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Visualizing Reaction Fronts and Transport Limitations in

Solid-State Li–S Batteries via Operando Neutron Imaging

Robert Bradbury, Georg F. Dewald, Marvin A. Kraft, Tobias Arlt, Nikolay Kardjilov,

Jürgen Janek, Ingo Manke,* Wolfgang G. Zeier,* and Saneyuki Ohno*

DOI: 10.1002/aenm.202203426

and natural abundance of sulfur-active

material,[3,4] solid-state Li–S batteries

have the potential to cause a paradigm

shift through simultaneous enhancement

of long-term stability and battery safety,

together with boosted energy density

(Wh kg−1).[5] Nevertheless, solid-state Li–S

batteries do not yet fulfil performance

expectations, and further improvement

is required.[6] Electrochemical reactions

require a low-impedance supply of ions

and electrons but sulfur-active materials

are ionically and electronically insulating.

Hence, for a functional cathode, the

sulfur-active materials need to be compos-

ited with ion and electron conductive addi-

tives. These composites provide sufficient

triple-phase boundaries where consecutive

conversion reactions take place in which

the active material can receive or donate

ions and electrons.[7] As a result, a func-

tional sulfur cathode often contains a high

interfacial area per unit volume (>104–106 cm−1).[8]

In particular, the rate-limiting step in solid-state Li–S bat-

teries is, at present, not well understood. Despite the very short

diffusion length and the substantial numbers of triple-phase

boundaries created after the compositing procedure, slow dif-

fusion of charge carriers in sulfur-active materials[9] and high-

impedance charge transfer over the electrolyte-active material

The exploitation of high-capacity conversion-type materials such as sulfur

in solid-state secondary batteries is a dream combination for achieving

improved battery safety and high energy density in the push toward a sustain-

able future. However, the exact reason behind the low rate-capability, bottle-

necking further development of solid-state lithium–sulfur batteries, has not

yet been determined. Here, using neutron imaging, the spatial distribution of

lithium during cell operation is directly visualized and it is shown that sluggish

macroscopic ion transport within the composite cathode is rate-limiting.

Observing a reaction front propagating from the separator side toward the

current collector confirms the detrimental influence of a low effective ionic

conductivity. Furthermore, irreversibly concentrated lithium in the vicinity

of the current collector, revealed via state-of-charge-dependent tomography,

highlights a hitherto-overlooked loss mechanism triggered by sluggish

effective ionic transport within a composite cathode. This discovery can be

a cornerstone for future research on solid-state batteries, irrespective of the

type of active material.

ReseaRch aRticle

1. Introduction

Solid-state batteries offer the potential for greater energy densi-

ties and increased safety and are therefore currently discussed

as an alternative to lithium-ion batteries.[1,2] By combining the

concepts of solid-state batteries with conversion-type lithium–

sulfur (Li–S) cells that have a high theoretical specific energy

The ORCID identification number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/aenm.202203426.

R. Bradbury, T. Arlt

Institute for Materials Science and Technologies

Technische Universität Berlin

Straße des 17, Juni 135, 10623 Berlin, Germany

R. Bradbury, N. Kardjilov, I. Manke

Helmholtz-Zentrum Berlin für Materialien und Energie (HZB)

Hahn Meitner Platz 1, D-14109 Berlin, Germany

E-mail: [email protected]

G. F. Dewald, J. Janek

Institute of Physical Chemistry

Justus-Liebig-University Gießen

Heinrich-Buff-Ring 17, D-35392 Gießen, Germany

G. F. Dewald, J. Janek

Center for Materials Research (LaMa)

Justus-Liebig-University Gießen

Heinrich-Buff-Ring 16, D-35392 Gießen, Germany

M. A. Kraft, W. G. Zeier

Institut für Energie- und Klimaforschung (IEK)

IEK-12: Helmholtz-Institut Münster

Forschungszentrum Jülich

48149 Münster, Germany

W. G. Zeier

Institute of Inorganic and Analytical Chemistry

University of Münster

Correnstrasse 30, 48149 Muenster, Germany

E-mail: wzeier@uni-muenster.de

S. Ohno

Department of Applied Chemistry

Graduate School of Engineering

Kyushu University

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

E-mail: [email protected]

VCH GmbH. This is an open access article under the terms of the Crea-

tive Commons Attribution License, which permits use, distribution and

reproduction in any medium, provided the original work is properly cited.

Adv. Energy Mater. 2023, 13, 2203426

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interfaces[10] have often been cited as the rate-limiting steps.[11]

In contrast, the sluggish ion transport within the composite

itself, which becomes exponentially worse at high potential,

strongly contributes to a high overpotential.[8] This retarded

ion transport is not limited to solid-state Li–S batteries and the

same transport limitations within the cathode composite are

found in intercalation-type solid-state batteries.[12,13] In other

words, irrespective of the cathode-active material used, a low

effective ionic conductivity of the cathode composite may be the

bottleneck for solid-state battery development. Electrochemical

impedance spectroscopy can provide details on the ionic trans-

port. However, a visualization of the overall spatial distribution

of transport limitations is essential for determining critical bot-

tlenecks toward high-loading and thick cathode design, as well

as fast charging/discharging. Thus, the development of a tech-

nique that can monitor cells operando and visualize the distri-

bution of lithium is needed.[14]

Notably, progress in the field of X-ray imaging allowed for

the elucidation of solid-state batteries while under opera-

tion.[15–18] Since X-ray attenuation coefficients monotonically

increase with atomic number, the identification of lithium, a

light element, using X-rays is challenging. Here, neutron-based

techniques appear promising due to the high neutron attenua-

tion of lithium compared to other elements that comprise the

cathode composite. Neutron radiography has been successfully

employed in the characterization of batteries with liquid elec-

trolytes[19–21] as has another nondestructive technique, neutron

depth profiling.[22–24] In solid systems, operando neutron dif-

fraction has been conducted to investigate the crystallization of

solid electrolytes and lithium transport in garnet-based solid-

state cells via depth profiling.[25,26] To the best of our knowledge,

operando neutron analysis has not yet, to date, been performed

on solid-state batteries.

2. Results and Discussion

In order to perform the operando neutron analysis on the solid-

state cell, a cell housing made from aluminium, with a hollow

interior was developed, as depicted in Figure 1a. To ensure

electrical insulation of the cell, the stack was prepared inside

a polyimide tube. The battery components were sandwiched

between two stainless steel stamps with O-ring seals to protect

the lithium-thiophosphate-based solid-state Li–S battery (In/Li

| Li6PS5Cl | S/C/Li6PS5Cl) from exposure to air and moisture

during cycling. A thick cathode configuration delivering an

areal capacity of almost 12 mAh cm−2 was employed to better

observe changes in the cathode composite layer (see Figures S1

and S2 in the Supporting Information for details of the cell

performance).

The neutron beam transmitted through the cell can visualize

the components with the level of contrast determined by the

degree of neutron absorption (see Table S1, Supporting Infor-

mation). Figure 1b shows a representative 2D neutron radio-

graph of the cell before cycling. Darker areas represent the

components with low neutron transmission or high absorption.

Among the elements comprising the cathode and electrolyte

regions, lithium possesses the highest neutron absorption coef-

ficient,[27] thus the contrast changes are primarily due to the

variations in lithium concentration. In the anode, the presence

of indium should be noted since it has an even higher neu-

tron absorption coefficient than lithium.[27] This is reflected in

Figure1b where the attenuation of the neutron beam is much

greater in the anode. However, since indium remains immobile

throughout the electrochemical cycling, mobile lithium can be

considered to be responsible for all changes in neutron attenu-

ation. In the pristine state, the attenuation difference between

the solid electrolyte and the cathode composite is less distinct

than for the anode due to the similar Li environments in both

components. However, the additional carbon and sulfur pre-

sent in the cathode alter the Li density sufficiently to enable

the two regions to be differentiated in the radiography images.

The neutron attenuation measured in the pristine (as-prepared)

and fully discharged cells show distinct differences (Figure1c).

These are most evident in the cathode, where the change from

the pristine to the fully discharged state is significant. It is con-

sistent with what is occurring in the cell during discharge—

there is a net loss in lithium from the anode and a net gain

Figure 1. Cell design, representative neutron radiography, and neutron attenuation. a) Schematic representation of the experimental setup for neutron

imaging. A custom-made cell optimized for neutron imaging was employed. b) The element-specific neutron attenuation allows differentiation of the

cell components from the steel and aluminium in the neutron radiogram. c) The position-dependent neutron attenuation quantified within the white

box in (b) before and after the initial discharge visualizes the variation in the distribution of the Li concentration. The plateau seen in the separator

giving neutron attenuation of about 2.35 cm−1 shows good agreement with the theoretical value of Li6PS5Cl (2.47 cm−1).

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at the cathode, while the solid electrolyte separator exhibits no

overall change.

Whereas in situ neutron tomography offers the opportunity

to quantify the lithium distribution through the cathode in 3D

at specific charge states, 2D radiography, conducted operando,

can visualize the changes in lithium distribution through the

cathode composite during cathode lithiation/delitiation. The

solid-state Li–S cell was cycled at 1.13mA cm−1 while in the neu-

tron beam (Figure 2a). Throughout the initial discharge images

were recorded, each with an exposure time of 10 s and a pixel

size of 13 µm (see Video S1 and Figure S3, Supporting Infor-

mation). After processing the images to account for the back-

ground and normalizing them to the initial pristine state, what

is now represented is the change in the number of transmitted

neutrons relative to the initial state. However, since the images

are a 2D representation of a cylindrical cell, the path taken by

the neutrons (lp) is not constant across the image. This can be

resolved by using the Beer–Lambert law (see the Supporting

Information) to convert neutron transmission to attenuation.

Neutron attenuation is dependent on lp and has units of cm−1.

To do this, it is necessary to consider each pixel in the image

individually and, working out from the center where lp is the cell

diameter, the path length for each can be calculated using simple

trigonometry. Compiling the images together in sequence, it

becomes clear how the attenuation changes with time (see Video

S2, Supporting Information), with a brighter area, reflecting

the local presence of more lithium. In order to focus attention

on the cathode composite only, a region between a point in the

solid electrolyte separator denoted by d0, and a point in the steel

current collector, denoted by dmax is displayed. Figure S4 in the

Supporting Information displays the attenuation change for the

cathode at 10% intervals in the depth of discharge (DoD).

If we consider how each pixel changes with time, taking

the derivative of the attenuation change offers a clear picture

of where the lithium is distributed in the cathode composite

and when it arrives, as shown in Video S3 in the Supporting

Information. The heatmap in Figure2b highlighting the rate of

neutron attenuation change (change in ∆AttnNeu as a function

Figure 2. Dynamics of lithium distribution visualized by operando neutron radiography. a) Voltage decrease plotted as a function of discharge capacity.

Each data point represents the degree of discharge (DoD). b) Neutron radiography image of the cathode composite at 10% DoD. The color scale

represents the rate of change in neutron attenuation with the same limits as (c) and Figure S5 in the Supporting Information. Neutron attenuation

and change in neutron attenuation as functions of time are displayed in the Supporting Information (Figures S2 and S3). In the vertical direction, the

image ranges from d0 to dmax.. c) Progression of the point of maximum rate of attenuation change (reaction front) toward higher d as DoD increases.

The region displayed for each DoD is taken from the equivalent dotted area in each image as illustrated in (b) for 10% DoD. The white lines represent

the cathode interfaces with the steel current collector (upper) and the solid electrolyte separator (lower), which were determined based on the data at

0% DoD in combination with the 3D tomography data. The black lines correspond to the position giving the maxima median rate of attenuation change

in (d). d) Median rate of attenuation change as a function of d (using the full-width data as shown in (b) and Figure S5, Supporting Information). The

dotted lines represent the interfaces as denoted by the white lines in (c).

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of time) shows that there is a distribution through the cathode

of the rate at which the attenuation changes. The maximum

rate of attenuation change progresses through the cathode with

time. Figure2c shows the propagation of this “reaction front”

through the cathode as a function of DoD (the full cathode

width images of the rate of change of attenuation are displayed

in Figure S5 in the Supporting Information at 10% intervals of

DoD). The shift in the maximum rate of change is highlighted

in Figure2d whereby the median rate of attenuation change is

displayed as a function of d for 10% DoD intervals. This forma-

tion of the reaction front propagating from the separator side

toward the current collector was also observed with the thinner

cathode configuration as shown in Figures S8–S10 in the Sup-

porting Information. The edges of the cell deform slightly due

to the soft polyimide sleeve used for cell insulation. This occurs

around the full circumference of the cell but is only clearly vis-

ible in Figure2b for the edges of the cell perpendicular to ori-

entation of the neutron beam. This and the limitation of the

instrument resolution are responsible for the broadening of

the peak shape as well as spill-over into the region of the steel

current collector when d> dCathode|CC and the separator when

d< dSE|Cathode. Variation in the rate of attenuation change is to

be expected across the cell throughout electrochemical cycling

but a given point in the cathode composite will experience the

greatest rate of change when the lithium front first arrives.

Notably, the volume change and the resulting shift in dSE|Cathode

during operando radiography were not trackable due to the

limited resolution and above-mentioned deformation. Despite

the known volume expansion of the cathode composite upon

discharging,[28] it is clear that this reaction front propagates

from the separator-side toward the current collector as the cell

discharges.

While operando neutron radiography provides informa-

tion regarding the dynamics of lithium transport through the

cathode composite during cycling, this insight is limited by the

2D nature of the technique, describing the lithium distribu-

tion only in terms of d. To acquire information on the homo- or

heterogenous distribution in the cathode area for a given d, a

different neutron imaging technique, neutron tomography, is

required. Although information on the dynamics will be lost

since tomography requires a series of images to be recorded

as the sample is rotated, these images, taken in situ, can be

reconstructed to create a 3D representation of the cell (see

Figure S6 in the Supporting Information showing the pristine,

discharged, and recharged states).

Through subtraction of the pristine state from subsequently

measured tomograms of fully discharged and recharged cells,

it is now possible to directly visualize changes to the lithium

distribution across the whole volume. Figure 3a shows two 3D

representations of the cell, one in a discharged (top) and the

other in a charged state (middle). Each tomogram has been

normalized to the pristine state to show only the changes in

Figure 3. In situ neutron tomography on the discharged and charged solid-state sulfur cathode. a) 3D tomography images of the discharged (top)

and recharged state (middle), normalized to the initial state to emphasize changes in the cell (cathode increase in attenuation—yellow to red, anode

decrease in attenuation—blue), along with the difference between them (bottom) that shows the location of the mobile lithium (green). b) Median

neutron attenuation change through the 200µm thick cathode composite in the discharged state normalized to the pristine state. The images range

from d0 at the edge of the solid electrolyte region to dmax on the current collector side of the cathode composite and each represents a 39µm thick

slice of the cathode. c) Same as (b) but for the recharged state. The color bar in both represents the range of attenuation change above that of the

background and is independent of colors used in (a). d) Total neutron attenuation change for 13 µm thick slices through the cathode composite for

the discharged (orange) and recharged states (blue). The latter represents the lithium trapped in the cathode. The difference between the two charged

states represents the mobile lithium (green). The data are displayed with error bars corresponding to the standard deviation.

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the cell affected during cycling. The anode shows the negative

attenuation change and the separator region has zero net

change in total lithium, similar to the steel stamp below the

anode and above the cathode. The separator region is denoted

in gray. The cathode, which is the focus of our in-depth analysis,

displays a notable difference between the two charge states. This

is clearly demonstrated in the third representation (bottom)

which shows the difference between the two charge states (the

anode difference is less noticeable due to percentage changes

to its overall attenuation being much less than the cathode).

Figure 3b,c displays the attenuation change in 39 µm thick

slices from d0 to dmax for the fully discharged and recharged

states, respectively. The difference in attenuation between the

discharged and charged states represents the mobile lithium

that is stripped from the cathode upon charging. The origin

of the nonzero rate of attenuation change observed beyond the

cathode|current collector interface in Figure2d, and attributed

to the deformation of the cathode outer edge, is also seen in the

final three images in both Figure3b,c. If we take each image

from the series and sum the attenuation change of all voxels

within the cell area, the differing behavior across the depth of

the cathode composite becomes even clearer (Figure3d). The

discharged state (orange plot) represents the lithium that has

accumulated in the cathode upon lithiation of S to form Li2S

during initial discharge. It is evident from Figure3c,d that the

lithium is heterogenously distributed throughout the cathode.

The attenuation from the recharged state (blue plot) represents

lithium that is trapped within the cathode composite, in the

form of Li2S that cannot be converted back to S. This trapped

lithium is observed across the cathode depth in Figure3d but

is notably greater at higher d, the current collector side of the

cathode composite. The mobile lithium that is not trapped in

the cathode and which is transported back toward the anode

when the cell is recharged can be quantified as the difference

between the lithium present in the cathode in the discharged

state and that which remains after the cell has been charged. In

Figure3d (green plot), this lithium can be shown to have been

located at, and lost from, the solid electrolyte separator side of

the cathode composite, at lower d.

Assuming that the cathode composite is homogenous and

has a substantially high interfacial area density, we can expand

a conventional “1D porous electrode theory,” also known as the

Newman model,[29] to solid-state sulfur cathodes. The model

was originally developed to describe the distribution of the reac-

tion current within an electrode that consists of a porous matrix

of a reactive material and a liquid electrolyte that had penetrated

into voids in the porous material. A dimensionless reaction rate

describes the distribution of the reaction current in the thick-

ness direction of the cathode composite, which corresponds to

the rate of attenuation change observed in this work. A detailed

description can be found in the Supporting Information.

When the effective ionic and electronic conductivities

(io

eff

and e

) of a cathode composite are sufficiently high

with respect to the applied current and thickness, the dis-

tribution of reaction current becomes uniform as shown in

Figure 4b. With fast supply of ions and electrons, charge

transfer and lithium diffusion within the active material

particles will remain as potential rate-limiting steps. In con-

trast, when the effective transport is insufficiently fast, the

reaction current distribution becomes nonuniform, as shown

in Figure4c,d, and the overvoltage due to the sluggish trans-

port within the electrolyte in the composite becomes non-

negligible. The degree of nonuniformity is parameterized by

δ ( ||

ion

eff

σσ











LI ), which is a function of the applied cur-

rent I, the cathode thickness L, effective conductivities, and

a parameter β, which is constant for a given battery chem-

istry. It is notable that with e

eff

σσ

, the model predicts a

nonuniform reaction rate with the formation of a reaction

front propagating from the separator-layer side toward the

current collector (Figure4c).[30] With comparable e

eff

and ion

eff

giving a high δ value, another reaction front from the current

collector side will form, and, lastly, the reaction front on the

separator side will diminish when ion

eff

σσ

 (Figure4d). Con-

sidering that the here-employed composite possesses effective

ionic and electronic conductivities of (4.5 × 10−3± 0.5) mS cm−1

and (20 ± 4) mS cm−1, respectively (see the Supporting Information),

the experimental results of operando neutron measurements,

visualizing the reaction front from the separator side toward

the current collector, agree well with theoretical prediction. The

pristine cell, visualized here via neutron measurements, would

correspond to δ∼ 2 × 102, predicting a severely uneven distribu-

tion of reaction current, as observed in the experiment. Details

of the calculation are shown in the Supporting Information.

The presence of a reaction front propagating toward the cur-

rent collector further implies an overlooked capacity loss mecha-

nism in a thick composite cathode. A reaction rate distribution

should be identical for both anodic and cathodic currents within

the model, as long as the given parameters are unchanged (see

Figure4c,d). However, in practice, an effective ionic conductivity

of the cathode composite experiences a substantial decrease

upon delithiation above 2V versus In/InLi (2.62vs Li+/Li),[8,31]

leading to a gradient in lithium concentration in the thickness

direction of the cathode composite. During the propagation of

the reaction front from the separator side, the effective ionic

conductivity of the composite decreases, and further delithia-

tion from the current collector side will be hindered. Conse-

quently, the lithium concentration gradient in the thickness

direction remains after the recharging process, as observed in

the in situ neutron tomography. This most likely accounts for

the commonly observed asymmetric overpotential between dis-

charge and charge after cycling.[8] Exemplary asymmetric poten-

tial profiles of a solid-state Li–S battery with the same cathode

composite are shown in Figure S7 in the Supporting Informa-

tion. Irrespective of the nature of active materials, the slow ionic

transport in composites has been shown in intercalation type

solid-state batteries as well, suggesting that the here-found phe-

nomena will also play a role in other types of solid-state batteries.

Therefore, a nonuniform reaction in the thickness direc-

tion caused by sluggish ion transport of the cathode composite

highlights the necessity of 1) a processing procedure that does

not sacrifice fast ion transport, 2) further development of

solid electrolytes possessing both high ionic conductivity and

electrochemical stability, and 3) a design principle of cathode

architecture boosting the effective ionic conductivity. The third

strategy includes the introduction of stable ion conductors

and 3D structure design of cathode composite to establish a

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“highway” for ion transport within the composite, which fur-

ther requires development of 3D transport modeling and visu-

alization techniques with higher spatial resolution.

3. Conclusions

Operando neutron radiography and in situ neutron tomog-

raphy have successfully been demonstrated and elucidate the

transport limitations in a solid-state sulfur cathode composite.

2D radiographs reveal a reaction front, propagating from the

separator-layer side toward the current collector upon the initial

discharge. 3D tomography visualizes residual lithium concen-

trated in the vicinity of the current collector after recharging.

Extending the porous electrode theory for solid-state batteries

corroborates that the sluggish effective lithium-ion transport in

composites is rate-limiting and leads to a nonuniform reaction

front.

This work peeks into the lithium dynamics inside composite

cathodes in solid-state batteries, showing that limitations exist

in cathode composites due to slow ionic transport. Observing

this reaction front urges further work in the design of cathode

composites for solid-state batteries in general because high

loading and thick cathode composites, as well as fast charging/

discharging, are needed for realistic implementation of solid-

state batteries.

Supporting Information

Supporting Information is available from the Wiley Online Library or

from the author.

Acknowledgements

This research was supported by the Federal Ministry of Education and

Research (BMBF) within the project LISZUBA under grant numbers

Figure 4. Extension of a porous electrode theory to solid-state sulfur cathodes. a) A schematic of the cathode composite with a thickness of L and an

applied current of I. A δ parameter is given by

σσ

∝+











ion

ff b) Porous electrode theory predicts a uniform reaction rate distribution with fast

effective carrier transport. c) Sluggish ion transport within the composite leads to a nonuniform reaction rate, forming a reaction front propagating

from the electrolyte-layer side toward the current collector. d) With low effective electronic conductivity, the reaction front propagates from the opposite

side. The reaction rate distribution becomes less steep with lower δ by increasing the rate-limiting transport.

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03XP0115A and 03XP0115C. S.O. gratefully acknowledges the Alexander

von Humboldt Foundation for partial financial support through a

Postdoctoral Fellowship.

Open access funding enabled and organized by Projekt DEAL.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

I.M., W.G.Z., and S.O. designed the project toward the transport

limitation. R.B., G.D., and S.O. designed the operando cell. G.D. and

S.O. synthesized materials, prepared the cathode composites, and

assembled the cells. R.B., G.D., T.A., and M.K. performed tomography.

R.B. and N.K. analyzed the data. R.B., N.K., W.G.Z., I.M., and S.O.

interpreted the data. R.B. and S.O. prepared the initial draft of the work.

All authors gave comments and edited the manuscript.

Data Availability Statement

The data that support the findings of this study are available from the

corresponding author upon reasonable request.

Keywords

composite electrodes, in situ neutron tomography, Li–S batteries,

operando neutron radiography, solid-state batteries

Received: October 10, 2022

Revised: February 24, 2023

Published online: March 20, 2023

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