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A KAIST research team has developed
a method capable of detecting minute variations in battery electrode thickness
that can contribute to thermal runaway with a precision equivalent to
approximately one ten-thousandth of the diameter of a human hair, all without
disassembling or damaging the battery. The technology is expected to improve
battery safety and quality by identifying invisible defects during the
manufacturing process.
A research team led by professor
Young-Jin Kim of the Department of Mechanical Engineering has developed a
technology that measures the thickness of lithium-ion battery electrodes in a
noncontact and nondestructive manner.
The study was led by Dr. Guseon
Kang of the KAIST Department of Mechanical Engineering, currently with the
Korea Institute of Industrial Technology, as the first author, with Kim serving
as the corresponding author. The research findings were published in Nature Communications on June 10.
Why thickness uniformity is critical
The technology combines terahertz
waves (electromagnetic waves in the spectral region between light and radio
waves) with an optical frequency comb, which divides the frequency of light
into evenly spaced intervals like the markings on a ruler and serves as a
reference for ultra-precise measurements.
The electrodes in lithium-ion
batteries, which are widely used in electric vehicles, are essential components
through which electric current flows. Even a slight variation in electrode
thickness can cause current to become concentrated in certain areas when
charging and discharging, generating heat. If the heat continues to accumulate,
it may lead to thermal runaway, a phenomenon in which a battery's internal
temperature rises rapidly and can result in a fire or explosion. Maintaining
uniform electrode thickness is therefore critically important during battery
manufacturing.
Existing inspection technologies,
however, have limitations when applied to production environments. X-ray
computed tomography can provide detailed images of internal structures, but its
relatively long inspection time makes it difficult to use on high-speed
production lines. Ultrasonic acoustic microscopy requires direct contact with a
liquid medium, while laser displacement sensors can perform rapid measurements
but have difficulty precisely analyzing structures inside an electrode.
Combining terahertz with a frequency comb
The research team overcame these
limitations by combining optical frequency comb and terahertz technologies. The
researchers first directed terahertz waves at a battery electrode and collected
signals generated as the waves were repeatedly reflected within the electrode.
They then used an optical frequency comb as a reference to analyze the signals
with exceptionally high precision and calculate the electrode thickness. This
enabled nanometer-scale measurements of the electrode's internal structure without
damaging the battery.
At the core of the technology is
Fabry–Pérot interference, a regularly spaced interference pattern produced as
terahertz waves repeatedly travel back and forth between the front and rear
surfaces of an electrode. Much like measuring length by reading the markings on
a ruler, the researchers precisely analyzed the interference pattern using the
optical frequency comb as a reference to determine the electrode thickness.
As a result, the team successfully
measured both the electrode thickness and its complex refractive index (a
material's optical property indicating how strongly it transmits and absorbs
electromagnetic waves) in a single measurement without requiring a separate
calibration process.
Precision at production-line speeds
The researchers validated the
technology using battery electrodes measuring between 50 and 150 micrometers in
thickness, comparable to the diameter of a human hair. With a measurement time
of just 0.2 seconds, the system detected thickness differences as small as 70.1
nanometers in the anode (approximately one fourteen-hundredth of the diameter
of a human hair) and 465.5 nanometers in the cathode. This measurement speed is
considered sufficient for use on rapidly moving battery production lines.
When the measurement time was
increased to 25.6 seconds, the precision improved further. The system
distinguished differences as small as 7.8 nanometers in the anode
(approximately one ten-thousandth of the diameter of a human hair) and 25.2
nanometers in the cathode. This represents up to a 100-fold improvement in
precision compared with conventional time-domain analysis methods, enabling the
detection of thickness variations that are completely invisible to the naked
eye.
The technology is not limited to
measuring thickness at a single point. It can generate a three-dimensional map
of thickness across an entire electrode and track gradual thickness variations
in real time during production. The researchers also confirmed that the system
could accurately measure an electrode tilted at an angle of approximately 45
degrees, demonstrating its potential for application to fast-moving, real-world
battery manufacturing lines.
Extending quality control to future batteries
The study is significant because it
presents a new inspection technology capable of identifying invisible
microscopic defects during production without disassembling or damaging
batteries. In addition to lithium-ion batteries, the technology is expected to
serve as a key quality-control tool for manufacturing next-generation all-solid-state batteries, which use solid electrolytes instead of liquid
electrolytes. By detecting defects at an early stage, the technology could
improve battery safety and quality while enabling more stable manufacturing
processes.
"This technology is an integrated metrology platform that can simultaneously measure electrode thickness and material properties without requiring a separate calibration process," said Kim. "We expect it to become a key technology for the real-time quality control of production lines for next-generation lithium-ion batteries and all-solid-state batteries."
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