Fear Conner PhD Thesis Report (v99).pdf
Reason: Contains unpublished data
until file(s) become available
MECHANISTIC ROLE OF THERMAL EFFECTS ON LITHIUM PLATING
In the pursuit to enable the rapid charging of lithium-ion batteries, lithium plating at the anode poses one of the most significant challenges. Additionally, the heat generation that accompanies high rate battery operation in conjunction with non-uniform cooling and localized heating at tabs is known to result in thermal inhomogeneity. Such thermal anomalies in the absence of proper thermal management can instigate accelerated degradation in the cell. This work seeks to elucidate the link between thermal gradients and lithium plating in lithium-ion batteries using a combined experimental and simulation-based approach. First, we experimentally characterize the lithium plating phenomenon on graphite anodes under a wide variety of charging rates and temperatures to gain mechanistic insights into the processes at play. An in operando detection method for the onset of dendritic lithium plating is developed. Lithium plating regimes are identified as either nucleate or dendritic, which exhibit vast differences in reversibility. An operando method to quantify lithium stripping based on the rest phase voltage plateau is presented. Next, a model is employed to provide fundamental insights to the thermo-electrochemical interactions during charging in scenarios involving an externally imposed in-plane and inter-electrode thermal gradient. The relative importance of in-plane vs. inter-electrode thermal gradients to charging performance and cell degradation is necessary to inform future cell design and cooling systems for large-format cells, which are crucial for meeting the energy requirements of applications like electric vehicles. While in-plane thermal gradients strongly influence active material utilization, the lithium plating severity was found to be very similar to an isothermal case at the same mean temperature. By contrast, inter-electrode thermal gradients cause a shifting on the solid phase potential at each electrode during charging, related to the increase or decrease in overpotential due to local temperature variation. An experiment is then performed on a commercial multi-layer pouch cell, in which it was found that applied thermal gradients provide a slight reduction in lithium plating severity and degradation rate when compared to an isothermal cell at the same mean temperature. The presence of a thermal gradient causes heterogeneous lithium plating deposition within the cell, with colder regions experiencing higher quantities of plating and larger thermal gradients leading to more severe heterogeneity.