Effect of electrode porosity on the performance of natural Brazilian graphite electrodes

Natural Brazilian graphite has been investigated as a reversible Li+-intercalating electrode for lithium-ion cells. Its performance is greatly influenced by the electrode preparation technique. A short compacting time (1 s or less), when pressing the powders, increases the specific capacity because the porosity remains sufficiently high. Low pressures (<200 kg/cm2), which create higher porosities, give limited capacities due to poor inter-particle electrical contact. Excellent results have been obtained by substituting pressed powders with slurry-derived electrodes. In this case, an electrode porosity of 25–30% can be measured from compacting pressures between 20 and 200 kg/cm2. With this technique, it is possible to obtain: (i) an efficiency of 85% in the first cycle; (ii) a high rate capability, and (iii) a high and constant capacity upon cycling, i.e. 340 mAh/g at 0.4 mA/cm2. Cycling produces an increase in graphite surface area, a phenomenon which deserves a closer attention in view of possible safety problems.