Effect of carbon coating on electrochemical performance of hard carbons as anode materials for lithium-ion batteries

Surface modification by a soft-carbon coating is used to improve the electrochemical performance of hard carbons as the negative-electrode (anode) material in lithium-ion batteries. The coating process involves simple heat-treatment of a mixture of coal-tar pitch and hard carbon powders at 1000 °C. The carbon coating significantly reduces the reaction of lithium with surface functional groups or absorbed species caused by air exposure. This is attributed to the effective suppression of the diffusion of both air and water into the hard carbon by the soft-carbon coating, and the better resistance of soft carbon to air. As a result, the charge–discharge coulombic efficiency during cycling, as well as during the first cycle, is improved.