Enabling nondestructive observation of electrolyte composition in batteries with ultralow-field nuclear magnetic resonance.

Rechargeable batteries represent a key transformative technology for electric vehicles, portable electronics, and renewable energy. Yet, there are few nondestructive diagnostic techniques compatible with realistic commercial cell enclosures. Many battery failures result from the loss or chemical degradation of the electrolyte. In this work, we present measurements through battery enclosures that allow quantification of electrolyte amount and composition. The study employs instrumentation and techniques developed in the context of zero-to-ultralow-field nuclear magnetic resonance (ZULF NMR), with quantum magnetometers as the detection elements (atomic optically pumped magnetometers, OPMs, and superconducting quantum interference devices, SQUIDs, used in this work). In contrast to conventional NMR methodology, which suffers from skin-depth limitations, the reduced resonance frequencies in ZULF NMR make battery housing and electrodes transparent to the electromagnetic fields involved. As demonstrated here through simulation and experiment, both the solvent and lithium-salt components of the electrolyte (lithium hexafluorophosphate, LiPF) signature can be quantified using our techniques. Further, we show that the ZULF-NMR apparatus and technique are compatible with measurements of pouch-cell batteries.