Practical Considerations for Operando 7Li Nuclear Magnetic Resonance in Coin and Pouch Cells

Abstract

Abstract Operando nuclear magnetic resonance (NMR) spectroscopy provides unique insight into chemical changes in batteries during use but has largely been limited to custom plastic cells. Unfortunately, the metal casings that house commercial batteries distort and shield radiofrequency (RF) fields required for excitation and detection in NMR. Here, we use operando7Li NMR spectroscopy to study metal-encased batteries in both coin and pouch cell formats without modifying the cell or casing. We benchmark NMR spectra obtained from stainless-steel coin cells and aluminum-laminate pouch cells to plastic coin cells and show that casing composition, auxiliary hardware, and sample placement strongly influence observed shift, signal intensity, and line shape due to bulk magnetic susceptibility effects and RF attenuation upon interacting with cell parts. These effects are most severe in stainless-steel coin cells (and in particular 304-grade stainless steel) but can be mitigated through the use of Al spacers while retaining the stainless-steel caps and springs. In light of these considerations, we leverage operando7Li NMR to track lithium plating/stripping and monitor the accumulation of electrochemically inactive “dead” lithium (Li0) during galvanostatic cycling in coin and pouch cells. In pouch cells, the lean electrolyte volume allows us to resolve changes in the 7Li electrolyte signal during formation that are consistent with electrolyte redistribution and wetting. Taken together, these results demonstrate how to alter cell susceptibility to improve NMR signals during operando measurements in commercially relevant form factors while also establishing the cell design constraints that limit quantitative interpretation.