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Non-uniform aging of cycled commercial LiFePO4//graphite cylindrical cells revealed by post-mortem analysis
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
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2014 (English)In: Journal of Power Sources, ISSN 0378-7753, 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.

Place, publisher, year, edition, pages
2014. Vol. 257, 126-137 p.
Keyword [en]
Battery aging, LiFePO4/graphite cells, Hybrid electric vehicle, Synchrotron material characterization, Electrode utilization
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-224557DOI: 10.1016/j.jpowsour.2014.01.105ISI: 000333780000017OAI: oai:DiVA.org:uu-224557DiVA: diva2:717989
Available from: 2014-05-19 Created: 2014-05-14 Last updated: 2015-03-11Bibliographically approved
In thesis
1. Structural Changes in Lithium Battery Materials Induced by Aging or Usage
Open this publication in new window or tab >>Structural Changes in Lithium Battery Materials Induced by Aging or Usage
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Li-ion batteries have a huge potential for use in electrification of the transportation sector. The major challenge to be met is the limited energy storage capacity of the battery pack: both the amount of energy which can be stored within the space available in the vehicle (defining its range), and the aging of the individual battery cells (determining how long a whole pack can deliver sufficient energy and power to drive the vehicle). This thesis aims to increase our knowledge and understanding of structural changes induced by aging and usage of the Li-ion battery materials involved.

Aging processes have been studied in commercial-size Li-ion cells with two different chemistries. LiFePO4/graphite cells were aged under different conditions, and thereafter examined at different points along the electrodes by post mortem characterisation using SEM, XPS, XRD and electrochemical characterization in half-cells. The results revealed large differences in degradation behaviour under different aging conditions and in different regions of the same cell. The aging of LiMn2O4-LiCoO2/Li4Ti5O12 cells was studied under two different aging conditions. Post mortem analysis revealed a high degree of Mn/Co mixing within individual particles of the LiMn2O4-LiCoO2 composite electrode.

Structural changes induced by lithium insertion were studied in two negative electrode materials: in Li0.5Ni0.25TiOPO4 using in situ XRD, and in Ni0.5TiOPO4 using EXAFS, XANES and HAXPES. It was shown that Li0.5Ni0.25TiOPO4 lost most of its long-range-order during lithiation, and that both Ni and Ti were involved in the charge compensation mechanism during lithiation/delithiation of Ni0.5TiOPO4, with small clusters of metal-like Ni forming during lithiation.

Finally, in situ XRD studies were also made of the reaction pathways to form LiFeSO4F from two sets of reactants: either FeSO4·H2O and LiF, or Li2SO4 and FeF2. During the heat treatment, Li2SO4 and FeF2 react to form FeSO4·H2O and LiF in a first step. In a second step LiFeSO4F is formed. This underlines the importance of the structural similarities between LiFeSO4F and FeSO4·H2O in the formation process of LiFeSO4F.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 75 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1227
Li-ion batteries, XRD, EXAFS, HAXPES
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
urn:nbn:se:uu:diva-243328 (URN)978-91-554-9165-9 (ISBN)
Public defence
2015-03-27, Å4001, Ångström laboratory, Uppsala, 09:15 (English)
Available from: 2015-03-04 Created: 2015-02-09 Last updated: 2015-03-12Bibliographically approved

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Eriksson, RickardHögstöom, Katarzyna CiosekGustafsson, TorbjörnEdström, Kristina
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