An equivalent circuit model for swelling in Prismatic Lithium-ion cells

Abstract

Abstract In this thesis, the expansion behavior of a Prismatic Lithium-ion cell is investigated via its external pressure changes. Experiments were done for 0% to 100% state-of-charge for both charge and discharge at various charging rates from 0.05C to 1.2C. An equivalent circuit with a single resistance was used to model the pressure behavior during charging, where the modeled resistance can be understood as a mechanical resistance of the cell casing to expand. It was shown to exhibit a clear trend for charging rate as well as state-of-charge, with values becoming bigger with both increasing charging rate and state-of-charge up to 0.5C. The model was evaluated for 0.3C with the absolute relative error between the modeled and experimental result being around 1% after 5% to 100% state-of-charge. Data from two research articles with similar pressure change experiments were used to validate the model. The first, [1], used charging rates comparable to those used in this thesis, and the other, [2], used charging rates above 1C. For the former, the modeled resistance values showed similar trends as presented here. For the latter however, the resistance values instead decreased with both state-of- charge and increasing charging rate, which was assumed to be due to lithium plating at higher state-of-charge at the higher charging rates as well as due to thermal effects.