Daniel Brandell

Molecular Dynamics Modeling of Proton Transport in Nafion and Hyflon Nanostructures

Classical molecular dynamics modeling studies at 363 K are reported of the local atomic-level and macroscopic nanostructures of two well-known perfluorosulfonic acid proton exchange polymer membrane materials: Nafion and Hyflon. The influence of the different side-chain lengths in the two polymers on local structure is relatively small: Hyflon exhibits slightly greater sulfonate-group clustering, while Nafion has more isolated side chains with a higher degree of hydration around the SO(3)(-) side-chain ends. This results in shorter mean residence times for water molecules around the end groups in Nafion. Hyflon also displays a lower degree of phase separation than Nafion. The velocities of the water molecules and hydronium ions are seen to increase steadily from the polymer backbone/water interface toward the center of the water channels. Because of its shorter side chains, the number of hydronium ions is approximately 50% higher at the center of the water channels in Hyflon, and their velocities are approximately 10% higher. The water and H(3)O(+) diffusion coefficients are therefore higher in the shorter side-chain Hyflon system: 6.5 x 10(-6) cm(2)/s and 25.2 x 10(-6) cm(2)/s, respectively; the corresponding values for Nafion are 6.1 x 10(-6) cm(2)/s and 21.3 x 10(-6) cm(2)/s, respectively. These calculated values compare well with experiment: 4 x 10(-6) cm(2)/s for vehicular H(3)O(+) diffusion.

Why PEO as a binder or polymer coating increases capacity in the Li-S system.

PEO, used either as a binder or a polymer coating, and PEGDME, used as an electrolyte additive, are shown to increase the reversible capacity of Li-S cells. The effect, in all three cases, is the same: an improved solvent system for the electrochemistry of sulfur species and suppression of cathode passivation on discharge. This constitutes a novel interpretation of the mechanistic behaviour of polyethers in the Li-S system, and sheds new light upon several previous studies.

Improving the electrochemical performance of organic Li-ion battery electrodes.

Dilithium benzenediacrylate was prepared and investigated as an example of a readily available organic electrode material for lithium-ion batteries. Its poor conductive properties were overcome by a method of carbon-coating in the liquid state, resulting in enhanced cycling performance, displaying a reversible capacity of 180 mA h g(-1).

Nanoporous carbon-based electrodes for high strain ionomeric bending actuators

Ionic polymer metal composites (IPMCs) are electroactive material devices that bend at low applied voltage (1–4 V). Inversely, a voltage is generated when the materials are deformed, which makes them useful both as sensors and actuators. In this paper, we propose two new highly porous carbon materials as electrodes for IPMC actuators, generating a high specific area, and compare their electromechanical performance with recently reported RuO2 electrodes and conventional IPMCs. Using a direct assembly process (DAP), we synthesize ionic liquid (Emi-Tf) actuators with either carbide-derived carbon (CDC) or coconut-shell-based activated carbon-based electrodes. The carbon electrodes were applied onto ionic liquid-swollen Nafion membranes using a direct assembly process. The study demonstrates that actuators based on carbon electrodes derived from TiC have the greatest peak-to-peak strain output, reaching up to 20.4 me (equivalent to>2%) at a 2 V actuation signal, exceeding that of the RuO2 electrodes by more than 100%. The electrodes synthesized from TiC-derived carbon also exhibit significantly higher maximum strain rate. The differences between the materials are discussed in terms of molecular interactions and mechanisms upon actuation in the different electrodes.

Interface layer formation in solid polymer electrolyte lithium batteries: an XPS study

The first characterization studies of the interface layer formed between a Li-ion battery electrode and a solid polymer electrolyte (SPE) are presented here. SPEs are well known for their electrochemical stability and excellent safety, and thus considered good alternatives to conventional liquid/gel electrolytes in high-energy density battery devices. This work comprises studies of solid electrolyte interphase (SEI) formation in SPE-based graphite|Li cells using X-ray photoelectron spectroscopy (XPS). SPEs based on high molecular weight poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt are studied. Large amounts of LiOH are observed, and the XPS results indicate a correlation with moisture contamination in the SPEs. The water contents are quantitatively determined to be in the range of hundreds of ppm in the pure PEO as well as in the polymer electrolytes, which are prepared by a conventional SPE preparation method using different batches of PEO and at different drying temperatures. Moreover, severe salt degradation is observed at the graphite–SPE interface after the 1st discharge, while the salt is found to be more stable at the Li–SPE interface or when using LiTFSI-based liquid electrolyte equivalents.

Benzenediacrylates as organic battery electrode materials: Na versus Li

This paper discusses investigations of a novel Na-based organic battery electrode material, disodium benzenediacrylate (Na2BDA) in comparison with its Li-ion homologue. Li2BDA has previously shown promising battery properties, such as stable cycling and good capacity retention. Na2BDA and Li2BDA are here successfully synthesized and characterized, and investigated as anode materials in prototype Na- and Li-ion battery cells. Moreover, different electrolytes are screened for the Na-battery material, and a useful operating voltage window is explored. Na2BDA is shown to possess a higher initial coulombic efficiency (91%) than the Li-homologue, which is uncommon for sodiated organic electrode materials. The Na-compound shows an initial capacity of 177.7 mA h g−1, which however decreases to ca. 50 mA h g−1 after 20–100 cycles depending on the cycling rate; a significantly lower capacity retention then that observed for Li2BDA. The capacity loss can primarily be explained by a decomposition mechanism of the Na2BDA compound.

Molecular dynamics simulation of the LiPF6·PEO6 structure

Molecular dynamics (MD) simulations have been performed for the crystalline LiPF6·PEO6 system at ambient temperature in an effort to model the detail of its atomic-level structure and dynamics. Start coordinates were taken from the neutron powder diffraction analysis of Gadjourova et al., Chem. Mater., 2001, 13, 1282 (ref. ). Polymer-chain conformation, Li+-ion coordination and thermal displacement parameters are compared with experimentally determined values; the differences found are rationalised in terms of differences between the infinite-chain models investigated (both experimental and theoretical) and the finite chain-length material studied.

Molecular dynamics simulation of the crystalline short-chain polymer system LiPF6·PEO6(Mw∼ 1000)

In an effort to probe the effect of chain length on the structure and properties of ionically conducting polymer electrolytes, the crystalline system LiPF6·PEO6 has been simulated at 293 K using the Molecular Dynamics Simulation (MDS) technique. The specific system studied is short-chain poly(ethylene oxide) with the formulation CH3–(OCH2CH2)22–OCH3; Mw = 1015, a commercially available mono-disperse short-chain PEO form resembling that studied experimentally (Stoeva et al., J. Am. Chem. Soc., 2003, 125, 4619, ). The methoxy chain-ends have been arranged to reproduce smectic and nematic models. Calculated Li+ ion coordination, polymer chain configuration, chain-end registry and diffraction profiles are compared both with experiment and with the results from earlier MD simulations of infinite PEO chain systems (Brandell et al., J. Mater. Chem., 2005, 15, 1422, ). The differences found are interpreted in the terms of chain-end effects and polymer relaxation.

At the polymer electrolyte interfaces: the role of the polymer host in interphase layer formation in Li-batteries

In this study, X-ray photoelectron spectroscopy was applied for compositional analysis of the interphase layers formed in graphite and LiFePO4 Li-battery half cells containing solid polymer electrolytes (SPEs) consisting of poly(trimethylene carbonate) (PTMC) and LiTFSI salt. Decomposition of PTMC was observed at the anode/SPE interface, indicating different reaction products than those associated with the more conventional host material poly(ethylene oxide). Degradation mechanisms of the PTMC host material at low potentials are proposed. Compared to the LiFePO4/PEO interface, the absence of LiOH - a result of water contamination - was generally seen when using hydrophobic PTMC as the polymer host. A clear correlation of moisture content with the constitution of interphase layers in Li polymer batteries could thus be concluded. At the SPE/LiFePO4 interface, good stability was seen regardless of the polymer host materials.

Ion transport in polycarbonate based solid polymer electrolytes: experimental and computational investigations

Among the alternative host materials for solid polymer electrolytes (SPEs), polycarbonates have recently shown promising functionality in all-solid-state lithium batteries from ambient to elevated temperatures. While the computational and experimental investigations of ion conduction in conventional polyethers have been extensive, the ion transport in polycarbonates has been much less studied. The present work investigates the ionic transport behavior in SPEs based on poly(trimethylene carbonate) (PTMC) and its co-polymer with ε-caprolactone (CL) via both experimental and computational approaches. FTIR spectra indicated a preferential local coordination between Li(+) and ester carbonyl oxygen atoms in the P(TMC20CL80) co-polymer SPE. Diffusion NMR revealed that the co-polymer SPE also displays higher ion mobilities than PTMC. For both systems, locally oriented polymer domains, a few hundred nanometers in size and with limited connections between them, were inferred from the NMR spin relaxation and diffusion data. Potentiostatic polarization experiments revealed notably higher cationic transference numbers in the polycarbonate based SPEs as compared to conventional polyether based SPEs. In addition, MD simulations provided atomic-scale insight into the structure-dynamics properties, including confirmation of a preferential Li(+)-carbonyl oxygen atom coordination, with a preference in coordination to the ester based monomers. A coupling of the Li-ion dynamics to the polymer chain dynamics was indicated by both simulations and experiments.