Thermal Characterization and Degradation Analysis of Lithium-Ion Batteries under Temperature Gradients

Abstract

Thermal management of lithium-ion batteries widely deployed across diverse applications from portable electronics to electric vehicles is crucial for performance, safety, and longevity. Key challenges include characterization of heat generation and optimal rejection of generated heat under different operating conditions. In electric vehicles, cell-to-pack architecture using large-format pouch cells was adopted to increase specific energy density. However, increased cell size inherently induces spatial temperature gradients due to extended current pathways, inhomogeneous current distribution, and uneven heat transfer by thermal management systems. Effects of these gradients on degradation have not been fully investigated. This dissertation addresses these gaps through complementary experimental and physics-based modeling to obtain the fundamental understanding of thermal phenomena. The first study is performed with LCO/C lithium-ion battery used in smartphones. Precise measurements of both reversible and irreversible heat at different C-rates, temperatures, and aging states are conducted using a developed calorimeter. An experimentally validated reduced-order electrochemical-thermal model analyzes heat generation, revealing the dynamics of detailed heat sources. Contact resistance and solid-phase diffusion limitation are the dominant sources of irreversible heat due to relatively thin current collectors and higher current density. Impedance analysis via distribution of relaxation times identifies resistance evolution during aging, linking SEI growth and Ohmic losses to increased heat generation. The second study focuses on the effects of temperature gradients on degradation in large-format automotive cells. A three-segmented calorimeter is developed to impose controlled longitudinal temperatures, where each segment represents a microcell with different operating temperatures. A corresponding quasi 3D framework modeling employing parallel-connected microcells is proposed to capture spatial electrochemical heterogeneity with reduced computational cost compared to 3D full-order models while maintaining accuracy. The model includes chemical degradation mechanisms such as side reactions and lithium plating. The study reveals that temperature gradients redistribute current toward tab regions, creating localized electrochemical stress that accelerates capacity fade and triggers lithium plating. Differential capacity analysis provides experimental evidence of the degradation mechanisms. These findings establish a comprehensive understanding of thermal behavior and degradation mechanisms across different battery formats under realistic operating conditions. For small-format cells, the validated model facilitates quantification of detailed heat sources, addressing thermal challenges in high energy density portable devices. For large-format cells, the calorimeter provides data at controlled in-plane temperature gradients, and the quasi-3D model predicts gradient-induced capacity fade in automotive applications. The complementary approaches inform format-specific thermal management system design grounded in fundamental thermal and degradation phenomena.