The Effect of LiFePO₄ Particle Size and Surface Area on the Performance of LiFePO₄/Graphite Cells

In an effort to better understand capacity loss mechanisms in LiFePO₄ (LFP)/graphite cells, this work considers carbon-coated LFP materials with different surface area and particle size. Cycling tests at room temperature (20 °C) and elevated temperatures show more severe capacity fade in cells with lower surface area LFP material. Measurements of Fe deposition on the negative electrode using micro X-ray fluorescence (μXRF) spectroscopy reveal more Fe on the graphite electrode from cells with low surface area. Measurements of parasitic heat flow using isothermal microcalorimetry show marginally higher parasitic heat flow in cells with low surface area. Cross-sectional SEM images of aged LFP electrodes show micro-fracture generation in large LFP particles, which are more prevalent in the low surface area material. Further, studies on the impact of vacuum drying procedures show that while Fe deposition can be inhibited by removing excess water contamination, the direct impact of Fe deposition on capacity fade is small. Despite the observed particle cracking, differential voltage analysis on aged cells suggested active material loss was not significant, leading to the conclusion that LFP particle fracture instead increases parasitic reaction rates leading to Li inventory loss.