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Electrochemical Studies of Aging in Lithium-Ion Batteries
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0003-4901-5820
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lithium-ion batteries are today finding use in automobiles aiming at reducing fuel consumption and emissions within transportation. The requirements on batteries used in vehicles are high regarding performance and lifetime, and a better understanding of the interior processes that dictate energy and power capabilities is a key to strategic development. This thesis concerns aging in lithium-ion cells using electrochemical tools to characterize electrode and electrolyte properties that affect performance and performance loss in the cells.

 

A central difficulty regarding battery aging is to manage the coupled effects of temperature and cycling conditions on the various degradation processes that determine the lifetime of a cell. In this thesis, post-mortem analyses on harvested electrode samples from small pouch cells and larger cylindrical cells aged under different conditions form the basis of aging evaluation. The characterization is focused on electrochemical impedance spectroscopy (EIS) measurements and physics-based EIS modeling supported by several material characterization techniques to investigate degradation in terms of properties that directly affect performance. The results suggest that increased temperature alter electrode degradation and limitations relate in several cases to electrolyte transport. Variations in electrode properties sampled from different locations in the cylindrical cells show that temperature and current distributions from cycling cause uneven material utilization and aging, in several dimensions. The correlation between cell performance and localized utilization/degradation is an important aspect in meeting the challenges of battery aging in vehicle applications.

 

The use of in-situ nuclear magnetic resonance (NMR) imaging to directly capture the development of concentration gradients in a battery electrolyte during operation is successfully demonstrated. The salt diffusion coefficient and transport number for a sample electrolyte are obtained from Li+ concentration profiles using a physics-based mass-transport model. The method allows visualization of performance limitations and can be a useful tool in the study of electrochemical systems.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , x, 72 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2014:16
Keyword [en]
aging, EIS modeling, electrolyte characterization, graphite, hybrid electric vehicles, impedance spectroscopy, LiFePO4, Li-ion batteries
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-145057ISBN: 978-91-7595-116-4 OAI: oai:DiVA.org:kth-145057DiVA: diva2:715990
Public defence
2014-05-28, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC 20140512

Available from: 2014-05-12 Created: 2014-05-07 Last updated: 2017-02-22Bibliographically approved
List of papers
1. Aging in Lithium-Ion Batteries: Experimental and Model Investigation of Harvested LiFePO4 and Mesocarbon Microbead Graphite Electrodes
Open this publication in new window or tab >>Aging in Lithium-Ion Batteries: Experimental and Model Investigation of Harvested LiFePO4 and Mesocarbon Microbead Graphite Electrodes
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2013 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 110, 335-348 p.Article in journal (Refereed) Published
Abstract [en]

This study investigates aging in LiFePO4/mesocarbon microbead graphite cells that have been subjected to either a synthetic hybrid drive cycle or calendar aging, at 22 C. The investigation involves detailed examination and comparison of harvested fresh and aged electrodes. The electrode properties are determined using a physics-based electrochemical impedance spectroscopy (EIS) model that is fitted to three-electrode EIS measurements, with input from measured electrode capacity and scanning electrode microscopy (SEM). Results from the model fitting provide a detailed insight to the electrode degradation and is put into context with the behavior of the full cell aging. It was established that calendar aging has negligible effect on cell impedance, while cycle aging increases the impedance mainly due to structural changes in the LiFePO4 porous electrode and electrolyte decomposition products on both electrodes. Further, full-cell capacity fade is mainly a consequence of cyclable lithium loss caused by electrolyte decomposition.

Keyword
Lithium-ion battery, Aging, EIS modeling, LiFePO4, Graphite
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-122395 (URN)10.1016/j.electacta.2013.05.081 (DOI)000329530300044 ()2-s2.0-84888320795 (Scopus ID)
Funder
StandUpSwedish Energy Agency
Note

 QC 20140120. Updated from "Accepted" to "Published".

Available from: 2013-05-20 Created: 2013-05-20 Last updated: 2017-12-06Bibliographically approved
2. Altered electrode degradation with temperature in LiFePO4/mesocarbon microbead graphite cells diagnosed with impedance spectroscopy
Open this publication in new window or tab >>Altered electrode degradation with temperature in LiFePO4/mesocarbon microbead graphite cells diagnosed with impedance spectroscopy
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2014 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 141, 173-181 p.Article in journal (Other academic) Published
Abstract [en]

Electrode degradation in LiFePO4 / mesocarbon microbead graphite (MCMB) pouch cells aged at 55 °C by a synthetic hybrid drive cycle or storage is diagnosed and put into context with previous results of aging at 22 °C. The electrode degradation is evaluated by means of electrochemical impedance spectroscopy (EIS), measured separately on electrodes harvested from the cells, and by using a physics-based impedance model for aging evaluation. Additional capacity measurements, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) are used in the evaluation. At 55 °C the LiFePO4 electrode shows increased particle/electronic conductor resistance, for both stored and cycled electrodes. This differs from results obtained at 22 °C, where the electrode suffered lowered porosity, particle fracture, and loss of active material. For graphite, only cycling gave a sustained effect on electrode performance at 55 °C due to lowered porosity and changes of surface properties, and to greater extent than at low temperature. Furthermore, increased current collector resistance also contributes to a large part of the pouch cell impedance when aged at increased temperatures. The result shows that increased temperature promotes different degradation on the electrode level, and is an important implication for high temperature accelerated aging. In light of the electrode observations, the correlation between full-cell and electrode impedances is discussed.

Keyword
battery aging;temperature;electrode impedance;EIS modeling;LiFePO4/MCMB graphite
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-145053 (URN)10.1016/j.electacta.2014.06.081 (DOI)000343022900023 ()2-s2.0-84905868311 (Scopus ID)
Funder
Swedish Energy Agency
Note

Updated from "Manuscript" to "Journal". QC 20141112

Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2017-12-05Bibliographically approved
3. Non-uniform aging of cycled commercial LiFePO4//graphite cylindrical cells revealed by post-mortem analysis
Open this publication in new window or tab >>Non-uniform aging of cycled commercial LiFePO4//graphite cylindrical cells revealed by post-mortem analysis
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2014 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 257, 126-137 p.Article in journal (Refereed) Published
Abstract [en]

Aging of power-optimized commercial 2.3 Ah cylindrical LiFePO4//graphite cells to be used in hybrid electric vehicle is investigated and compared for three different aging procedures; (i) using a simulated hybrid electric vehicle cycle within a narrow SOC-range, (ii) using a constant-current cycle over a 100% SOC-range, and (iii) stored during three years at 22 degrees C. Postmortem analysis of the cells is performed after full-cell electrochemical characterization and discharge. EIS and capacity measurements are made on different parts of the disassembled cells. Material characterization includes SEM, EDX, HAXPES/XPS and XRD. The most remarkable result is that both cycled cells displayed highly uneven aging primarily of the graphite electrodes, showing large differences between the central parts of the jellyroll compared to the outer parts. The aging variations are identified as differences in capacity and impedance of the graphite electrode, associated with different SEI characteristics. Loss of cyclable lithium is mirrored by a varying degree of lithiation in the positive electrode and electrode slippage. The spatial variation in negative electrode degradation and utilization observed is most likely connected to gradients in temperature and pressure, that can give rise to current density and potential distributions within the jellyroll during cycling.

Keyword
Battery aging, LiFePO4/graphite cells, Hybrid electric vehicle, Synchrotron material characterization, Electrode utilization
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-145047 (URN)10.1016/j.jpowsour.2014.01.105 (DOI)000333780000017 ()2-s2.0-84894504735 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20140509

Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2017-12-05Bibliographically approved
4. Uneven film formation across depth of porous graphite electrodes from cycling in commercial Li-ion batteries
Open this publication in new window or tab >>Uneven film formation across depth of porous graphite electrodes from cycling in commercial Li-ion batteries
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(English)Manuscript (preprint) (Other academic)
Keyword
aging, graphite, EIS modeling, Li-ion batteries, SEI
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-145054 (URN)
Note

QS 2014

Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2014-05-20Bibliographically approved
5. Quantifying mass transport during polarization in a Li Ion battery electrolyte by in situ 7Li NMR imaging
Open this publication in new window or tab >>Quantifying mass transport during polarization in a Li Ion battery electrolyte by in situ 7Li NMR imaging
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2012 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 134, no 36, 14654-14657 p.Article in journal (Refereed) Published
Abstract [en]

Poor mass transport in the electrolyte of Li ion batteries causes large performance losses in high-power applications such as vehicles, and the determination of transport properties under or near operating conditions is therefore important. We demonstrate that in situ 7Li NMR imaging in a battery electrolyte can directly capture the concentration gradients that arise when current is applied. From these, the salt diffusivity and Li + transport number are obtained within an electrochemical transport model. Because of the temporal, spatial, and chemical resolution it can provide, NMR imaging will be a versatile tool for evaluating electrochemical systems and methods.

Keyword
Electrolytes, Magnetic resonance imaging, Transport properties
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-103644 (URN)10.1021/ja305461j (DOI)000308574800006 ()2-s2.0-84866398560 (Scopus ID)
Funder
Swedish Research CouncilStandUp
Note

QC 20150624

Available from: 2012-10-18 Created: 2012-10-17 Last updated: 2017-12-07Bibliographically approved

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