Korea Advanced Institute of Science and Technology (KAIST) said on the 23rd it developed a technology that distinguishes minute thickness nonuniformity in battery electrodes, which can cause thermal runaway, down to about one ten-thousandth the thickness of a human hair without disassembling or damaging the battery.
The electrodes of lithium-ion batteries, widely used in electric vehicles, are key components through which electricity flows. The electrodes of lithium-ion batteries, widely used in electric vehicles, are key components through which electricity flows. Even a very slight difference in electrode thickness can concentrate current in specific areas during charging and discharging, generating heat. If this heat continues to build up, thermal runaway can occur, so it is important in battery manufacturing to keep electrode thickness uniform.
However, existing inspection technologies had limits for use on production lines. X-ray computed tomography (CT) can closely examine the interior, but the inspection time is long, making it difficult to apply to fast production lines. Ultrasonic acoustic microscopes require contacting the sample with liquid, and laser displacement sensors can measure quickly but have difficulty precisely analyzing the inside of the electrode.
The research team led by Professor Kim Young-jin of the KAIST Department of Mechanical Engineering directed terahertz waves at battery electrodes to capture signals created by multiple internal reflections, then used an optical frequency comb as a reference to analyze these signals with ultrahigh precision and calculate electrode thickness. This enabled nondestructive measurements of the interior down to the nanometer level.
The core of this technology is using Fabry-Pérot interference, a pattern of evenly spaced fringes created as terahertz waves make multiple round trips between the front and back faces of the electrode. Like reading a ruler's markings to measure length, the team precisely analyzed these fringes using an optical frequency comb as a reference to calculate the electrode thickness. As a result, they measured not only thickness but also the complex refractive index at once without separate calibration.
The team validated the technology on battery electrodes 50–150 micrometers (µm) thick, similar to a single human hair. With a brief 0.2-second measurement, they detected minute thickness differences equivalent to about one 1,400th of a hair (70.1 nanometers for the anode and 465.5 nanometers for the cathode), a level usable even on fast-moving battery production lines.
When the measurement time was extended to 25.6 seconds, precision improved further. For the anode, they distinguished down to 7.8 nanometers, about one ten-thousandth the thickness of a hair, and measured 25.2 nanometers for the cathode. The team said this offers up to 100 times higher precision than conventional methods, capable of detecting tiny thickness differences invisible to the naked eye.
Professor Kim Young-jin said, "This technology is an integrated metrology platform that can simultaneously measure electrode thickness and material properties without separate calibration," adding, "it will serve as a key technology for real-time quality control on production lines not only for next-generation lithium-ion batteries but also for solid-state batteries."