STUDIES OF LITHIUM INSERTION IN BALLMILLED SUGAR CARBONS

Hard carbons were prepared by pyrolyzing sugar precursors at 1000 degr ees C. The sugar carbons have a microporous structure and large specif ic capacity (greater than or equal to 550 mAh/g) for lithium insertion in carbon/Li electrochemical test cells. Powders of sugar carbon were then treated by high-impact ballmilling either in argon or air. These carbon samples were characterized by x-ray diffraction, small-angle x -ray scattering, thermogravimetric analysis, chemical analysis, and Br unauer-Emmett-Teller surface area measurements. The structure of the b allmilled powders was different from that of the original sugar carbon s. As milling proceeds in argon or in air, the graphene layers initial ly become more stacked (as indicated by changes in the 002 diffraction peak), the nanoscopic or microscopic pores are rapidly eliminated, an d the number of macropores or mesopores increases. Upon further millin g, the 002 diffraction peak weakens again, as the carbon structure bec omes more disordered. We explain these trends with a qualitative model . Thermogravimetric analysis and chemical composition analysis on the air-milled samples confirm that the materials contain substantial oxyg en, suggest that oxygen-containing surface functional groups are forme d and show that the amount of the functional groups increases with mil ling time. Carbons ballmilled in argon atmosphere needed to be slowly exposed to air and kept cool or they burst into flames when brought in to contact with air. This implies that the milling created broken carb on-carbon bonds, which are highly reactive, in the material. Studies o f ballmilled carbon/Li coin cells showed that ballmilled carbons have large reversible specific capacities of more than 600 mAh/g for lithiu m insertion. However, the cells demonstrated large hysteresis compared to that of unmilled sugar carbon/Li cells. We propose that the mechan ism for quasi-reversible lithium insertion in ballmilled carbons may i nvolve (i) reactions of Li atoms at the edge of small graphene sheets, (ii) intercalation in cases where stacked layers remain, and (iii) re actions with surface functional groups where they exist. It was found that hysteresis in the ballmilled carbons is only weakly dependent on temperature and cycling rate.