Abstract
Hard carbons were prepared by pyrolyzing sugar precursors at 1000°C. The sugar carbons have a microporous structure and large specific capacity (⩾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 Brunauer‐Emmett‐Teller surface area measurements. The structure of the ballmilled powders was different from that of the original sugar carbons. As milling proceeds in argon or in air, the graphene layers initially become more stacked (as indicated by changes in the 002 diffraction peak), the nanoscopic or microscopic pores are rapidly eliminated, and the number of macropores or mesopores increases. Upon further milling, the 002 diffraction peak weakens again, as the carbon structure becomes 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 oxygen, suggest that oxygen‐containing surface functional groups are formed and show that the amount of the functional groups increases with milling time. Carbons ballmilled in argon atmosphere needed to be slowly exposed to air and kept cool or they burst into flames when brought into contact with air. This implies that the milling created broken carbon‐carbon bonds, which are highly reactive, in the material. Studies of ballmilled carbon/Li coin cells showed that ballmilled carbons have large reversible specific capacities of more than 600 mAh/g for lithium insertion. However, the cells demonstrated large hysteresis compared to that of unmilled sugar carbon/Li cells. We propose that the mechanism for quasi‐reversible lithium insertion in ballmilled carbons may involve (i) reactions of Li atoms at the edge of small graphene sheets, (ii) intercalation in cases where stacked layers remain, and (iii) reactions 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.