Determining parasitic reaction enthalpies in lithium-ion cells using isothermal microcalorimetry

Typical measurements of parasitic heat flow in lithium-ion cells can determine the magnitude of heat flow but cannot differentiate between different types of reactions. This causes difficulty when comparing cells that contain different coatings and solvent systems, which may affect parasitic reaction pathways. In this work, a new technique to measure the rate and net enthalpy change (△H) of parasitic reactions in lithium ion cells is introduced. Isothermal microcalorimetry and precision current measurements are used to make in-situ, non-destructive measurements of heat flow and parasitic current during high-voltage holds. The results are used to determine △H of parasitic processes in NMC532/graphite pouch cells. Effects of positive electrode coating, solvent, and upper cutoff voltage are explored. Solvent systems of organic carbonates with and without methyl acetate, and fluorinated carbonates are also investigated. Each solvent system yielded a unique trend of △H with increased voltage, and results were consistent between pair cells. Cells with organic carbonates showed consistent △H between voltages, while cells containing methyl acetate showed decreasing H. Cells containing fluorinated carbonates showed increasing △H with increased voltage. The observed trends in △H and solvent systems as well as future developments of this method are discussed.