Thermal Management for High-Capacity Large Format Li-Ion Cells

Li-ion batteries are known to have significant heating at high C-rates, such as 5C, 10C, and higher as demanded in automotive and other applications. The cell temperature will increase dramatically and additional cooling is required.  In order to cool down the core temperature to keep the Li-ion cells within the optimum operating temperatures of 20-40ºC, convective natural cooling, forced air cooling, and forced air or liquid cooling through conducting plates have been utilized.

The state-of-the-art cooling methods utilize the large surface areas and try to cool the battery from the surface. However, the cell is composed of multiple layers of cathode/separator/anode as a jelly-roll or in stacks.  Heat conduction through these layers is highly undesirable as the thermal resistance between the layers and the low thermal conductivity of the separators, which are typically polymeric membranes.  Although copper and aluminum are used as current collectors, the effective thermal conductivity through the layers is about 1-2W/mK as the effective conductance is in series (significantly lower than Al/Cu collectors).  Cooling from the surface is very ineffective creating a gradient through the thickness with increase in the middle layer temperature during high C-rate discharge.  These conditions have limited current Li-ion battery designs to have large surface area and small in thickness (6-8 mm) and ~5C maximum discharge rate. However, if the thickness of the batteries could be increased without increasing the surface area, for example high capacity cells could be produced with only 50 cells for a battery of 24 kWh.  This would reduce the cost of the overall battery pack because of easier battery management and significant reduction of the peripherals related to connections and system complexity. In addition, more compact packing of the cells would be possible, thus increasing overall energy density of the battery pack.

In this paper, we will present a novel thermal management idea where we directly access the current collectors for thermal management. We have employed an integrated modeling/experimental study to understand the heat generation and dissipation within the cell at high discharge rates and arrived at configurations that can effectively remove the heat from the cell by creating minimal temperature gradients. In addition, one can use the same thermal tabs to heat the cells for low-temperature applications. We will present this improved understanding of cell thermal performance and results from our experiments and modeling studies that show excellent control over the cell temperature. Moderating the cell temperature (both in mean and gradients) with this new approach can help decouple the cell electrochemistry from thermal management with the following advantages:

1. Develop very thick and high-capacity cells
2. Improve life time of the cells
3. Improve safety characteristics because of more effective heat removal
4. Very high-C discharge without compromising safety or lifetime
5. Low temperature applications