The Cell Cooling Coefficient As a Design Tool to Optimize Thermal Management of Lithium-Ion Cells in Battery Packs

Abstract

Lithium-ion cells and battery packs are not typically designed for optimised thermal management. As a result, almost every cell in use worldwide is performing below optimum levels and degrading needlessly fast. The root cause of the problem is the lack of information surrounding the thermal performance of cells. A cell manufacturer cannot market a cell with excellent thermal performance if there is no robust and recognised metric to define thermal performance. As a result, the cell manufacturer will optimise against metrics that are recognised, namely energy density. Cells are packed with active material in order to optimise energy density, and the consequence is very poor thermal properties. Furthermore, a battery pack designer cannot down select an appropriate cell for their battery pack based on thermal performance, if there is no way to compare thermal performance of many different cell models. The battery industry is trapped, unable to move away from very energy dense cells which look great on a datasheet, but ultimately lead to poor battery pack operation and rapid degradation.Cell Cooling Coefficients (CCCs) have been developed to define a cell’s heat rejection capability. In this presentation, the CCCs are used to indicate how thermal performance varies with geometric change in five different lithium-ion pouch cells. It is found that the performance of ‘surface cooling’ is reduced dramatically by increasing the pouch cell’s electrode-stack thickness. The results highlight the design limitations of the current generation of lithium-ion pouch cells, which are almost universally surface cooled. They must have large aspect ratios, long and wide to create a large surface and very thin to limit the build up of thermal gradients across the electrode-stack. This is hindering for battery pack innovation, limiting the maximum battery pack energy density that can be achieved. Across the automotive industry, the ‘mass efficiency’ of a battery pack is about 60%, i.e., only 60% of the mass is lithium-ion cells, and most of the remaining mass is made up by the heavy and cumbersome thermal management systems that are required to overcome the limitations of the cells. This analysis is not limited to just pouch cell design; the Tesla Model 3, seen by most as the market-leading electric vehicle, has a battery pack with just 65% mass efficiency.The performance of ‘tab cooling’ is unaffected by electrode-stack thickness. Tab cooling is limited by the cross-sectional area of the tabs, which are shown to be a significant thermal bottleneck. In this study, increasing the tab thickness by 34% yields a tab cooling performance enhancement of 20%, with a specific energy reduction of just 0.7%. Simple redesign could drastically improve tab cooling capability for most cells, enhancing cell thermal performance and in turn reducing the requirements for the battery pack thermal management system. Energy density at cell level may reduce by a small amount, but this will be made up many times over at the battery pack level, where smaller thermal management systems will lead to higher battery pack mass efficiency. Furthermore, tab cooling is known to benefit the performance and lifetime of cells, when the thermal bottleneck is not a limiting factor.The battery industry must embrace innovation and implement tab cooling in the automotive market, grid storage and beyond. There is no downside; the incremental advancements in cell chemistry will continue to trickle down to the end-user, regardless of the thermal management system that is chosen.