The Effect of Different Li(Ni1-x-yMnxCoy)O2 Positive Electrode Materials and Coatings on Parasitic Heat Flow as Measured by Isothermal Microcalorimetry, Ultra-High Precision Coulometry and Long Term Cycling

Abstract

Isothermal microcalorimetry is used to investigate the effect of different Li(Ni1-x-yMnxCoy)O2 materials (NMC442, NMC532, NMC622) and coatings (Al2O3 and a proprietary high voltage coating) on parasitic reactions that occur in Li-ion pouch type cells. NMC/graphite pouch cells were prepared with a typical organic carbonate-based electrolyte containing a well-known additive blend and were tested up to 4.4 V at 40°C. A new method of extracting the parasitic heat flow during both charge and discharge is introduced. Differences between charge and discharge parasitic heat flow yielded more insight into the behavior of high voltage parasitic reactions. Ultra-high precision coulometry, long-term charge discharge cycling, in-situ gas measurements, and electrochemical impedance spectroscopy were also used to compare the observed heat flow to well-known performance metrics. All coated cell types performed significantly better than uncoated NMC442/graphite cells. It was found that the magnitude of the parasitic heat flows did not correlate as expected to the precision coulometry results nor to the long term cycling results. In particular, cells with Al2O3-coated NMC622 had the highest parasitic heat flow among the cells with coated electrodes but competed for best performance in the cycling tests. The possible reasons for this unexpected result are explored.