Surface-Scan MRI Diagnostics of Li-Ion Cells: Boosting the Sensitivity with High-Performance Unilateral RF Sensors

Li-ion battery screening mitigates risks associated with uncontrolled release of electrochemical energy referred to as thermal runaway. Surface-Scan Magnetic Resonance Imaging (MRI) is a battery diagnostic concept based on the quantification of magnetic field distributions near the outer surface of Li-ion cells. This "inside-out" MRI technique enables ultra-fast in operando artifact-free mapping of severely distorted magnetic fields. Detection of mechanical defects and subtle magnetic field variations at early stages of cell degradation can be challenging. In this work, we develop a specialized radio frequency sensor for highly efficient battery screening applications and provide new physicochemical insights into mechanisms of premature degradation of commercial cells. The sensor prototype constructed as a flat solenoid encapsulating a thin layer of silicon-based detection medium has an optimized filling factor of 1 and a Q-factor of 100. Placing a cell in direct contact with the working surface of the sensor ensures the detection of the strongest magnetic field perturbations. The proposed technology provides an ultimate sensitivity to a variety of battery degradation mechanisms and is suitable for a high-throughput screening of large capacity pouch cells. Surface-Scan MRI analysis demonstrates an ultimate sensitivity to mechanical defects and reveals a highly non-uniform spatial distribution of state-of-charge and Li-ion transport in common commercial pouch cells.