In recent years, the development of High-Nickel type batteries with high Ni content, which have high energy density, has been accelerating in Li-ion batteries for EVs in order to extend the cruising range. Increasing the nickel content not only increases the cruising range and enables battery miniaturization, but also reduces the use of cobalt, which is expensive and subject to large price fluctuations. However, high density requires high quality control, and it is important to eliminate small metallic contaminants in the manufacturing process. Metallic particles with external dimensions of less than 100 microns cannot be detected by conventional X-ray imaging systems. Therefore, a highly sensitive detection system for small metallic contaminants is required. To address this issue, we have developed an inspection system for small metallic contaminants in lithium-ion battery components using a high-temperature superconducting quantum interference device (SQUID). There are two types of SQUIDs: magnetometers and gradiometers. Magnetometers have high sensitivity, but when the inspection target is uniformly magnetized, the background noise becomes large, making it difficult to inspect the target metallic contaminants. On the other hand, the gradiometer is more resistant to background noise than the magnetometer under such conditions. Therefore, in many cases, gradiometers are used in metallic magnetic foreign object inspection systems to cancel out background magnetic noise from uniformly magnetized objects to be inspected.
We first describe the development of a micro-metallic contaminant inspection system equipped with a SQUID gradiometer. For practical use, the detection width must be expanded to at least 65 mm by employing multiple sensors. This paper presents an 8-channel High-Tc SQUID roll-to-roll system for inspection of lithium-ion battery anodes with a width of 65 mm. A special microscopic-type cryostat was developed and eight SQUID gradiometers were mounted on it. Furthermore, a horizontal magnetization system was developed to reduce background noise. As a result, the system successfully detected iron particles as small as several tens of microns in diameter on an actual lithium-ion battery anode with a width of 70 mm. This system is effective for detecting metallic contaminants in lithium-ion battery anode sheets.
Next, several planar gradiometers were designed and their sensitivity distribution was investigated for optimization. The gradiometers were fabricated using a 200 nm thick YBa2C3O7-ythin film deposited on a SrTiO3 bi-crystal substrate (tilt angle = 36.8°, thickness = 0.5 mm). The gradiometers were completed using Ar ion milling after silver was deposited on selected areas of the thin film and patterning by photolithography. The three designed gradiometers are directly coupled type, with two identically shaped pickup coils connected to a centrally located SQUID by a differential coupling scheme. the dimensions a × b of the pickup coils of the three gradiometers are different, with (A) 3.0 × 8.75, (B) 4.3 × 6 .0, and (C) 4.32 x 8.75.
A special apparatus was fabricated to measure these distributions. This apparatus consists of a SQUID microscope-type liquid nitrogen indirectly cooled cryostat and a motor-driven XYZ stage. Contour plots of the signal intensity distribution for each gradiometer were drawn. The most sensitive peak point appeared near the center of the pickup coil. The peak magnitude decreased with an increase in the area of the pickup coil. As a result, the peak (B) in the 4.3 x 6.0 dimension showed the largest magnitude. It was found that the difference in peak values can be explained by the imbalance in inductance between the pickup coil and the SQUID.
[1] Saburo Tanaka, Takashi Izu, Ryo Ohtani, and Kanji Hayashi, “Design of a High-Tc SQUID Planer Gradiometer and Evaluation of Sensitivity Distribution”, IEEE Transactions on Applied Superconductivity, 33, No.5 1600204 (2023).
[2] S. Tanaka, Y. Kitamura, Y. Uchida, Y. Hatsukade, T. Ohtani and S. Suzuki, “Development of Metallic Contaminant Detection System using Eight-Channel High-Tc SQUIDs”, IEEE Trans. Appl. Supercond., 23, 1600404 (2013).