Degradation study for 18650 NMC batteries at low temperature

Abstract

This research focuses on the identification and quantification of lithium-ion battery degradation indicators at low temperatures. We studied the cycling aging of 18,650 commercial NMC lithium-ion batteries at 10 °C. For this purpose, we carried out life cycle tests and performed Galvanostatic Intermittent Titration Technique (GITT) tests in the voltage range for the charge and discharge processes for different States of Health (SoH). The diffusion time constant and the ohmic overpotential were determined from GITT for different SoH. As the batteries degrade both parameters increase. A degradation mechanism associated with faradaic effects (increase in ohmic drop overpotential) and a thermodynamic effect associated with changes in the active material (diffusional time constant) is observed. Furthermore, we performed Electrochemical Voltage Spectroscopy studies through Incremental Capacity (IC) curves. IC curves peaks and valleys are associated with battery phase transformations due to aging phenomena. Each peak has a unique peak height, area, and position associated with a degradation mode. This research focuses on the IC curves derived from the discharge capacities at 3A and OCP (Open Circuit Potential) to study the thermodynamic and faradaic effects separately. The peak at 3.6 V and 3.4 V for the 3A and OCP studies, respectively, is the main feature for the detection of the degradation mechanism. The valley in the OCP curves intensifies with the decrease in SoH and is shifted to lower potentials, an effect that is not observed in the 3A curves. So, we could associate this with thermodynamic effects, Loss of Lithium Inventory (LLI) and Loss of Active Material (LAM) of both the positive electrode and the negative electrode.