RESEARCH ARTICLE
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Impact of the Solid Electrolyte Particle Size Distribution in
Sulfide-Based Solid-State Battery Composites
Eva Schlautmann, Alexander Weiß, Oliver Maus, Lukas Ketter, Moumita Rana,
Sebastian Puls, Vera Nickel, Christine Gabbey, Christoph Hartnig, Anja Bielefeld,
and Wolfgang G. Zeier*
their intrinsic energy density limits are
approaching, and new technologies need
to be explored to shift these limits further. A possible alternative technology
is solid-state batteries, shown exemplarily in Figure 1. Here, the organic electrolyte is replaced by a solid electrolyte,
for example, Li6 PS5 Cl. These systems
are promising as they are expected to
be able to operate over a wide temperature range and their ability to be employed in contact with lithium metal[1]
or silicon[2,3] as anode material helps
to achieve higher energy densities.[4]
An important performance-limiting
factor of lithium-ion battery systems is
their maximum areal loading.[5] To develop cathodes with high areal capacities
in solid-state batteries, high areal loadings of cathode active material (CAM) are
necessary.[6] However, fast ion and electron transport must be ensured to allow complete utilization of the CAM.[7]
Minnmann et al. showed how high loadings of CAM result in a
tortuous pathway for the mobile lithium-ions, which leads to a
drop in ionic conductivity compared to the bulk material. They
further show that smaller solid electrolyte particles should increase the ionic conductivity and, as a result, increase the rate
All solid-state batteries are promising, as they are expected to offer increased
energy density over conventional lithium-ion batteries. Here, the
microstructure of solid composite electrodes plays a crucial role in
determining the characteristics of ionic and electronic pathways.
Microstructural aspects that impede charge carrier transport can, for
instance, be voids resulting from a general mismatch of particle sizes. Solid
electrolyte materials with smaller particle size distribution represent a
promising approach to limit the formation of voids and to match the smaller
active materials. Therefore, a systematic investigation on the influence of the
solid electrolyte particle size on the microstructural properties, charge carrier
transport, and rate performance is essential. This study provides an
understanding of the influence of the particle sizes of Li6 PS5 Cl on the charge
carrier transport properties and their effect on the performance of solid-state
batteries. In conclusion, smaller Li6 PS5 Cl particles optimize the charge
transport properties and offer a higher interface area with the active material,
resulting in improved solid-state battery performance.
1. Introduction
Access to sufficient electrochemical energy storage is essential
for a successful energy and mobility transition from fossil fuels
to renewable energies. So far, lithium-ion batteries with liquid
electrolytes are the most widely used battery system. However,
E. Schlautmann, O. Maus, L. Ketter, M. Rana, W. G. Zeier
Institute of Inorganic and Analytical Chemistry
University of Münster
Corrensstrasse 28/30, 48149 Münster, Germany
E-mail: wzeier@uni-muenster.de
A. Weiß, A. Bielefeld
Center for Materials Research
Justus Liebig University
Heinrich-Buff-Ring 17, 35392 Giessen, Germany
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/aenm.202302309
© 2023 The Authors. Advanced Energy Materials published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution
and reproduction in any medium, provided the original work is properly
cited.
A. Weiß, A. Bielefeld
Institute of Physical Chemistry
Justus Liebig University
Heinrich-Buff-Ring 17, 35392 Giessen, Germany
S. Puls, W. G. Zeier
Institut für Energie- und Klimaforschung
IEK-12: Helmholtz-Institut Münster
Forschungszentrum Jülich
Corrensstrasse 46, 48149 Münster, Germany
V. Nickel, C. Gabbey, C. Hartnig
AMG Lithium GmbH
Industriepark Höchst, Building B852, 65926 Frankfurt am Main, Germany
DOI: 10.1002/aenm.202302309
Adv. Energy Mater. 2023, 2302309
2302309 (1 of 9)
© 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH
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Figure 1. Scheme of a solid-state battery with tortuous charge transport pathways and microstructures of electrode composites with different solid
electrolyte particle sizes.
capability.[8] In addition, void spaces are present in solid-state
electrodes. In contrast to the liquid electrolyte, solid electrolyte
does not infiltrate small pores and the formation of voids leads
to a decrease in the ionic conductivity. This was demonstrated
by Bielefeld et al. by simulation and comparison of different geometry models of typical CAM arrangements.[9,10] Rana et al.[11]
explored the particle size compatibility of CAM and solid electrolytes for an optimal packing density. Shi et al.[12] used microstructure simulations to show how CAM utilization depends
on different ratios of CAM and solid electrolyte particle sizes, focusing on the influence on electrochemical performance. However, it is still open to how the charge transport correlates to void
space formation and homogeneity as a function of the solid electrolyte particle size and how this impacts the electrochemical performance.
To answer these questions, this work systematically alters the
particle size of Li6 PS5 Cl in cathode composites and focuses on
changes in the microstructure, charge transport, and overall electrochemical performance. LiNi0.83 Co0.11 Mn0.06 O2 (NCM811) is
used as CAM together with Li6 PS5 Cl of different particle size
distributions as the catholyte. The Li6 PS5 Cl is first examined
for structural differences at different particle sizes with X-ray
diffraction and pair distribution function analyses. Rate performance tests and long-term cycling are performed on half cells
to investigate the electrochemical performance of the composite electrodes. The effective electronic and ionic charge transport is determined by impedance spectroscopy using a transmission line model. Finally, the microstructure of the composite electrodes is modeled, and the resulting current density distributions are simulated and compared with the experimental results. This work underlines the importance of adjusting the particle sizes of the solid electrolyte to design an opti-
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mized microstructure for charge transport in solid-state battery
composites.
2. Results and Discussion
To assess the influence of the solid electrolyte particle size distribution on the effective transport in solid-state battery composites, first, the Li6 PS5 Cl needs to be assessed in terms of potential structural and transport differences for different particle
sizes. Figure 2a shows the volumetric particle size distribution
of the different Li6 PS5 Cl samples. The particle size distribution
range of the studied solid electrolytes covered 40, 20, 11, and
4 μm. To emphasize on the complex nature of the particle size
distributions, the prepared composites are herein referred to as
S (D50vol = 4 μm), M (D50vol = 11 μm), L (D50vol = 20 μm),
and XL (D50vol = 40 μm), respectively. This particle size range
was selected to investigate the optimum size ratio of the solid
electrolyte with the CAM, which has a D50 value of 3.4 μm (see
Figure S1, Supporting Information). Additional scanning electron microscopy images and a detailed discussion of the particle size distributions can be found in Figures S2, S16–S18 (Supporting Information). Figure 2b presents the impedance spectra of the Li6 PS5 Cl. Due to the high ionic conductivities, no
process besides the blocking behavior of the electrodes can be
seen and the data is fitted with an R-CPE equivalent circuit.
The total ionic conductivities at room temperature of the different solid electrolytes calculated from the impedance spectra are
2.2 mS cm−1 . The only exception is the 20 μm Li6 PS5 Cl, which
has a conductivity of 2.8 mS cm−1 . Nevertheless, this can be
considered within the measurement and sample uncertainty.[13]
Temperature-dependent impedance measurements show similar
activation barriers for ionic transport (see Figure S3, Supporting
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Figure 2. Characterization of Li6 PS5 Cl with different particle sizes. a) Volumetric cumulative particle size distribution measured by a laser diffraction
particle size analyzer. b) Impedance spectroscopy to determine the ionic conductivity, c) X-ray diffraction patterns, and d) pair distribution functions
suggest no strong differences in the structure and transport at different particle sizes.
Information). All these ionic conductivities are within the range
of the Li6 PS5 Cl argyrodites of 10−3 –10−2 S cm−1 ,[14–16] suggesting no particular influence of the particle size distribution on the
ionic conductivity.
Powder X-ray diffraction (Figure 2c) indicates phase purity of
the Li6 PS5 Cl and no differences in the average structure and lattice parameters can be found for the different particle sizes (see
Figure S4, Supporting Information). As different particle sizes
may exhibit differences in the local structure of the materials,
pair distribution function analyses were performed. The pair distribution functions are found in Figure 2d and their quantitative
analyses by small-box modeling can be found in Figure S5 (Supporting Information). Despite the different particle sizes, no difference in the local structure or coherency can be found. Clearly,
no impact of the particle size of Li6 PS5 Cl on the ionic conductivity as well as the average and local structure is evident. This can
also be demonstrated by the similar Raman spectra (Figure S6,
Supporting information S6).
To investigate the effects of the different particle sizes of
Li6 PS5 Cl in the NCM composite electrode, cathode composites
with 70 wt.% NCM811 and 30 wt.% Li6 PS5 Cl are produced. Scanning electron microscopy cross-sections with energy-dispersive
X-ray spectroscopy (EDS)-mapping of the pressed composites
from Figure 3a indicate a more homogeneous distribution of
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Li6 PS5 Cl for smaller particle sizes compared to larger particle
sizes. Another magnification of the SEM images can be found
in Figure S7 (Supporting Information). Figure 3b shows the
modeled microstructure using the experimental microstructural
inputs. Further details of the simulations and how the digital microstructures are derived can be found on Pages S9–S16
(Supporting Information). The modeled microstructure also indicates a better distribution of Li6 PS5 Cl with smaller particle
sizes of the solid electrolyte. A decrease in void space with decreasing particle size is shown in Table S1 (Supporting Information). The different composites are used to build half-cells with
Li5.5 PS4.5 Cl1.5 as the separator and In/LiIn alloy as the counter
electrode. Li5.5 PS4.5 Cl1.5 was used as the separator because it has
a conductivity of 8 mS cm−1 that ensures a low resistance of the
separator and only a low effect on the performance of the cell.[14]
The cells are either charged and discharged at various C-rates or
at 0.1 C for 50 cycles. Triplicates of each measurement were made
to assure reproducibility (see Figures S8 and S9, Supporting Information) and the resulting cell performance data are shown in
Figure 4, together with their standard deviation.
Figure 4a,b shows the charge and discharge curves at 0.05 C
(j = 0.107 mA cm−2 ) and 1 C (j = 2.14 mA cm−2 ) for the
NCM811 composites with different particle sizes of Li6 PS5 Cl.
The general shape of the potential curve during charging and
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Figure 3. a) Scanning electron microscopy with EDS-mapping of the pressed composites with different particle sizes of Li6 PS5 Cl and b) modeled
microstructures indicate a more homogeneous distribution of solid electrolyte.
discharging at 0.05 C is identical and similar to the literature.[17]
The composite S exhibits the highest initial charge capacity at
247 mAh g−1 (±24 mAh g−1 ). The initial charge capacities of the
composites M and L are similar at 212 mAh g−1 (±4 mAh g−1 ).
The composite XL has the lowest initial charge capacity at
205 mAh g−1 (±4 mAh g−1 ). Solid-state cells with a sulfide solid
electrolyte and with a high nickel-content NCM typically show
initial capacities of more than 200 mAh g−1 .[18] A similar trend
can be observed for the discharge capacities. The composite S
has a discharge capacity of 198 mAh g−1 (±18 mAh g−1 ). The
discharge capacities of the composites M and L are similar at
165 mAh g−1 (±3 mAh g−1 ). The composite XL shows the lowest discharge capacity at 152 mAh g−1 (±3 mAh g−1 ). The difference in the charge and discharge capacity of the initial cycle is often reported and stems from structural rearrangements, chemomechanical effects, and loss of available lithium.[19] The Coulombic efficiencies of the composites are shown in Figures S8 and S9
(Supporting Information). For the charge and discharge curves at
1.0 C, the shape of the discharge curves changes as the particle
sizes increase. The overpotential increases with increasing particle size of Li6 PS5 Cl. This may be due to kinetic limitations such
as lithium-ion transport,[20] but needs to be corroborated below.
Figure 4c presents the discharge capacity over 50 cycles. No difference can be seen in the capacity retention for the composites
with different particle sizes of Li6 PS5 Cl. The initial decrease in
capacity can likely be explained by the formation of solid electrolyte interphase of the decomposition reactions.[21] The resulting phosphate- and sulfate-like species are redox active, which
could explain the small increase in the capacity after ten cycles of
the S and M composites.[21,22] Figure 4d shows that all composites
have constant capacities at all C-rates. However, the rate capability decreases with increasing particle size, which, at this point, we
attribute to the higher overpotential shown in Figure 4b. Clearly,
the reason for the lower overpotential and with-it better cycling
and rate-capability of the smaller catholyte particle sizes needs to
be evaluated.
To do so, electrochemical impedance spectroscopy and DC polarization measurements were carried out to determine the effective conductivity of the cathode composite. Triplicate measure-
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ments of the impedance spectroscopy can be found in Figures
S10 and S11 (Supporting Information). The results of the DC
polarization as a support of the impedance analyses are shown
in Figures S12 and S13 (Supporting Information). The resulting
Nyquist plots of the impedance spectra are shown in Figure 4.
The cathode composite consists of two phases that conduct different species, namely lithium-ions that travel through the Li6 PS5 Cl,
and electrons that are conducted by the CAM. In addition, charge
transfer also takes place at the interface between the phases.
Therefore, a transmission line model is required to describe
the electrochemical system. This is commonly used to describe
porous membranes[23] and was developed by Siroma et al.[24] for
composite electrodes. Recently Minnmann et al.[8] extended it to
sulfide-NCM systems. For these systems, the use of the transmission line model measured in half cells is limited due to the overall
low cell resistances. This results in limitations of the resolution
of the impedances. When cells with higher resistances are used,
the transmission line model can also be used on half cells as has
recently been shown by Hendriks et al.,[7] Miß et al.[25] and Rana
et al.[11]
Figure 5a shows the impedance data of the composites with
different particle sizes of Li6 PS5 Cl in an ionic blocking symmetric cell setup with the resulting “T-type” transmission line model
that helps to extract the effective electronic transport of the composites. The cell impedance ZCC of the ionic blocking cell is described as
ZCC
)
( √
zion +zel
√
−1
cosh L
2z2el zint
zion zel
zint
(𝜔) =
L+
×
( √
)
3
(
)
zion +zel
zion + zel
sinh L
zion + zel 2
z
(1)
int
It depends on the impedance of the electron-conducting phase
zel and the impedance of the ion-conducting phase zion as well as
the impedance of the interface between these two phases zint . L
is the thickness of the composite electrode.[24]
The electronic impedance zel is represented by the electronic
bulk resistance rel and an in-series connected electronic interfacial impedance zel,int of the NCM particles, as previously reported
by Minnmann et al.[8] This electronic interfacial impedance is
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Figure 4. Cell performance tests of composite electrodes with different solid electrolyte particle sizes in half-cell setups. a) First cycle charge and
discharge curves at 0.05 C (j = 0.107 mA cm−2 ). b) Charge and discharge curve at 1 C (j = 2.14 mA cm−2 ). c) Discharge capacities for 50 cycles at 0.1 C
(j = 0.214 mA cm−2 ). d) Discharge capacities of rate-performance test. All data show that with decreasing particle size distribution, higher capacities
and better rate-performance can be obtained.
based on the electronic charge transfer across the NCM–NCM
interface and is represented by a parallel R-CPE unit. The ionic
impedance can be described by an ohmic resistance rion . While
other transmission line model approaches have shown the ionic
path as a resistor in parallel with a constant phase element when
using a solid electrolyte with lower ionic conductivity, the fast
ionic transport of the sulfide electrolyte in this work requires only
a resistor equivalent circuit element.[7] For the system, a nonfaradaic behavior can be assumed as there is no charge transfer
between the ion-conducting and electron-conducting phases in
the symmetric cell. This is represented by a CPEint for the zint. [8]
Figure 5b shows the impedance spectra and the “T-type” transmission line model of the symmetric electronic blocking cells.
Since it represents the ionic pathway, the ionic and electronic
pathways are interchanged. The ionic, electronic, and interface
impedances are represented by the equivalent circuit elements
previously described in Figure 5a. Additionally, the equivalent circuit has a resistor in series with a parallel R-CPE. This represents
the bulk resistance from the electrolyte and the impedance of the
interface of the electrolyte-In/LiIn electrode.
The effective electronic and ionic resistance Rel and Rion are
obtained as follows:
(
)
Rel = L rel,bulk + rel,int
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Rion = L rion
(3)
Comparing the so-determined electronic and ionic resistances
from the impedance analyses with the values determined by DC
polarization measurements (Figures S12 and S13, Supporting Information) shows the validity of both approaches, see Figure S14
(Supporting Information).
From Figure 6a it is evident that the experimental effective ionic conductivity in the composite electrode is enhanced
with decreasing particle size of Li6 PS5 Cl. The experimental
conductivities are similar for the composites L and XL at
0.16 mS cm−1 (±0.03 mS cm−1 ) but increase to 0.25 mS cm−1
(±0.01 mS cm−1 ) in the composite S. Figure 6c shows the simulated current distribution through the composites and illustrates
that this increase in ionic conductivity can be attributed to a more
homogeneous distribution of Li6 PS5 Cl in the composite that
leads to a more uniform distribution of the ionic current density.
volume fraction ⋅bulk conductivity
, is
Here, the tortuosity factor 𝜅, with 𝜅 =
effective conductivity
decreasing with smaller particle sizes (Figure S15, Supporting Information) that is a result of a significantly smaller void space in
composite S than the other composites (see Table S1, Supporting
Information). Despite the same composition in terms of weight
fraction, the overall volume occupied by Li6 PS5 Cl is larger, and
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Figure 5. Nyquist plots of impedance data of composites with different particle sizes of Li6 PS5 Cl and the transmission line model. Impedance spectra measured in a) ionic blocking symmetric cells with electron-electron “T-type” connection for measuring the effective electronic transport and in
b) electronic blocking symmetric cells with “T-type” ion–ion connection for measuring the effective ionic transport.
that in turn has a positive effect on the effective ionic conductivity. Figure 6c further illustrates that the current distribution
in the composites L and XL is rather local. The larger Li6 PS5 Cl
particles exhibit a smaller surface area, so the possibility to form
a well-connected conduction network is smaller. Practically, the
ionic current is forced to pass through a few particle-to-particlepoint contacts that can be regarded as bottlenecks.
Compared to the effective ionic conductivity, its electronic
equivalent shows less dependence on the Li6 PS5 Cl particle size
with 0.40 mS cm−1 (±0.06 mS cm−1 ) for the composite XL and
0.38 mS cm−1 (±0.15 mS cm−1 ) for the composite S, in Figure 6a.
While Minnmann et al[8] experimentally observed a reduction of
the effective electronic conductivity with decreasing solid electrolyte particle size. In contrast, the simulated conductivities decrease with the solid electrolyte particle size. The simulated electronic current distribution in the composites is shown in Figure
S21 (Supporting Information) and exhibits the same trend as the
ionic current distribution. The composite S possesses less voids
and a more homogeneous distribution of NCM811 and Li6 PS5 Cl
that is generally favorable for both, ionic and electronic pathways.
It is unclear why this effect is not evident in the experiment. At
this stage, we assume this to stem from surface effects on the
NCM leading to different resistances between NCM particles as
any changes due to cycling-based decomposition or contact losses
can be ruled out for these symmetric cells.
Taking all experimental data, Figure 6b compares the attainable capacity at 0.05 and 1 C with the ratio of the ionic and electronic conductivities. An increase in capacity can be observed
when the conductivity is balanced and closer to unity. It becomes evident that for a high-performance electrode not only fast
charge transport but also balanced charge transport is essential
as an electrochemical reaction requires both electrons and ions
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to be supplied at comparable rates.[26] A similar effect was recently seen by Hendriks et al. using a LiMn2 O4 /Li3 InCl6 solidstate battery.[7] Concerning the attainable capacity, apart from
the effective conduction pathways, the active interface area also
acts in favor of composites with smaller particle sizes. Due to
their higher surface area, small particle-containing composites
offer more possibilities for contacts and, thus, a higher interface
area. This is underlined by the specific active interface area of
142.7 × 105 m−1 for the composite S that is lowered by a factor of
3.5 for the composite M and decreases to 16.5 × 105 m−1 for the
composite XL (see Figure S22, Supporting Information).
3. Conclusion
This study shows that the particle size distribution of Li6 PS5 Cl
in solid-state battery composites is a key factor in determining
the overall cell performance. In the studied range, the particle
size distribution has no impact on the ionic conductivity and
the structure of pure Li6 PS5 Cl. However, it significantly affects
the performance of NCM811 composite electrodes. Smaller particle sizes of Li6 PS5 Cl result in higher capacities and better rate
performance of the composite electrodes. Smaller Li6 PS5 Cl particle sizes ensure a larger contact interface, which provides more
opportunities for contact between solid electrolytes and CAM.
The effective ionic conductivity in the composite electrode is improved with decreasing particle size by a more homogeneous distribution of Li6 PS5 Cl, resulting in a more uniform distribution of
ionic current density. In addition, the smaller void and larger volume fraction occupied by Li6 PS5 Cl in composites with smaller
particle sizes contributed to the higher effective ionic conductivity and the lower ionic tortuosity factor.
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Figure 6. a) Effective electronic (blue) and ionic (yellow) conductivity of composites with different particle sizes of Li6 PS5 Cl measured with impedance
spectroscopy and compared to simulated conductivities. The electronic conductivity keeps constant while the ionic conductivity increases with smaller
particle size. b) Capacity of composite electrodes at C/20 and 1 C versus the ratio of ionic and electronic conductivity measured by impedance spectroscopy. The initial charge capacity is increasing when the ionic and electronic conductivity are more balanced. c) Comparison of the effective ionic
conductivity from the flux-based simulation for all composites. The small Li6 PS5 Cl particles show a uniform current distribution, while the composite
XL exhibits a localized ionic current flow.
This work highlights the importance of balanced charge transport for high-performance electrodes in solid-state batteries, in
which both fast and balanced transport of electrons and ions
are critical for efficient electrochemical reactions. In the end,
this needs to be achieved by tailoring the composite properties
whenever active materials, additives, or catholyte materials are
changed so that an optimum matching of particle sizes and transport parameters is achieved.
4. Experimental Section
Synthesis of Solid Electrolyte: Li6 PS5 Cl (provided by AMG Lithium) was
used as a catholyte and Li5.5 PS4.5 Cl1.5 as the separator. All synthetic steps
were performed under inert conditions (O2 < 0.1 ppm, H2 O < 0.5 ppm).
The Li5.5 PS4.5 Cl1.5 was synthesized by a solid-state synthesis route as previously reported.[14] A stoichiometric amount of lithium chloride LiCl (LiCl,
anhydrous, Alfa Aesar, 99%) was hand-ground with a mortar and pestle for
15 min. Lithium sulfide (Li2 S, Alfa Aesar, 99.99%) and diphosphorus pentasulfide (P2 S5 , Sigma) were added and ground for another 15 min. The
resulting mixture was densified into pellets and put in a silica ampoule
with a carbon coating, which was then sealed under a vacuum. In the first
reaction step, the ampoule was kept in the furnace for 72 h at 450 °C at
a heating rate of 100 °C h−1 , followed by natural cooling after the reac-
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tion. The ampoule was then opened, and the obtained pellets were handgrounded for 15 min. The powder was pressed into pellets again and the
first reaction step was repeated. After the second heating step, the pellets were ground, and the powder was characterized with X-ray diffraction
and impedance spectroscopy. For the Li6 PS5 Cl, the analysis of the particle
sizes was performed with a static laser diffraction particle size analyzer
from Sympatec Helos.
X-Ray Diffraction: The phase purity of the synthesized Li5.5 PS4.5 Cl1.5
and the Li6 PS5 Cl with different particle sizes was analyzed by X-ray powder diffraction using a StadiP from STOE in Debye-Scherrer geometry with
Cu-K𝛼 radiation (𝜆1 = 1.545051 Å). The samples were analyzed in sealed
0.5 mm capillaries. The measurements were taken at an angle range of
2𝜃 = 10°–70° and a step size of 0.1 and 15 s per step.
Pair Distribution Function Analyses: The Li6 PS5 Cl samples were measured in a sealed glass capillary with an STOE StadiP diffractometer (Ag
K𝛼 1 radiation: 𝜆 = 0.55941 Å, Ge 111 monochromator) in Debye-Scherrer
geometry with four Dectris MYTHEN2 1K detectors. The scattering data
were recorded over a Q-range of 0.8–20.5 Å–1 for 24 h. PDFgetX3 was used
for data reduction with a Q-range cutoff of Qmax = 18 Å−1 .[27] Small box
modeling was performed using TOPAS Academic V7 where 1) scalefactor, 2) correlated motion factor, 3) lattice parameters, 4) atomic positions,
and 5) isotropic atomic displacement parameters were subsequently refined over an r-range of 1.5–40 Å.
Scanning Electron Microscopy: The CAM, Li6 PS5 Cl samples, and the
surface of the pressed composites were analyzed by scanning electron
microscopy with EDS. The samples and the pressed composites were attached to the sample holder with double-sided carbon tape and coated
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with gold. The scanning electron microscopy was carried out at Carl Zeiss
AURIGA CrossBeam working station (accelerating voltage of 3 kV) and an
In-lens detector. The EDS was performed with an X-Max 80mm2detector
(accelerating voltage 15 kV).
Preparation of Electrode Composites: Cathode composites were prepared by mixing lithium nickel cobalt manganese oxide (, MSE supplies,
dried overnight in a B-585 oven (Büchi) in dynamic vacuum at 250 °C) and
the respective Li6 PS5 Cl (AMG Lithium) in a weight ratio of 70:30. The two
components were transferred into a 15 mL ZrO2 cup with 3 mm sized zirconia spheres and mixed using a frequency ball mill (Fritsch pulverisette
23 Mini Mill) at a frequency of 15 Hz for 15 min.
Electrochemical Cell Assembly: For all electrochemical measurements
a PEEK lined, airtight press cell with stainless steel stamps with a surface
area of 0.785 cm2 as current collectors were used.[28] For the half cells,
60 mg of Li5.5 PS4.5 Cl1.5 was filled as the separator in the cell. A hand press
was used to press the separator between the two stainless steel stamps
in a uniformly distributed layer. Composite (12 mg), corresponding to an
area loading of the CAM of 10.7 mg cm−2 , was layered on one side of the
separator. Particular attention was given to ensure an even and complete
coverage of the composite on the separator. The separator and the composite layer were densified with a uniaxial press with 3 tons for 3 min. After
pressing, an indium foil (chemPUR, 100 μm thickness, 99.99%) with 9 mm
diameter was placed on the other side of the separator. 1.5 mg of lithium
(Li, abcr, 99.8%) was pressed into a foil and was then placed on the indium
foil. The cell was closed airtight. For electrochemical measurements, the
cell was placed in a metal frame to apply a pressure of 50 MPa and was
left for 6 h at 25 °C for equilibration.
Electrochemical Characterization: Long-term cycling tests were performed using the MACCOR in a climate chamber maintained at 25 °C.
The half cells were cycled in a voltage window of 2.0 to 3.7 V versus In/LiIn
with a C-rate of 0.1 C for 50 cycles. Rate performance tests were performed
at a Biologic-VMP300 potentiostat in a climate chamber at a constant
25 °C. The applied charge and discharge currents were varied from C/20
(j = 0.107 mA cm−2 ), C/10 (j = 0.214 mA cm−2 ), C/5 (j = 0.428 mA cm−2 ),
C/2 (j = 1.070 mA cm−2 ) and 1 C (j = 2.140 mA cm2 ) in a voltage window
from 2.0–3.7 V versus In/LiIn.
Effective Transport Characterization: Symmetric cells were prepared for
the charge transport measurements. For effective electronic transport,
steel stamps were used as ionic-blocking electrodes on both sides. 100 mg
of the composite was filled in the press cell and uniformly distributed. The
cell was closed airtight and pressed with the uniaxial press with 3 tons
for 3 min. For the determination of effective ionic transport, electronically
blocking electrodes on both sides were used. For this, layers of 80 mg of
Li5.5 PS4.5 Cl1.5 were placed on both sides of the 100 mg prepressed composite. The cell was closed airtight and pressed in the uniaxial press with
3 tons for 3 min. Then indium foil (9 mm diameter) and 1.5 mg lithium
foil were placed on both sides of the solid electrolyte layers. The cells were
placed in a metal frame to apply a pressure of 50 MPa and were left for 6 h
at 25 °C for equilibration.[29]
To determine the effective electronic conductivity, impedance spectroscopy was performed on the symmetric cell with ionic blocking electrodes. Electrochemical impedance spectroscopy (EIS) was performed at
25 °C with a Biologic VMP 300 potentiostat at a perturbation amplitude
of 10 mV and in a frequency range from 10 mHz to 7 MHz. After the
impedance measurement, a subsequent DC polarization was performed
to corroborate the AC data. Here, a voltage from 45 to 50 mV was applied
in 5 mV steps. To reach an equilibrium state, each voltage step was kept for
2 h. For measuring the effective ionic conductivity, symmetrical cells with
electronic blocking electrodes were used. First, impedance spectroscopy
was performed with the same conditions as for electronic conductivity.
The subsequent DC polarization measurement was performed in a voltage
range from 0.5 to 5 mV. The voltage steps were 0.5 mV. For the adjustment
of the equilibrium, each voltage step was measured for 5 h to ensure full
polarization. The impedance data were evaluated with a transmission line
model using RelaxIS 3 software.
Microstructure Modeling and Simulation: The simulation of ion flux
through composite electrodes requires the reconstruction of microstructure models that consist of voxels. These models are generated based on
Adv. Energy Mater. 2023, 2302309
2302309 (8 of 9)
the scanning electron microscopy images and EDS mappings of the pristine CAM (Figure S1, Supporting Information), Li6 PS5 Cl particles (Figure
S2, Supporting Information), and the various composites (Figure S7, Supporting Information). The simulations and microstructure generations are
performed on a Dell Precision 3650 Tower (96 GB RAM, 11th Gen Intel
Core i7-11700 CPU) with the GeoDict[30] Modules GrainGeo, ProcessGeo,
ConductoDict, and MatDict.
The microstructure models are constructed based on the particle size
distributions and particle shapes of all electrode components, their volume fractions, the overall void space, and the homogeneity of the particle/component distribution (see Table S1, Figure S16, Pages S10 and S11,
Supporting Information). To model the cathode composite, both CAM and
Li6 PS5 Cl particles are randomly distributed within the model volume of
(300 μm)3 for the larger Li6 PS5 Cl particles and (90 μm)3 for the smallest
particles at a resolution of 600 and 300 nm voxel−1 , respectively. For each
composite, three microstructure models are generated to provide an average. The flux simulations are performed as introduced previously[10] with
a potential difference of 1 and 0.3 V for the smallest Li6 PS5 Cl particles. As
the material bulk conductivity, 2.2 mS cm−1 (Li6 PS5 Cl) and 5.2 mS cm−1
(CAM) were employed. The simulation assumed that the ionic current was
carried by the solid electrolyte, while the electronic current was carried
by the CAM. Since the CAM has a high Young’s modulus of 194 GPa,[31]
the resulting overlap between the CAM and Li6 PS5 Cl was assigned to the
CAM.[32] Before the simulation, the structure was cleansed from residual
CAM particles within the Li6 PS5 Cl substructure. For a detailed explanation
see Figures S19 and S20, and Pages S13–S15 (Supporting Information).
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
The authors acknowledge financial support from the ANISSA and FestBatt
projects, funded by the Bundesministerium für Bildung und Forschung
(BMBF, projects 05K22PMA and 03XP0430F). O.M. and L.K. acknowledge
financial support by the International Graduate School for Battery Chemistry. Characterization, Analysis, Recycling, and Application (BACCARA),
which is funded by the Ministry for Culture and Science of North Rhine
Westphalia, Germany. M.R. acknowledges funding from the Alexander von
Humboldt Foundation. A.B. acknowledges financial support by the Hessian Ministry of Higher Education, Research, and the Arts (HMWK).
Open access funding enabled and organized by Projekt DEAL.
Conflict of Interest
Three authors are employed by the company that provided the samples
and performed the particle size distribution analyses.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Keywords
effective transport, microstructure, particle size distribution, solid-state
batteries, sulfide solid electrolytes, transmission line modeling
Received: July 19, 2023
Revised: August 23, 2023
Published online:
© 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH
16146840, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202302309, Wiley Online Library on [05/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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