Enhancing the real-time detection of phase changes in lithium-graphite intercalated compounds through derivative operando (dOp) NMR cyclic voltammetry

Robust and reliable diagnostic tools that can be employed under operating conditions are crucial to understanding performance and failure mechanisms in battery processes. The operando spectrum of a battery often consists of a strongly overlapping mixture of time dependent and independent resonances due to the compositional complexity. Here we report a new method called derivative operando (dOp) that improves the resolution of operando nuclear magnetic resonance (NMR) spectra by removing time independent signals and further distinguishes between time dependent signals associated with the formation and removal of species. This approach not only provides better resolution but also more clearly reveals correlations between resonances and the chemical transformations occurring at a specific potential. With the dOp-NMR method we detect the formation of lithium graphite intercalation compounds (GICs), including the signatures of LiC and its precursors, which have been previously undetected. We also observe a clear correlation of the dOp Li NMR spectra of lithium metal dendrites on the counter electrode with the chemistry of the working electrode.