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

Li-ion cells based on layered transition metal oxides (LTMO) demonstrate the best overall performance to date. A detailed understanding of ion transport and charge storage mechanisms in these cathode materials is key to improved design, performance, and safety of cells. The magnetism of LTMO-based materials depends on the concentration and the type of the intercalant. This phenomenon provides a source of sensitive magnetic resonance imaging (MRI) contrast for studies of Li-ion cell function and failure mechanisms. Surface-scan MRI is a nondestructive operando technique designed for artifact-free mapping of strongly inhomogeneous magnetic fields near various portable devices and battery cells. Recent experiments revealed nonuniform distributions of current density and magnetic susceptibility in common Li-ion pouch cells. Further analysis of the surface-scan MRI data suggests the coexistence of several magnetic phases and the presence of transient Li concentration gradients in the cathode. These hypotheses are validated herein through the observation of propagating magnetic susceptibility fronts in Li_xCoO₂ cathodes of resting state pouch cells. We show evidence for the cathode lithium distribution to follow the areas of high current densities, which is a surprising result, given that the cathode generally has very high conductivity. Furthermore, equalization of the lithiation levels is a slow process happening over several days. Such observations of structural varieties and solid-state ion transport are possible in any material with pronounced intercalation-dependent magnetic properties. The methodology described in this work is a powerful tool for the analysis of kinetic phenomena in a wide range of pouch cells.