2, University of California, Riverside, Riverside, California, United States
3, University of California, Riverside, Riverside, California, United States
Engineering higher-performing lithium-ion batteries may be key to meeting the ever-growing demand for energy storage. Commonly used in battery cathodes because of its high theoretical capacity, thermal stability, and ecological friendliness, lithium iron phosphate (LFP) still has one major flaw, its low conductivity, both electronic and ionic. Technology is limited by this property of LFP, but several methods have been developed to increase battery performance, such as carbon additives and controlled porosity. Previous studies have established the importance that these two factors play in battery performance, and this project investigates the relationship between performance and pore size.
In lieu of a more common carbon black/graphite additive, graphene was chosen in this research due to its higher surface area and electronic conductivity. Additionally, four polymers (polyethylene terephthalate, polyvinyl chloride, polystyrene, and polypropylene) were tested as porous media, all with different particle surface-area-to-volume ratios, allowing us to manipulate pore surface area while keeping porosity (%) constant. Cathode slurries were mixed with LFP as the active material and with varying polyvinylidene difluoride (PVDF) to graphene weight ratios in an N-methylpyrrolidone (NMP) solvent. Then, the mixtures were spread on non-conducting plates, dried, and their electronic conductivities were measured using a four-point probe.
The PVDF/Graphene ratio yielding the highest conductivity was determined, and then slurries were prepared with the added polymers in order to achieve porosities between 20% and 40%. The slurries were spread onto non-conducting plates, dried, and calcinated. The four-point probe method was used to determine each sample’s conductivity, and coin cell batteries were assembled. Lastly, an Arbin battery tester was used to perform cycling tests on each of the prepared batteries. Imaging of each cathode material was also taken using a scanning electron microscope (SEM), and the crystallinity of the material was analyzed by dispersive spectroscopy (EDS).