Operando EC-AFM Imaging of Li-02
Battery Solid-Electrolyte Interphase
ASYLUM RESEARCH
It is commonly understood that batteries areelectrochemical devices that feature electricalcurrents, internal ion flows, and chemical reactions.
It may be less obvious that batteries also undergo
structural changes during their charge anddischarge cycles. Yet these changes have a criticaleffect on battery performance and especially battery
lifetime. One reason these structural changes are
sometimes overlooked is that they are surprisingly
difficult to measure. After all, for most batteryarchitectures it is not possible to “look” inside with
an operando measurement, that is, a measurementthat takes place during actual operation. Most
microscopy techniques, including light microscopy
and electron microscopy, cannot visualize what
happens inside an operational battery. Atomic
force microscopes (AFMs), however, provide asolution wherein model battery systems can bemeasured during operation. AFMs like the Asylum
Research MFP-3D and Cypher ES are available withelectrochemical cells, which enables AFM imaging
while also allowing the electrochemical reaction
to take place. This mode of operation is known asElectrochemical AFM imaging, or just EC-AFM.
For lithium (Li) battery technologies, we know
from post-operation analysis that a chemistrydependent solid-electrolyte interphase (SEI)

generally forms and dissolves during discharge andcharge cycles, respectively. In the schematic of ageneralized Li battery in Figure 1, the interfaces ofinterest for SEI formation are highlighted. However,
little is known about the structural evolution ofthis SEI as a function of time, charge state, andcycle number. Crucially, SEI structure influences
current density, battery capacity, achievable
output voltages, operational lifetime, and evensafety. So, understanding SEI evolution will help
make better, longer-lasting, safer batteries.
Other issues make this research even more complex.
Lithium metal is unstable in environments containing
oxygen, nitrogen, water, and other volatile andreactive molecules. So for safety and proper battery
operation, Li batteries are usually assembled andtested in dry, argon-filled gloveboxes. Also, theproprietary solvents and electrolyte blends used inbatteries (organic carbonates, ionic liquids, organicsoluble salts, etc.) can be toxic, flammable, still
under development, or poorly characterized.
Successfully navigating these challenges requires
an AFM with a well-controlled environment tosuccessfully image the dynamics of electrodes andcharge-transfer interfaces in batteries in realistic
operating conditions. Enter the Cypher ES AFM withFigure 1: Schematic depicting primary battery components, current, and ion motion during charge and discharge. The difficult to characterizeinterfaces of interest, where the SEI forms, are indicated by the yellow boxes.

© Oxford Instruments Asylum Research Inc.

Operando EC-AFM Imaging of Li-O2 Battery Solid-Electrolyte Interphase1

its accompanying Electrochemical Cell (EC cell) from
Oxford Instruments Asylum Research.1,2 As shown inFigure 2, the Cypher is straightforward to integratewith an argon glovebox, and its environmental
chamber allows for further atmospheric control
locally around the sample. Meanwhile, the ECspecific probe holder lets the AFM probe dip
down into an electrolyte bath, making it possible
to image coin-cell-type battery geometries.
One of Asylum’s experienced users, Dr. Kumar
Virwani used exactly this instrumentation atIBM Almaden Research Center to obtain theelectrochemical AFM (EC-AFM) data presented
below. He took advantage of the intense reactivityof Li towards oxygen and water to investigate Li-O2
batteries modeled after those in a previous study.3
In Figure 2, one may notice that oxygen seems to
be flowing directly into the glovebox. That would
normally run counter to the whole purpose of aglovebox and create a hazardous situation. However,the fully-sealed Cypher ES sample chamber andtubing keep the in-flowing oxygen separate fromthe glovebox atmosphere, even at positive oxygenpressure relative to the glovebox interior. This
allowed Dr. Virwani to cycle Li-O2 batteries
for up to a week and monitor the operando evolutionof the cathodic SEI as a function of time and charge
state. Figure 3 shows EC-AFM images of thecathode surface of such a battery at various
points in a single discharge/charge cycle lasting
about ten hours. In these measurements, theprobe is a non-perturbative observer of thetopography; it remains electrically floating
and uninvolved in the chemical reaction.
These data suggest that, in the early stages ofdischarge, the SEI, a mixed-valence LixOy material,
first nucleates in solution as large spherical/
toroidal structures (Figure 3B), followed by thenucleation of smaller nanostructures on the cathodesurface (Figure 3C). The spherical/toroidal shapeof the large deposits is consistent with previous
scanning electron microscopy results4 and is inpart mediated by H2O molecules in the solvent.5
Cathodic nanostructures continue to increase insize during discharge, completely filling the spaces
between large deposits by the time discharging
is complete (Figires 3D-G). Upon charging, thestructures initially persist (Figure 3H), then begin todissolve (Figure 3I), and finally disappear by the time
50% state of charge is reached (Figures 3J and K).
Dr. Virwani’s article discusses these conclusions inmuch greater depth. It also shows: (1) EC-AFM
data acquired with varying water concentration, (2)compelling videos of the SEI evolution obtained
through extended operando EC-AFM imaging,(3) impedance spectroscopy, and, (4) SEI volumetricanalysis. 6

Figure 2: Li-O2 battery testing with a Cypher ES in an argon glovebox. (A) Photograph of the Cypher ES integrated into an Ar-filled Mbraun
glovebox. (B) Schematic of the battery cell and sealed sample chamber used in these experiments. Pure O2 flows into the AFM but does not
escape into the glovebox atmosphere due to the hermetic seal of the sample chamber.

2 Operando EC-AFM Imaging of Li-O2 Battery Solid-Electrolyte Interphase© Oxford Instruments Asylum Research Inc.

Figure 3: Li-O2 battery cycling and operando EC-AFM images. In the center is the discharge/charge voltage cycle plotted against the capacity,surrounded by tapping-mode images of the cathode SEI topography obtained at the timepoints A-K indicated in the cycle. The cathode waspolished glassy carbon, the anode was Li metal, the solvent was tetraglyme, the electrolyte was 1 M LiNO3, and the discharge/charge rate
was 5 μA, corresponding to a full cycle time of ~10 h. The scan size is 3 μm x 3 μm and the height scale is 300 nm for all images.

Examples of similar operando battery imaging arerare. This is no doubt limited by the fact that mostAFMs cannot be configured with experimental
conditions that mimic real battery chemistry.
However, examples are emerging now that this
EC-AFM capability is present on the Cypher ES. In
one recent notable example, sodium oxide (NaO2)
batteries were analyzed with EC-AFM on the Cypher
ES. It was found that the stability of the NaO2
discharge products in the electrolyte could help
explain the high cyclability of the battery.7 These
measurements were possible thanks to the exquisite
control of Cypher ES with the EC cell accessory andsuggest this technique could readily be extended
to other sensitive electrochemical measurements
requiring extraordinary environmental control.

© Oxford Instruments Asylum Research Inc.

Our society is becoming increasingly reliant onmobile devices and batteries and electric motors
are gradually supplanting internal combustion
images in vehicles and machinery for their reduced
environmental impact. These changes have madebatteries with improved capacity, safety, andeconomy more important than ever before. Asylum
Research AFMs provide a unique capability to study
battery systems operando, providing new insightsthat will drive new energy storage innovations.

Operando EC-AFM Imaging of Li-O2 Battery Solid-Electrolyte Interphase3References
1. Learn more about Cypher ES here: https://www.oxford-instruments.com/products/
atomic-force-microscopy-systems-afm/
asylum-research/high-resolution-fastscanning-afm
2. Download the EC Cell Data Sheet andsee the benefits here: https://afm.oxinst.
com/products/cypher-nanoelectricalaccessories/cypher-electrochemistry-cell
3. Wen, R. et al. In Situ AFM Imaging of Li‑O2
Electrochemical Reaction on Highly Oriented
Pyrolytic Graphite with Ether-BasedElectrolyte. Journal of the American ChemicalSociety 2013, 135 (29), 10870-10876.
https://doi.org/10.1021/ja405188g
4. Zheng, H. et al. New Insight inUnderstanding Oxygen Reduction and
Evolution in Solid-State Lithium–OxygenBatteries Using an in Situ Environmental
Scanning Electron Microscope. Nano Letters
2014, 14 (8), 4245–4249.
https://doi.org/10.1021/nl500862u

Find the best AFM
for your research!
AFM.oxinst.com
AFM.info@oxinst.com+1-805-696-6466

5. Aetukuri, N. B. et al. Solvating additives drive
solution-mediated electrochemistry andenhance toroid growth in non-aqueous Li–O2batteries. Nature Chemistry 2015, 7, 50–56.
https://doi.org/10.1038/nchem.2132
6. Virwani, K. et al. In situ AFM visualizationof Li–O2 battery discharge products duringredox cycling in an atmosphericallycontrolled sample cell. Beilstein Journal ofNanotechnology 2019, 10, 930-940.
https://doi.org/10.3762/bjnano.10.94
7. Ansari, Y. et al. A Highly Stable Sodium–
Oxygen Battery Using a MechanicallyReinforced Membrane. Advanced Energy
Materials 2018, 8, 1802603.
https://doi.org/10.1002/aenm.201802603

ASYLUM RESEARCH