Capacity degradation of lithium-ion cell: The role of free carbon black content in the slurry and drying induced cracks in LiFePO4 electrode

Abstract

Conductive additives, such as carbon black (CB), are incorporated into LiFePO4 (LFP) electrodes to address their low electronic conductivity. Within the electrode, a portion of CB remains as free carbon while the remaining amount attaches itself to the LFP particles. We have attempted to quantify the free carbon content in an LFP/CB/binder electrode composition via rheology studies. A low carbon content leads to an increased CB-binder domain formation thereby reducing electronic conduction. An increase in the carbon content is found to induce cracks and form islands within the electrode at the time of solvent evaporation and first cycling. Here we demonstrate the impact of free carbon content on the changes in the percentage of electronically well-connected and ill-connected active materials and its effect on the capacity in the nominal voltage zone during cycling studies. The electrode with 2.06 vol% CB exhibited a higher nominal zone capacity compared to electrodes with higher and lower CB content throughout 200 cycles of charge and discharge at 1C. Finally, the influence of premixing time, which affects the percentage of free CB, on the cyclic stability for a specific volume percentage of CB is investigated.