<i>In situ</i>nuclear magnetic resonance investigations of lithium ions in carbon electrode materials using a novel detector

Abstract

The reversible electrochemical process (insertion/extraction) of lithium ions in graphitic carbon was monitored in situ for the first time by 7Li nuclear magnetic resonance (NMR) spectroscopy using a novel NMR apparatus. The compression coin cell battery imager is a simple device that combines the functions of an electrochemical cell and an NMR detector. A series of 7Li NMR spectra obtained for a blend of spherical and flaky disordered graphitic carbon particles revealed two distinct chemical shift signatures for the lithium ions that were inserted and extracted in the first electrochemical cycle. The lithium signal at ~50 ppm is consistent with the interplane sites for lithium ions on the sixfold axis between two stacked aromatic carbon rings aligned in registry. The second predominant lithium signal at ~12 ppm occurs in the chemical shift region reported for high-stage lithiated graphite and a dispersion of lithium-ion sites found in disordered carbon matrices. In addition, we observed chemical shift signatures similar to those assigned to Li-7 nuclei in lithium oxide, lithium carbonate, lithium alkyls, and lithium alkoxides that occur near 0 ppm and represent lithium nuclei that are irreversibly bound in the electrode/electrolyte interphase. An increase in intensity in the spectral region that is normally associated with irreversibly bound lithium was observed during the first discharge cycle, as anticipated. However, the same peaks in the spectrum unexpectedly diminished during the subsequent charge cycle, suggesting that the interphase between the carbon electrode and the electrolyte is built up over several cycles.