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The Complex Nature of the Electrode/Electrolyte Interfaces in Li-ion Batteries: Towards Understanding the Role of Electrolytes and Additives Using Photoelectron Spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The stability of electrode/electrolyte interfaces in Li-ion batteries is crucial to the performance, lifetime and safety of the entire battery system. In this work, interface processes have been studied in LiFePO4/graphite Li-ion battery cells. 

The first part has focused on improving photoelectron spectroscopy (PES) methodology for making post-mortem battery analyses. Exposure of cycled electrodes to air was shown to influence the surface chemistry of the graphite. A combination of synchrotron and in-house PES has facilitated non-destructive interface depth profiling from the outermost surfaces into the electrode bulk. A better understanding of the chemistry taking place at the anode and cathode interfaces has been achieved. The solid electrolyte interphase (SEI) on a graphite anode was found to be thicker and more inhomogeneous than films formed on cathodes. Dynamic changes in the SEI on cycling and accumulation of lithium close to the carbon surface have been observed.   

Two electrolyte additives have also been studied: a film-forming additive propargyl methanesulfonate (PMS) and a flame retardant triphenyl phosphate (TPP). A detailed study was made at ambient and elevated temperature (21 and 60 °C) of interface aging for anodes and cathodes cycled with and without the PMS additive. PMS improved cell capacity retention at both temperatures. Higher SEI stability, relatively constant thickness and lower loss of cyclable lithium are suggested as the main reasons for better cell performance. PMS was also shown to influence the chemical composition on the cathode surface.

The TPP flame retardant was shown to be unsuitable for high power applications. Low TPP concentrations had only a minor impact on electrolyte flammability, while larger amounts led to a significant increase in cell polarization. TPP was also shown to influence the interface chemistry at both electrodes.

Although the additives studied here may not be the final solution for improved lifetime and safety of commercial batteries, increased understanding has been achieved of the degradation mechanisms in Li-ion cells. A better understanding of interface processes is of vital importance for the future development of safer and more reliable Li-ion batteries.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 74 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1129
Keyword [en]
Li-ion battery, LiFePO4/graphite cell, interface, electrolyte additives, solid electrolyte interphase (SEI), photoelectron spectroscopy (PES), synchrotron
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-219336ISBN: 978-91-554-8890-1 (print)OAI: oai:DiVA.org:uu-219336DiVA: diva2:699240
Public defence
2014-04-11, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-03-20 Created: 2014-02-26 Last updated: 2014-04-29
List of papers
1. Consequences of air exposure on the lithiated graphite SEI
Open this publication in new window or tab >>Consequences of air exposure on the lithiated graphite SEI
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2013 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 105, 83-91 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, consequences of air exposure on the surface composition of one of the most reactive lithium-ion battery components, the lithiated graphite, was investigated using 280-835 eV soft X-ray photoelectron spectroscopy (SOXPES) as well as 1486.7 eV X-ray photoelectron spectroscopy (XPS) (similar to 2 and similar to 10 nm probing depth, respectively). Different depth regions of the solid electrolyte interphase (SEI) of graphite cycled vs. LiFePO4 were thereby examined. Furthermore, the air sensitivity of samples subject to four different combinations of pre-treatments (washed/unwashed and exposed to air before or after vacuum treatment) was explored. The samples showed important changes after exposure to air, which were found to be largely dependent on sample pre-treatment. Changes after exposure of unwashed samples exposed before vacuum treatment were attributed to reactions involving volatile species. On washed, air exposed samples, as well as unwashed samples exposed after vacuum treatment, effects attributed to lithium hydroxide formation in the innermost SEI were observed and suggested to be associated with partial delithiation of the surface region of the lithiated graphite electrode. Moreover, effects that can be attributed to LiPF6 decomposition were observed. However, these effects were less pronounced than those attributed to reactions involving solvent species and the lithiated graphite. 

Keyword
XPS, SEI, Graphite, Air, LiPF6
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-208068 (URN)10.1016/j.electacta.2013.04.118 (DOI)000322414800012 ()
Available from: 2013-09-24 Created: 2013-09-23 Last updated: 2017-12-06Bibliographically approved
2. The Buried Carbon/Solid Electrolyte Interphase in Li-ion Batteries Studied by Hard X-ray Photoelectron Spectroscopy
Open this publication in new window or tab >>The Buried Carbon/Solid Electrolyte Interphase in Li-ion Batteries Studied by Hard X-ray Photoelectron Spectroscopy
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2014 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 138, 430-436 p.Article in journal (Refereed) Published
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-219332 (URN)10.1016/j.electacta.2014.06.129 (DOI)000341464000054 ()
Available from: 2014-02-26 Created: 2014-02-26 Last updated: 2017-12-05
3. Comparing anode and cathode electrode/electrolyte interface composition and morphology using soft and hard X-ray photoelectron spectroscopy
Open this publication in new window or tab >>Comparing anode and cathode electrode/electrolyte interface composition and morphology using soft and hard X-ray photoelectron spectroscopy
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2013 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 97, 23-32 p.Article in journal (Refereed) Published
Abstract [en]

Electrode/electrolyte interface depth profiling was performed on lithiated graphite and delithiated lithium iron phosphate electrodes after electrochemical cycling in a balanced full cell configuration containing a carbonate based LiPF6 electrolyte. The profiling was performed by synchrotron radiation based hard X‑ray photoelectron spectroscopy, HAXPES, and soft X‑ray photoelectron spectroscopy, SOXPES. In this way, the probing depth was varied over a wide range in the order of 2-50 nm. Both more surface and more bulk sensitive investigations than possible using traditional in-house X‑ray photoelectron spectroscopy (XPS) could thus be performed. The composition and morphology of the lithiated graphite anode/electrolyte interface (solid electrolyte interphase, SEI) and the delithiated lithium iron phosphate cathode/electrolyte interface (solid permeable interface, SPI) were compared. In the vicinity of the highly reductive graphite active material in the SEI, low binding energy components like Li2O were found while no obvious composition gradients were observed in the SPI. Both in the cathode SPI and the anode SEI, significant amounts of C-O and P‑F containing compounds were found to deposit during cycling. Evidence for mixing of the porous binder and other SEI/SPI components was observed in both the anode and cathode electrode/electrolyte interfaces. The lithiated graphite SEI was estimated to be of the order of two tens of nanometers, while the cathode SPI thickness was estimated to a few nanometers only. 

National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-196469 (URN)10.1016/j.electacta.2013.03.010 (DOI)000319024500004 ()
Available from: 2013-03-08 Created: 2013-03-08 Last updated: 2017-12-06
4. The influence of PMS-additive on the electrode/electrolyte interfaces in LiFePO4/graphite Li-ion batteries
Open this publication in new window or tab >>The influence of PMS-additive on the electrode/electrolyte interfaces in LiFePO4/graphite Li-ion batteries
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2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 45, 23476-23486 p.Article in journal (Refereed) Published
Abstract [en]

The influence of a film-forming additive, propargyl methanesulfonate (PMS), on electrochemical performance and electrode/electrolyte interface composition of LiFePO4/graphite Li-ion batteries has been studied. Combined use of in-house X-ray photoelectron spectroscopy (XPS) and soft and hard X-ray photoelectron spectroscopy (PES) enabled nondestructive depth profiling at four different probing depths in the 2-50 nm range. Cells cycled with PMS and LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC) were compared to a reference sample cycled without PMS. In the first cycle, PMS cells showed a higher irreversible capacity, which is explained by formation of a thicker solid electrolyte interphase (SEI). After three cycles, the SET thicknesses were determined to be 19 and 25 nm for the reference and PMS samples, respectively. After the initial cycling, irreversible losses shown by the PMS cells were lower than those of the reference cell. This could be attributed to a different SET composition and lower differences in the amount of lithium between lithiated and delithiated electrodes for the PMS sample. It was suggested that PMS forms a triple-bonded radical on reduction, which further reacts with the electrolyte. The PMS additive was shown to influence the chemical composition at the positive electrode/electrolyte interface. Thicker interface layers with higher C-O and smaller LiF contributions were formed on LiFePO4 cycled with PMS.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-197151 (URN)10.1021/jp4045385 (DOI)000327110500005 ()
Available from: 2013-03-18 Created: 2013-03-18 Last updated: 2017-12-06
5. Aging of electrode/electrolyte interfaces in LiFePO4/graphite cells cycled with and without PMS additive
Open this publication in new window or tab >>Aging of electrode/electrolyte interfaces in LiFePO4/graphite cells cycled with and without PMS additive
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2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 24, 12649-12660 p.Article in journal (Refereed) Published
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-219334 (URN)10.1021/jp502732j (DOI)000337783900009 ()
Available from: 2014-02-26 Created: 2014-02-26 Last updated: 2017-12-05Bibliographically approved
6. Impact of the flame retardant additive triphenyl phosphate (TPP) on the performance of graphite/LiFePO4 cells in high power applications
Open this publication in new window or tab >>Impact of the flame retardant additive triphenyl phosphate (TPP) on the performance of graphite/LiFePO4 cells in high power applications
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2014 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 256, 430-439 p.Article in journal (Refereed) Published
Abstract [en]

This study presents an extensive characterization of a standard Li-ion battery (LiB) electrolyte containing different concentrations of the flame retardant triphenyl phosphate (TPP) in the context of high power applications. Electrolyte characterization shows only a minor decrease in the electrolyte flammability for low TPP concentrations. The addition of TPP to the electrolyte leads to increased viscosity and decreased conductivity. The solvation of the lithium ion charge carriers seem to be directly affected by the TPP addition as evidenced by Raman spectroscopy and increased mass-transport resistivity. Graphite/LiFePO4 full cell tests show the energy efficiency to decrease with the addition of TPP. Specifically, diffusion resistivity is observed to be the main source of increased losses. Furthermore, TPP influences the interface chemistry on both the positive and the negative electrode. Higher concentrations of TPP lead to thicker interface layers on LiFePO4. Even though TPP is not electrochemically reduced on graphite, it does participate in SEI formation. TPP cannot be considered a suitable flame retardant for high power applications as there is only a minor impact of TPP on the flammability of the electrolyte for low concentrations of TPP, and a significant increase in polarization is observed for higher concentrations of TPP.

National Category
Engineering and Technology
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-219330 (URN)10.1016/j.jpowsour.2014.01.022 (DOI)000333724100057 ()
Available from: 2014-02-26 Created: 2014-02-26 Last updated: 2017-12-05Bibliographically approved

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Ciosek Högström, Katarzyna

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