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A Molecular Dynamics Study of Short-Chain Ordering in Crystalline LiPF6PEO6
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry.
2007 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 48, no 21, 6448-6456 p.Article in journal (Refereed) Published
Abstract [en]

Molecular dynamics (MD) simulations have been made of the crystalline short-chain LiPF6·PEO6 system to probe structural ordering for different chain-end arrangements for a methyl-terminated monodisperse poly(ethylene oxide) (EO23 Mw = 1059) host polymer. Five different start structures have been studied, two "smectic" and three "nematic", to represent different types of relative alignment of the end-groups between adjacent PEO chains, and different chain-end coordination situations to the Li-ions. One particular situation is found to result effectively in Li-ion bridging between PEO chains along the chain axes, thereby creating continuous ion transport pathways across the chain breaks. This situation is also found to give rise to Li+-PF6- ion-pairing and Li-O coordination instabilities in the end-group regions, where coordination to Li-ions would appear to have a more radical influence on local structure than the issue of smectic vs. nematic end-group alignment. It could be that such structural situations involving bridging Li-ions (in both smectic and nematic arrangements) are a necessary condition for the promotion of Li-ion transport in the chain direction. Comparison of simulated and experimental XRD profiles is concluded to be an inappropriately crude and uncertain technique for distinguishing between possible short-chain ordering models.

Place, publisher, year, edition, pages
2007. Vol. 48, no 21, 6448-6456 p.
Keyword [en]
End-group ordering, LiPF6·PEO6, Polymer electrolytes
National Category
Chemical Sciences
URN: urn:nbn:se:uu:diva-95854DOI: 10.1016/j.polymer.2007.08.051ISI: 000250492000031OAI: oai:DiVA.org:uu-95854DiVA: diva2:170213
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2016-05-13Bibliographically approved
In thesis
1. Ordering in Crystalline Short-Chain Polymer Electrolytes
Open this publication in new window or tab >>Ordering in Crystalline Short-Chain Polymer Electrolytes
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polymer electrolytes are the most obvious candidates for safe "all-solid" Li-ion batteries and other electrochemical devices. However, they still have relatively poor ionic conductivities, which limits their wider adoption in commercial applications. It has earlier been the conventional wisdom that only amorphous phases of polymer electrolytes show usefully high ionic conduction, while crystalline forms are insulators. However, this has been challenged in the last decade by the discovery of highly organized, low-dimensional ion-conducting materials. Specifically, the crystalline phases of LiXF6.PEO6 exhibit higher ionic conductivities than their amorphous counterparts, with the Li-ion conduction taking place along the PEO channels. Polymer chain-length and chain-end registry has emerged as potentially significant in determining ionic conduction in these materials.

Molecular Dynamics simulations have therefore been made of short-chain, monodisperse (Mw~1000), methoxy end-capped LiPF6.PEO6 to examine relationships between ion conduction and mode of chain-ordering. Studies of smectic and nematic arrangements of PEO chains have revealed that ion-transport mechanisms within the smectic planes formed by cooperative chain-end registry appear to be more suppressed by ion-pairing than in-channel conduction. Disorder phenomena in the chain-end regions emerge as a critical factor in promoting Li-ion migration across chain-gaps, as does the structural continuity of the PEO channels.

Simulations incorporating ~1% aliovalent SiF62- dopants further suggest an increase in Li-ion conduction when the extra Li-ions reside within the PEO channels, with the anion influencing charge-carrier concentration through enhanced ion-pair formation.

XRD techniques alone are shown to be inadequate in ascertaining the significance of the various short-chain models proposed; atomistic modelling is clearly a helpful complement in distinguishing more or less favourable situations for ion conduction.

Though providing valuable insights, it must be concluded that this work has hardly brought us significantly closer to breakthroughs in polymer electrolyte design; the critical factors which will make this possible remain as yet obscure.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. 50 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 303
Atomic and molecular physics, polymer electrolytes, molecular dynamics, ionic conductivity, crystalline ordering, polymer chain length, smectic, nematic, Atom- och molekylfysik
urn:nbn:se:uu:diva-7853 (URN)978-91-554-6885-9 (ISBN)
Public defence
2007-05-11, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2010-06-09Bibliographically approved

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