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Modelling the Polymer Electrolyte/Li-Metal Interface by Molecular Dynamics simulations
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
Univ Fed Fluminense, Dept Fis Quim, Inst Quim, Outeiro Sao Joao Batista S-N, BR-24020150 Niteroi, RJ, Brazil..
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.ORCID iD: 0000-0002-8019-2801
2017 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 234, p. 43-51Article in journal (Refereed) Published
Abstract [en]

Solid polymer electrolytes are considered promising candidates for application in Li-metal batteries due to their comparatively high mechanical strength, which can prevent dendrite formation. In this study, we have performed Molecular Dynamics simulations to investigate structural and dynamical properties of a common polymer electrolyte, poly(ethylene oxide) (PEO) doped with LiTFSI salt in the presence of a Li metal surface. Both a physical (solid wall) and a chemical (slab) model of the Li (100) surface have been applied, and the results are also compared with a model of the bulk electrolyte. The average coordination numbers for oxygen atoms around the Li ions are ca. 6 for all investigated systems. However, the calculated Radial Distribution Functions (RDFs) for Li+-(OPEO) and Li+-(OTFSI) show sharper peaks for the Li slab model, indicating a more well-defined coordination sphere for Li+ in this system. This is clearly a surface effect, since the RDF for Li+ in the interface region exhibits sharper peaks than in the bulk region of the same system. The simulations also display a high accumulation of TFSI anions and Li+ cations close to interface regions. This also leads to slower dynamics of the ionic transport in the systems, which have a Li-metal surface present, as seen from the calculated mean-square-displacement functions. The accumulation of ions close to the surface is thus likely to induce a polarization close to the electrode.

Place, publisher, year, edition, pages
2017. Vol. 234, p. 43-51
Keywords [en]
Li-battery, Polymer Electrolyte, Li-metal, Molecular Dynamics
National Category
Materials Chemistry Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-321177DOI: 10.1016/j.electacta.2017.03.030ISI: 000398328800006OAI: oai:DiVA.org:uu-321177DiVA, id: diva2:1094870
Funder
Swedish Energy Agency, 39036-1Carl Tryggers foundation Swedish Research Council, 2014-5984Available from: 2017-05-11 Created: 2017-05-11 Last updated: 2019-08-05Bibliographically approved
In thesis
1. Modelling the Molecular World of Electrolytes and Interfaces: Delving into Li-Metal Batteries
Open this publication in new window or tab >>Modelling the Molecular World of Electrolytes and Interfaces: Delving into Li-Metal Batteries
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lithium metal batteries (LMBs) are potential candidates for powering portable electronic devices and for electromobility. However, utilizing the reactive Li metal electrode means tackling serious challenges in terms of safety risks. A better understanding of electrolytes and solid electrolyte interphase (SEI) formation are highly important in order to improve these issues.

In this thesis, density functional theory (DFT) and molecular dynamics (MD) are used to explore novel electrolyte systems and the interfacial chemistry of electrolyte/Li metal surfaces. In the first part, the electronic structure and possible decompositions pathways of organic carbonates at the Li metal surface are investigated, which provide information about initial SEI formation. Computed X-ray photoelectron spectroscopy (XPS) for these interfacial compounds is used as a tool to find likely electrolyte decomposition pathways and are supported by direct comparison with the experimental results. The electronic structure and computed XPS spectra of electrolyte solvents and the LiNO3 additive on Li metal by DFT provide atomistic insights into the interphase layer.

Solid polymer electrolytes (SPEs) are promising electrolytes to be used with the Li metal electrode. In the second part of the thesis, MD simulations of poly(ethylene oxide) (PEO) doped with LiTFSI salt/Li metal interface demonstrate the impact of the surface on the structure and dynamics of the electrolyte. A new interfacial potential model for MD simulations is also developed for the interactions at the SPE/metal interface, which can better capture this chemical interplay. Moreover, the approach to improve the ionic conductivity of SPEs by adding side-chains to the backbone of polymers is scrutinized through MD simulations of the poly(trimethylene carbonate) (PTMC) system. While providing polymer flexibility, a hindering effects of the side-chains on Li+ ion diffusions through reduced coordination site connectivity is observed.

In the final part, different polymer hosts interacting with Li metal are explored, and rapid decomposition of polycarbonates and polyester on the surface is seen. The complexes of these polymers with LiTFSI and LiFSI showed significant changes in the computed electrochemical stability window and salt degradations. Lastly, Li2O was obtained by DFT calculations as a thermodynamically stable layer on the surface of the Li metal oxidized by PEO.

The modelling studies performed in this thesis highlight the applicability of these techniques in order to probe the SEI and electrolyte properties in LMBs at the atomistic level.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 81
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1830
Keywords
Li-metal battery, solid polymer electrolyte, density functional theory, molecular dynamics simulation, solid electrolyte interphase
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-390066 (URN)978-91-513-0703-9 (ISBN)
Public defence
2019-09-20, Polhemsalen, 10134, Ångstrom Laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-08-30 Created: 2019-08-05 Last updated: 2019-09-17

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Ebadi, MahsaAraujo, C. MoysesBrandell, Daniel

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