In Situ NMR Tracks Real-Time Li Ion Movement in Hybrid Supercapacitor–Battery Device

Lithium ion capacitors (LICs) are emerging as promising energy storage devices; thus, understanding their electrochemistry is of great interest. Here we report the study of a novel LIC by employing in situ nuclear magnetic resonance spectroscopy (NMR) as a nondestructive tool, revealing the sequence of electrochemical processes in it. We have performed in situ 7Li NMR experiments on the LIC by simultaneously cycling the LIC pouch cell in the voltage range 2.0–4.0 V. NMR spectra recorded for multiple cycles reveal the 7Li NMR signals arising from different parts of the capacitor. By employing a combination of in situ 7Li NMR, component isolation, and Gaussian–Lorentzian peak fitting, we investigate the resonances arising from the Li metal from stabilized lithium metal powder (SLMP), free ions in electrolyte, the solid electrolyte interface layer (SEI), intercalated lithium in carbon anode, and the Li ions in the electric double layer on cathode. The recorded in situ 7Li NMR spectra showed that the charge and discharge processes caused electrochemical reactions, resulting in considerable repetitive changes in peak intensities and chemical shifts over multiple cycles. Further cycle experiments revealed contributions from individual electrodes. This series of experiments aid in the visualization of the Li ion transfer mechanisms in LICs.