Advanced Thermal Conductivity Characterization with Implications for Thermal Control Strategies during Fast Charging

Abstract

Extreme fast charging lithium ion batteries require aggressive thermal management, which keeps the maximum cell temperature below abusive thresholds without derating the charging power. The importance of thermal management is further increased for many new cell designs with improved energy density which often brings along weaker thermal performance. For instance, reducing the volume fraction of electrochemically inactive materials like the current collectors reduces the thermal conductivity and increases the heat generation. Aggressive cooling is achieved by increasing the heat convection coefficients between the cell surface and the heat transfer medium. With high heat convection coefficients, the internal thermal conductivity of the electrode-separator-composite determines the maximum cell temperature. Consequently, the thermal conductivity needs to be characterized accurately for fast charging investigations, which includes dependencies on parameters like temperature or compression load at the cell surface. Therefore, this work presents thermal conductivity measurements at different cell temperatures and compression loads with their impact on fast charging.The thermal conductivity of a large-format NMC-111 graphite cell with a flat-wound jelly roll and prismatic PHEV2 hardcase made of aluminum alloy is measured at temperatures between -10 and 50 °C and at external compression loads between 37 and 74 kPa. This compression range is defined by the manufacturer at the largest cell surfaces to counter swelling of the jelly roll. Based on the guarded heater principle, a precise thermal conductivity test bench is designed and validated by a stainless steel reference material. For deriving the thermal conductivity of the electrode-separator-composite from the full-cell measurements, the thermal conductivity of the hardcase has to be compensated. For this purpose, a fast and simple technique for measuring the thermal conductivity of the hardcase by using electrical resistance measurements and applying theories like the law of Wiedemann–Franz is introduced.According to the measurement result, the thermal conductivity increases by 13.6% at 20 °C when the compression load rises from 37 to 74 kPa, which is mainly attributed to reduced thermal contact resistances between the cell layers. At constant compression and rising mean temperature, the thermal conductivity decreases by more than -1% per °C compared to the value at 20 °C. Both findings affect the cell internal temperature rise during aggressive cooling and therefore the power-derating events due to overheating. Based on these findings, implications for thermal control strategies during fast charging are discussed.Figure: Thermal conductivity of NMC-G electrode-separator-composite in through-plane direction at different cell temperatures and compression loads at the two largest surfaces of a prismatic PHEV2 cell. Figure 1