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

Abstract

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.

Operando Magnetic Resonance Imaging Reveals Phase Transitions Driven by Nonuniform Cathode Lithiation in Li-Ion Pouch Cells

“…A strong dipolar pattern ( B ST ca. ± 100 ppm) forming at the tip of an extended Ni-plated tab is the dominant feature reported previously for similar pouch cells. − …”

“…The unilateral RF sensor is a flat RF coil encapsulating a thin layer of proton-rich polymer (detection medium) . Such a design is optimal for the sensitive imaging of flat samples.…”

“…The unilateral RF sensor is a flat RF coil encapsulating a thin layer of proton-rich polymer (detection medium). 14 Such a design is optimal for the sensitive imaging of flat samples. The proton signal of the detection medium enables a dramatic sensitivity gain by comparison with direct NMR spectroscopy of charge carriers ( 7 Li, 23 Na).…”

“…In the presence of a uniform external magnetic field, components of the cell with sufficient magnetism will generate detectable magnetic field perturbations outside the cell. Magnetic field components associated with state-of-charge (SoC) dependent magnetic susceptibility (χ) and direct current (DC) density (referred to as B SoC and B DC , respectively) have been identified as sensitive markers of electrochemical processes. − B SoC and B DC depend on the overall design of the cell (geometry), and on the electromagnetic properties of the cell materials . Accurate measurements of B SoC and B DC maps and magnetic field simulations ,, suggest that cathode (de)­lithiation can occur nonuniformly throughout the pouch cell.…”

“…Magnetic field components associated with state-of-charge (SoC) dependent magnetic susceptibility (χ) and direct current (DC) density (referred to as B SoC and B DC , respectively) have been identified as sensitive markers of electrochemical processes. − B SoC and B DC depend on the overall design of the cell (geometry), and on the electromagnetic properties of the cell materials . Accurate measurements of B SoC and B DC maps and magnetic field simulations ,, suggest that cathode (de)­lithiation can occur nonuniformly throughout the pouch cell. A nonuniform distribution of SoC and current density can be associated with several adverse effects such as mechanical deformations, temperature and pressure gradients, as well as rapid local degradation of active materials.…”