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Abstract
In the first part of this dissertation, thin and dense Li2O2 layers were electrochemically
grown on glassy carbon electrodes. The electronic and ionic transport of these layers was
studied with a combined approach of electrochemical impedance spectroscopy and atomic
force microscopy measurements. Impedance spectroscopy allowed the determination of
the layers’ ionic and electronic resistances, as well as the layers’ geometric and double
layer capacitances during growth. Finally, after finishing the electrochemical experiment,
the thickness of the Li2O2 layers was determined by means of atomic force microscopy.
As the final thickness of the layers and their geometric capacitance were known, the
permittivity could be calculated. In return, this allowed the monitoring of the layers’
time-dependent growth, the thickness-dependent decrease of the ionic conductivity, and
the time dependent-increase of the resistances.
The exponential increase of both the ionic and the electronic resistances with the
thicknessgave strong indication that the obstruction of the charge transport could not be
explained based on the coverage of insulating Li
2O2
on the glassy carbon electrode. The
thickness of the layers is proportional to the charge. The latter would be proportional to
the surface coverage if the layers grew in a non-uniform way. Hence, one would expect
a hyperbolic dependence of the electronic resistance on the thickness in the log-plane
for a surface-coverage dependent scenario. Furthermore, by analyzing the slopes of the
thickness-dependent increase of resistance, tunneling was ruled out as a mechanism for
charge transport through the layers. However, it was pointed out that field-dependent,
non-linear charge transport of both, lithium ions and electron holes, is a likely explanation
for the observed phenomenon. Non-linear Lithium ion migration is caused by strong
gradients in the electric field, whereas electron holes are subject to non-linear effects
because of a strong gradient in chemical potential.
In the second part of this dissertation, the ionic transport in a solid-state composite
anode was studied. These anodes contained different volume fractions of the glassy solid
electrolyte Li7P2S8I (bulk ionic conductivity σion,electrolyte
= (0, 75 ± 0, 1) mS · cm−1),
the active material lithium titanate (Li7Ti5O12), and the conducting additive Super
C-65 (carbon black). While the volume fraction of the conducting additive was kept
constant throughout the whole study, the volume fractions of the active material and
the solid electrolyte were varied. The ionic transport was probed by transmission-linetype-measurements and by measurement of the stationary Li
+
-current under electron
blocking conditions. For volume fractions of the solid electrolyte ε ≥ 0, 4 both methods
were in good agreement. Furthermore, in this porosity region the tortuosity/porosity
relation behaved as predicted by the Bruggeman relation. However, there was a strong
deviation from the Bruggeman relation at lower volume fractions of the solid electrolyte
ε. Moreover, the transmission-line model turned out to be inaccurate for low ε. In this
publication it was demonstrated that all solid state lithium ion batteries can be built
such that their performance is comparable to liquid electrolyte counterparts. The ionic
conductivity of an all solid composite electrode can be as high as systems that employ
commercial electrodes and liquid electrolytes, if highly conductive materials are used.
In the third part of this dissertation, an all solid state composite cathode was examined
with two different methods. First the stationary Li+-current through the electrode was
measured under electron blocking conditions by means of impedance spectroscopy. Here,
the ionic resistance of the composite cathode was determined. Using optical microscopy,
the thickness of the electrode was determined and an effective tortuosity was calculated.
Furthermore, the expected tortuosity for an electrode with spherical particles and the
same volume fractions of electrolyte ε was calculated by means of the Bruggeman relation. The experimentally determined effective tortuosity τ
ef f
deviated by 25% from the
calculated value. In order to find an explanation for this deviation, the electrode was examined by means of focused-ion-beam/scanning-electron-microscopy, which generated a
tomography. The investigated volume was reconstructed in a computer-based approach.
A geometrical analysis and simulations were conducted on the reconstructed volume.
Random-walk simulations allowed the determination of the tracer diffusion coefficient in
the electrolyte phase within the composite electrode. The effective tortuosity was determined by comparing diffusion inside the electrode with unhindered diffusion in the bulk
electrolyte. The effective tortuosity found by simulation was in very good agreement with
the experimentally determined one. However, there was a lot of void space in the reconstructed volume of the electrode. These voids were artificially filled with electrolyte in
another random-walk simulation. This way the effective tortuosity was calculated for a
void-free electrode. On this instance, the effective tortuosity was in good agreement with
the Bruggeman relation, but was significantly smaller than the one that accounted for
the voids. Furthermore, the geometric analysis of the reconstructed sample showed that
important characteristic lengths are shortened by the presence of voids. Altogether, it
was shown that voids significantly limit the power of all solid state composite electrode
because highly tortuous conducting paths are created. This problem does not exist in
liquid cells because the electrolyte permeates through the porous electrodes. In order to
build all solid state batteries that outperform their liquid counterparts, the void problem
must be solved.
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Contributors
Supervisor:
Dates
Created: 2018Issued: 2018-12-10Updated: 2018-12-10
Faculty
Fachbereich Chemie
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
Keywords
TortuositätImpedanzspektroskopietortuositytrnasmission line modelall solid state batteries,impedance spectroscopyTransmission-Line ModelFestkörperbatterienIonentransportion transport
DFG-subjects
Transmission-Line ModelFestkörperbatterienImpedanzspektroskopieTortuositätIonentransport
DDC-Numbers
540
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Kaiser, Nico: Untersuchungen zu Transportlimitierungen in Batterieelektroden. : Philipps-Universität Marburg 2018-12-10. DOI: https://doi.org/10.17192/z2018.0121.