Electron Paramagnetic Resonance Monitoring of Sodium Clustering and Its Effect on the Sodium Storage of Biowaste-Derived Carbons

Biowaste-derived carbons store large amounts of sodium as many competing reactions of adsorption, intercalation, and pore filling take place. Herein, we report electron paramagnetic resonance (EPR) spectroscopy as a comprehensive method to monitor the interaction of sodium with carbons used as electrodes in sodium-ion cells. It is shown that EPR can distinguish signals due to sodium intercalation and sodium cluster growth. Applying the correlation between the EPR line width and the metallic particle dimension, the sodium clusters are quantified regarding their size. In terms of the EPR, we discuss the sodiation mechanism and sodium storage performance of carbons derived from spent coffee grounds. The smallest sodium clusters (below 2 nm) are formed on closed-pore carbons at a potential of 0.01 V, while the largest clusters (around 200 nm) occur on hydrogen-rich carbons at 0.05 V at the earliest. The small sodium clusters generated at around 0.01 V give extra capacity, while sodium intercalation occurring between 0.5 and 0.1 V ensures good cycle stability.