Measuring Parasitic Heat Flow in LiFePO4/Graphite Cells Using Isothermal Microcalorimetry

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Isothermal microcalorimetry has previously been used to probe parasitic reactions in Li-ion batteries, primarily studying Li[NixMnyCo1-x-y]O2 (NMC) positive electrode materials. Here, isothermal microcalorimetry techniques are adopted to study parasitic reactions in LiFePO4 (LFP)/graphite cells. Features in the heat flow from graphite staging transitions were identified, and the associated heat flow was calculated using simple lattice-gas mean-field theory arguments, finding good agreement with experimentally measured values. Parasitic heat flow was measured in LFP/graphite pouch cells with different electrolyte additives. In an electrolyte without additives, a massive parasitic heat flow was measured suggesting a shuttle reaction unique to the LFP/graphite system. In cells containing electrolyte additives, parasitic heat flow agreed well with long-term cycling results, confirming the value of this technique to rank the lifetime of LFP/graphite cells with different electrolyte additives. Finally, comparing cells with and without unwanted water contamination, it was found that the parasitic heat flow was similar or slightly higher in cells where water was intentionally removed before cycling, seemingly contradicting long-term cycling results. It is concluded that the presence of water (at the 500 ppm level) may slightly reduce parasitic reactions, but at the expense of a more resistive SEI layer.