Determination of the Time Dependent Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Isothermal microcalorimetry has been previously used to determine the voltage dependent parasitic heat flow for individual lithium ion cells. While a decrease in parasitic heat flow over time was observed, no attempts were made to quantify the time dependence. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the components of the total heat flow were isolated as functions of time and state of charge. By fitting the measured total heat flow to a simple empirical model, the polarization and entropic heat flows were determined as a function of state of charge, while the heat flow resulting from parasitic chemical reactions was determined as functions of both state of charge and time. The time and state of charge dependent parasitic heat flow was determined for high voltage LiCoO 2 /graphite and Li[Ni 0.4 Mn 0.4 Co 0.2 ]O 2 (NMC442)/graphite pouch cells, with particular emphasis on high voltage operation. The effects of electrolyte additive blends containing combinations of vinylene carbonate, prop-1-ene sultone, tris(trimethylsilyl)phosphite, and methylene methanedisulfonate are also shown.