Electrochemical, textural and microstructural effects of mechanical grinding on graphitized petroleum coke for lithium and sodium batteries

A graphitized coke material obtained from petroleum residua was mechanically ground at different milling times between 0 and 100 h. Electrochemical reactions with both lithium and sodium are significantly altered as a function of grinding time. Short-time ball milling of graphite (1 and 5 h) induces a limited decrease in particle size and an increase in microstrain content. Simultaneously, alkali metal intercalation and electrolyte decomposition are hindered, and thus the irreversible and reversible capacities decrease. For longer milling time (up to 100 h), average crystallite size decreases and particles adopt a lamellar shape. Simultaneously, the irreversible capacity increases and correlates with an increase of the resistance, as obtained by impedance spectroscopy. Ex-situ XRD shows that extensively ground graphite samples need a higher discharge specific capacity to reach the formation of n-stages as compared to non-ground graphite, this being indicative of lithium incorporation in energetically different sites to the interlayer space. Sodium storage capacity increases with prolonged grinding time. This effect is shown here for the first time for graphitized cokes.