Loss verification of Li-ion battery cells from thermal flow measurements

Abstract

Abstract This master’s thesis focuses on heat generation in Lithium-ion battery cells a crucial aspect affecting their performance, lifespan, and overall safety. A custommade isothermal heat conduction calorimeter, integrated with heat flow sensors (Peltier elements), was used to measure heat flow from the lithium-ion cells. Initial experiments involved calibration and sensitivity testing using a 1.25 W resistor mat. Further heat generation experiments were conducted on lab-scale 0.25 mAh LiFePo4/Graphite cells and an 850 mAh lithium-polymer commercial pouch cell. These cells underwent electrochemical characterization tests, including formation cycles and electrochemical impedance spectroscopy (EIS), under various charge/discharge rates and temperatures. The calorimeter displayed reasonable sensitivity up to 6 mW, but higher noise levels were detected below this. Heat generation experiments were unsuccessful on the lab cells due to low dissipation of heat. However, measurements from the commercial cell agreed with established literature. The experimental results found that both irreversible (from internal cell impedance) and reversible (from electrochemical reactions) heat generation contribute to overall heat generation. Irreversible heat generation showed exothermic behaviour, especially during high current rates (especially in discharges direction), while reversible heat displayed both endothermic and exothermic behaviour and was more dominant at lower current rates (and is dependent on the state-of-charge). Interestingly, it was found that at low current rates endothermic behaviour was prominent for low state-of-charge levels while charging. Ambient temperatures significantly affected irreversible heat contributions due to resistance changes, but they did not notably impact reversible heat.