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Assessing Heat Generation in Lithium-ion Batteries: An Experimental Methodology
Cell characterization
Characterization methods

Lithium-ion battery research has been conventionally driven by power and energy density targets without much consideration given to the thermal performance in the cell.[1,2] This has been shown to lead to suboptimal cells, which are prone to internal thermal gradients with accelerated and varying rates of degradation. Cell behaviour is strongly dominated by temperature, since it affects impedance exponentially.[3] Therefore, a badly designed cell from a thermal management perspective will lead to reduced power, less usable capacity, and ultimately reduced energy density at pack level. Despite the strong dependency of lithium-ion cell performance on temperature, battery suppliers and manufacturers seldom provide the temperature characteristics of a cell.
In response to this need, a new metric to evaluate the thermal performance of lithium-ion cells independent of their chemistry, form factor or manufacturer has been recently developed.[2] The cell cooling coefficient, or CCC, which can be measured empirically via standardized experiments, quantifies the heat rejection capability of a cell for a particular thermal management method, i.e. via the tab[4] or surface cooling.[5] Whilst the CCC can describe the heat rejection capabilities of a cell for a thermal strategy with a single parameter, the quantification of the heat generation rate of a cell is a much more complex issue since it varies significantly with operation variables. It is crucial to know how good a cell is at rejecting heat, but it is equally important to know how the cell generates it and how this heat generation varies with operational parameters to have a complete assessment of the thermal performance of a cell.
In the present study, an empirical methodology is developed to quantify the heat generation capabilities of a cell across the whole range of operation conditions. The empirical methodology allowed the construction of heat generation rate maps which gave a full insight on how the most important operational variables (frequency, C-rate, SOC and temperature) influence the thermal performance of the cell. The functionality of these maps in combination with the CCC is demonstrated in an academic example, informing on the requirements of the thermal management system to maintain the cell below a safe temperature.

1. Francfort, J. & Walkowicz, K. US Drive Electrochemical Energy Storage Technical Team Roadmap. (2017).
2. Offer, G., Patel, Y., Hales, A., Bravo Diaz, L. & Marzook, M. Cool metric for lithium-ion batteries could spur progress. Nature 582, 485–487 (2020).
3. Troxler, Y. et al. The effect of thermal gradients on the performance of lithium-ion batteries. J. Power Sources 247, 1018–1025 (2014).
4. Hales, A. Bravo Diaz, L., Marzook, M. W., Patel, Y. & Offer, G. J. The cell cooling coefficient: A standard to define heat rejection from lithium-ion batteries. J. Electrochem. Soc. 166, A2383–A2395 (2019).
5. Hales, A., Marzook, M. W., Bravo Diaz, L., Patel, Y. & Offer, G. J. The Surface Cell Cooling Coefficient : A Standard to Define Heat Rejection from Lithium Ion Battery Pouch Cells. J. Electrochem. Soc. 167, 020524 (2020).

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Alastair Hales, Waseem Mohamed Marzook, Yatish Patel, Gregory J. Offer

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