A nanoscale look at how lithium batteries work
DR. JÖRG SCHWARTZ, JOERG.SCHWARTZ@PHOTONICS.COM
OAK RIDGE, Tenn. – A new type of scanning probe microscopy that can visualize
electrochemical strain has enabled scientists to study the movement of ions in the
cathode material of lithium-ion batteries, an approach that not only offers a better
understanding of the ions’ mechanisms but that also could help improve their
The technique, called electrochemical strain microscopy (ESM),
was developed by researchers at the Center for Nano-phase Materials Sciences at
the US Department of Energy’s Oak Ridge National Laboratory (ORNL).
A new technique called electrochemical strain microscopy maps how lithium ions flow through a battery’s cathode material. This 1 x 1-μm composite image demonstrates how regions on a cathode surface display varying electrochemical behaviors when probed with ESM. Courtesy of Oak Ridge National Laboratory.
Lithium-ion batteries have a number of advantages over other types
of rechargeable batteries, including a good capacity-to-weight ratio, no memory
effect and a slow loss of charge. They are not only highly popular for consumer
electronics products, but also critical parts for future electric cars or as buffers
for renewable yet noncontinuous energy sources such as solar cells.
Their main components are the anode and cathode electrodes separated
by an electrolyte. The lithium ions move from the anode to cathode through the electrolyte
during charge and discharge, producing electric work. The movement of lithium ions
into and out of electrodes is central to the charge capacity and to the power of
lithium-ion batteries; therefore, the processes of insertion (or intercalation)
and extraction (de-intercalation) of ions are areas of active research.
The researchers say that, although the process has been extensively
studied at the device level, it remains virtually unknown at the nanoscale level
of grain clusters, single grains and defects.
One method used to date is atomic force microscopy (AFM) to study
how the surface morphology of the electrodes changes while the battery is charging
or discharging. Static strains can be derived from this and electronic currents
mapped across the electrode surfaces. However, a dynamic study of the intercalation
processes, strain charge and ion transport at the level of single-grain boundaries
and dislocations in the electrodes is not possible with standard AFM alone.
This is exactly what the researchers have done with ESM, which
they reported on in Nature Nanotechnology
5, pp. 749 to 754, published online Aug.
29, 2010. By using the tip of an AFM to concentrate an oscillating electric field
onto the cathode of a lithium-ion battery, they triggered lithium ions to intercalate
and de-intercalate in a small volume underneath the biased tip. This resulted in
periodic changes of the cathode volume and a strain at its surface. The strain was
then measured by the same AFM tip, leading to a map of the lithium intercalation
and transport processes.
The method showed highly anisotropic expansion of the battery
with sub-100-nm resolution, which they correlated to local lithium diffusivity and
to the microstructure of LiCoO2, one of the most used cathode materials for lithium-ion
batteries. “We can provide a detailed picture of ionic motion in nanometer
volumes, which exceeds state-of-the-art electrochemical techniques by six to seven
orders of magnitude,” researcher Sergei Kalinin said.
“Very small changes at the nanometer level could have a
huge impact at the device level,” added his colleague Nina Balke. “Understanding
the batteries at this length scale could help make suggestions for materials engineering.”
Although the initial focus was on lithium-ion batteries, the team
expects that its ESM technique can be applied to measure other electrochemical solid-state
systems, including other battery types, fuel cells and similar electronic devices
that use nanoscale ionic motion for information storage.