EPR, NMR, and electrochemical studies of surface-modified carbon microbeads

Nongraphitic carbon microbeads were prepared by hydrothermal treatment of a glucose aqueous solution at 180 degrees C. Impregnation of glucose-derived carbon microbeads with citric acid was used to control the microbeads' surface and electrochemical properties. The microspherical sample thus obtained exhibited a highly disordered nanocrystalline structure and a low BET surface area. The electrochemical performance of carbon microbeads was evaluated from discharge/charge experiments in lithium and sodium cells, as well as from electrochemical impedance spectroscopy measurements. The intercalation of Li and Na in surface-modified carbon microbeads was explored by EPR and NMR spectroscopy. It was shown that similar mechanisms operate for lithium and sodium insertion into disordered carbons. The observed Curie-like behavior and the change in intensity of the EPR signals as a function of cell voltage agreed well with lithium insertion into different sites of disordered carbon structures forming paramagnetic centers. Microspherical carbon samples with high reversible capacities in both Li and Na cells commonly exhibit low concentrations of paramagnetic centers.