The influence of electrolyte and graphite type on the PF6- intercalation behaviour at high potentials

Several graphite types were investigated in respect to their PF6- intercalation ability in various electrolytes during electrochemical cycling at positive potentials up to 5.5 V vs. Li/Li+. It revealed that the electrochemical behaviour is related to the structural changes of the graphite particles, especially in terms of exfoliation and surface film formation, as deduced from post-mortem scanning electron microscopy (SEM). Small particle size SFG6 graphite in ethylene carbonate/dimethyl carbonate electrolyte showed reversible cycling and retention of the structure. With other tested electrolytes, the graphite could not be cycled reversibly under the given conditions. High crystalline, heat-treated SFG44 graphite exfoliated even at less positive potentials (4.8 V vs. Li/Li+) whatever electrolyte was used. These results provide guidance about the applicability of different graphites as a conductive additive for positive electrodes of lithium-ion batteries, in order to maintain the structural integrity of the electrode during cycling.