Lithium iron phosphate (LiFePO4, LFP) is the driving technology for battery energy storage applications due to its high theoretical capacity (170 mAh g-1), cost-effectiveness, long cycle life, good thermal stability, and environmental friendliness in comparison to other traditional cathode materials such as lithium cobalt oxide (LiCoO2) and lithium manganese oxide (LiMn2O4). However, key limitations of LFP are a low intrinsic electronic conductivity (~10-9 S cm-1) as well as limited ionic conductivity at room temperature. Several methods have been used to enhance electronic conductivity, such as carbon coating, super-valence ion doping on the Li-site, and nano-networking of electronic conductive metal-rich phosphides. On the other hand, enhancement of ionic conductivity has been researched to a less extend.
Theoretically, Chen et al. determined that the best conductivity for LFP cathode is achieved by a combination of 30% active material, 7.5% graphite, 10.15% carbon black, and 12.35% polyvinylidene fluoride (PVDF) by volume, to achieve a 40% porosity. In this work, we used polyvinyl chloride (PVC) and Polystyrene (PS) powders to experimentally optimize the size of pores and control the porosity of prepared cathodes. LFP cathodes are prepared in a slurry composed of lithium iron phosphate powder as the active material, carbon black and graphite as conductive additives, PVDF as a binder, and N-methyl-2-pyrrolidone (NMP) as the solvent. To create a matrix of pores, the cathodes are heated at 300°C and 360°C to evaporate PVC and PS, respectively. To calculate pore volume of the cathodes, their morphology is characterized by a scanning electron microscope (SEM) an image processing. To determine battery performance, coin-cells were assembled with cathodes containing various percentage of porosity. Cycling testing was performed using an Arbin tester.