Microscopic Behavior of Active Materials Inside a TCNQ-Based Lithium-Ion Rechargeable Battery by in Situ 2D ESR Measurements

Real-time spectroscopic measurements in rechargeable batteries are important to understand the electrochemistry of the batteries at the molecular level and improve relevant functionalities. We have applied in situ two-dimensional (2D) electron spin resonance (ESR) spectroscopy to a well-known organic lithium-ion battery, which is composed of 7,7,8,8-tetracyanoquinodimethane (TCNQ) as the cathode-active material and a lithium metal anode electrode. The TCNQ rechargeable battery is suitable for investigating electrochemistry in the battery in terms of behavior of electron spin at microscopic levels on both the cathode and anode electrodes. We have discussed two-stage oxidation/reduction reactions of TCNQ, Li deposited/stripped process and their resulting dendritic and/or mossy microstructures, clearly elucidating the cause of the cell capacity degradation upon the charge-discharge cycles. The observed in situ ESR spectra showed that the degradation of the cell capacity was due to the elution of the active molecules, which caused the increase of ion conductivity by the substitution of the electrolyte solution for the adsorbed active materials on the conductive carbon surface. To discriminate paramagnetic species during the charge-discharge process, the generalized 2D correlation spectroscopy has been applied to characterize time-dependent in situ ESR spectra. The correlation analysis with in situ ESR helps us identify the paramagnetic species occurring in the battery cell in a straightforward manner.