This study investigates the degradation of parallel-connected lithium-ion battery packs under thermal gradients. It identifies two main degradation modes: convergent degradation with homogeneous temperatures and divergent degradation driven by thermal gradients. Divergent degradation is attributed to cathode impedance growth, which increases over time and leads to heterogeneous current and state-of-charge distributions. Experimental data, including current distribution measurements, decoupled impedance measurements, and degradation mode analysis, support these conclusions. The study highlights the critical role of capturing cathode degradation in parallel-connected batteries, which is essential for battery pack developers. The research shows that thermal gradients significantly affect battery performance. A 30°C thermal gradient leads to a doubling of the degradation rate, and heterogeneous temperature packs experience a 50% reduction in capacity loss at 1000 cycles. High-temperature cells degrade slower than reference cells, attributed to imbalanced currents in parallel strings. The study also finds that cathode impedance growth is inversely correlated with temperature, leading to divergent degradation in parallel-connected batteries. The study uses a 1S2P pack configuration with deliberately mismatched cell impedance to show that a 40% reduction in lifetime can occur when comparing balanced and imbalanced packs. The results indicate that increasing thermal gradients are increasingly detrimental to battery performance and lifetime. The study also finds that the rate of degradation is linked to the applied thermal gradient, with a 10% increase in capacity fade rate when comparing ±25°C thermal gradients. The study uses electrochemical impedance spectroscopy (EIS) and hybrid pulse power characterisation (HPPC) to measure resistance growth and identify degradation modes. The results show that cathode degradation is the dominant mechanism at intermediate temperatures, with significant changes in cathode charge transfer resistance leading to capacity loss. The study also finds that the degradation of high-temperature cells is primarily due to SEI layer formation, while low-temperature cells experience cathode particle fracture and consequent pulverisation. The study concludes that understanding and managing cathode degradation is critical for battery pack design and operation. The findings highlight the importance of minimizing thermal gradients and optimizing average cell temperature to maximize energy availability in parallel-connected batteries. The study provides a mechanistic model and a publicly available aging dataset to support further research and development in the field of lithium-ion battery technology.This study investigates the degradation of parallel-connected lithium-ion battery packs under thermal gradients. It identifies two main degradation modes: convergent degradation with homogeneous temperatures and divergent degradation driven by thermal gradients. Divergent degradation is attributed to cathode impedance growth, which increases over time and leads to heterogeneous current and state-of-charge distributions. Experimental data, including current distribution measurements, decoupled impedance measurements, and degradation mode analysis, support these conclusions. The study highlights the critical role of capturing cathode degradation in parallel-connected batteries, which is essential for battery pack developers. The research shows that thermal gradients significantly affect battery performance. A 30°C thermal gradient leads to a doubling of the degradation rate, and heterogeneous temperature packs experience a 50% reduction in capacity loss at 1000 cycles. High-temperature cells degrade slower than reference cells, attributed to imbalanced currents in parallel strings. The study also finds that cathode impedance growth is inversely correlated with temperature, leading to divergent degradation in parallel-connected batteries. The study uses a 1S2P pack configuration with deliberately mismatched cell impedance to show that a 40% reduction in lifetime can occur when comparing balanced and imbalanced packs. The results indicate that increasing thermal gradients are increasingly detrimental to battery performance and lifetime. The study also finds that the rate of degradation is linked to the applied thermal gradient, with a 10% increase in capacity fade rate when comparing ±25°C thermal gradients. The study uses electrochemical impedance spectroscopy (EIS) and hybrid pulse power characterisation (HPPC) to measure resistance growth and identify degradation modes. The results show that cathode degradation is the dominant mechanism at intermediate temperatures, with significant changes in cathode charge transfer resistance leading to capacity loss. The study also finds that the degradation of high-temperature cells is primarily due to SEI layer formation, while low-temperature cells experience cathode particle fracture and consequent pulverisation. The study concludes that understanding and managing cathode degradation is critical for battery pack design and operation. The findings highlight the importance of minimizing thermal gradients and optimizing average cell temperature to maximize energy availability in parallel-connected batteries. The study provides a mechanistic model and a publicly available aging dataset to support further research and development in the field of lithium-ion battery technology.