|Fundamental understanding of the heat generation process of lithium-ion batteries during operations is crucial for securing lifespan and safety by the cost-effective and efficient design of thermal management systems. Heat generation rate (HGR) of lithium-ion batteries varies at different operation conditions, such as charge and discharge rates, state of charge (SOC), temperatures and degradation conditions. In the first part of this work, a low-cost and high-performance multifunctional calorimeter is firstly developed. The calorimeter uses the thermoelectric assemblies (TEAs) as hardware and accompanied with a feedback loop, which enables the dynamic measurement of HGR and the active temperature control. The HGR performance of a large format lithium-ion cells is measured and compared as a function of charge and discharge rates, SOC, temperatures and degradation conditions, and the associated energy efficiency is analyzed. These works are presented as the first and second part of the dissertation.
The sources of HGR within lithium-ion cells are predominantly classified as reversible heat and irreversible heat. The reversible heat is generated by a change in entropy during the electrochemical reactions and can be estimated using the entropy coefficient. The irreversible heat is caused by the resistances that represent concentration, activation, and Ohmic polarizations. In the third part, we developed several novel experimental techniques that facilitate the fast and accurate characterization of the two heat source terms, which include (1) accelerated equilibration method, (2) hybridized time-frequency domain analysis (HTFDA) method, and (3) improved frequency-domain calorimetric method, and (4) wavelet-transform based simultaneous and continuous characterization method. The results are compared with those measured by the conventional experimental methods, and show advantages with respect to measurement time and accuracy.
In order to further explore the heat generation mechanism within lithium-ion cells, an electrochemical-thermal life model is developed and validated. The electrochemical model describes the cell’s internal reaction mechanisms such as the mass transport, charge conservation, and electrochemical kinetics; the degradation model describes the aging mechanisms including the solid electrolyte interphase (SEI) layer formation and lithium plating; while the associated HGRs are modeled by the coupled thermal model. Based on the developed model, the heat generation and the associated mechanism can be analyzed for both fresh and aged cells. This work is presented in the fourth part of the dissertation.
As a closing work to the dissertation, we further proposed an improved battery electrochemical model by considering a SOC-dependent diffusion coefficient lithium ions in cathode. The model has been validated to show a drastic increase of the accuracy in predicting the terminal voltage, while maintaining low computational time. The work may provide guidelines for further improvement and optimization of the battery model.