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A Quest for the Unseen: Surface Layer Formation on Li4Ti5O12 Li-Ion Battery Anodes
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The electric vehicle itself today outlives its battery, necessitating battery replacement. Lithium titanium oxide (LTO) has, in this context, been suggested as a new anode material in heavy electric vehicle applications due to intrinsic properties regarding safety, lifetime and availability.

The work presented here is focused on the LTO electrode/electrolyte interface. Photoelectron spectroscopy (PES) has been applied to determine how and if the usage of LTO could prevent extensive anode-side electrolyte decomposition and build-up of a surface layer. The presence of a solid electrolyte interphase (SEI) comprising LiF, carbonates and ether compounds was found in half-cells utilizing a standard ethylene:diethylcarbonate electrolyte with 1 M LiPF6. Via testing of symmetrical LTO-LTO cells, the stability of the formed SEI was put in to question. Moreover, the traditional polyvinylidene difluoride (PVdF) binder was replaced by more environmentally benign carboxylmethyl cellulose (CMC) and polyacrilic acid (PAA) binders in LTO electrodes, and it was found that CMC helped to form a more stable surface-layer that proved beneficial for long term cycling.

Following the half-cell studies, full-cells were investigated to observe how different cathodes influence the SEI of LTO. The SEI in full-cells displayed characteristics similar to the half-cells, however, when utilizing a high voltage LiNi0.5Mn1.5O4 cathode, more electrolyte decomposition could be observed. Increasing the operational temperature of this battery cell generated even more degradation products on the LTO electrodes. Mn was also found on the anode when using Mn-based cathodes, however, it was found in its ionic state and did not significantly affect the composition or behavior of the observed SEI layer. Furthermore, by exchanging the electrolyte solvent for propylene carbonate, the thickness of the SEI increased, and by replacing the LiPF6 salt for LiBF4 the stability of the SEI improved. Thus is it demonstrated that such a passivation can be beneficial for the long-term surface stability of the electrode. These findings can therefore help prolong the lifetime of LTO-based battery chemistries.

Place, publisher, year, edition, pages
Uppsala: Uppsala University, 2017. , p. 67
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1576
Keywords [en]
SEI, LTO, XPS, PES, Surface Layer, Titanate
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-331349ISBN: 978-91-513-0105-1 (print)OAI: oai:DiVA.org:uu-331349DiVA, id: diva2:1148939
Public defence
2017-12-01, Häggsalen, Ångström, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2017-11-10 Created: 2017-10-13 Last updated: 2018-03-07
List of papers
1. Depth profiling the solid electrolyte interphase on lithium titanate (Li4Ti5O12) using synchrotron-based photoelectron spectroscopy
Open this publication in new window or tab >>Depth profiling the solid electrolyte interphase on lithium titanate (Li4Ti5O12) using synchrotron-based photoelectron spectroscopy
2015 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Journal of Power Sources, Vol. 294, p. 173-179Article in journal (Refereed) Published
Abstract [en]

The presence of a surface layer on lithium titanate (Li4Ti5O12, LTO) anodes, which has been a topic of debate in scientific literature, is here investigated with tunable high surface sensitive synchrotron-basedphotoelectron spectroscopy (PES) to obtain a reliable depth profile of the interphase. LijjLTO cells with electrolytes consisting of 1 M lithium hexafluorophosphate dissolved in ethylene carbonate:diethyl carbonate (LiPF6 in EC:DEC) were cycled in two different voltage windows of 1.0e2.0 V and 1.4e2.0 V. LTO electrodes were characterized after 5 and 100 cycles. Also the pristine electrode as such, and an electrode soaked in the electrolyte were analyzed by varying the photon energies enabling depth profiling of the outermost surface layer. The main components of the surface layer were found to be ethers, PeO containing compounds, and lithium fluoride.

Keywords
Li-ion batteries, LTO, PES, XPS, Surface layer, SEI
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-331106 (URN)10.1016/j.jpowsour.2015.06.038 (DOI)
Funder
StandUp
Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-01-03
2. Different Shades of Li4Ti5O12 Composites: The Impact of the Binder on Interface Layer Formation
Open this publication in new window or tab >>Different Shades of Li4Ti5O12 Composites: The Impact of the Binder on Interface Layer Formation
Show others...
2017 (English)In: ChemElectroChem, ISSN 2196-0216, Vol. 4, no 10, p. 2683-2692Article in journal (Refereed) Published
Abstract [en]

Replacing the traditional PVdF(-HFP) electrode binder by water-soluble alternatives can potentially render electrode fabrication more environmentally benign. Herein, the surface layer formation of stored and cycled samples of two water-based Li4Ti5O12 composites employing either poly(sodium acrylate) (PAA-Na) or sodium carboxymethyl cellulose (CMC-Na) as binders are studied by X-ray photoelectron spectroscopy. In all three formulations, the surface layer composition formed upon storage differed notably from the solid-electrolyte interphase (SEI) layer formed on cycled samples. The surface layer under open-circuit conditions seems to originate mostly from the electrolyte salt (LiPF6) degradation. The comparison with cycled samples after 10 and 100 cycles shows a continuous build-up of an SEI layer on PAA-Na and PVdF-HFP electrodes. In contrast, on CMC-Na containing electrodes the SEI composition remains nearly unchanged. The results correlate well with the electrochemical behavior.

National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-331111 (URN)10.1002/celc.201700395 (DOI)000412892600036 ()
Funder
StandUpSwedish Research Council, 20123837
Available from: 2017-10-10 Created: 2018-02-05 Last updated: 2018-02-06Bibliographically approved
3. Manganese in the SEI layer of Li4Ti5O12 studied using combined NEXAFS and HAXPES techniques
Open this publication in new window or tab >>Manganese in the SEI layer of Li4Ti5O12 studied using combined NEXAFS and HAXPES techniques
Show others...
2016 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 6, p. 3206-3213Article in journal (Refereed) Published
Abstract [en]

A combination of hard X-ray photoelectron spectroscopy (HAXPES) and near edge X-ray absorption fine structure (NEXAFS) are here used to investigate the presence and chemical state of crossover manganese deposited on Li-ion battery anodes. The synchrotron based experimental techniques-using HAXPES and NEXAFS analysis on the same sample in one analysis chamber-enabled us to acquire complementary sets of information. The Mn crossover and its influence on the anode interfacial chemistry has been a topic of controversy in the literature. Cells comprising lithium manganese oxide (LiMn2O4, LMO) cathodes and lithium titanate (Li4Ti5O12, LTO) anodes were investigated using LP40 (1 M LiPF6, EC:DEC 1:1) electrolyte. LTO electrodes at lithiated, delithiated, and open circuit voltage (OCV-stored) states were analyzed to investigate the potential dependency of the manganese oxidation state. It was primarily found that a solid surface layer was formed on the LTO electrode and that this layer contains deposited Mn from the cathode. The results revealed that manganese is present in the ionic state, independent of the lithiation of the LTO electrode. The chemical environment of the deposited manganese could not be assigned to simple compounds such as fluorides or oxides, indicating that the state of manganese is in a more complex form.

National Category
Other Chemical Engineering
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-267788 (URN)10.1021/acs.jpcc.5b11756 (DOI)000370678700012 ()
Funder
Swedish Energy Agency
Available from: 2015-11-26 Created: 2015-11-26 Last updated: 2017-12-01Bibliographically approved
4. Understanding the capacity loss in LNMO-LTO lithium-ion cells at ambient and elevated temperatures
Open this publication in new window or tab >>Understanding the capacity loss in LNMO-LTO lithium-ion cells at ambient and elevated temperatures
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract
Keywords
High voltage spinel, cross-talk, electrode interactions, LNMO-LTO, Mn dissolution
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-331117 (URN)
Funder
StandUp
Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-02-05
5. Surface Layer Formation on Li4Ti5O12 Electrodes in Li-ion Cells with Propylene Carbonate-Based Electrolyte
Open this publication in new window or tab >>Surface Layer Formation on Li4Ti5O12 Electrodes in Li-ion Cells with Propylene Carbonate-Based Electrolyte
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The increasing usage of lithium titanate (Li4Ti5O12; LTO) as anode material in Li-ion batteries creates challenges and possibilities for the electrolytes. Traditional Li-ion battery electrolytes are tailored primarily for chemistries that use graphite as anode, and are therefore not optimized for LTO electrodes. In this study, propylene carbonate is, together with LiPF6 salt, investigated as an alternative electrolyte system in such batteries. The LTO surface is investigated with photoelectron spectroscopy after the formation cycle and after 10 cycles to characterize the decomposition products. The results show the presence of a surface layer formed on the LTO electrode irrespective of counter electrode used, but with varying thickness. Contrary to conventional ethylene carbonate based electrolytes, no manganese crossover could be observed when using either lithium manganese oxide or lithium nickel manganese oxide/lithium cobalt oxide composite as cathodes in LTO cells.

Keywords
Li-ion battery, LTO anode, Interface layer, Propylene carbonate, Photoelectron spectroscopy
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-331116 (URN)
Funder
StandUp
Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-02-05

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