Untersuchungen zum Ionentransport in konzentrierten Flüssigelektrolyten für Lithiumionenbatterien
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Philipps-Universität Marburg
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Abstract
Based on the work of Wohde et al., the objective of this research was the investigation
of ion transport and its limitations within both liquid electrolytes and complete battery
cells. Furthermore, interionic interactions and their influence on ion transport parameters
should be examined more closely. Based on this, the theoretical model following the
Onsager formalism should be extented in order to determine the Onsgar transport
coefficients. Knowledge of these coefficients yields nearly all transport parameters as well
as fundamental insight into the microscopic dynamics within the electrolyte solutions.
In the first part of this thesis, the possibility of cation-anion-anticorrelations was added
to the combined formalism of Onsager’s reciprocal relations and linear response theory.
This may cause a lower lithium ion transference number under anion blocking conditions
compared to the related transport number, as well. Based on this, the Onsager formalism
was expanded by additional transport parameters forming a system of equations
which can be used to calculate the Onsager transport coefficients as well as the thermodynamic
factor inserting several measured variables. This system was used to analyse the
well-known electrolyte G4/LiTFSI 1:1. Thus, a strong cation-anion-anticorrelation was
found to cause the very low lithium ion transference number under anion blocking conditions.
Combining these findings with the work of Kashyap et al. leads to the conclusion
that in ideal QILs the lithium ion transference number under anion blocking conditions
must be zero due to the total conservation of momentum. The presence of unbound solvent
molecules or the exchange of complexing molecules among themselves may cause
a transfer of momentum leading to a measurable transference number higher than zero.
These results could be approved by MD simulations. The experimental prove is still missing
at the moment due to great instabilities of the corresponding electrolytes combined
with lithium metal.
The second part discusses the reproduction of the above mentioned results using another
QIL in addition with the attempt to eliminate conservation of momentum as a limitating
factor of ion transport by adding a third ionic species and using IL/salt mixtures. The
chosen electrolytes were characterised electrochemically by determining the ionic conductivity,
the lithium ion transference number under anion blocking conditions and the salt
diffusion coefficient. Analysis of the ionic conductivities shows an Arrhenius behaviour
whereas a decrease with increasing salt concentration for the IL/salt mixtures can be
seen. The QILs show ionic conductivities in the range of the concentrated IL/salt mix-tures.
The examined salt diffusion coefficients exhibit the same trend. For the analysed
QIL, a very low lithium ion transference number under anion blocking conditions could
be determined according to the expectations. Thus, the insight granted by the first part of
this thesis seems to be an intrinsic and universal feature of QILs. In contrast, the IL/salt
mixtures show much higher transference numbers which are even increasing with increasing
salt concentration. Also, their transport and transference numbers show only slight
differences. Therefore, it is concluded that there are interionic interactions present in the
IL/salt mixtures but they are not influencing the transport as much as it is done in case
of the QILs. These findings lead to the conclusion that the conservation of momentum
is not a limiting factor any more for ion transport under anion blocking conditions in an
electrolyte system containing three ionic species.
In the third part, the parameters which limit ion transport and the charging process
within a typical commercial lithium ion battery cell were investigated. Using a mathematical
model by Huang et al. it could be shown that the low frequency resistance of
a commercial lithium ion battery cell is dominated by the lithium ion transport under
anion blocking conditions within the composite electrodes. With a small change to the
transmission line model it is possible to monitor the same low frequency limit. This can be
achieved by substitution of the ionic conductivity by the lithium ion conductivity under
anion blocking conditions as well as considering the chemical capacitance within the active
particle phase. Thus, the overall cell resistance could be estimated for all the electrolytes
even though they are not fully characterised and could not be used within the complete
model. In the following, it could be shown that the QILs are very resistive and can
only achieve charging rates of about 0.3C, whereas the IL/salt mixtures can be charged
with rates up to nearly 1C. Therefore, the IL/salt mixtures may be used as alternative
electrolytes within a lithium ion battery as long as fast charging options are not required.
Review
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Dates
Created: 2019Issued: 2020-07-09Updated: 2020-07-09
Faculty
Fachbereich Chemie
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
Keywords
Lithium ion battery electrolyte solution ion transport conductivity transference number electrochemical impedance spectroscopy Onsager formalism
DFG-subjects
Lithium-Ionen-Batterie Elektrolytlösung Ionentransport Leitfähigkeit Überführungszahl Elektrochemische Impedanzspektroskopie
DDC-Numbers
540
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Sälzer, Fabian (0000-0002-0449-7478): Untersuchungen zum Ionentransport in konzentrierten Flüssigelektrolyten für Lithiumionenbatterien. : Philipps-Universität Marburg 2020-07-09. DOI: https://doi.org/10.17192/z2019.0534.
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This item has been published with the following license: In Copyright