Assessing Heat Generation in Lithium-ion Batteries: An Experimental Methodology

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Lithium-ion batteries are becoming increasingly important for ensuring sustainable mobility, and are now the technology of choice for electric vehicles. Research into lithium-ion batteries is intensive and widespread; in order to develop advanced materials required for the technology to meet the demands of the market.
However, resources are committed principally to the enhancement of power capability and specific energy at the cell level (Wh.g-1) with far less consideration given to how to remove heat. This has been shown to lead to suboptimal cells which are prone to large internal thermal gradients which cause accelerated and inhomogeneous degradation.
Cell behaviour is strongly dominated by temperature, since it affects impedance exponentially.1 A badly designed cell from a thermal management perspective will lead to reduced power, less usable capacity, and ultimately reduced useable energy at pack level. Despite the strong dependency of lithium-ion cell performance on temperature, the information battery suppliers and manufacturers provide regarding the effect of temperature on battery performance is not standardised, and is rarely sufficient to characterise a given battery’s performance across the full temperature operating range, as defined by the same manufacturer. Additionally, there is insufficient information on typical cell spec sheets to inform how easy it is to reject heat from a cell, nor sufficient information on heat generation to guide pack design.
In response to this need, a new metric to evaluate the ability of lithium-ion cells to reject heat, independent of their chemistry, form factor or manufacturer has been recently developed.2 The cell cooling coefficient, or CCC (W K-1), 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 tab3 or surface cooling.4 The evaluation of a given cell’s thermal performance using this metric avoids the use of unjustifiable assumptions required to calculate the Biot number or the thermal conductance of the cell. These conventional heat rejection measures are not suitable for its application in lithium-ion cells since they do not consider the anisotropic thermal conductivity and the uneven heat generation characteristic of these electrochemical devices. The CCC methodology has been demonstrated to be able to determine the rate of heat rejection from a cell which is generating heat at a constant rate, and is not imposed to transient thermal conditions, using a specified thermal management method (i.e. surface cooling or tab cooling). The apparatus for the CCC is low-cost and the experiments can be conducted with any battery cycler and in any battery laboratory, making the CCC apparatus concept an attractive alternative for long term experimental studies, where isothermal calorimeters are not available or financially justifiable.
The design of a battery thermal management system should consider the heat rejection capability of a cell among many other factors, such as the cooling method strategy, size, weight, material, and amount of coolant, all of which have an influence on the cost and performance of the system. However, one of the most important battery characteristics that must be understood for the design of an adequate thermal management system is the heat generation rate of the battery. A capability for the battery to effectively reject heat is important, but the battery manufacturer should also focus on minimising the rate of heat generation – this will reduce the burden on the thermal management method and reduce the sensitivity of the battery’s heat rejection capability on overall battery performance.
In this study, an empirical method to measure the irreversible heat generation of a lithium-ion battery in the form of heat generation rate maps is presented.5 Heat generation was measured as a function of frequency, current, state of- charge (SOC) and temperature, resulting in 4D maps of heat generation. The results were highly consistent with previous literature on the subject. The heat generation maps can be informative on their own or combined with the cell cooling coefficient (CCC) and used for cell selection and as a design tool to define the requirements of the thermal management system for different operating conditions.
The rate of heat generation was most sensitive to current and temperature. Regarding current, the heat generation rate map showed a quadratic dependency, consistent with literature. In terms of temperature, lowering temperature rapidly increases the heat generation because of increased overpotentials. The exponential trend observed can be explained by the Arrhenius relationship of the ohmic and charge transfer resistances with temperature. The influence of frequency on the heat generation rate was noticeable, but less significant. Three major regions of behaviour were observed: 1) Low frequency region with high heat generation (diffusion dominated), 2) Plateau region (diffusion and migration coexist) and 3) High frequency region with low heat generation (migration dominated). In terms of SOC, the map shows that the heat generation rate increases at very low SOCs, remaining constant at all other SOCs studied. This is due to the diffusion impedance and charge transfer resistance increasing with decreasing SOC. This is not expected to be the same for every cell as it will be a function of the materials and electrode balancing.
Finally, a worked example is set out to demonstrate how the obtained heat generation maps can be used in combination with the CCC in the early stages of a battery pack thermal management system design. Three operational points at differing C-rate were compared in terms of heat generation rates and the requirements needed from the thermal management system.
The heat generation maps are easily obtained with the described empirical methodology using equipment that can be found in most electrochemical labs. They provide an insight into the thermal performance of the cell across a desired range of operation conditions. The validity of the observations in the heat generation maps holds beyond the specific choice of electrode materials, geometry and physical design, as long as physicochemical interactions are of a similar nature. Therefore, the methodology can be applied to evaluate any lithium-ion cell, independently of its chemistry, format or physical design. Consequently, battery pack designers will gain a valuable technique enabling cell to cell comparison from a thermal perspective in order to select the most appropriate cell for an application. The uptake of the heat generation maps by cell manufacturers and its inclusion on the specification sheets would inform their potential customers on the thermal performance of their cells enhancing competition, not only competing on power and energy density criteria.
The present study provides heat generation maps as a tool alongside the cell cooling coefficient to summarise how easy or hard it is to thermally manage any type of cell in any type of application. This is a critical advancement. Uptake of the methodology will allow engineers within the battery industry to optimize thermal management systems with a simple analysis method at a much earlier stage in battery pack design than was previously possible, saving considerable time and money. A better understanding of how easy or hard it is to thermally manage cells could also incentivise cell manufacturers to optimise both heat rejection and heat generation together.

References
1. Troxler, Y. et al. The effect of thermal gradients on the performance of lithium-ion batteries. J. Power Sources 247, 1018–1025 (2014).
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. Hales, A. et al. The cell cooling coefficient: A standard to define heat rejection from lithium-ion batteries. J. Electrochem. Soc. 166, A2383–A2395 (2019).
4. 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 The Surface Cell Cooling Coefficient : A Standard to Define Heat Rejection from Lithium Ion Battery Pouch Cells. J. Electrochem. Soc. 167, 020524 (2020).
5. Bravo Diaz, L., Hales, A., Marzook, M. W., Patel, Y., Offer G. J. Measuring Irreversible Heat Generation in Lithium-ion Batteries: An Experimental Methodology, J. Electrochem. Soc. (2022).

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