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Role of the LiPF6 Salt for the Long-Term Stability of Silicon Electrodes in Li-Ion Batteries: A Photoelectron Spectroscopy Study
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
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2013 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 25, no 3, 394-404 p.Article in journal (Refereed) Published
Abstract [en]

Silicon presents a very high theoretical capacity (3578 mAh/g) and appears as a promising candidate for the next generation of negativeelectrodes for Li-ion batteries. An important issue for the implementation ofsilicon is the understanding of the interfacial chemistry taking place duringcharge/discharge since it partly explains the capacity fading usually observedupon cycling. In this work, the mechanism for the evolution of the interfacialchemistry (reaction of surface oxide, Li−Si alloying process, and passivationlayer formation) upon long-term cycling has been investigated byphotoelectron spectroscopy (XPS or PES). A nondestructive depth resolved analysis was carried out by using both soft Xrays(100−800 eV) and hard X-rays (2000−7000 eV) from two different synchrotron facilities. The results are compared withthose obtained with an in-house spectrometer (1486.6 eV). The important role played by the LiPF6 salt on the stability of thesilicon electrode during cycling has been demonstrated in this study. A partially fluorinated species is formed upon cycling at theoutermost surface of the silicon nanoparticles as a result of the reaction of the materials toward the electrolyte. We have shownthat a similar species is also formed by simple contact between the electrolyte and the pristine electrode. The reactivity betweenthe electrode and the electrolyte is investigated in this work. Finally, we also report in this work the evolution of the compositionand covering of the SEI upon cycling as well as proof of the protective role of the SEI when the cell is at rest.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2013. Vol. 25, no 3, 394-404 p.
Keyword [en]
lithium-ion batteries, silicon, alloy, SEI, XPS, PES, synchrotron
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
URN: urn:nbn:se:uu:diva-196593DOI: 10.1021/cm303399vISI: 000315018500016OAI: oai:DiVA.org:uu-196593DiVA: diva2:610408
Available from: 2013-03-11 Created: 2013-03-11 Last updated: 2013-04-29Bibliographically approved
In thesis
1. Insights in Li-ion Battery Interfaces through Photoelectron Spectroscopy Depth Profiling
Open this publication in new window or tab >>Insights in Li-ion Battery Interfaces through Photoelectron Spectroscopy Depth Profiling
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Compounds forming alloys with lithium, such as silicon or tin, are promising negative electrode materials for the next generation of Li-ion batteries due to their higher theoretical capacity compared to the current commercial electrode materials.

An important issue is to better understand the phenomena occurring at the electrode/electrolyte interfaces of these new materials. The stability of the passivation layer (SEI) is crucial for good battery performance and its nature, formation and evolution have to be investigated. It is important to follow upon cycling alloying/dealloying processes, the evolution of surface oxides with battery cycling and the change in surface chemistry when storing electrodes in the electrolyte.

The aim of this thesis is to improve the knowledge of these surface reactions through a non-destructive depth-resolved PES (Photoelectron spectroscopy) analysis of the surface of new negative electrodes. A unique combination utilizing hard and soft-ray photoelectron spectroscopy allows by variation of the photon energy an analysis from the extreme surface (soft X-ray) to the bulk (hard X-ray) of the particles. This experimental approach was used to access the interfacial phase transitions at the surface of silicon or tin particles as well as the composition and thickness/covering of the SEI.

Interfacial mechanisms occurring upon the first electrochemical cycle of Si-based electrodes cycled with the classical salt LiPF6 were investigated.

The mechanisms of Li insertion (LixSi formation) have been illustrated as well as the formation of a new irreversible compound, Li4SiO4, at the outermost surface of the particles. Upon long cycling, the formation of SiOxFy was shown at the extreme surface of the particles by reaction of SiO2 with HF contributing to battery capacity fading.

The LiFSI salt, more stable than LiPF6, improved the electrochemical performances. This behaviour is correlated to the absence of SiOxFy upon long-term cycling. Some degradation of LiFSI was shown by PES and supported by calculations.

Finally, interfacial reactions occurring upon the first cycle of an intermetallic compound MnSn2 were studied. Compared to Si based electrodes, the SEI chemical composition is similar but the alloying process and the role played by the surface metal oxide are different.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 200 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1041
Lithium-ion batteries, negative electrodes, silicon, MnSn2, SEI, PES, XPS, synchrotron
National Category
Materials Chemistry Physical Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
urn:nbn:se:uu:diva-197250 (URN)978-91-554-8662-4 (ISBN)
Public defence
2013-05-24, Amphithéâtre de l'IPREM, 2 avenue du Président Pierre Angot, Pau, France, 10:00 (English)
Available from: 2013-05-03 Created: 2013-03-20 Last updated: 2013-08-30Bibliographically approved

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Philippe, BertrandRensmo, HåkanEdström, Kristina
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