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Avoiding ageing: Surface degradation of commercial electrode materials in lithium-ion batteries
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry. Uppsala universitet.ORCID iD: 0000-0002-2736-9145
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The battery market today expands rapidly, not least for electric vehicles. But to compete against the combustion engine, the cost of batteries must be reduced. After years of usage, the batteries degrade and need to be exchanged, increasing the cost over the vehicle lifecycle. This can be mitigated by tailoring the usage conditions and the battery materials. Understanding and avoiding ageing can be key to a more sustainable transport system. This thesis contains studies on degradation processes in Li-ion batteries utilizing the LiNixMnyCozO2 (NMC) cathode material, and suggests strategies for the improvement of battery life time.

When cycling different negative electrodes – including graphite, lithium foil and lithium titanium oxide (LTO) – against NMC electrodes, only minor capacity fading was observed in the NMC-LTO and NMC-graphite cells, in contrast to the NMC-Li-metal cells. The capacity fading for Li-metal cells was determined to be caused by degradation products formed at the lithium foil which thereafter diffused to the NMC electrode, leading to a higher resistance. Commercial NMC/LiMn2O4-graphite cells were also investigated after cycling in limited state of charge (SOC)-intervals. The cycle life was far longer in the low-SOC cell than in the high-SOC cell. Photoelectron spectroscopy revealed increased manganese dissolution in the high-SOC cell, likely causing a less stable solid electrolyte interphase layer on the negative electrode. This, in turn, limits the capacity. How temperature influence ageing in NMC-LTO was analysed in cells cycled at -10 °C, 30 °C and 55 °C. It was found that the initial side reactions at the LTO electrode limited the cell capacity, but that these also stabilized the NMC electrode. At 55 °C, excessive side reactions at LTO caused capacity fading due to loss of active lithium. At -10 °C, high cell resistance limited the capacity. Switching to a PC based electrolyte allowed stable low temperature cycling, although it was found that PC degraded and formed thick electrode surface layers. Also sulfolane-based electrolytes were investigated, showing thinner surface layers than the EC containing reference electrolyte at high potentials, thus indicating a more stable electrolyte system.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 72
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1802
Keywords [en]
Li-ion battery, Ageing, Photoelectron spectroscopy, Nickel Manganese Cobalt Oxide
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-381548ISBN: 978-91-513-0639-1 (print)OAI: oai:DiVA.org:uu-381548DiVA, id: diva2:1303904
Public defence
2019-06-05, Room 4001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-05-09 Created: 2019-04-11 Last updated: 2019-06-17
List of papers
1. How the Negative Electrode Influences Interfacial and Electrochemical Properties of LiNi1/3Co1/3Mn1/3O2 Cathodes in Li-Ion Batteries
Open this publication in new window or tab >>How the Negative Electrode Influences Interfacial and Electrochemical Properties of LiNi1/3Co1/3Mn1/3O2 Cathodes in Li-Ion Batteries
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2017 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 164, no 13, p. A3054-A3059Article in journal (Refereed) Published
Abstract [en]

The cycle life of LiNi1/3Co1/3Mn1/3O2 (NMC) based cells are significantly influenced by the choice of the negative electrode. Electrochemical testing and post mortem surface analysis are here used to investigate NMC electrodes cycled vs. either Li-metal, graphite or Li4Ti5O12 (LTO) as negative electrodes. While NMC-LTO and NMC-graphite cells show small capacity fading over 200 cycles, NMC-Li-metal cell suffers from rapid capacity fading accompanied with an increased voltage hysteresis despite the almost unlimited access of lithium. X-ray absorption near edge structure (XANES) results show that no structural degradation occurs on the positive electrode even after >200 cycles, however, X-ray photoelectron spectroscopy (XPS) results shows that the composition of the surface layer formed on the NMC cathode in the NMC-Li-metal cell is largely different from that of the other NMC cathodes (cycled in the NMC-graphite or NMC-LTO cells). Furthermore, it is shown that the surface layer thickness on NMC increases with the number of cycles, caused by continuous electrolyte degradation products formed at the Li-metal negative electrode and then transferred to NMC positive electrode.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-338161 (URN)10.1149/2.0711713jes (DOI)000418409800021 ()
Funder
Swedish Energy Agency, 37725-1; 40495-1StandUp
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2019-04-11Bibliographically approved
2. Temperature dependence of electrochemical degradation in LiNi0.33Mn0.33Co0.33O2/Li4Ti5O12 cells
Open this publication in new window or tab >>Temperature dependence of electrochemical degradation in LiNi0.33Mn0.33Co0.33O2/Li4Ti5O12 cells
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2019 (English)In: Energy Technology, ISSN 2194-4288, Vol. 7, no 9Article in journal (Refereed) Published
Abstract [en]

Aging mechanisms in lithium-ion batteries are dependent on the operational temperature, but the detailed mechanisms on what processes take place at what temperatures are still lacking. The electrochemical performance and capacity fading of the common cell chemistry LiNi1/3Mn1/3Co1/3O2 (NMC)/Li4Ti5O12 (LTO) pouch cells are studied at temperatures 10, 30, and 55 degrees C. The full cells are cycled with a moderate upper cutoff potential of 4.3 V versus Li+/Li. The electrode interfaces are characterized postmortem using photoelectron spectroscopy techniques (soft X-ray photoelectron spectroscopy [SOXPES], hard X-ray photoelectron spectroscopy [HAXPES], and X-ray absorption near edge structure [XANES]). Stable cycling at 30 degrees C is explained by electrolyte reduction forming a stabilizing interphase, thereby preventing further degradation. This initial reaction, between LTO and the electrolyte, seems to be beneficial for the NMC-LTO full cell. At 55 degrees C, continuous electrolyte reduction and capacity fading are observed. It leads to the formation of a thicker surface layer of organic species on the LTO surface than at 30 degrees C, contributing to an increased voltage hysteresis. At 10 degrees C, large cell-resistances are observed, caused by poor electrolyte conductivity in combination with a relatively thicker and LixPFy-rich surface layer on LTO, which limit the capacity.

Keywords
aging, lithium-ion batteries, photoelectron spectroscopy
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-381545 (URN)10.1002/ente.201900310 (DOI)
Available from: 2019-04-11 Created: 2019-04-11 Last updated: 2019-10-10Bibliographically approved
3. Investigation of dimethyl carbonate and propylene carbonate mixtures for LiNi0.6Mn0.2Co0.2O2-Li4Ti5O12 cells
Open this publication in new window or tab >>Investigation of dimethyl carbonate and propylene carbonate mixtures for LiNi0.6Mn0.2Co0.2O2-Li4Ti5O12 cells
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2019 (English)In: Chemelectrochem, E-ISSN 2196-0216, Vol. 6, no 13, p. 3429-3436Article in journal (Refereed) Published
Abstract [en]

It has recently been shown that ethylene carbonate (EC) experience poor stability at high potentials in lithium-ion batteries, and development of electrolytes without EC, not least using ethyl methyl carbonate (EMC), has therefore been suggested in order to improve the capacity retention. In this context, we here explore another alternative electrolyte system consisting of propylene carbonate (PC) and dimethyl carbonate (DMC) mixtures in NMC-LTO (LiNi0.6Mn0.2Co0.2O2, Li4Ti5O12) cells cycled up to 2.95 V. While PC experience wettability problems and DMC has difficulties dissolving LiPF6 salt, blends between these could possess complementary properties. The electrolyte blend showed superior cycling performance at sub-zero temperatures compared to EC-containing counterparts. At 30 degrees C, however, the PC-DMC electrolyte did not show any major improvement in electrochemical properties for the NMC-LTO cell chemistry. Photoelectron spectroscopy measurements showed that thin surface layers were detected on both NMC (622) and LTO electrodes in all investigated electrolytes. The results suggest that both PC and EC will react on the electrodes, but with EC forming thinner layers comprising more carbonates. Moreover, the electrochemical stability at high electrochemical potentials is similar for the studied electrolytes, which is surprising considering that most are free from the reactive EC component.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-381546 (URN)10.1002/celc.201900672 (DOI)000475512500026 ()
Funder
Swedish Energy Agency, 37725-1StandUp
Available from: 2019-04-11 Created: 2019-04-11 Last updated: 2019-08-19Bibliographically approved
4. Sulfolane based EC-free electrolytes in high voltage cells
Open this publication in new window or tab >>Sulfolane based EC-free electrolytes in high voltage cells
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-381547 (URN)
Available from: 2019-04-11 Created: 2019-04-11 Last updated: 2019-04-11
5. Influence of state-of-charge in commercial LiNi0.33Mn0.33Co0.33O2/LiMn2O4-graphite cells analyzed by synchrotron-based photoelectron spectroscopy
Open this publication in new window or tab >>Influence of state-of-charge in commercial LiNi0.33Mn0.33Co0.33O2/LiMn2O4-graphite cells analyzed by synchrotron-based photoelectron spectroscopy
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2018 (English)In: Journal of Energy Storage, ISSN 2352-152X, Vol. 15, p. 172-180Article in journal (Refereed) Published
Abstract [en]

Degradation mechanisms in 26 Ah commercial Li-ion battery cells comprising graphite as the negative electrode and mixed metal oxide of LiMn2O4 (LMO) and LiNi1/3Mn1/3Co1/3O2 (NMC) as the positive electrode are here investigated utilising extensive cycling at two different state-of-charge (SOC) ranges, 10–20% and 60–70%, as well as post-mortem analysis. To better analyze these mechanisms electrochemically, the cells were after long-term cycling reassembled into laboratory scale “half-cells” using lithium metal as the negative electrode, and thereafter cycled at different rates corresponding to 0.025 mA/cm2 and 0.754 mA/cm2. The electrodes were also analyzed by synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES) using two different excitation energies to determine the chemical composition of the interfacial layers formed at different depth on the respective electrodes. It was found from the extensive cycling that the cycle life was shorter for the cell cycled in the higher SOC range, 60–70%, which is correlated to findings of an increased cell resistance and thickness of the SEI layer in the graphite electrode as well as manganese dissolution from the positive electrode.

Keywords
Li-ion battery, Commercial cells, Battery ageing, Photoelectron spectroscopy
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-338184 (URN)10.1016/j.est.2017.11.010 (DOI)000426619500015 ()
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2019-04-11Bibliographically approved

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