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A molecular dynamics study of ion-conduction mechanisms in crystalline low-Mw LiPF6·PEO6
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: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 17, no 37, 3938-3946 p.Article in journal (Refereed) Published
Abstract [en]

Molecular dynamics (MD) simulation has been used to probe ion-conduction mechanisms in crystalline LiPF6.PEO6 for smectic- and nematic-ordered models of methyl-terminated short-chain monodisperse poly(ethylene oxide) chains with the formula CH3-(OCH2CH2)23-OCH3; Mw = 1059. The effect of aliovalent substitution of the PF6- anion by ca. 1% SiF62- has also been studied. External electric fields in the range 3-6 x 106 V m-1 have been imposed along, and perpendicular to, the chain direction in an effort to promote ion transport during the short timespan of the simulation. Ion-migration barriers along the polymer channel are lower for the nematic models than for the smectic, with anions migrating along the channels more readily than Li-ions. Ion mobility within the smectic interface could also be confirmed, but at a higher field-strength threshold than along the chain direction. Li-ion migration within the smectic plane appears to be suppressed by ion pairing, while Li-ion transport across the smectic gap is facilitated by uncoordinated methoxy end-groups. Interstitial Li-ions introduced into the PEO channel through SiF62- doping are also shown to enhance Li-ion conduction.

Place, publisher, year, edition, pages
2007. Vol. 17, no 37, 3938-3946 p.
Keyword [en]
Doping, Interstitials, Energy gap, Pairing, Diffusion, Interfaces, Ion mobility, Polymers, Diffusion barriers, Digital simulation, Electric field effects, External fields, Polyethylene glycols, Theoretical study, Ionic conduction, Molecular ions, Molecular dynamics method
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-92859DOI: 10.1039/b706938cISI: 000249553200010OAI: oai:DiVA.org:uu-92859DiVA: diva2:166164
Available from: 2005-03-31 Created: 2005-03-31 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Understanding Ionic Conductivity in Crystalline Polymer Electrolytes
Open this publication in new window or tab >>Understanding Ionic Conductivity in Crystalline Polymer Electrolytes
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polymer electrolytes are widely used as ion transport media in vital applications such as energy storage devices and electrochemical displays. To further develop these materials, it is important to understand their ionic conductivity mechanisms.

It has long been thought that ionic conduction in a polymer electrolyte occurs in the amorphous phase, while the crystalline phase is insulating. However, this picture has recently been challenged by the discovery of the crystalline system LiXF6∙PEO6 (X=P, As or Sb) which exhibits higher conductivity than its amorphous counterpart. Their structures comprise interlocking hemi-helical PEO-chain pairs containing Li+ ions and separating them from the XF6- anions.

The first Molecular Dynamics (MD) simulation study of the LiPF6∙PEO6 system is presented in this thesis. Although its conductivity is too low for most applications at ambient temperature, it can be enhanced by iso- and aliovalent anion doping.

It is shown that the diffraction-determined structure is well reproduced on simulating the system using an infinite PEO-chain model. The Li-Oet coordination number here becomes 6 instead of 5; minor changes also occur in the polymer backbone configuration. The crystallographic asymmetric unit and diffraction profiles are also reproduced. On simulating a shorter-chain system (n=22), more resembling the real material, the structure retains its double hemi-helices, but the polymer adopts a more relaxed conformation, facilitating the formation of Li+-PF6- pairs.

Infinite-chain simulation shows the ionic conduction to be dominated by anion motion, in contrast to earlier NMR results. The effects of doping are also reproduced. Shortening the polymer chain-length has the effect of raising the transport number for lithium, thereby bring it into better agreement with experiment. It can be concluded that it is critical to take polymer chain-length and chain-termination into account when modelling ionic conductivity mechanisms in crystalline polymer electrolytes.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2005. 63 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 34
Keyword
Chemistry, Polymer electrolyte, molecular dynamics, conductivity mechanism, aliovalent anion substitution, smectic, nematic, polymer chain length, Kemi
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-5734 (URN)91-554-6204-9 (ISBN)
Public defence
2005-04-21, Häggsalen, Ångström Laboratory, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2005-03-31 Created: 2005-03-31 Last updated: 2010-03-03Bibliographically approved
2. 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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 303
Keyword
Atomic and molecular physics, polymer electrolytes, molecular dynamics, ionic conductivity, crystalline ordering, polymer chain length, smectic, nematic, Atom- och molekylfysik
Identifiers
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
Opponent
Supervisors
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2010-06-09Bibliographically approved

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