Effects of Surface Coating on Gas Evolution and Impedance Growth at Li[NixMnyCo1-x-y]O2Positive Electrodes in Li-Ion Cells

The effects of surface coatings on Li[NixMnyCoz]O2 (NMC, x+y+z = 1, x:y:z = 4:4:2 (NMC442), x:y:z = 5:3:2 (NMC532), x:y:z = 6:2:2 (NMC622)) electrodes in pouch cells and pouch bags, containing electrolyte with a certain additive blend, were systematically studied. Ex-situ gas measurements, gas chromatography coupled with a thermal conductivity detector and electrochemical impedance spectra were used to study the reactions that occurred. The results obtained from pouch bag experiments at elevated temperature indicate that the LaPO4 surface coating did not impede impedance growth at the NMC442 surface and did not reduce gas generation at 60°C while the Al2O3 surface coating effectively prevented impedance growth at the NMC622 surface at 60°C. The coating procedures and the underlying NMC materials were different so it is difficult to conclude that the Al2O3 coating is more effective than the LaPO4 coating in any situation. Hydrogen was only detected in pouch cells rather than pouch bags suggesting a crosstalk between the lithiated graphite and delithiated NMC electrodes as has been proposed before by the Gasteiger group. That is, species created at the positive electrode migrate to the negative, and react there to produce hydrogen.