Ultrafast Inside-Out NMR Assessment of Rechargeable Cells

Rechargeable battery cells are notoriously difficult to analyze. Conductive casings and the close spacing between electrode layers prevent the penetration of radiofrequency into the active compartment, and thus preclude direct nuclear magnetic resonance studies of cells unless they are specifically designed for such studies. Recently, an inside-out magnetic resonance imaging (MRI) method was developed that allowed measuring the magnetic field distributions in the volume surrounding the cells, and inferring internal parameters, such as the state of charge and current distributions. While the imaging approach provides a potentially very detailed picture of internal mechanisms, it can often be sensitive to background gradients and can be slow. In this work, an alternative approach is presented, which is based on the acquisition of free induction decays in the sample volume surrounding the cells. The signals encode intrinsic battery properties via the induced magnetic fields from the battery materials. A large range of cells were studied with different cathode materials, electrolyte amounts and cycle numbers (age). The spectroscopic signatures from these studies are shown to provide strong classification power for cathode materials. In addition, the derived principal components follow distinct pathways as a function of state of charge. The method is simple and fast (completes in less than a second), and requires only minimal hardware.