Determination of the Voltage Dependence of Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Abstract

Isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition. However, the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells has not previously been determined. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the heat flow sources (polarization, entropy and parasitic reactions) as a function of state of charge is isolated. The measured heat flow data is fit using a model where each contribution is a function of state of charge (typically linear over a narrow voltage range), allowing for the extraction of the voltage dependency of the parasitic heat flow for an individual cell. The parasitic heat flow as a function of voltage is determined for LiCoO₂/graphite and Li[Ni_1/3Mn_1/3Co_1/3]O₂(NMC)/graphite pouch cells. The effect of the addition of 2% VC to the electrolyte in NMC/graphite cells is also shown, where a significant reduction in parasitic heat flow is observed. The absolute parasitic heat flow for individual cells also allows for cross-chemistry comparisons, where it is observed that at high potentials, the parasitic heat flow for the LiCoO₂/graphite cells is reduced compared to NMC/graphite cells.