Lifetime of Li-ion Secondary Batteries and Measurement of Key Parameters for Internal Temperature Profile Assessment

Abstract

The thesis’ purpose was to contribute to the understanding of thermal gradients within Li-ion secondary batteries.

A first step was the determination of thermal conductivities of various electrode materials and separators. Some materials were extracted from commercial cells and some were directly bought from the manufacturers. The thermal conductivity was measured under varying conditions as a function of pressure - with and without electrolyte solvent. In case of the electrodes, we were able to differentiate between the active material of the electrodes and the whole electrode including the current collector.

A second step was answering the question to which extend the thermal conductivity of electrodes may change as the battery ages. These studies were carried out for a battery with a lithium nickel manganese cobalt oxide | hard carbon chemistry. The same cell was undergoing lifetime studies, which included measurements of the change in ohmic resistance of the cell.

To give an idea about the impact of the thermal conductivity values on thermal gradients, they were used in a simple 1D model. The model was used for 2 main reasons: First, to be able to compare different cell chemistries at different rates.

Second, to compare a pristine and an aged cell. With respect to the second, we wanted to determine the importance of the change in ohmic resistance and the change in thermal conductivity for the thermal profile of a Li-ion secondary battery.

Finally, we developed a method for the measurement of single electrode entropy changes, which can act as a heat source or heat sink within a Li-ion battery during operation. We measured the entropy change for a lithium cobalt oxide electrode.