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@PHDTHESIS{Fritsch:472121,
author = {Fritsch, Charlotte},
othercontributors = {Ehrenberg, Helmut and Powell, Annie},
title = {{M}icrostructural characterization and multiscale ionic
conductivity in lithium and sodium-based solid state
electrolytes},
school = {Karlsruhe Institute of Technology},
type = {Dissertation},
publisher = {KIT},
reportid = {PUBDB-2021-04844},
pages = {156},
year = {2021},
note = {Dissertation, Karlsruhe Institute of Technology, 2021},
abstract = {In order to meet the increasing need for energy storage
systems in consumer devices, electric vehicles and
stationary energy storage devices, the existing battery
technologies are constantly being further developed. An
important criterion for the performance of the battery is
its energy density. Even if the cathode accounts mostly for
the weight of a lithium ion battery cell, there is a
promising possibility of weight saving on the anode side by
replacing graphite with pure lithium metal. To this end, an
electrolyte needs to be developed that is stable against the
potential of the pure metal. Liquid electrolytes cannot meet
this requirement and also involve safety risks as
ammability. Solid-state electrolytes are supposed to enable
the use of a lithium-metal electrode. Additionally, due to
the scarcity of resources for lithium, sodium-based
technologies are being developed for use as stationary
energy storage devices. Thiophosphates feature the highest
ionic conductivities among all solid electrolytes. In many
cases, amorphous thiophosphates offer even higher
conductivities than their crystalline analogues and their
structure can differ, too. A structural investigation of
amorphous compounds may not be possible with a standard
X-ray analysis. However their local structure has to be
enlightened so that a reproducible synthesis can take place.
The solid electrolyte Na2P2S6 was synthesized via ball
milling in an amorphous state with subsequent
crystallization. The structure of the crystalline phase
differs markedly compared to the corresponding amorphous
phase. A combination of XRD-PDF analysis and 23Na/31P MAS
NMR spectroscopy measurements indicate that single PS30-4
tetrahedra and corner-sharing tetrahedra are transformed to
edge-sharing-tetrahedra during crystallization of amorphous
Na2P2S6 to crystalline Na2P2S6. Impedance spectroscopy shows
that amorphous Na2P2S6 has a conductivity of 5.710-8 S cm-1
which is three orders of magnitude higher than crystalline
Na2P2S6 (2.6 10-11 S cm-1). The higher conductivity can also
be recovered by ball milling crystalline Na2P2S6, inducing a
reamorphization. Lithium Lanthanum Zirconium oxide (LLZO)
exhibits a high ionic conductivity and stability against
lithium metal and is therefore a promising candidate as
solid state electrolyte. Yet, specifcations on its
conductivity are often not reliable and spread widely.
Attempts are made to attribute the differences in reported
conductivities to the different substituents, sintering
times or surface passivations. A microstructural comparison
of four differently substituted samples is performed to
elucidate the reasons for the different conductivities.
X-ray diffraction revealed that commercial LLZO samples
crystallize in the hydrogarnet structure (space group No.
220), which is described for the first time with a
substituent on Zr and La sites. Ball milling of Al3+, Nb5+,
Ta5+ and W6+ substituted LLZO results in a phase
transformation from the garnet structure into the
hydrogarnet structure with a lower symmetry. The
distribution of lithium ions in the hydrogarnet structure
differs from that in the garnet structure which was
investigated with 6Li MAS NMR and neutron diffraction. A
targeted conversion of the hydrogarnet structure into the
garnet structure is proved by calcining the material at 1100
°C for 10 h. With high-temperature X-ray diffraction, an
low thermal expansion of the hydrogarnet unit cell is
observed in comparison to the greater expansion of the
garnet unit cell. The ionic mobility of Li ions in the
hydrogarnet structure is examined by means of NMR, in
particular line shape analysis, relaxometry and pulsed-field
gradient NMR. This combination of techniques shows that the
mobility of lithium is significantly reduced on small length
scales. In combination with the structural analysis, this
can be traced back to the high occupancy of the Li3 position
in the hydogarnet structure, blocking long-range lithium
diffusion. However, it was not possible to access the
long-range mobility of Li in the hydrogarnet structure (at
25 °C). Therefore, the contribution of the ceramic
component to the total ionic conductivity of polymer
composite electrolytes is evaluated. The question whether
the long-range lithium mobility in the hydrogarnet structure
is lower compared to the garnet structure is assessed
without the necessity for sintering the LLZO to pellets.
Impedance spectroscopy shows a conductivity of 1.2 10-6 S
cm-1 for a composite electrolyte with a hydrogarnet
structure and 3.4 10-6 S cm-1 for a composite electrolyte
with a garnet structure. The higher Li mobility of the
garnet-based composite electrolyte compared to the
hydrogarnet-based electrolyte is verified with PFG-NMR
measurements of the diffusion coeffcient: 6.1 10-14 m2 s-1
(garnet), resp. 1.1 10-14 m2 s-1 (hydrogarnet). The measured
activation energy of dffusion is also higher in the
hydrogarnet composite. The conductivity results measured
with impedance spectroscopy are compared with commercial
composite electrolytes; a SiO2-ceramic-polymer and a purely
polymer-based electrolyte. The next step from optimizing a
solid state electrolyte in terms of ionic conductivity is to
look at its compatibility with the electrodes, here the
cathode. It is tested whether ball milling of LLZO with the
established cathode material Lithium Nickel Cobalt Manganese
oxide (NCM) results in a good contact of the two materials
and consequently a low Li ion diffusion barrier. The
interface between LLZO and NCM is investigated by X-ray
diffraction, 6Li MAS NMR and transmission electron
microscopy. A model system consisting of LLZO and NCM is
characterized with impedance spectroscopy for a lithium
diffusion barrier sandwiched between an auxiliary
electrolyte in order to separate the ionic conductivity from
the electrical. An evaluation of only the ionic conductivity
apart from the electrical conductivity is not possible due
to the high electrical conductivity of the auxiliary
electrolyte. The electrolyte Lithium Aluminum Titanium
Phosphate (LATP) is examined crystallographically against
the background of upscaling of the synthesis. If the process
is upscaled, local inhomogeneities of the educts can be
expected in a way that varying educt contents have an effect
on the product. This applies especially to phosphoric acid,
the concentration of which cannot be specified precisely due
to its hygroscopy. A Rietveld refinement analysis against
X-ray diffraction data of LATP with varying phosphoric acid
content during synthesis is performed. An excess of
phosphoric acid leads to the formation of AlPO4, which
impedes the ionic conductivity. Insufficient phosphoric acid
causes the formation LiTiOPO4. TiO2 is formed in this
material after a second sintering step. The findings in this
work contribute the understanding of structural changes in
solid electrolytes during processing and thus contribute to
the improvement of future solidstate batteries.},
keywords = {solid state electrolyte (Other) / garnet electrolyte
(Other) / hydrogarnet (Other)},
cin = {DOOR ; HAS-User / KIT},
cid = {I:(DE-H253)HAS-User-20120731 / I:(DE-H253)KIT-20130928},
pnm = {6G3 - PETRA III (DESY) (POF4-6G3)},
pid = {G:(DE-HGF)POF4-6G3},
experiment = {EXP:(DE-H253)P-P02.1-20150101},
typ = {PUB:(DE-HGF)11},
doi = {10.5445/IR/1000133273},
url = {https://bib-pubdb1.desy.de/record/472121},
}
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