Dependence of the morphology of graphitic electrodes on the electrochemical intercalation of lithium ions

We have studied the effects of several parameters that influence the electrochemical intercalation of lithium ions into various carbonaceous materials: massive samples of pyrographite PGCCL (Le Carbone Lorraine), bulky pitch-based graphitized carbon fibres P100-S (Amoco) and divided natural graphite powder UF4 (Le Carbone Lorraine). The electrochemical Li+ intercalation has been achieved in electrolytic solutions composed of a solvent, ethylene carbonate and a conducting salt, LiClO4. We have shown previously that such an electrolyte allows the intercalation of unsolvated lithium ions up to the richest stage-I LiC6 composition without apparent solvent decomposition. The electrochemical behaviour of the electrodes in such electrolytes was followed either by chronopotentiometry (galvanostatic charge/discharge cycles) or by cyclic voltammetry. The use of micro-computers, able to conduct the experiments by imposition of charge or potential steps followed by cell relaxations, has allowed to obtain data on the kinetics of Li+ intercalation. The electrochemical behaviour of the graphitic electrode is strongly dependent on its morphology. Moreover, the decrease of the size of the crystalline domains during prolongated cyclings has been shown particularly in massive pyrographite samples. Such an electrochemical grinding of the electrode has obviously a positive effect on its performances characterized by a noticeable increase in the maximum x composition reached (x refers to the LixC6 composition). It appears also that the use of poly(vinylidene difluoride) (PVDF) leads to side reactions that have a negative effect on the performances of the electrodes.