Abstract

Understanding the degradation mechanisms of Li-ion batteries is essential to gain insights about battery aging. The primary research area of this thesis is the positive electrode, NMC811. The purpose of the thesis is to understand how low and elevated temperatures affect the aging of NMC811, by considering the effects on resistance. The aim of the research is to investigate the degradation mechanisms of NMC811. Here, three-electrode Li-ion pouch cells are assembled with LiNi8Mn1Co1O2 (NMC811) as the positive electrode, graphite as the negative, gold wire as the reference electrode, and LiPF6 as the electrolyte. The positive electrode impedance is recorded at temperatures –10, 22, and 40 ºC. Also, symmetric and half cells are built for validation measurements. The Nyquist diagrams are fitted through equivalent circuits to determine the cells’ impedance at voltages 3.8 and 3.0 V vs Li+/Li. The resistances observed and analyzed in this project are the high-frequency resistance, the contact resistance, the charge transfer resistance, and the resistance due to the electrode–electrolyte interphase. By comparing these resistances, it is observed that the charge transfer resistance has the highest dependence on the ambient temperature. The increase in charge transfer resistance at –10 ºC is suggested to depend on the Ni-rich electrode, which tends to contribute to volume changes in the electrode, affecting the intercalation and de-intercalation of Li-ions. The resistance reduces significantly at 40 ºC, due to the loss of lithium inventory in the active material. This thesis has thus shown that temperature has a significant effect on cell internal resistance, especially on the electrode–electrolyte interface, which describes the charge transfer reactions. For future studies, the transmission line model can be used to provide more information about the porous electrode.