visualizing battery reactions and processes by using in
TRANSCRIPT
Review
Visualizing Battery Reactionsand Processes by Using In Situand In Operando MicroscopiesYutong Wu1 and Nian Liu1,*
The Bigger Picture
The demand for high-
performance energy storage
systems has increased
dramatically over the past few
decades, leading to a paradigm
shift from traditional fossil fuels to
sustainable energy sources. As
energy storage components for
electric vehicles and mobile
electronic devices, batteries have
Based on traditional electronic, X-ray, and optical microscopies, novel tools and
methodologies have been developed to monitor batteries in situ or in oper-
ando. This review surveys the recent development of several types of in situ
and in operando microscopy and their utilization in studying batteries. The
article is organized mainly by the in situ and in operando techniques used
(X-ray microscopy, electron microscopy, scanning probe microscopy, etc.)
rather than the chemical reactions that govern battery behavior. We compare
different methodologies to illustrate their advantages and disadvantages in
the study of different systems and problems and, at the end of the article,
discuss generalized principles for the selection of methodologies and possible
future directions.
been a well-studied
interdisciplinary topic across
chemistry, material science, and
engineering. Monitoring the
phenomena inside batteries—
e.g., volume and pressure
changes, side reactions,
temperature effects, and material
transport during the operation
process—with powerful tools and
methodologies allows for rapid
advances in battery design and
paves the way for battery
optimization. This article reviews
the recent applications (mostly
from 2015 to 2017) of in situ and in
operando microscopy in battery-
related studies and gives insights
on possible future directions.
INTRODUCTION
Portable energy storages such as batteries have the potential to replace traditional
energy sources in the near future. Novel materials and methodologies have been
developed to improve battery performance, and discovering and understanding
the phenomena and fundamentals inside the battery are necessary for promoting
battery performance to a higher level.1 Traditional ex situmicroscopy and spectros-
copy have been used extensively for studying batteries to obtain spatial and chem-
ical information on the materials inside. However, ex situmethods are limited by the
need to remove samples from their native environments (introducing artifacts) and
by the time delay between the event and the characterization. Therefore, it is ideal
to get real-time information on reactions and processes inside batteries without
disassembling them.
The Latin phrase ‘‘in situ,’’ as opposed to ‘‘ex situ,’’ means ‘‘on site,’’ i.e., the battery
does not have to be cracked open for analyzing the materials inside, which poten-
tially provides more reliable information than the ex situ scenario. ‘‘In operando’’
means ‘‘during operation,’’ so ideally the battery is being monitored in real time.
For example, pausing the battery periodically to take slow measurements and
then restarting it is an ‘‘in situ’’ experiment. Measuring a battery periodically during
its operation is an ‘‘in operando’’ measurement. In situ and in operando microscopy
allow researchers to obtain more representative information that would otherwise
be missing or misleading. Correlating microscopy with electrochemical data (cyclic
voltammetry, galvanostatic cycling, etc.) obtained on the same device at the same
time can avoid sample heterogeneity and allow a direct cause-and-effect relation-
ship to be built, leading to clear conclusions. On the other hand, in situ and in oper-
ando experiments require special design, development, and modifications of the
batteries and probing equipment to perform multiple measurements in the same
438 Chem 4, 438–465, March 8, 2018 ª 2017 Elsevier Inc.
device and/or at the same time, so the experimental conditions for modified batte-
ries can deviate from the common operating conditions. For example, to probe bat-
teries under transmission electron microscopy (TEM), researchers use an ionic liquid
or dry cell setup, which is significantly different from a commercial battery.2
Different continuous-monitoring microscopies, including electron, X-ray, scanning
probe, and optical microscopy, are compared with respect to their resolution, chem-
ical and spatial information, dimensionality, and straightforwardness. Electron mi-
croscopy uses an electron beam to create an image for the sample, X-ray microscopy
magnifies the sample with electromagnetic radiation, scanning probe microscopy
(SPM) provides surface information measured by a tip, and optical microscopy uti-
lizes transmitted light to probe the sample. Optical microscopy has the lowest
spatial resolution limited by the diffraction of light, and electron microscopy has
the highest spatial resolution; X-ray and atomic force microscopy (AFM) lie in the
middle. Optical microscopy is the easiest to access and operate, and the battery
design process is more straightforward; it could be simply built on a glass slide.
This review focuses on microscopy and imaging; recent developments on spectros-
copy can be found elsewhere.3 Imaging techniques based on different principles are
introduced, along with a representative novel battery design for each imaging tech-
nique, as well as the scientific understanding and importance to the battery commu-
nity. Insights on the choice of appropriate imaging techniques and possible future
research directions are provided. A recent review article focuses on lithium-ion bat-
tery (LIB) characterization,4 another review discusses in situ electrochemistry for
rechargeable batteries,5 and another review talks about in situ 1D nanomaterial.6
Here, we focus on in situ and in operando microscopy and include various battery
systems.
1School of Chemical and BiomolecularEngineering, Georgia Institute of Technology,Atlanta, GA 30332, USA
*Correspondence: [email protected]
https://doi.org/10.1016/j.chempr.2017.12.022
ELECTRON MICROSCOPY
Scanning Electron Microscopy
For scanning electron microscopy (SEM), an electron beam is focused onto the
surface of a sample to probe the morphology and the structure. The advantages
of using SEM are that (1) 2D morphology over high spatial resolution can be
obtained and (2) different chemical components in the material can be identified
with the help of energy-dispersive X-ray spectroscopy (EDX) analysis. Continuous
monitoring under SEM requires the electron beam to focus on the materials inside
the battery; the electron beam cannot penetrate the battery and probe the pro-
cesses inside if the battery is constructed in a traditional way, and at the same
time, the electrodes and electrolytes need to be in a vacuum environment. A typical
electrolyte choice for SEM is ionic liquids because of low volatility. For the reasons
mentioned above, material selection and battery structure modification need to
be considered for in situ or in operando SEM experiments.
In situ SEM can be helpful for observing the effect of different additive chemicals on a
specific process. Recently, Rong et al.7 combined SEM with electrochemical
scanning technology to definitively visualize Li dendrite growth and dissolution.
The detailed design of an in situ SEM battery and the results of real-time dendrite
monitoring are shown in Figures 1A and 1B. The authors demonstrated the impor-
tance and benefits of the Li2S8 and LiNO3 additives on Li dendrite suppression
inside a liquid cell in a straightforward manner. Combining in situ SEM and theoret-
ical calculations, the article confirmed that Li polysulfides help limit dendrite growth,
in agreement with previous ex situ results.8 SEM is advantageous over both optical
Chem 4, 438–465, March 8, 2018 439
Figure 1. Battery Cell Design and Results of In Situ SEM
(A and B) Schematic of the in situ SEM setup for monitoring real-time Li dendrite growth (A) and in situ SEM results of Li plating and stripping with false
color (scale bars: 20 mm) (B). Reprinted with permission from Rong et al.7 Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(C and D) In situ SEM results for a Li-S battery with specified layers (C) and the effect of a silica-loaded electrolyte on the battery (right) and the EDX data
(left) (D). Reprinted from Marceau et al.9
microscopy and TEM because it probes detailed features with a higher resolution
than optical systems. SEM does not lead to significant damage or artifacts in the
sample like TEM, and the sample is kept in an adequate amount of electrolyte to
mimic the situation in a typical battery. By sealing with epoxy, the problem of volatile
liquid is solved as well. Most importantly, the author successfully demonstrated that
LiNO3 added to the electrolyte forms a solid electrolyte interphase (SEI), but for
Li2S8, the chemical prevents dendrite formation by etching away ‘‘dead’’ Li rather
than the generally believed SEI mechanism. By obtaining the chemical potential
via computation, the author explains that the Li atom prefers to stay in Li2S8 instead
of Li metal to be more stable. This work shows that in situ microscopy is powerful,
revealing true information on operating batteries, whereas ex situ analysis only
probes the start and end product, which makes it harder to uncover the true mech-
anism. Also, this work provides a novel way to evaluate electrolyte additives for Li
dendrite suppression. With the sealed liquid cell system, liquid with a higher vapor
pressure can potentially be analyzed by in situ SEM. For example, electrochemical
platting and stripping of other metal anodes (Na, Zn, etc.) could be done in the
same platform.
Different from focusing only on a single process, in operando SEM can be used for
studying and observing different phenomena that former researchers may not have
found or paid attention to. Marceau et al.9 analyzed a Li/solid polymer electrolyte/
sulfur cell and recorded specific indications of the different material layers. The
440 Chem 4, 438–465, March 8, 2018
analytical results for in situ SEM and EDX are shown in Figures 1C and 1D. A voltage
profile ranging from 1.6 to 2.8 V at a C/20 rate and a charge capacity of 85%, as well
as a comparison of the discharge capacity of the cell initially charged outside the
SEM chamber, are reported; elemental sulfur can be visualized after 2.3 V. In this
study, the authors used an EDX mapping technique to precisely identify the compo-
nents of different material layers inside the battery. Combined with UV-visible
(UV-vis) spectroscopy technology, a number of processes going on in the battery
are monitored, including polysulfide dissolution, formation of an insulating layer
and sulfur particles after charging, and polysulfide transport. The authors observed
that a solid-state electrolyte cannot completely suppress the loss of active sulfur
material. This study highlights that further modification of the polymer electrolyte
is needed to prevent active material loss.
In situ SEM can provide information on themorphology and size change of materials.
Chen et al.10 analyzed the charging and discharging reactions of the silicon anode
in LIBs with a binder-free electrode setup. They found that the size and shape of
the Si material determined its morphology during cycling. The cutoff voltage
was �3.88 and �2.40 V versus LiCoO2 at a rate of 0.12 C, but the first cycle was
partially irreversible. Excluding the binder material and other additives during the
in situ SEM process is very important because the existence of other materials can
hinder the active material being probed, making it difficult to distinguish materials
with similar properties without the help of EDX mapping. The existence of other
materials also introduces extra components into the EDX mapping, which can
lead to inaccurate percentage counting. In situ TEM can be conducted according
to Chen’s work to focus on the size effect of a single silicon particle.
A recent study analyzed the Si anode reaction inside a modified coin cell by using in
situ SEM.11 The voltage profile versus LiCoO2 was similar to that of a normal cell, but
the capacity was higher. During the first charging process, Li-induced volume expan-
sion caused cracks and drastic changes in the morphology of the anode by directly
probing into the coin cell, but the change in the remaining cycles was moderate. The
authors point out the importance of modifying the stiff binder to avoid internal stress
and enhance the performance.
Several limitations of in situ SEM are as follows: (1) The complicated and unstandard-
ized procedure for making suitable batteries makes it difficult to compare the perfor-
mance of even similar batteries. The fabrication process can also introduce artifacts
that can accelerate the degradation of the battery materials and affect the EDX
result. (2) The intrinsic properties of SEM limit the information that researchers can
get from the image. For example, the performance of coated materials on a battery
can be difficult to justify, and the penetration depth into the material can vary
dramatically with different strengths of electron beam energy, which makes the
situation very complicated; one area could have all coated particles, whereas
another region at a different depth could have no coated particles at all. (3) The
use of an electron beam of several keV makes it incompatible with e-beam-sensitive
materials.
Transmission Electron Microscopy
Different from that in SEM, the electron beam used in TEM has a significantly higher
voltage, which allows the beam to pass through the sample and generate a projec-
tion of the sample. An insight on material composition can be gathered by
combining TEM with electron diffraction and other analytical techniques based on
TEM.12 Pioneered by Huang et al.13 on electrochemical lithiation of a single SnO2
Chem 4, 438–465, March 8, 2018 441
Figure 2. Different Processes of Material Probing with In Situ TEM
(A and B) Schematics of a typical open cell (A) and a typical sealed liquid cell (B) for in situ TEM. Reprinted from Wu and Yao.15
(C) In situ TEM of the electrochemical lithiation of Si-C yolk-shell anode material. Reprinted with permission from Liu et al.18 Copyright 2012 American
Chemical Society.
(D and E) Schematic of the in situ TEM micrometer-level fluoride-ion battery design (top, half cell; bottom; full cell; cross sections also shown) (D) and
SEM images (a and c, fabricated half and full cells; b, d, and e, interfaces) (E). Reprinted with permission from Fawey et al.19 Copyright 2016 Wiley
Periodicals, Inc.
nanowire electrode in 2010, in situ TEM has been extensively developed for the
study of electrochemical processes. A 2015 review summarized the early works on
in situ TEM for batteries.14 Here, we mainly update this with recent developments
since 2015. In situ TEM setup can be divided into two main categories: an open
cell system and a sealed liquid cell system, as shown in Figures 2A and 2B.15 The
open cell system can be further divided into those that require no electrolyte and
others that use a solid-state or non-volatile low vapor pressure liquid electrolyte.
Open cells are limited for study of bulk material properties, such as Li diffusion,
mechanical stress, and phase transformation, but the resolution is higher and the ex-
periments can be conducted more easily. Also, open cells generally do not have the
expected current and voltage profiles of a typical battery. The reason for developing
liquid cells in addition to open cells is that, compared with open cells, which use a
point attached solid or ionic liquid electrolyte, liquid cells use the same electrolytes
(volatile liquids) as typical LIBs. In addition, the solid electrodes are immersed in
liquid electrolyte, which resembles the testing conditions used by most researchers
and allows the analysis of reactions inside the cell, on the surface, and on interfacial
properties, such as the SEI formed inside a typical testing condition, an important
battery component that significantly affects the performance of most LIBs. Also,
the sealed liquid cell has the ability to be cycled, despite the practical experimental
difficulties. The disadvantages of sealed liquid cells are the low resolution and
442 Chem 4, 438–465, March 8, 2018
increased difficulty in conducting experiments. A nice review on the current status
and future developments of in situ TEM16 and another review about the achieve-
ments and major problems to be solved for liquid cells17 can be found elsewhere.
In situ TEM is especially powerful in elucidating phase transitions inside the battery.
Chen et al.20 studied the lithiation and delithiation of FeS nanosheets and recorded
the morphology change and phase transformation . The voltage for the first five
cycles was stable and in a similar range to that reported for a typical cell, except
the first cycle showed a loss of capacity for the SEI formation. The analysis revealed
that FeS underwent an irreversible transformation into Li2S and Fe nanoparticles dur-
ing the first lithiation; during the delithiation process, the reaction followed a
different mechanism whereby Li1.13FeS2 was formed, and transformation to Fe and
Li2S was found. Sodiation behavior of single-crystal and poly-crystal ZnO nanowire
anodes in a sodium-ion battery (SIB) was probed by Xu et al.21 The voltage range
reported was from 0.01 to 2 V versus Na/Na+. The electrochemical sodiation reac-
tion was successfully visualized step by step at the atomic level. The article also
reveals that chemically identical materials with different crystalline forms can behave
differently, indicating that the control of microstructures is important in battery fabri-
cation. Leenheer et al.22 studied the phase boundary propagation in Li-alloying elec-
trodes and showed that the solid-solid phase transformation at the propagating
phase boundary caused significant stresses, leading to performance degradation.
The voltage versus Li is reported with different alloys; the voltages are higher than
typical voltages with beam-induced overpotential. Three different material, Si, Al,
and crystalline Au were evaluated, and it was concluded that for different host
materials, the nucleation and growth at the phase front are different. Traditionally,
a simple outside-in diffusion-limited model has been used for describing the reac-
tion of Si electrodes. For Al and Au, however, grains that are nucleated stop growing
at a certain size, whereas new grains start the process with the existence of an unfa-
vored local strain. This case study reveals the importance of analyzing phase bound-
aries for designing batteries and gives a good explanation why attempts to model
the phase behavior by other scientists previously were not accurate. New in situ
microscopy techniques allow battery reactions to be visualized in novel ways,
providing new evidence for verifying existing models and formulating new models,
which could lead to advances in battery science and technology. In another study,23
rapid and reversible movement of the phase boundary was visualized in a LiMn2O4
nanowire electrode during cycling, which explained the excellent rate capability and
reversibility of the LiMn2O4 cathode. Cyclic voltammetry was performed in relation
to Li/Li+ cycled between 2.7 and 5.2 V with pA level current.
The atomic-scale reaction mechanism can be revealed by in situ TEM. Su et al.24
analyzed a ZnFeO4 anode for a LIB with in situ TEM and found that intercalation of
the Li ion into ZnFeO4 induced the formation of Fe and Zn nanocrystallites inside
a Li2O matrix with volume expansion. During delithiation, Fe and Zn were oxidized
and Li2O disappeared but ZnFeO4 was not formed. It will be interesting to image
other mixed transition-metal oxides and compare them with ZnFeO4. Leenheer
et al.25 studied the plating and stripping mechanism of a Li-metal anode and
concluded that during Li stripping, the dissolution is initiated from the Li grains at
discrete points. They also found that an electron beam can induce changes in Li’s
microstructure, emphasizing that attention must be paid to beam-induced artifacts
when drawing conclusions from in situ TEM. Utilizing the electron energy-loss
spectroscopy (EELS) tool in TEM, Holtz et al.26 successfully revealed the lithiation
and delithiation dynamics of a LiFePO4 electrode. This work opens the door for
the study many other intercalation electrodes with a two-phase mechanism. A very
Chem 4, 438–465, March 8, 2018 443
recent work27 imaged the morphology of the product in a Li-O2 battery with a solid-
state electrolyte in which the reaction mechanism remained unclear. It showed that
the initial discharge product was LiO2, which subsequently decomposed into Li2O2
and O2, creating a hollow nanostructure after the release of O2. The formation of the
hollow structure can be explained with a diffusion-limited hypothesis. With the help
of selected-area electron diffusion, it was determined that LiO2 decomposes to
Li2O2 and O2. During charge, the hollow structure shrinks and collapses. This is
the first time that the Li-O2 reaction pathway has been monitored at the atomic level.
With aberration-corrected environmental TEM, the reaction can be conducted un-
der an O2 environment. This platform can potentially be used to study many other
catalytic reactions involving O2 at atomic level. A recent study on the lithiation of
a SnS2 anode in a LIB showed that, after the initial Li intercalation, crystalline SnS2goes through a two-step reaction. First, amorphous Li sulfide and metallic tin were
formed irreversibly. Second, the rate-determining step was a reversible transforma-
tion of metallic Sn to Li-Sn alloys. Discovery of a self-assembled framework, which
confines local individual Sn particles and prevents particle agglomeration during
the cycling, has also been reported.28 The effect of nitrogen doping on anatase
TiO2 nanotubes as anodes in a LIB has been studied.29 The results indicate that
nitrogen doping increases the conductivity while keeping the structural and chemi-
cal properties the same. A three-step lithiation mechanism was also revealed; after
the initial Li intercalation, there was a phase transformation from anatase TiO2 to
orthorhombic Li0.5TiO2 and finally to polycrystalline tetragonal LiTiO2. In situ TEM
is outstandingly helpful for probing the reactions of core-shell materials, especially
for carbon-coated materials, because it is easy to distinguish the light amorphous
carbon shell from the darker core. Liu et al.18,30 visualized the expansion of Si core
inside the carbon shell without damaging the shell during lithiation (Figure 2C).
Similarly, battery reactions of other core-shell materials can be visualized in a similar
manner by in situ TEM for the study or confirmation of the reaction mechanism.
Thermodynamic properties can have a significant influence on battery performance.
Recently, Hwang et al.31 studied the thermal stability of a P2-NaxCoO2 cathode for
SIBs and discovered that the surface of NaxCoO2 became porous and decomposed
to Co3O4, CoO, and Co because of reduction of Co at elevated temperature. Using
in situ TEM combined with electron diffraction clearly resolved the structural change
upon the increase in temperature and the local composition. Because a significant
increase in temperature can happen under battery abuse or thermal runaway, the
study of electrodes, electrolytes, and their reactions under very high or very low
temperatures is a promising future research direction.
In situ cell fabrication is an important part of a successful in situ electron microscopy
experiment. Fawey et al.19 provided a reliable method of fabricating a micrometer-
sized solid fluoride-ion battery, and the micro-battery was also analyzed under SEM
to show the structure of the whole battery. The schematics of the full cell and the half
cell, as well as the SEM images, are shown in Figures 2D and 2E. In the article, Fawey
et al.19 also discussed the challenges of the cell, such as contamination, redeposition
of the metal, porosity tuning, resistance, and current leakage, and provided guid-
ance for the readers.
Combining multiple in situ techniques for probing a battery is another interesting
direction. Sacci et al.32 studied SEI formation and the growth of Li-metal nanoclus-
ters in a LIB by using a novel in situ electrochemical scanning transmission electron
microscopy (STEM) platform combined with annular dark-field STEM imaging.
Current as small as several nanoAmperes was measured and correlated to the
444 Chem 4, 438–465, March 8, 2018
reactions. Furthermore, in situ measurement of material deposition thickness and
mass was achieved by annular dark-field STEM.
Researchers have also used all-solid-state batteries for in situ TEM studies. A quan-
titative study on the interfacial impedance inside a LiCoO2/LiPON/Si thin film
battery revealed that the interfacial impedance between the LiCoO2/LiPON inter-
face was created by chemical changes instead of space charge effects.33 Another
study34 of high-voltage delithiation using a LiCoO2/LLZO/Au cell showed that
different from a cell with a liquid electrolyte, single LiCoO2 crystals become nano
polycrystals connected by coherent twin boundaries and antiphase domain bound-
aries. A Li migration pathway for both before and after polycrystallization is also
provided. The two articles not only show the atomic scale differences between
complete solid-state batteries and batteries with liquid electrolytes but also give
suggestions and insights on all-solid-state TEM cell design.
Although in situ TEM is already a powerful tool for studying battery reactions with
high resolution, there are several areas that need further development. First,
because of the complexity of in situ TEM cells and the setup, researchers often
need to develop their own design and protocols, which limits the routine use of
the tool. Second, degradation of materials under extremely high electron beam
energy may lead to artifacts in the result. Third, the in situ cells need further optimi-
zation to increase the performance, e.g., the existence of high overpotential is
needed as a driving force for solid-state electrodes leads to inaccurate reaction
kinetics. Also, for a battery with ionic liquid (e.g., a single nanowire battery), the
interface between the liquid and electrodes is much smaller than that of commercial
batteries, in which the electrodes are usually flooded in electrolyte. For a sealed
liquid cell system, probing liquid under TEM can lead to poor contrast and inaccu-
racy of the image. Fourth, the intrinsic limitations stemming from TEM still exist,
i.e., the image is a 2D projection, which does not reveal the 3D geometry of the
material (3D tomography is still too slow for in operando studies), and it is difficult
to distinguish between materials with similar properties without the help of electron
diffraction. The high cost of in situ TEM battery fabrication is another big obstacle for
broader accessibility.35
X-RAY MICROSCOPY
X-ray is a type of electromagnetic radiation with a wavelength of 0.01–10 nm. X-ray
sources are usually divided into two categories on the basis of energy: soft X-rays
with <2,000 eV energy require less acquisition time and beam damage but pene-
trate less into the sample, and hard X-rays with >4,000 eV energy require more
acquisition time and beam damage and penetrate deeper into the sample. Soft
X-rays require a liquid about 1 mm thick, similar to TEM, but have the ability to image
samples that are tens to hundreds of nanometers thick. On the other hand, hard
X-rays allow the use of plastic pouch cells with liquid electrolyte hundreds of micro-
meters thick, which are relatively easy to fabricate, but enough contrast can be
obtained only for samples thicker than about 5 mm. Two major methods, scanning
probe and full field, are usually used for X-ray microscopy. For full-field microscopy,
the zone plate is equipped downstream of the sample, providing a single-shot high-
resolution image with a large field; scanning probe mode involves multiple scanning
steps with the zone plate placed upstream of the sample. Different characterization
methods such as X-ray fluorescence and X-ray diffraction can be coupled for a small
sample area. Full-field mode induces beam damage to a larger area, whereas scan-
ning probe mode damages a specific small spot. The acquisition times for the two
Chem 4, 438–465, March 8, 2018 445
methods are of similar magnitude. A detailed review on X-ray microscopy can be
found elsewhere,36 and information on X-ray microscopy, such as X-ray penetration
lengths, can be found in the CXRO database.37
Transmission X-Ray Microscopy
Transmission X-ray microscopy (TXM) uses electromagnetic radiation to probe the
sample up to the atomic level on the basis of zone plate scanning. It can image
thicker samples than TEM because X-rays penetrate through most samples, and
TXM overcomes in situ TEM’s limitation of high spatial resolution within a small field
of vision. Another advantage of TXM is that it can be combined with X-ray absorption
near-edge structure (XANES) to obtain chemical information about the material.2
Also, compared with TEM, TXM experiments can be conducted under gaseous
environments. There are two main categories of TXM: scanning probe X-ray micro-
scopy is called scanning transmission X-ray microscopy (STXM), and full-field micro-
scopy is called TXM.
Researchers have demonstrated that in operando TXM can be used to extract infor-
mation from both deep within batteries and on their surface. Cheng et al.38 showed
the capability of in operando hard X-ray TXM to probe Li dendrite growth with a
unique in situ plastic battery setup. They visualized Li dendrite growth dynamics
on a Cu surface, as shown in Figures 3A and 3B. Both the counter and reference elec-
trodes were Li metal wrapped on polished Cu wire, and the voltage profile is related
to in operando images at specific temporal points under various current densities.
This work gives insight on Li dendrite growth on different substrates as well as
how to design an in operando TXM battery in an easy and accessible way.
TXM can provide both morphological and chemical information on battery reaction
kinetics. Recent developments have made the spatial resolution high enough to
clearly image single-particle level transformation. Lim et al.39 design a liquid cell
by using soft X-ray STXM to study the insertion reaction of LiFePO4 at single-particle
resolution (Figures 3C and 3D), which could be adapted to other materials. The
counter and reference electrodes were Li foils, the voltage profile cycling at different
rates ranged from about 2 to 4.5 V, and a plateau was observed around 3.4 V. The
voltage profile was similar to that of a typical LiFePO4 coin cell at low rates but devi-
ated from it at high rates. They revealed that heterogeneities in the surface reaction
rate is one of the key components during battery operation. Higher rates improve
LiFePO4 performance during lithiation but account for nonuniformities during the
delithiation process. This work confirms that elastic strain affects phase separation
and controls the equilibrium of Li distribution for insertion. Analyzing the phase
change at various currents revealed that Li distribution during delithiation was less
uniform than during lithiation. Therefore, the authors could improve the cycle life
of LiFePO4 by increasing the lithiation rate to overcome the nonuniformity in the
material, providing important guidance to the battery community. TXM has also
been used to visualize phase separation in batteries. Li et al.40 imaged LiFePO4 par-
ticles cycled at various current densities with soft X-ray STXM and revealed that,
unexpectedly, the current density does not vary much when the global electrode
cycling rate changes; only the number of particles participating in reaction changes.
This new understanding provides important guidelines for the design and engineer-
ing of LiFePO4 and possibly other cathode materials.
Combined with other tools such as neutron powder diffraction, TXM can be used to
understand the morphology, crystal structure, and phase change, thereby corre-
lating chemical information to electrochemical information. The article by Chen
446 Chem 4, 438–465, March 8, 2018
Figure 3. Schematics of Apparatus on In Operando X-Ray Microscopy
(A and B) Schematic and picture of an in operando TXM setup (A) and in operando TXM results of a Li plating process, correlated to the electrochemical
data (B). Reprinted with permission from Cheng et al.38 Copyright 2017 American Chemical Society.
(C and D) In operando TXM design for probing the LiFePO4 electrochemical reaction (C) and results of the lithiation and delithiation of LiFePO4 (D) from
Lim et al.39 Reprinted with permission from AAAS.
et al.41 provides a methodology for studying battery reaction initiation and degrada-
tion and ideas for combining different tools with hard X-ray TXM. It also points out
that suppressing phase separation and lattice change help to retain battery perfor-
mance. Ohmer et al.42 studied the phase evolution of a micrometer-scale LiFePO4
single crystal with soft X-ray STXM. The method of battery fabrication in the article
gives insights to researchers who are interested in in situ STXM at the single-particle
level, and the strategy can be adapted to the study of other batteries. Li et al.43 com-
bined soft X-ray STXM with TEM to probe the lithiation sequence of a LiFePO4
battery and drew the important conclusions that directly probing the reaction steps
Chem 4, 438–465, March 8, 2018 447
helps to understand the rate-limiting reaction and that conductive additive loading
plays an important role in controlling the lithiation process.
TXM can also be adapted to probe pore-size changes within materials, and compared
with TEM, TXM offers a bigger field of view instead of imaging a single pore. With a
high specific capacity, Sn has the potential to replacegraphite as the negative electrode
in LIBs, yet Sn alloyingwith Li acceleratesmaterial degradation. Cook et al.44 concluded
that only increasing porosity is not an efficient way to promote battery performance; the
connections between pores, pore distribution, and particle size all need to be consid-
ered. Also, the article revealed that other alloying metal electrodes similar to Sn may
be monitored under in situ hard X-ray TXM, and future research directions can focus
on how to tune the porosity directly under monitoring.
TXM is compatible with systems that involve gases, which is an advantage over
typical TEM techniques. Olivares-Marın et al.45 studied an ether-based Li-O2 battery
by using oxidation-state-sensitive full-field soft X-ray TXM. The article concluded
that strategic control of the discharged product layer with a small surface-to-volume
ratio is necessary for maintaining good rechargeability. Also, TXM shows that the de-
posits during the reaction are mushroom-cap shaped instead of toroidal shaped,
which has never been discovered before. The article provides a representative
example for the study of metal-air batteries.
The limitations of TXM are as follows: (1) TXM introduces radiation damage to X-ray
sensitive samples. (2) Compared with electron microscopy, contrast mechanisms
and analytical tools of TXM are still limited. (3) At the moment, research articles
are mostly focused on very few model materials (e.g., LiFePO4). Expansion to other
material systems and combining TXM with other tools are needed. (4) Because the
methodology is still in an early-access state, a general protocol has not yet been
developed for comparing results across research labs.
X-Ray Tomography
X-ray tomography (XRT) collects a series of 2D projections of the sample when the
sample rotates nearly 180� and combines them to form a 3D structure. A 3D compo-
sition profile of a single molecule can be simulated and created with the help of XRT,
which is advantageous over other microscopies. Researchers can also combine XRT
with multiple X-ray spectroscopies to understand batteries from different aspects
both chemically and spatially. A review of the application of XRT specifically to Li
batteries can be found elsewhere.46
In an early work, Harry et al.47 probed dendrite growth on a Li-polymer symmetrical
cell with micrometer-scale hard X-ray XRT to show that contrary to intuition, early-
stage Li dendrites form under the surface of the electrode. This pioneer work not
only expanded our understanding of Li dendrite formation but also demonstrated
the power of XRT in the study of battery reactions in situ with deep penetration
and high resolution. This successful demonstration provoked further utilization of
XRT in battery in situ diagnosis. Another pioneer work by Ebner et al.48 demon-
strated the power of hard X-ray XRT in characterizing core-shell lithiation, crack
initiation and growth along preexisting defects, and irreversible distortion of SnO
electrodes with quantitative visualization of their volume change. Both articles
provided new opportunities for XRT in battery diagnosis.
After these early works, researchers have used XRT to diagnose other battery
systems. Wang et al.49 studied the sodiation and desodiation processes with
448 Chem 4, 438–465, March 8, 2018
nanoscale hard X-ray XRT by tracking Sn 3D structure changes in a SIB. The cell was
cycled between 0.005 and 1 V with a current density of 5 mA/g versus Na/Na+;
although the voltage range was narrower than in a typical battery, the plateaus
at specific voltages matched the data reported previously. It was long believed
that because Na ions have a larger radius, the volume expansion during the reac-
tion is more dramatic and the diffusion rate is slower than that of Li. Surprisingly,
however, Wang et al. did not find obvious pulverization or structural failure during
the desodiation processes after the first cycle, and most of the damage occurred
during sodiation. Also, the critical fracture size was identified to be between 0.5
and 1.6 mm, larger than the critical fracture size of silicon when alloying with Li.
Moreover, when comparing (de)sodiation and (de)lithiation of the same material
(Sn), the authors showed that structural degradation occurs predominantly during
sodiation and delithiation. The result suggests that alloying electrodes in SIBs do
not necessarily suffer from fractures induced by volume change more than those
in LIBs, and making nanostructured Sn anodes for SIBs could potentially solve
the fracture problem. This work demonstrates the power of in situ XRT in shaping
the research direction of battery sciences. Yermukhambetova et al.50 analyzed Li-S
batteries with in situ hard X-ray XRT and found that there are unavoidable mass
transport limitations with thick sulfur electrodes. During cycling, sulfur agglomer-
ates to become larger particles and occupies more volume. Zielke et al.51 focused
on Si electrodes in LIBs. The in situ hard X-ray XRT results are complementary to
those of previous researchers while revealing that lithiation happens only if Si par-
ticles have at least 40% of surface and electrical surface contact. This article also
called for the need to optimize the entire electrode to increase active material uti-
lization. In a recent report on the morphological change of a Si anode in an LIB with
20 keV hard X-ray, the 3D model clearly indicated that more intense structural
changes and disconnections of Si particles occur at the bottom part of the elec-
trode and mainly in the transverse direction.52
In addition to solid electrodes, in situ XRT can also be used to monitor gas bubble
and channel formation and distribution in batteries. Sun et al.53 visualized gas-bub-
ble evolution in a LIB with hard X-ray, which provides guidance on managing gas
evolution and enhancing the safety of batteries. The cyclic voltammetry (CV) and gal-
vanostatic cycling with potential limitation (GCPL) results for Li/Li+ are in the same
range as for a typical battery. The technique can also be used to study gas evolution
in aqueous batteries.
Apart from traditional battery systems, researchers have developed novel in situ XRT
tools for studying flow batteries and fuel cells. Jervis et al.54 introduced a detailed
special design for redox flow batteries. Despite the complexity of the flow battery
itself, the article successfully demonstrated the capability of in situ hard X-ray XRT
for a flow battery and claimed that the battery can be used under both lab X-ray sour-
ces and synchrotron sources, which will potentially make this technique more acces-
sible and widely used.
Different from the typical research focus, Eberhardt et al.55 used in operando hard
X-ray XRT to image themovement and redistribution of phosphoric acid in a polymer
membrane fuel cell, which is difficult to study with conventional techniques. The
influence of current density and membrane doping level was found to affect the
migration of phosphoric acid in the membrane. With the help of in operando XRT,
the pore- and particle-size distributions on the gas diffusion layer were clearly visu-
alized. The results showed that XRT can visualize small guest molecules or ions inside
a solid electrolyte, which opens up many research opportunities.
Chem 4, 438–465, March 8, 2018 449
Schroder et al.56 demonstrated the capability of in situ hard X-ray XRT for studying
various battery systems, including zinc-oxygen, sodium-oxygen, and metal-sulfur
batteries. The article provides general guidelines for using XRT to study batteries,
e.g., how to design batteries for XRT, what aspects need to be considered during
the experiments, and what problems researchers might encounter during the anal-
ysis. The article is helpful for those who are interested in in situ XRT probing on
batteries from scratch. The article also shows that although XRT can give a better
understanding on phenomena such as dendrite formation and electrolyte transition
in the gas diffusion layer with good spatial resolution, attention should be given to
the sample complexity, heterogeneity, and reproducibility.
A potential limitation of XRT is that organic solvents, which are often used in LIBs,
can be sensitive to the X-ray beam, which generates reactive radicals. Also, synchro-
tron X-ray sources with much higher intensity, which are required for getting
real-time chemical information, are not as accessible as laboratory X-ray sources.
Like other microscopy techniques, specially designed batteries are typically needed
to make sure the X-ray beam is not blocked during rotation to get 3D tomography.
Careful alignment of the sample rotation axis is necessary for XRT to provide an
accurate 3D image.
SCANNING PROBE MICROSCOPY
SPM uses a micro- to nano-level tip to detect either the current or the static force on
the sample surface, providing detailed surface information by ‘‘touching and
feeling’’ the surface. It is a less explored tool for in situ imaging of batteries but
has unique capabilities to reveal characteristics that are hard to reveal by other tech-
niques. Commonly used AFM and modified SPM suitable for electrochemical
measurements are discussed below. Two recent reviews summarizing the history
of SPM and its challenges, as well as information acquisition from SPM, can be found
elsewhere.57,58
Atomic Force Microscopy
With a probe tip continuously touching the sample surface, AFM provides a 3D topo-
graphic image. Compared with other microscopy techniques mentioned above, in
situ AFM has fewer requirements for sample preparation and pretreatments. Also,
AFM experiments can be conducted in ambient air or a liquid environment. The
tool is often combined with other microscopy and spectroscopy to provide informa-
tion on the objects from different aspects.
In an early demonstration, Balke et al.59 used ex situ AFM to study grain clusters,
single grains, and defects in LIB electrode material. It was revealed that at specific
grains and grain boundaries, Li-ion diffusivity was higher; this understanding gave
insight on optimizing grain boundaries to improve battery performance. This work
raised much interest in developing methodologies to study battery materials by in
situ and in operando AFM.
Combined with other tools, such as spectroscopies, AFM can be used to study
interfacial properties and dynamics, as well as various reaction kinetics. Lang
et al.60 studied the nanoscale Li2S cycling process. The results showed that the reac-
tivity of Li2S depends highly on its structure. Insoluble Li2S2 accumulation at the
interface causes a decrease in battery performance. The results were confirmed
with Raman spectroscopy. AFM can also probe metal-oxygen batteries by revealing
the discharge product morphology. Liu and Ye61 studied the growth and
450 Chem 4, 438–465, March 8, 2018
Figure 4. Design and Results of In Situ AFM
(A and B) Schematic of a sodium-ion battery for in situ AFM (A) and in situ AFM results before and after passivation for a sodium-ion battery (B).
Reprinted with permission from Lacey et al.62 Copyright 2015 American Chemical Society.
(C) In situ AFM images of a HOPG electrode cycled at a scanning rate of 0.5 mV/s between 3.0 and 0.0 V in 1 mol/L LiPF6 in FEC/DMC. Reprinted with
permission from Shen et al.64 Copyright 2015 American Chemical Society.
morphological changes of Li2O2 on a gold electrode during the oxygen reduction
reaction (ORR) and oxygen evolution reaction (OER) processes in a DMSO-based
electrolyte solution. The water concentration during the ORR reaction was found
to influence the shape of Li2O2, and the ORR or OER reaction was irreversible in
the DMSO solution.
In situ AFM has been successfully utilized for studying SEI in batteries. Lacey et al.62
designed an in situ cell to study SEI formation on a MoS2 electrode in a SIB, as shown
in Figures 4A and 4B. The reference electrode used was Na metal, and the cutoff
voltages were 0.4 V (the voltage before sodiation) and 1 V (at which no sodiation
occurs). The cutoff voltages were similar to those of a typical SIB. SEI formation
and permanent material wrinkling induced by strain were observed directly at low
potential. This is the first time that in situ AFM has been used for studying SIBs.
The SEI formation mechanism in SIBs is similar to that in an ethylene carbonate
(EC)/Li/graphite system, except the SEI is thinner in SIBs. This observation leads to
speculation that the SEI in SIBs is denser or more insulating, which needs to be
Chem 4, 438–465, March 8, 2018 451
further verified. This work highlights the importance of interfacial reactions and
structures in affecting battery performance and pioneers the use of in situ AFM for
studying the SEI. The low vapor pressure of an ethylene-carbonate-based electrolyte
allows an open cell configuration, which can also be adopted for studying SEI in both
sodium and Li-ion solid-state electrolyte systems but is difficult for aqueous batteries
because of the high vapor pressure of water. A recent study compared the stability of
the SEI layer formed on an Fe3O4 electrode from fluoroethylene carbonate (FEC)-
and EC-based electrolytes in LIBs.63 Although the FEC-based SEI is more stable,
an optimized electrolyte with suitable carbon coating techniques is necessary for
the formation of an intact SEI layer to prevent air-bubble evolution and dendrite
growth on metal oxide electrodes.
Combing AFM with X-ray photoelectron spectroscopy (XPS), Shen et al.64 studied
the SEI on a highly ordered pyrolytic graphite (HOPG) electrode and compared
EC- and FEC-based electrolytes. The results for in situ AFM on FEC are shown in Fig-
ure 4C. The CV results are related to real-time AFMmeasurements and show that for
both EC and FEC electrolytes, there are top and bottom layers for the SEI, but for
FEC the SEI is thick and dense, which cannot be easily removed by the AFM probing
tip. Although the SEI consumes a larger amount of Li, it prevents dendrite formation.
The article suggests that researchers should balance the SEI properties carefully to
improve LIB performance.
Variants of Traditional AFM
Variants of traditional AFM have recently been utilized for studying electrochem-
istry.65 The lithiation- or delithiation-induced transition between Li4Ti5O12 and
Li7Ti5O12 was first studied by conductive AFM by Verde et al.66 The voltage reported
in the article lies in a similar range of 1–2.6 V versus Li/Li+ to the previously reported
data. The former compound is conductive, whereas the latter is insulating. Different
from conventional wisdom, it was clearly demonstrated that the transition between
two materials occurs via percolation channels within single grains instead of grain
boundaries, which means that the current is not distributed evenly across the
surface. Another important discovery is that different from what was believed previ-
ously, the Li4Ti5O12 reaction forms an SEI layer. This novel research platform can pro-
vide insight on other electrode materials that have a change of conductivity during
cycling, providing strategies for studying zero-strain material in which there is hardly
ever any volume or morphology change during the reaction, as well as materials with
insulating to conducting behavior. Using another variant of AFM, Yang et al. map-
ped the temperature dependence of electrode materials.67 This article provides
complimentary data to the conclusion that the diffusivity of Li ions in a
Li1.2Co0.13Ni0.13Mn0.54O2 cathode increases as the temperature increases. In the
future, piezoresponse force microscopy68 could be applied to image electrode
materials that have ferro- and piezoelectric properties. A recent article presents
the study of Li intercalation in graphite particles in LIBs via two different methods
of AFM.69 With the first method, the authors revealed the detailed structures of
the graphite particles by charging the applied fields. By periodically injecting ions
into the graphite, they could distinguish active particles from inactive particles
with the second method. The two methods provide strategies for researchers to
analyze the same material from different perspectives.
The limitations of SPM also apply to in situ SPM. For example, it is usually difficult to
accurately measure steep trenches or walls, the speed of image formation is slow,
and an open cell configuration has to be used, which makes it difficult to study sys-
tems in volatile liquid electrolytes. Also, compared with other microscopy, AFM has
452 Chem 4, 438–465, March 8, 2018
a higher risk of the probe tip scraping the sample away or damaging the battery as
well as the tool. Another limitation is that AFM provides only topographic informa-
tion, and it needs to be combined with other tools such as various spectroscopies to
get a comprehensive understanding of the sample. Modifications on the AFM tool
instead of the samples are often needed for in situ and in operando purposes, which
can be a potential barrier for wide deployment of the tool.
Scanning Electrochemical Microscopy
Different from conventional SPM, scanning electrochemical microscopy (SECM) is
capable of not only surface analysis but also bulk material analysis by applying
different modes according to the interests. More importantly, as the name suggests,
SECM is powerful for measuring electrochemical activity. The surface analysis
depends on the feedback mode. The most frequently used feedback mode uses a
tip that provides positive feedback when it encounters a conductive surface area,
whereas negative feedback is detected with an insulating surface area. This feature
can monitor the formation of a very thin SEI, which cannot be detected by traditional
AFM. The feedback mode could be used to analyze the insulation caused by gas
bubble evolution and detachment. Bulk analysis consists of several different modes:
substrate generation/tip collection (SG/TC) mode, scanning electrochemical cell mi-
croscopy mode (SECCM) and scanning ion conductance microscopy (SICM) mode.
SG/TCmode cannot distinguish some overlapping reactions inside LIBs, but SECCM
and SICM use a nano pipette with electrolyte in it to monitor the surface electro-
chemical activity. In SICM, both the sample and pipette are immersed in the electro-
lyte, yet the resolution and sensitivity are reduced. SECCM is suitable for aqueous
electrolyte whereas SICM is suitable for organic electrolyte. Researchers have also
attempted to monitor Li-ion activity by using a Hg/Pt ultramicrolectrode with
SECM. The results are summarized and discussed in another review.70 SECM’s
unique capability to obtain spatially resolved electrochemical information deserves
more attention in the battery community.
OPTICAL MICROSCOPY
Traditional Light Microscopy
Optical microscopy is the oldest, simplest, and most accessible technique among
those described in this review, yet it is also the most underutilized technique. It is
best used for monitoring and discovering interesting reactions, phase changes, or
phenomena at the microscale. The differences in optical absorption for different
materials allow a sample of nanometer-level thickness to be imaged through a milli-
meter-thick glass housing. But for electron and X-ray microscopies, the contrast in
absorption requires a very thin housing for thin samples, such as for TEM and soft
X-ray, whereas a thicker housing can be used only for thicker samples such as for
hard X-ray. With optical microscopy, the battery can be cycled under ambient
conditions with different gas species and electron- and radiation-sensitive materials
inside the battery. This advantage also opens the door to various aqueous batteries,
which would be difficult to study otherwise. The setup of a planar glass battery and
the structure of the electrodes are closest to a typical battery, which means that
analysis of the battery can be directly applied to the most widely used battery
models. Also, when it is coupled with RGB analysis and simulation, different and
more general information can be obtained from optical microscopy for modeling
the entire battery rather than focusing on a few atoms or molecules.71–74
Harris et al.75 claimed to be the first group to report direct in situ measurements of
Li transport in an operating cell in 2009. They suggested that transport phenomena
can be controlled by liquid-phase diffusion. Also, the authors pointed out that
Chem 4, 438–465, March 8, 2018 453
Figure 5. Design and Results of In Operando Light Microscopy
(A) Schematic of a glass cell for optical microscopy. Reprinted from Sano et al.77
(B and C) Schematic of an in operando Li-S cell and the imaging setup (B) and in operando optical microscopy results for a Li-S cell (C). Reprinted from
Sun et al.78
the color of the images might not represent the true color of the sample and
that other spectroscopy tools are needed to confirm the results. This pioneering
work and battery design has been adopted for other battery systems, such as
aqueous batteries.76 A typical planar cell design is shown schematically in
Figure 5A.77
454 Chem 4, 438–465, March 8, 2018
By using a cylindrical battery design and imaging from the side, Sun et al.78 success-
fully visualized soluble polysulfide formation in a Li-sulfur (Li-S) battery. The battery
design and in operando results comparing sulfur and sulfur encapsulated with
poly(3,4-ethylenedioxythiophene) (PEDOT) are shown as Figures 5B and 5C. The
cell was cycled between 1.7 and 2.6 V with 10 mA versus Li/Li+, and a plateau at lower
than typically reported values between 2.1 and 2.6 V was obtained because of the
low current density used. The article provides a simple methodology for battery
analysis and helps in understanding the ‘‘shuttle effect’’ in Li-S batteries. It is fairly
straightforward to apply the methodology to other battery systems.
Optical properties of materials usually change upon electrochemical reaction, which
can serve as a contrast mechanism for imaging. Xiong et al.79 studied the insertion of
Li+ into MoS2 nanosheet, and the lithiation and delithiation processes are clearly
visualized. They were the first group to provide the live process of Li intercalation
intoMoS2 flakes. They cycled the cell from 2 to 1 V versus only Li/Li+ to avoid irrevers-
ible reaction. The evidence helps elucidate the optical changes of the material upon
Li intercalation, as well as the reactive boundary propagation in MoS2 and other 2D
materials. Takachi and Moritomo80 successfully studied the Li+ deintercalation
phase separation behavior in cobalt hexacyanoferrate (LixCo[Fe(CN)6]0.9) films and
concluded that phase separation does not exist in the films but that the external
strain influences the performance. Both examples above demonstrate the unique
contrast mechanism that stems from the materials’ optical properties. However,
the true color of the sample could be distorted by the contrast mechanism and could
be different from what is seen on the camera.
Because of the minimum requirements for the sample and device, optical micro-
scopy can be easily correlated with other in situ microscopy. Wan et al.81 studied
Li-MoS2 batteries by using correlated in situ optical microscopy and TEM. The opti-
cal results provided in situ visualization on the lithiation of a MoS2 flake with a large
field of view, whereas TEM provided atomic structural information. A thickness
dependence of the resistance of MoS2 was observed: flakes thicker than 30 nm
had a dramatically lower resistance. Also, fast lithiation, i.e., shorting the battery
at the first cycle, significantly increased the battery performance, which is counter-
intuitive. Both phenomena can be attributed to the hypothesis that a conducting
Mo network is formed, which benefits the performance of sulfide electrodes. The
thickness-dependence study requires multiple experiments with different sample
thicknesses, which, if done with in situ TEM or TXM, is too tedious. Yet, in situ optical
microscopy has simpler sample and device preparation and was chosen by the
authors to demonstrate the experiment.
Lithium dendrite growth on nano-thick nitrogen-doped amorphous carbon films was
studied by Zhang et al.82 with in situ confocal microscopy, and the visual results
showed that nitrogen-doped amorphous carbon films prevented Li dendrite growth
by SEI formation. Different from typical in situ optical microscopy cells, their cell
used a geometry similar to that of a coin cell, with an additional glass window on
the side, which made the observation conditions close to a practical battery. Kuwa-
bara and Sato83 reported real-time dendrite growth on the positive electrode within
an LIB during the charging process because of Cu contamination. The results
improve our understanding of contamination behavior in batteries and provide in-
sights on future battery fabrication. Kabzinski et al.84 also used in situ confocal micro-
scopy to study lead-acid batteries at different temperatures. The article focuses
mainly on sample preparation procedures, with the help of in situ optical micro-
scopy; different washing, drying times, and temperatures are tested, and the
Chem 4, 438–465, March 8, 2018 455
process is optimized. The results provide insights on future laboratory and industrial
battery fabrication procedures and prove that in situ optical microscopy is a simple
but powerful monitoring tool for process optimization.
Bai et al.85 utilized a glass capillary cell, rather than a conventional glass slide cell, to
visualize Li dendrite growth. They showed that Sand’s capacity is the key parameter
governing the growth behavior of Li. When the Li deposition capacity was below
Sand’s capacity, Li dendrite growth was reaction limited, grew from roots, and did
not penetrate a nano-porous ceramic separator. Above Sand’s capacity, transport-
limited dendrite growth at the sharp tips penetrated the separator easily. The
suggested Li whisker growth mechanism is different from Cu and Zn, and the authors
suggest maximizing Sand’s capacity and replacing separators with ceramic separa-
tors to prevent shorting and extend battery life. This work demonstrates the useful-
ness of in situ optical microscopy in probing battery reactions in liquid electrolytes.
The advantage of low beam damage with optical microscopy is particularly powerful
for time-lapse experiments. Long exposure, frequent sampling, and long-term
observation can be used without concerns about beam damage. Wu et al.86
designed a highly sensitive bifunctional separator that can detect Li dendrite and
provide a signal and voltage drop to avoid dendrite penetration and cause safety
issues. The authors carried out in situ optical microscopy to investigate the perfor-
mance of the separator. Aetukuri et al.87 fabricated a flexible Li+-conductive ceramic
membrane to suppress Li dendrite growth. By comparison under in situ optical
microscopy, they showed that for a Li symmetrical cell with LiTFSI electrolyte, the
dendrite grew dramatically within the typical setup after 6 min, but for the specially
designed membrane there was no dendrite even after 40 min.
In a recent study, Han et al. visualized the Br2 droplet emulsion phenomenon by
using a Pt ultra-microelectrode immersed in electrolyte by MEPBr.88 CV between
0.4 and 1.1 V was conducted versus Ag/AgCl, and current of less than 2 mA was
similar to the value reported previously. Although the electrochemical reaction
has been well studied and used in redox flow batteries to prevent Br2 permeation
through the battery membrane and self-discharging process, this is the first time
that it has been visualized by in situmethods. The Br2 droplets are formed heteroge-
neously and adsorbed onto the electrode. A chronoamperogram study resulting in a
current surge up to 7.38 times the baseline value during droplet collision with the
electrolyte indicated that electrochemistry at the droplet-electrode interface plays
an important role in the reaction.
Wojcik et al.89 used optical microscopy to quantitatively study the redox reaction
dynamics of graphene because optical microscopy provides a higher signal contrast
and spatial-temporal resolution than other in situ techniques. They showed that the
oxidation reaction operates with a micro-flower-like pattern going outward, whereas
the reduction of graphene oxide is a pseudo-first-order homogeneous reaction.
Also, pH is a key variable for the redox reaction.
Fluorescence microscopy has been used for studying the properties of different car-
bon papers.90 The model battery used in this article was an aqueous quinone flow
cell in which the redox module was controlled by UV light. It was concluded that
the electrochemical reaction in the porous carbon papers was heterogeneous on
the length scale of tens of micrometers tomillimeters, which could provide ameasur-
able effect on various electrochemical systems. The proposed method provides in-
sights on strategies for evaluating the performance of a variety of porous materials.
456 Chem 4, 438–465, March 8, 2018
Morphological change is not the only information that optical microscopy can
probe. Wan et al.91 used the change in transmittance to image the intercalation of
Na into reduced graphene oxide (RGO). The voltage profile versus Na/Na+ was in
the range of 0 to 2 V, similar to what has been reported for a typical SIB. The slightly
larger layer distance of RGO than of graphene allows Na ion intercalation. After Na
was intercalated, the transmittance of RGO increased from 36% to 79%, and the
sheet resistance decreased from 83,000 to 311 U/sq. The superior optical and elec-
tronic properties, together with the higher chemical stability than Li-RGO, show that
Na-RGO has great potential in optoelectronic applications. This fabrication of new
material and the discovery of its properties are enabled by optical microscopy’s
ability to probe optical properties such as color changes and transmittance, which
is difficult for other microscopies.
Also, because of the low beam damage, in situ optical microscopy is extremely
powerful at investigating processes in liquid electrolytes, such as evaluating the ef-
fect of different electrolyte additives on dendrite suppression. Banik and Akolkar92
studied the effect of a branched polyethylenimine additive on Zn dendrite forma-
tion. A more stable current was achieved temporally with an increase in the concen-
tration of a polyethylenimine additive at �1.57 V versus Hg/HgO. The results clearly
indicate that the additive is functional.
It would be interesting to investigate a sealed cell setup rather than the open cell
setup used in this work; a sealed cell would be closer to a practical cell involving
complicated gas evolution and the swelling problem. From this work, it is evident
that other tools such as ex situ SEM or TEM are often needed to complement the res-
olution limit of optical microscopy and provide nanoscale or atomic information.
A comprehensive study of dendrite formation in LiS batteries has been reported,93
and the effect of various electrolytes on dendrite formation was evaluated.
Markers and sensors have been developed to relate optical imaging contrast with
material properties. Roscher et al.76 developed a method of probing the state of
charge of a LiFePO4 cathode precisely by adding transparent conductive oxides
(electrochromic ITO) as a marker. This is an interesting direction to pursue because
it expands what can be visualized. However, the introduction of markers should not
affect the intrinsic electrochemical reaction.
Quantitative imaging also makes it possible to confirm modeling and predictions.
Thomas-Alyea et al.94 successfully observed the distribution of Li within graphite
by in situ optical microscopy and combined porous-electrode theory and Cahn-Hill-
iard phase-field theory to describe the flux of Li within the graphite. The model
matched the experimental results well. It was also shown that particles with low resis-
tance reacted first and had a higher risk of degradation.
The optical microscopymethodsmentioned above aremainly based on visualization
characteristics; chemistry-sensitive transmission and adsorption methods such as
in situ Raman and Fourier-transform infrared microscopies are also being used.
Raman microscopies are discussed later. There are comprehensive discussions on
this topic in other review articles mentioned in the Introduction.
The biggest limitation of optical microscopy is spatial resolution. This could be
compensated by combining it with other ex situ tools such as electron microscopy.
And the resolution problem is beginning to be addressed by fluorescence super-res-
olution microscopy (SRM), which is discussed in the next subsection. In addition to
Chem 4, 438–465, March 8, 2018 457
resolution, a change in color can provide information but might not reflect the sam-
ple’s true color. Overall, optical microscopy is an underused yet powerful tool for
battery science, especially for delicate material systems. Even though most existing
works involve imaging of dendrite formation or growth and color change, more
breakthrough discoveries can be expected with optical microscopy.
Super Resolution Microscopy
SRM, originally proposed for the higher-resolution study of details smaller than
Abbe’s diffraction limit of light (�0.2 mm) in the biology field, has hardly been
mentioned or applied to battery studies, but it could can be an interesting topic
in the future for the battery community.
SRM is usually used to image biological fine details down to 10–20 nm resolution.
Representative techniques include stimulated emission depletion (STED), total in-
ternal reflection fluorescence microscopy (TIRFM), and single-molecule localiza-
tion.95,96 The STED technique uses the stimulated emission phenomena to quench
an exciting laser beam with an annular laser beam to get a nanometer-level beam
volume; the desired sample is then depicted with a higher resolution. The principle
of the single-molecule localization technique relies on the fact that fluorescence can
be turned on and off. It uses a photo-switchable method, such as fluorescent dyes or
genetically encoded proteins, to label a sample and excite a subset of fluorophores
that are at least 0.2 mm apart at a time and then stack all the time-resolved data
together to enhance the resolution. Both of these techniques have developed
from far-field microscopy, which penetrates to greater depth than surface-focused
near-field microscopy. For near-field microscopy such as TIRFM, an area near the
interface between two different media is excited selectively with an evanescent
wave to eliminate background fluorescence and improve the resolution; the detec-
tion depth is limited to �200 nm. For biological science, SRM makes it possible to
have nanometer resolution without damaging live samples, and it is being used
increasingly in applications.
For the battery community, novel powerful microscopy could bring new understand-
ing on fundamental electrochemistry and new performance breakthroughs. Ideally,
electron microscopies can provide extremely high resolution, yet the high energy
required by high resolution induces severe damage and artifacts to the sample.
In situ cells often differ from the common battery geometries used by most
researchers. SRM, on the other hand, could become a unique and powerful micro-
scopy tool for studying batteries.
The most attractive advantage of SRM over other in situ microscopies is its use
of light as the probe. Compared with an electron beam and X-rays, light intro-
duces less radiation damage to the sample, especially liquid electrolytes and
soft materials in the electrodes. Light can also operate in ambient conditions; no
vacuum is needed. The device preparation for in situ or in operando experiments
is simple and convenient; usually a planar glass cell is sufficient. Third, no synchro-
tron facility is needed, which makes the equipment much more accessible and
available.
According to the principle of electrochemistry—electron gain and loss—special
chemicals that can be excited by fluorescence during electron gain or loss can be
added to the electrolyte.97–99 With this mechanism, a single molecular electrochem-
ical reaction can be captured. Chen et al.100–103 have done a series of pioneering
works on TIRFM probing single catalytic events by using fluorophores that turn on
458 Chem 4, 438–465, March 8, 2018
after redox reaction. One of their early works used the fact that Au colloidal (�6 nm)
is a catalyst to turn on the fluorescence in resorufin by reducing the dye from resa-
zurin with NH2OH. From the data collected, the surface restructuring time for the
Au catalyst was calculated to be 40–150 s. Three different types of catalytic
reactions for a single Au colloidal were discovered, revealing nano-level heteroge-
neity. With the same method, for example, single Au colloidal catalysts with larger
sizes can be analyzed for their different reactivity toward the same reaction as well
as the catalytic dynamics. A recent work highlights the power of SRM in providing
strategies for sub-particle-level catalyst design.101 The catalytic dynamics of a single
nano rod TiO2 catalyst for the OER was studied by TIRFM, and it was suggested that
catalyst reactivity was optimized by the selective blocking and deposition of sites
with different overpotentials and onset potentials. Although the work is not directly
related to batteries, future studies on air batteries could be carried out with a similar
setup.
The drawbacks of SRM are as follows: (1) The limited choice of fluorescent dyes limits
the electrochemical reactions that can be monitored, and there are other restric-
tions, such as the pH of the electrolyte and the electrochemical potential of the
reaction. (2) Image acquisition and processing for SRM could take longer than other
techniques because of the architecture of the data. In summary, SRM and other
biological science tools applied to in situ and in operando studies on batteries could
yield many new research opportunities.
THOUGHTS ON THE CHOICE OF APPROPRIATE IN SITU OR INOPERANDO MICROSCOPY
Choosing the appropriate microscopy for battery and material characterization not
only leads to new findings on battery fundamentals but also helps researchers
verify their ideas and work in a convincing manner. However, the first and most
important task is to understand the advantages and disadvantages of the micro-
scopy. In operando optical microscopy can be helpful for probing phenomena
over the entire area of the electrodes with color, but the resolution is limited.
Combining these factors, researchers can take advantage of optical microscopy
to do a first-step troubleshooting process if the experimental data cannot provide
a clear solution. TEM and X-ray microscopy can provide information on the nano-
scale or even structure changes and reactions of a single atom, which can be used
for detailed material characterization. SPM and SEM provide information on the
surface and general morphology of a micrometer-level battery and can be used
to probe defects.
Below, we present a case study of different microscopies on Si anodes in a LIB
to illustrate what information each microscopy can provide and hopefully guide
readers on selecting the appropriate microscopies for studying different problems
on battery materials. A corresponding schematic is shown in Figure 6.
In situ SEM104 reveals morphological changes in Si and SiO after cycling for a wide
range of particle sizes (0.1–20 mm). The cross-sectional image and elemental map-
ping after charging illustrates the strength of SEM over other microscopies. It is
concluded that for Si particles of 2 mm and larger, cracking is observed, whereas
small particles around 0.1 mm sinter together during cycling and then crack. Simply
replacing Si with SiO cannot solve the problem, because the oxygen in SiO is lost
during cycling. SEM enables the analysis of a wide range of feature sizes. The energy
for SEM is weaker energy than for TEM, which allows SEM to probe different depths
Chem 4, 438–465, March 8, 2018 459
Figure 6. Information from Different Microscopies on a Si Anode
(A) Information on the morphology and cross section by in situ SEM. Reprinted from Hovington et al.104
(B) 2D nano-level chemical composition and structure by in situ TEM. Scale bar: 50 nm. Reprinted with permission from He et al.105 Copyright 2014
American Chemical Society.
(C) 2D and 3D SEI information revealed by in situ AFM. Reprinted with permission from Tokranov et al.106 Copyright 2014 American Chemical Society.
(D) Chemical state changes probed by in situ Raman microscopy. Reprinted from Nanda et al.107
(E) 3D reconstruction from 2D information by in situ XRT. Republished with permission of the Electrochemical Society from Tariq et al.;108 permission
conveyed through Copyright Clearance Center, Inc.
(F) Information on 2D tomographic slices by in situ TXM. Scale bar: 100 mm. Reprinted from Sun et al.109
in the sample. Yet, SEM cannot reach very high resolution, and EDX analysis might
not differentiate elements with similar properties, such as Na and K. Operating in a
vacuum also limits the method.
In situ TEM is successful in tracing the fate of different coating materials on Si and
provides insight on the selection of battery coating materials.105 The results show
that Si particles coated with alucone by molecular layer deposition (MLD) lead to
the formation of Li2O upon lithiation, which is not the case with native oxides. After
evaluation of the electrical and mechanical functions, it can be concluded that for Si
particles smaller than 150 nm in diameter, MLD-coated Si particles overcome the
problems of particle deactivation and incomplete delithiation caused by native
oxides. The 2D projection clearly reveals the nano-level reaction mechanism and
460 Chem 4, 438–465, March 8, 2018
structure up to several nanometers. With both EDX and electron diffraction patterns,
spatially resolved chemical information is revealed in real time or quasi real time.
TEM provides very detailed structural and chemical information with extremely
high resolution, and this composition-determining method is more accurate than
SEM. The drawbacks with TEM are that it provides only 2D information, samples
have to be loaded on very thin grids, the beam damages the sample, it requires
vacuum or quasi vacuum operation conditions, and the sample holder and sample
preparation are complicated.
Techniques based on X-rays are used most often for long-term in situ microscopy.
The degradation mechanism of Si half cells can be studied by TXM.109 2D electrode
slices are recorded for both the charge and discharge processes; when the slices
are combined with attenuation curves, it is claimed for the first time that Si particles
go through deactivation processes during the first delithiation, and there are inac-
tivated Si particles that do not participate in the reaction. TXM can provide 2D in-
formation in different topographical slices and mapping of chemical composition.
On the other hand, the spatial resolution of TXM limits its use when extremely high
resolution is desired.
XRT can reconstruct 3D structures and has been used to elucidate the degradation
mechanisms of Si and other alloy anodes.108,110 It has been quantitatively analyzed
that the increase in contact resistance causedby lithiation-induced fracturing is signif-
icant: 44.1%of the current collector area is lost during operation at 100 mAwithin 1 hr.
The change in shape was reconstructed in 3D step by step, and the stress-induced
change in angular shape was obtained and compared with modeling results. The
unique information provided by X-ray microscopies includes 3D reconstruction and
the power to quantitatively model the data, which provides indispensable informa-
tion for material characterization. The drawback of XRT is also its spatial resolution.
SEI is critical for the performance of anode materials. A fundamental challenge for
Si anodes is that the large change in volume upon cycling makes the SEI on the
surface fracture, re-form, and accumulate. Tokranov et al.106 used in situ AFM to
study the relationship between SEI formation and Li insertion into a Si anode,
revealing that faster rates in the initial cycling generate a thinner and smoother
SEI. AFM is unique and powerful for surface analysis, providing 2D information
on the surface front and boundary changes and on surface roughness and
morphology and 3D surface-thickness information. SPMs are especially powerful
for SEI studies. A potential disadvantage of using SPMs for in situ microscopy is
slow data acquisition.
In situ Raman optical microscopy provides a different perspective on Si-based
anodes.107 Lithiation-induced stress is studied by monitoring Raman shift, and a
tensile-to-compressive transition in the Si core has been verified.111 Complete
disappearance of the Raman signal during the first discharge step indicates the
amorphorization of Si and the formation of Li-Si alloys. Broadly speaking, optical
microscopies can provide microscale morphological color and chemical-composi-
tion information to some degree. Confocal microscopy can even provide 3D recon-
struction. The limitations of optical methods are the spatial resolution and the depth
of penetration of light in microscale materials.
In summary, a detailed comparison between various in situ and in operandomicros-
copies is shown in Table 1.112 The comparison is only for individual microscopies and
does not include microscopies that are used in combination.
Chem 4, 438–465, March 8, 2018 461
Table 1. Summary of Characteristics for Each Typical In Situ and In Operando Microscopy
SEM TEM Soft X-Ray Hard X-Ray Scanning Probe Optical Super-resolutionOptical
Resolution (nm) <�10 <�0.2 <�30 <�35 depends on tipsharpness, <�several pm
<�0.2 mm <�10 nm
Sample thickness medium low low high only surface medium medium
Radiation damage medium high medium medium low low low
Ease of use medium low high low medium high low
Accessibility/cost high/high high/high low/medium medium/medium medium/medium low/low high/medium
3D capable no no no yes no yes yes
Compatibility withvolatile liquidelectrolytes
for liquid cell for liquid cell for liquid cell yes yes yes yes
CONCLUSION AND OUTLOOK
Various in situ and in operandomicroscopies are reviewed with regard to basic prin-
ciples and applications in the study of different battery materials. Novel techniques
and methodologies, new discoveries, and their implication for batteries are high-
lighted for each category of microscopy. Their limitations and future directions are
also discussed. Overall, with the capability of directly visualizing reactions and pro-
cesses, in situ and in operando microscopy are expected to continue to be indis-
pensable tools in the development and optimization of battery materials. Despite
the practical issue that it is extremely difficult for researchers to design in situ and
in operando batteries close to the typical cell models used in the battery community
nowadays, we encourage researches in the field to compare the current and voltage
signature in in situ and in operando batteries with those of typical batteries such as
coin cells and pouch cells, so that a strong foundation can be constructed for re-
searchers to compare results and evaluate their reliability.
The technique of usingmicroscopy to probe batteries is dynamic and ever changing.
Despite many years of advances in this field, there are numerous opportunities for
continued research and development. For example, it would be significant to trace
the movement of single Li atoms inside solids and across interfaces and boundaries.
One could achieve this by combining different microscopy techniques for a suitable
specimen and environment. Oshima et al.113 visualized Li atomic columns in LiV2O4
and LiMn2O4. This visualization method can be adapted for studying the movement
of Li atoms within different compounds. Imaging interfacial reactions inside batteries
is challenging and could also be an opportunity for future research. These reactions
and processes are highly beam sensitive, the detection volume is small, and they
also need high resolution to be resolved. The possible solution could come from
correlative microscopy. In situ cell design is less discussed but is often critical to
the success of an in situ experiment. Novel design is always needed for extracting
more information and resembling a common battery setup. Rather than pushing
the limit of techniques, finding new reactions or processes to image, such as side
reactions and gas evolution, is also important. The tools, methods, and designs dis-
cussed in this review could be adapted to other research fields such as catalysis and
environmental science.
ACKNOWLEDGMENTS
N.L. acknowledges support from faculty startup funds from the Georgia Institute of
Technology.
462 Chem 4, 438–465, March 8, 2018
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