Improving Battery Performance with AFM
ASYLUM RESEARCH
Batteries power a lot of our modern technologies
from cell phones to laptops. In the late 20th century,
development in battery technology was mainly
motivated by the rise of such portable consumer
electronics, which required batteries of high energy
density.1 This gave rise to the wide use of rechargeable
nickel-cadmium batteries; but by the 1990s, lithium
ion batteries (LIBs) had taken over as the dominant
technology. Lithium has low molecular weight,
low density, and it intercalates into various layeredmaterials, resulting in a high volumetric energy density(200 W h L-1) twice that of competing systems at
the time. The last three decades saw an impressive
increase in capacity to more than 400 W h L-1,
which spurred the rapid reduction in size of portabledevices we witnessed over this period.  ​
LIBs are based on the transport of Li ions across thebattery cell; for example, during discharge, lithium
ion flows from the anode to the cathode, providing
current that powers devices.2 Lithium metal oxides
like LiCoO2 (LCO) are favored cathode materials
due to their high chemical potential (vs. L/L+), whilegraphite is favored as a low voltage anode material
for its layered structure, natural abundance, and low
cost.3 It was also important to develop non-aqueous
electrolyte solutions such as propylene carbonate
or ethylene carbonate that allowed for reversible
lithium intercalation with minimal capacity fade.4
Critical in this development is understanding the roleof the solid electrolyte interface (SEI), which forms apassivating layer that protects the metallic form oflithium. Performance of the SEI directly impacts theaging of battery cells, and the total number of chargedischarge cycles.​To improve overall performance, all aspects of thebattery device need to be understood and optimized.
The cathode and anode materials, the separator, andthe electrolyte, including the formation of the SEI, areall potential targets for engineering to improve energy
and power densities, as well as aging.   ​
Atomic force microscopes (AFMs) provide a way tovisualize battery materials at micro and nanometer
resolutions in order to evaluate their texture andmorphology–to better understand, for example,
how their processing can impact performance. In
addition, AFMs can be used in nanoelectrical andnanomechanical characterization to provide insighton how material properties can be engineered at thenanoscale. Unlike other characterization tools, AFMs
can also be operated within inert atmosphere (e.g., ina glovebox), and measurements can be done in situ orin environments (e.g., in electrolytes) relevant to theoperation of the battery cell. ​The following examples show how AFMs can beused to evaluate materials from different parts of thebattery cell to provide critical information in improving
their overall performance.​

Aging of LiCoO2 (LCO) Cathodes​
A continuing challenge in battery research is the
reduction in capacity fade over a larger number ofcharge/discharge cycles. Understanding aging ofbattery materials at the microscopic level is key toimproving their performance in this regard. AFMs
can monitor the evolution of critical properties withhigh spatial resolution, which can give insight into
the degradation of material performance. AFM has
been used to study the evolution of contact stiffness
and surface potential of lithium cobaltate (LCO) thinfilms as a function of charge/discharge cycle, which
has yielded insights on the aging behavior of thisimportant cathode material. 5
Fig. 1 shows how LCO thin film roughness,grain size, and contact stiffness evolve over
100 charge/discharge cycles. AFM nanomechanics,
typically utilizes indentation techniques which are slow
and plastically deform the sample. Here, researchers
used AM-FM Viscoelastic Mapping mode, a tapping
mode based technique proprietary to Asylum
Research, which obtains both surface morphology
and contact stiffness simultaneously. Imaging istherefore fast and high resolution, as well as gentle
on the sample because it relies on monitoring small
frequency shifts to map out contact stiffness.
Surface roughness and grain size increases with thenumber of charge/discharge cycles, while contact
stiffness decreases. Furthermore, while roughness
and grain size increase linearly, there is a more rapid
decrease in contact stiffness in the first 10 cycles,
which becomes less pronounced at higher cycle
numbers.
Changes in grain size has been shown to be
induced by changes in the structure of LCO due tothe intercalation and deintercalation of lithium ions
during the charge/discharge process. The change© Oxford Instruments Asylum Research Inc. Improving Battery Performance with AFM 1

Nc= 0

Nc= 10

Nc= 50

Nc= 100

C

D

E

F

G

H

Cycle Number

Stiffness (mV/m)Grain Size (nm)Surface Roughness (nm)B

Stiffness (mV/m)A

Cycle Number

Figure 1: Evolution in surface morphology (A-D) of LCO cathode films, and their contact stiffness (E-H), measured by AM-FM viscoelastic
mapping mode, as a function of the number of charge/discharge cycles. The bottom graphs show that while grain size and surface
roughness increased linearly, contact stiffness decreased more dramatically in the first 10 cycles.

in dimensions is anisotropic, and because the grains
are bounded by each other, stress is generated
between grains. Local changes in lithium ion content
also produce defects which results in stress. These
accumulate over the number of cycles resulting in
fracture and agglomeration into bigger grains, whichis confirmed in roughness and grain size trends.

In addition to contact stiffness, surface potential,
measured by Kelvin Probe Force Microscopy(KPFM), also evolves over 100 charge/discharge
cycles. Fig. 2 shows a similar trend where grain size
increases linearly while surface potential dramaticallydecreases in the first 10 cycles, followed by a more
gradual decrease until 100 cycles.

It is known that the lithiated form of LCO is stiffer thanthe delithiated form. Before cycling, the edges of thegrains are less stiff than the center, and indeed theseare expected to have more lithium ion content. Whenthe battery is charged, the cathode is delithiated,and subsequently re-lithiated during discharge. A
decrease of stiffness over time indicates that lithiation
is not completely reversible, resulting in reduction incapacity. Furthermore, it is also known that the SEI,
which forms during the first few cycles, is responsible
for elastic modulus degradation of cathode films. Thismay be the reason for the initial dramatic decrease incontact stiffness observed during the first 10 cycles.

The work function of LCO is higher in the lithiated form,
than in the delithiated form. Since the intercalation/
deintercalation is not completely reversible, thesurface potential decreases over the number ofcycles since both phases become present and coexist
in the film. Surface electrochemical reactions andcoarsening of the nanograins can also contributeto the lower surface potential. The surface potential
distribution also becomes more uniform over thenumber of cycles, indicating a more even distributionof lithium ions with higher cycle numbers.

2 Improving Battery Performance with AFM © Oxford Instruments Asylum Research Inc.

Nc= 0

Nc= 10

Nc= 50

Nc= 100

C

D

E

F

G

H

Cycle Number

Counts Number

Cycle Number

Surface Potential (mV)Grain Size (nm)Surface Roughness (nm)B

Surface Potential (mV)A

Surface Potential / V

Figure 2: Evolution in surface morphology (A-D) of LCO cathode films, and their surface potential (E-H), measured by KPFM, as a functionof the number of charge/discharge cycles. Graphs show similar trend with contact stiffness, where grain size increased linearly while
contact potential decreased more dramatically in the first 10 cycles. In addition, the contact potential distribution shows a decrease invariation with the number of cycles.

Lithiation/Delithiation of Li4Ti5O12 Anodes
Understanding anode materials with nanoscopic
detail can also help improve the overall performanceof battery cells. Lithium titanate (LTO) has become acommercially viable alternative to graphite despite
having comparatively lower capacity and higher
voltage requirements. Advantages include its ability
to accommodate three lithium ions in its spinelframework to form Li7Ti5O12 without volume change,
operation within the electrolyte stability window
and thus not reliant upon the formation and stabilityof the SEI, high reversibility, i.e., high coulombic
efficiency even at high current rates, ease of synthesis,
inexpensiveness, and eco-friendliness. 6, 7

A major drawback is its low electronic conductivity,
which requires engineering (e.g. coating with carbon)
to improve performance. However, it is known that
Li7Ti5O12 or the lithiated phase is highly conductive,
from spectroscopic and electrochemical studies,and that even a small amount of lithiation candramatically increase bulk electronic conductivity.7
It is thus important to understand how this phasetransformation occurs, but because there is no
volume change between the two phases, and theirlattice parameters are nearly identical, diffraction
techniques are unable to physically differentiate them.
However, since one phase is insulating, while the otheris conducting, current measurements with AFM are
ideal to track the nanoscale changes in phase during
the lithiation/delithiation process that results in theinsulator-to-metal transition.

© Oxford Instruments Asylum Research Inc. Improving Battery Performance with AFM 3

Fig. 3 shows current maps of LTO overlaid on surface
topography, obtained by conductive AFM (CAFM). Notethe area scanned is 1 µm x 1 µm, and the z-scaling is
100 nm. The series of images correspond to points
in the electrochemical profile of the first cycle, where
the pristine film (A) is first discharged to 50% (B), thendischarged to (1.5 V), discharged to 1.0 V, charged to50%, and finally charged to 2.0 V.
Before cycling, the LTO is completely insulating, as
we expect from Li4Ti5O12. As the LTO is discharged(lithiated), the morphology remains the same, but
the average current goes up from 0.12 µA µm-2,
when discharged to 50%, then to 0.58 µA µm-2,
when discharged to 1.5 V. Following total lithiation,i.e., completion of the two-phase reaction, nearly all
grains are conducting, as we expect from Li4Ti5O12. It
had been hypothesized that LTO by itself, forms the
conductive phase first near the current collector, thenevenly propagates upward, towards the interface
with the electrolyte. But CAFM measurements
show that current is clearly localized within discretegrains and not evenly distributed across the film.
This demonstrates that the transition from insulating
to conducting phase proceeds instead via a small
number of percolation channels between the current
collector and the electrolyte.

A

B

C

D

In addition, when the film is discharged beyond 1.5 V
to the LTO cutoff voltage of 1.0 V, current decreased
to 0.32 µA µm-2 and the morphology significantly
changes, i.e., the average particle size increased. It isthought that the SEI may have formed, despite the factthat LTO is not known to form the SEI, perhaps fromthe high surface area of the thin film. Or overlithiation
may have occurred at the surface which may form
non-conductive species.

Formation of the Solid Electrolyte
Interphase (SEI)
The solid electrolyte interphase (SEI) has been thesubject of great interest because its formation andstability directly impact battery life, safety, andperformance. 7, 8 The SEI is a passivating layer that
results from the reduction of the electrolyte on theanode surface. Once formed, further electrolyte
decomposition is prevented, stabilizing the electrode/electrolyte interface. The SEI needs to be electricallyinsulating and impermeable to electrolyte but allow
for lithium diffusion, mechanically stable against stressfrom changes in electrode volume, and insoluble/inert
in the electrolyte at the electrode working potentials
and temperatures. 7 A poorly formed SEI can result in

E

F

Voltage (V)

A
F
B

E

C

D
Capacity (µAh cm-2)•

Figure 3: Conductive AFM current maps of LTO anode films overlaid on topography at various states of charge from (a) pristine,(b) discharged 50% , (c) discharged to 1.5 V , (d) discharged to 1.0 V, (e) charged 50% , (f) charged to 2.0 V.

4 Improving Battery Performance with AFM © Oxford Instruments Asylum Research Inc.

capacity fade and reduction in power density, suchthat understanding and tuning the SEI formation is
critical in improving overall battery performance.

the white box), shows the nucleation and growth of the
SEI layer in real time. The image number indicates the
position in the voltammogram during the first cycle.

Unlike other measurement techniques that require
high vacuum environments, AFMs can be operated
under a variety of environmental conditions making it
ideal to image within the sample’s relevant working
environment. In this example, an electrochemicalcell was used in order to understand the formationof the SEI on the basal plane of HOPG during
electrochemical cycling.7 Nucleation and growth of
the SEI within a commercial battery grade electrolyte
solution (1 M LiPF6 in 50/50 v/v ethylene carbonate/
dimethyl carbonate) could therefore be observed inreal-time. In addition, the AFM was also placed inside
the glovebox so that measurements could be done in
an inert atmosphere as required by the air-sensitive
reagents.

The white arrows in image #4, corresponding toa prominent dip in the voltammogram, indicates
nucleation sites of the SEI. At 0.8 V the SEI was
detected along the step edges, and at 0.5 V along
the basal plane, correlating with the current dip
between 0.8-0.3 V. Beyond 0.4 V (image #5), the SEI
formed a uniform layer which remained stable until
the end of the cycle. Note that after the SEI formed,
the morphology remained the same during the second
cycle, while the voltammogram showed a muchsmaller dip between 0.8-0.3V.

Fig. 4 shows the electrochemical cell illustrating how
the HOPG electrode can be imaged in situ within theelectrolyte solution during electrochemical cycling.

This example demonstrates how the AFM can beused to study the complex formation of the SEI
under realistic operational conditions, which can beused to understand how they can be engineered to
produce the optimal properties for improving battery
performance and lifetime.

The 15-µm images show the surface topography of the
HOPG electrode before and after cycling, and shows
the SEI layer forming at the end of the cycle. The seriesof 4-µm images within the 15-µm images (indicated by

1

4

7

2

5

8

3

6

9

5 µm

Figure 4: Topographic images of HOPG recorded at different points on the voltammograms during cycling in the presence of 1 M
LiPF6 in 50/50 v/v EC/DMC electrolyte solution and at 2 mV/s cycling rate. The 1-9 series shows changes in a 4 µm section of the
15 µm survey scans (top, left); white arrows indicate locations of SEI nucleation. The bottom is a homebuilt setup for facile in situ
measurement of SEI nucleation and growth on different anode substrates.

© Oxford Instruments Asylum Research Inc. Improving Battery Performance with AFM 5

References
1. Schipper, Florian, and Doron Aurbach. “A brief
review: past, present and future of lithium ion
batteries.” Russian Journal of Electrochemistry 52, no.
12 (2016): 1095-1121.​
2. US Department of Energy (https://www.energy.gov/
eere/articles/how-does-lithium-ion-battery-work)​
3. Kumar, Pushpendra,
Hatem Abuhimd, Wandi Wahyudi, Mengliu Li, Jun
Ming, and Lain-Jong Li. “Two-Dimensional Layered
Materials for Energy Storage Applications.” ECS
Journal of Solid State Science and Technology 5, no.
11 (2016): Q3021.​
4. Li, Qi, Juner Chen, Lei Fan, Xueqian Kong,and Yingying Lu. “Progress in electrolytes forrechargeable Li-based batteries and beyond.” Green
Energy & Environment 1, no. 1 (2016): 18-42.​
5. Wu, Jiaxiong, Shan Yang, Wei
Cai, Zhuanfang Bi, Guangyi Shang, and Junen
Yao. “Multi-characterization of LiCoO2 cathode films
using advanced AFM-based techniques with highresolution.” Scientific reports 7, no. 1 (2017): 1-9.​
6. Aravindan, Vanchiappan, Yun‐Sung Lee, andSrinivasan Madhavi. “Research progress on negativeelectrodes for practical Li‐ion batteries: beyond
carbonaceous anodes.” Advanced Energy Materials
5, no. 13 (2015): 1402225.​

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7. Verde, Michael G., Loïc Baggetto, Nina Balke,
Gabriel M. Veith, Joon Kyo Seo, Ziying Wang,
and Ying Shirley Meng. “Elucidating the phasetransformation of Li4Ti5O12 lithiation at thenanoscale.” ACS nano 10, no. 4 (2016): 4312-4321.​
8. Verma, Pallavi, Pascal Maire, and Petr Novák.
“A review of the features and analyses of thesolid electrolyte interphase in Li-ion batteries.”
Electrochimica Acta 55, no. 22 (2010): 6332-6341.​
9. Luchkin, Sergey Yu, Svetlana A. Lipovskikh, Natalia
S. Katorova, Aleksandra A. Savina, Artem M.
Abakumov, and Keith J. Stevenson. “Solid-electrolyte
interphase nucleation and growth on carbonaceous
negative electrodes for Li-ion batteries visualizedwith in situ atomic force microscopy.” Scientific
Reports 10, no. 1 (2020): 1-10.​

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6 Improving Battery Performance with AFM © Oxford Instruments Asylum Research Inc.