Development of Metallic Contaminant Inspection System Using High-Tc rf-SQUID for Li-ion Battery Liquid Components

The use of a high-Tc (HTS) rf-superconducting quantum interference device (SQUID) for inspecting the liquid component of lithium-ion (Li-ion) batteries is proposed. It is possible that a small amount of metal can become mixed in with components of Li-ion batteries during the manufacturing process and contaminate them, with a resultant fire risk, Thus, the development of a highly sensitive inspection system is required. In this article we describe a test system, we developed using a single channel HTS rf-SQUID consisting of a cryostat, a polyethylene tube, and a permanent magnet. The SQUID needs to be installed as close to the battery liquid as possible to obtain more sensitive readings. Therefore, we designed a microscope-type SQUID cryostat in which the SQUID can approach the target as close as 1 mm. In the system, the liquid was not flowed through the polyethylene tube, but a small metallic sample stack on a fishing line was moved by a motor. A stainless steel cut wire with a nominal size of φ50 μm was prepared. After magnetizing the sample, the stray magnetic field of the sample was measured by the SQUID. The peak-to-peak signal scaled inversely with the cube of the stand-off distance. It was found that a signal of the sample with φ50 μm at a distance of 26 mm could be detected if the signal-to-noise ratio (SNR) > 3 was supposed as the threshold of the inspection. The dependence on the sample size was also investigated, using stainless steel samples with different sizes (φ50-φ160 μm). The peak-to-peak signal scaled with the cube of the diameter.