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Si negative electrodes for Li-ion batteries: Aging mechanism studies by electrochemistry and photoelectron spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focusing on the challenges when using Si as a possible new negative electrode material in Li-ion batteries. The overall aim is to contribute to a general understanding of the processes in the Si electrode, to identify aging mechanisms, and to evaluate how they influence the cycling performance. Another objective is to investigate how photoelectron spectroscopy (PES) can be used to analyze these mechanisms.

LiPF6 based electrolytes are aggressive towards the oxide layer present at the surface of the Si particles. With the use of fluoroethylene carbonate (FEC) as an electrolyte additive the cycling performance is improved, but the oxide layer is still affected. A recently developed salt, lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), is shown not to have any detrimental effects on the oxide. The SEI with FEC and vinylene carbonate (VC) as contains a high concentration of LiF and polymeric carbonate species and this composition seems to be beneficial for the cycling performance, but the results indicate that additional aging mechanisms occur. Therefore, electrochemical analysis is performed and confirms a continuous SEI formation. However, it also reveals a self-discharge mechanism and that a considerable amount of Li is remaining in the Si material after standard cycling.

PES is used in this work to analyze the SEI-layers as well as the surface and the bulk of the Si material. With this technique it is hence possible to distinguish changes in the Si material as a function of lithiation. To improve the data interpretation of PES spectra, a range of battery electrode model systems are investigated. These results show shifts of the SEI peaks relative to the electrode specific peaks as a result of the SEI thickness and the presence of a dipole layer. Also other electronically insulating composite electrode components show relative peak shifts as a function of the electrochemical potential.

To summarize, these studies investigate a number of well recognized aging mechanisms in detail and also establish additional processes contributing to aging in Si electrodes. Furthermore, this work highlights phenomena that influence data interpretation of PES measurements from battery materials.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 67 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1362
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-281694ISBN: 978-91-554-9533-6 (print)OAI: oai:DiVA.org:uu-281694DiVA: diva2:915755
Public defence
2016-06-02, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2016-05-09 Created: 2016-03-29 Last updated: 2016-05-12
List of papers
1. Improved Performance of the Silicon Anode for Li-Ion Batteries: Understanding the Surface Modification Mechanism of Fluoroethylene Carbonate as an Effective Electrolyte Additive
Open this publication in new window or tab >>Improved Performance of the Silicon Anode for Li-Ion Batteries: Understanding the Surface Modification Mechanism of Fluoroethylene Carbonate as an Effective Electrolyte Additive
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2015 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 27, no 7, 2591-2599 p.Article in journal (Refereed) Published
Abstract [en]

Silicon as a negative electrode material for lithium-ion batteries has attracted tremendous attention due to its high theoretical capacity, and fluoroethylene carbonate (FEC) was used as an electrolyte additive, which significantly improved the cyclability of silicon-based electrodes in this study. The decomposition of the FEC additive was investigated by synchrotron-based X-ray photoelectron spectroscopy (PES) giving a chemical composition depth-profile. The reduction products of FEC were found to mainly consist of LiF and -CHF-OCO2-type compounds. Moreover, FEC influenced the lithium hexafluorophosphate (LiPF6) decomposition reaction and may have suppressed further salt degradation. The solid electrolyte interphase (SEI) formed from the decomposition of ethylene carbonate (EC) and diethyl carbonate (DEC), without the FEC additive present, covered surface voids and lead to an increase in polarization. However, in the presence of FEC, which degrades at a higher reduction potential than EC and DEC, instantaneously a conformal SEI was formed on the silicon electrode. This stable SEI layer sufficiently limited the emergence of large cracks and preserved the original surface morphology as well as suppressed the additional SEI formation from the other solvent. This study highlights the vital importance of how the chemical composition and morphology of the SEI influence battery performance.

National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:uu:diva-253257 (URN)10.1021/acs.chemmater.5b00339 (DOI)000353176100041 ()
Available from: 2015-05-26 Created: 2015-05-25 Last updated: 2017-12-04Bibliographically approved
2. A hard X-ray photoelectron spectroscopy study on the solid electrolyte interphase of a lithium 4,5-dicyano-2- (trifluoromethyl)imidazolide based electrolyte for Si-electrodes
Open this publication in new window or tab >>A hard X-ray photoelectron spectroscopy study on the solid electrolyte interphase of a lithium 4,5-dicyano-2- (trifluoromethyl)imidazolide based electrolyte for Si-electrodes
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2016 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 301, 105-112 p.Article in journal (Other academic) Published
Abstract [en]

This report focuses on the relatively new salt, lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), and its functionality together with a silicon based composite electrode in a half-cell lithium ion battery context. LiTDI is a promising alternative to the commonly used LiPF6 salt because it does not form HF which can decompose the oxide layer on Si. The formation of a solid electrolyte interphase (SEI) as well as the development of the active Si-particles are investigated during the first electrochemical lithiation and de-lithiation. Characterizations are carried out at different state of charge with scanning electron microscopy (SEM) as well as hard x-ray photoelectron spectroscopy (HAXPES) at two different photon energies. This enables a depth resolved picture of the reaction processes and gives an idea of the chemical buildup of the SEI. The SEI is formed by solvent and LiTDI decomposition products and its composition is similar to SEI formed by other carbonate based electrolytes. The LiTDI salt or its decomposition products are not in itself reactive towards the active Si-material and no unwanted side reactions occurs with the active Si-particles. Despite some decomposition of the LiTDI salt, it is a promising alternative for electrolytes aimed towards Si-based electrodes.

Keyword
Lithium 4, 5-dicyano-2-(trifluoromethyl); imidazolide; Silicon negative electrode; Solid electrolyte interphase; Hard x-ray photoelectron spectroscopy
National Category
Natural Sciences Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-261159 (URN)10.1016/j.jpowsour.2015.09.112 (DOI)000365060500014 ()
Funder
VINNOVA, P37446-1EU, FP7, Seventh Framework ProgrammeEU, FP7, Seventh Framework ProgrammeEU, FP7, Seventh Framework Programme
Available from: 2015-08-31 Created: 2015-08-31 Last updated: 2017-12-04Bibliographically approved
3. SEI formation and Interfacial Stability of a Si electrode in a LiTDI-salt Based Electrolyte with FEC and VC Additives
Open this publication in new window or tab >>SEI formation and Interfacial Stability of a Si electrode in a LiTDI-salt Based Electrolyte with FEC and VC Additives
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-281673 (URN)
Available from: 2016-03-29 Created: 2016-03-29 Last updated: 2016-05-12
4. Aging mechanisms of composite nano-Si electrodes in a Li-ion battery half-cell
Open this publication in new window or tab >>Aging mechanisms of composite nano-Si electrodes in a Li-ion battery half-cell
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-281688 (URN)
Available from: 2016-03-29 Created: 2016-03-29 Last updated: 2016-05-12
5. Electric potential gradient at the buried interface between Lithium-ion battery electrodes and the SEI observed using photoelectron spectroscopy
Open this publication in new window or tab >>Electric potential gradient at the buried interface between Lithium-ion battery electrodes and the SEI observed using photoelectron spectroscopy
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2016 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 7, no 10, 1775-1780 p.Article in journal (Refereed) Published
Abstract [en]

The buried interface between the bulk electrode material and the solid electrolyte interphase (SEI) in cycled Li-ion battery anodes is suggested to incorporate an electric potential gradient. This suggestion is based on photoelectron spectroscopy (PES) results from different anode materials that all show relative binding energy shifts between the components of the SEI and the active anode. Implications of this electric potential gradient on binding energy reference points in PES as well as on charge-transfer kinetics in Li-ion batteries are discussed. Specifically, we show that the separation of surface layer and bulk material spectral contributions (depth profiling) is crucial for consistent data interpretation. We conclude that previous interpretations of lithiation as cause for changes in PES spectra may need to be revised.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-281687 (URN)10.1021/acs.jpclett.6b00391 (DOI)000376421200004 ()27104985 (PubMedID)
External cooperation:
Funder
EU, FP7, Seventh Framework Programme, 608575StandUpSwedish Research Council, VR-2012-4681
Available from: 2016-03-29 Created: 2016-03-29 Last updated: 2017-11-30Bibliographically approved
6. The Influence of Electrochemical Cycling and Side Reactions on Battery Electrode Materials Analyzed by Photoelectron Spectroscopy
Open this publication in new window or tab >>The Influence of Electrochemical Cycling and Side Reactions on Battery Electrode Materials Analyzed by Photoelectron Spectroscopy
(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-281686 (URN)
Available from: 2016-03-29 Created: 2016-03-29 Last updated: 2016-05-12

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