Effect of Mechanical Grinding on the Lithium Intercalation Process in Graphites and Soft Carbons

Abstract

The effects of mechanical grinding on morphology and electrochemical performance of graphite and soft carbon powders with respect to lithium insertion were studied. The morphology of the milled graphitic powders was found to depend strongly upon the nature of the interactions (e.g., impact or shear) generated by the two kinds of mixer mills used. For the same milling time, crystallite size was smallest and the density of defects highest for graphitic powders that were ball-milled using impact interactions. The specific surface area of the milled samples does not increase indefinitely with increased milling time, but there is a critical milling time (mc) beyond which the specific surface area goes through a maximum (graphite) or levels off for cokes. By controlling milling conditions, graphite and soft carbon powders with well-defined morphology, d-spacings, surface area, and crystallite size can be made. The reversible (reversible amount of inserted Li) vs. irreversible capacity (irreversible lithium loss between the first discharge and charge) was measured for various C/Li cells using various tailor-made graphite and soft carbon powders. A direct correlation between the irreversible capacity of the milled samples and their specific surface area was observed, consistent with catalytically induced reduction of the electrolyte. For milling times greater than mc, the irreversible capacity remains constant or even decreases while the reversible capacity still increases. With mechanical grinding, both graphite and coke samples having irreversible capacity of 328 mAh/g for a reversible capacity of 708 mAh/g (∼Li2C6) were obtained