structural and Electrochemical Characterization of Glassy Carbon Prepared from Silicon‐Doped Polymethacrylonitrile/Divinylbenzene Copolymer

There has been a great deal of discussion on the potential role of heteroatoms in carbons intended for lithium ion intercalation. Previous observations have been mixed, ranging from beneficial to detrimental effects on intercalation capacity, irreversible losses, and charge efficiencies. The introduction or substitution of silicon into a carbon matrix prepared by chemical vapor deposition has demonstrated a positive effect. The present study concerns substitutional effects in disordered carbons synthesized from polymer precursors. Solid‐state nuclear magnetic resonance, photoacoustic IR spectroscopy, X‐ray fluorescence, and high‐resolution transmission electron microscopy were used to follow the modified carbon synthesis. Silicon is successfully incorporated into the polymer matrix by copolymerizing the precursors with tetravinylsilane. On oxidative stabilization and pyrolysis, however, the Si‐C connectivity is replaced by Si‐O coordination. These silicon species appear to be discrete, and no evidence for either a crystalline or amorphous secondary phase is seen. These silicon species have a profound effect on the electrochemical performance. Irreversible loss processes are greatly increased, and charge efficiencies are decreased when compared to nonsubstituted controls.

Electrochemical and spectroscopic evaluation of lithium intercalation in tailored polymethacrylonitrile carbons

Disordered polymethacrylonitrile (PMAN) carbon monoliths have been studied as potential tailored electrodes for lithium ion batteries. A combination of electrochemical and surface spectroscopic probes have been used to investigate irreversible loss mechanisms. Voltammetric measurements show that Li intercalates readily into the carbon at potentials 1V positive of the reversible Li potential. The coulometric efficiency rises rapidly from 50% for the first potential cycle to greater than 85% for the third cycle, indicating that solvent decomposition is a self-limiting process. Surface film composition and thickness, as measured by x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS), does not vary substantially when compared to more ordered carbon surfaces. Li{sup +} profiles are particularly useful in discriminating between the bound states of Li at the surface of solution permeable PMAN carbons.

Effect on Performance of Composition of Li-Ion Carbon Anodes Derived from PMAN/DVB Copolymers

The effects on electrochemical performance of the nitrogen content of disordered carbons derived from polymethacryonitrile (PMAN)-divinylbenzene (DVB) copolymers were examined in galvanostatic cycling tests between 2 V and 0.01 V vs. Li/Li+ in lM LiPF 6 /ethylene carbonate (EC)-dimethyl carbonate (DMC). The first-cycle reversible capacities and coulombic efficiencies increased with increase in the level of nitrogen for samples prepared at 700°C. However, the degree of fade also increased. Similar tests were performed on materials that were additionally heated at 1,000° and 1,300°C for five hours. Loss of nitrogen, oxygen, and hydrogen occurred under these conditions, with none remaining at the highest temperature in all cases but one. The pyrolysis temperature dominated the electrochemical performance for these samples, with lower reversible and irreversible capacities for the first intercalation cycle as the pyrolysis temperature was increased. Fade was reduced and coulombic efficiencies also improved with increase in temperate. The large irreversible capacities and high fade of these materials makes them unsuitable for use in Li-ion cells.

The effects of near surface and bulk microstructure on lithium intercalation of disordered, ``hard'' carbons

Disordered carbons were synthesized at 700 C from methacrylonitrile-divinylbenzene precursors. The disorder, even at the free surface, was confirmed with TEM. These powdered carbons were subjected to rapid surface heating by a pulsed infrared laser. While the bulk structure remained essentially unchanged, there was substantial surface reconstruction to a depth of 0.25 {micro}m after heating (5.9 W average power at 10Hz, 10 ns pulse width, 1,064nm wavelength). The surface ordering appears similar to the bulk microstructure of carbons isothermally annealed at 2,200 C (i.e., turbostratic). Improvements were observed in first cycle irreversible loss, rate capability, and coulombic efficiencies of the reconstructed carbons, relative to the untreated carbon.

Adsorption of polystyrene onto noble metals from theta solvents

Abstract The adsorption of polystyrene onto noble metals from 3.3% solutions in five different theta solvents was measured on six noble metals at temperatures a few degrees above the theta temperatures. The amounts adsorbed (in mg m −2 ) on any given metal were of the same order of magnitude from all five of the theta solvents, but decreased two or three orders of magnitudes with a change of noble metals, in the following order: gold⪢silver⪢palladium, silver-palladium ⪢platinum⪢iridium. The amounts adsorbed on gold from cyclohexane at 60°C decreased seven-fold with a forty-fold decrease of molecular weight, as predicted by Scheutjens and Fleer, but there was no molecular weight dependence to the weak adsorption observed on platinum and iridium. A direct method was used to determine the amount of polymer adsorbed on these metal powders. After the polymer solution and metal powder were equilibrated, the powder was washed free of excess polymer solution with fresh theta solvent and then the adsorbed polystyrene was removed from the metal by rinsing in chloroform, a very good solvent, in which the amount of polystyrene was determined by UV spectroscopy. With the high molecular weight polystyrene (1.8·10 6 ) the differences in duplicate samples were about 2%, but with the lower molecular weight polystyrene (5·10 4 ) they were about 25%. It is proposed that the high reproducibility of adsorption results observed with the high molecular weight polymer indicates negligible desorption during rinsing with the theta solvent, but that this rinsing step was less reproducible with the low molecular weight polymer.