Extreme fast charging lithium-ion batteries require aggressive thermal management. High heat generation has to be compensated with active cooling to keep the cell temperature below hazardous thresholds. The importance of thermal management is further increased for many new cell designs. Improvements in energy density often bring 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, 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 battery states like temperature or external compression pressure.
Therefore, this work presents thermal conductivity measurements for a large-format NMC-111 graphite cell with a flat-wound jelly roll and prismatic PHEV2 hardcase made of aluminum alloy. The mean temperature is varied between -10 and 50 °C for each test and the external pressures is adjusted either to 37 or 74 kPa. This pressure 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 was 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. This compensation requires a precise knowledge of the hardcase conductivity. To obtain this quantity, electrical resistance measurements and theories like the law of Wiedemann–Franz were applied.
According to the measurement result, the thermal conductivity increases by 12% at 20 °C when the compression pressure 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 conductivity at 20 °C. Both findings affect the cell internal temperature rise during aggressive cooling and therefore the thermal management of the battery system. Based on these findings, implications for thermal modeling and battery system design are discussed.
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