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Insights in Li-ion Battery Interfaces through Photoelectron Spectroscopy Depth Profiling
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1041
Keyword [en]
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
Identifiers
URN: urn:nbn:se:uu:diva-197250ISBN: 978-91-554-8662-4 (print)OAI: oai:DiVA.org:uu-197250DiVA: diva2:615942
Public defence
2013-05-24, Amphithéâtre de l'IPREM, 2 avenue du Président Pierre Angot, Pau, France, 10:00 (English)
Opponent
Supervisors
Available from: 2013-05-03 Created: 2013-03-20 Last updated: 2013-08-30Bibliographically approved
List of papers
1. Nanosilicon Electrodes for Lithium-Ion Batteries: Interfacial Mechanisms Studied by Hard and Soft X-ray Photoelectron Spectroscopy
Open this publication in new window or tab >>Nanosilicon Electrodes for Lithium-Ion Batteries: Interfacial Mechanisms Studied by Hard and Soft X-ray Photoelectron Spectroscopy
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2012 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 24, no 6, 1107-1115 p.Article in journal (Refereed) Published
Abstract [en]

Largely based on its very high rechargeable capacity, silicon appears as an ideal candidate for the next generation of negative electrodes for Li-ion batteries. However, a crucial problem with silicon is the large volume expansion undergone upon alloying with lithium, which results in stability problems. Means to avoid such problems are largely linked to the understanding of the interfacial chemistry during charging/discharging. This is especially of great importance when using nanometric silicon particles. In this work, the interfacial mechanisms (reaction of surface oxide, Li-Si alloying process, and passivation layer formation) accompanying lithium insertion/extraction into Si/C/CMC composite electrodes have been scrutinized by Xray photoelectron spectroscopy (XPS). A thorough nondestructive depth-resolved analysis was carried out by using both soft X-rays (100-800 eV) and hard X-rays (2000-7000 eV) from two different synchrotron facilities compared with in-house XPS (1487 eV). The unique combination utilizing hard and soft X-ray photoelectron spectroscopy accompanied with variation of the analysis depth allowed us to access interfacial phase transitions at the surface of silicon particles as well as the composition and thickness of the SEI (electrode/electrolyte interface layer).

Keyword
lithium-ion batteries, silicon, alloy, SEI, XPS, PES, synchrotron
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-173341 (URN)10.1021/cm2034195 (DOI)000301947000020 ()
Funder
StandUp
Available from: 2012-04-24 Created: 2012-04-23 Last updated: 2017-12-07Bibliographically approved
2. Role of the LiPF6 Salt for the Long-Term Stability of Silicon Electrodes in Li-Ion Batteries: A Photoelectron Spectroscopy Study
Open this publication in new window or tab >>Role of the LiPF6 Salt for the Long-Term Stability of Silicon Electrodes in Li-Ion Batteries: A Photoelectron Spectroscopy Study
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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
Keyword
lithium-ion batteries, silicon, alloy, SEI, XPS, PES, synchrotron
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-196593 (URN)10.1021/cm303399v (DOI)000315018500016 ()
Available from: 2013-03-11 Created: 2013-03-11 Last updated: 2017-12-06Bibliographically approved
3. Improved performances of nanosilicon electrodes using the salt LiFSI: A photoelectron spectroscopy study
Open this publication in new window or tab >>Improved performances of nanosilicon electrodes using the salt LiFSI: A photoelectron spectroscopy study
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2013 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 135, no 26, 9829-9842 p.Article in journal (Refereed) Published
Abstract [en]

Silicon is a very good candidate for the next generation of negative electrodes for Li-ion batteries, due to its high rechargeable capacity. An important issue for the implementation of silicon is the control of the chemical reactivity at the electrode/electrolyte interface upon cycling, especially when using nanometric silicon particles. In this work we observed improved performances of Li//Si cells by using the new salt lithium bis(fluorosulfonyl)imide (LiFSI) with respect to LiPF6. The interfacial chemistry upon long-term cycling was investigated by photoelectron spectroscopy (XPS or PES). A nondestructive depth resolved analysis was carried out by using both soft X-rays (100–800 eV) and hard X-rays (2000–7000 eV) from two different synchrotron facilities and in-house XPS (1486.6 eV). We show that LiFSI allows avoiding the fluorination process of the silicon particles surface upon long-term cycling, which is observed with the common salt LiPF6. As a result the composition in surface silicon phases is modified, and the favorable interactions between the binder and the active material surface are preserved. Moreover a reduction mechanism of the salt LiFSI at the surface of the electrode could be evidenced, and the reactivity of the salt toward reduction was investigated using ab initio calculations. The reduction products deposited at the surface of the electrode act as a passivation layer which prevents further reduction of the salt and preserves the electrochemical performances of the battery.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2013
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-198221 (URN)10.1021/ja403082s (DOI)000321541800045 ()
Available from: 2013-04-10 Created: 2013-04-10 Last updated: 2017-12-06Bibliographically approved
4. MnSn2 electrodes for Li-ion batteries: Mechanisms at the nano scale and electrode/electrolyte interface
Open this publication in new window or tab >>MnSn2 electrodes for Li-ion batteries: Mechanisms at the nano scale and electrode/electrolyte interface
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2014 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 123, 72-83 p.Article in journal (Refereed) Published
Abstract [en]

We have investigated the reaction mechanisms occurring upon the first discharge/charge cycle of a MnSn2//Li electrochemical cell, by using bulk- and surface-sensitive characterization techniques (Xray Diffraction, Sn-119 Mossbauer spectroscopy, magnetic measurements, X-ray photoelectron and Auger spectroscopies). Compared to other tin-transition metal alloys, MnSn2 displays an original behaviour. Lithium insertion into MnSn2 particles results in a nanocomposite consisting of Li7Sn2 phase, and of Mn nanoparticles which are immediately oxidized at their surface. Lithium extraction from this nanocomposite leads to the formation of magnetic MnSn2 particles and to our knowledge it is the first time such a mechanism is observed in tin-based intermetallic electrode materials due to electrochemical reaction with Li. The solid electrolyte interphase (SEI) is formed at the beginning of the first discharge and its thickness slightly increases upon further lithium insertion. A partial re-dissolution process occurs upon lithium extraction from the material, while its chemical composition is very stable over the whole cycle.

Place, publisher, year, edition, pages
elsevier: , 2014
Keyword
Lithium-ion batteries, MnSn2, Tin, Intermetallics, Mössbauer, XPS, Magnetism
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-198264 (URN)10.1016/j.electacta.2014.01.010 (DOI)000334898800010 ()
Available from: 2013-04-11 Created: 2013-04-11 Last updated: 2017-12-06Bibliographically approved
5. Electrochemical performances and mechanisms of MnSn2 as anode material for Li-ion batteries
Open this publication in new window or tab >>Electrochemical performances and mechanisms of MnSn2 as anode material for Li-ion batteries
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2013 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 244, 246-251 p.Article in journal (Refereed) Published
Abstract [en]

A synthesis method consisting of a mechanical ball milling activation process followed by a sinteringheating treatment is proposed to obtain MnSn2 as anode material for Li-ion batteries. This two-stepapproach strongly reduces the amount of bSn impurities and provides a better material morphology.This improves the electrochemical performances, even at high C-rate, as shown from the comparisonbetween electrode materials obtained with and without this preliminary activation process. The electrochemicalreactions have been followed at the atomic scale by in situ 119Sn Mössbauer spectroscopy.The first discharge is a restructuring step that transforms the pristine material into Mn/Li7Sn2 nanocompositewhich should be considered as the real starting material for cycling. The delithiation of thisnanocomposite is characterized by two plateaus of potential attributed to the de-alloying of Li7Sn2 followedby the back reaction of Mn with poorly lithiated LixSn alloys, respectively. The composition and thestability of the solid electrolyte interphase were characterized by X-ray photoelectron spectroscopy.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-197842 (URN)10.1016/j.jpowsour.2013.01.110 (DOI)
Conference
16th International Meeting on Lithium Batteries (IMLB)
Available from: 2013-04-04 Created: 2013-04-04 Last updated: 2017-12-06Bibliographically approved

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