(Keynote) Thermal Performance Optimisation at Cell Level, to Achieve Better, Longer Lasting Battery Packs

Abstract

Temperature is a major issue in the operation of lithium-ion batteries, with both high and low temperatures causing problems. Cold batteries are inefficient, while hot batteries are at risk of thermal runaway and fire, making it crucial to maintain a safe and optimal temperature range. Temperature gradients can negatively impact battery performance, reducing accessible energy and increasing the degradation rate. This is a critical issue for electric vehicles (EVs), which can experience reduced driving range and deteriorating performance over time.Despite these challenges, current lithium-ion cell designs prioritize maximizing specific energy, which can negatively impact thermal performance. Thermal management systems are used to extract heat from poorly designed cells, but they are a significant parasitic load, demanding on average 8 kW in typical operation. Further, they can introduce uneven distribution of current throughout the battery pack, leading to diminished performance and accelerated degradation. The limited performance of these systems contributes towards range anxiety, which is a major barrier to the widespread adoption of electric vehicles in the UK, Europe, and the USA.To address these challenges, the authors propose the use of a new metric, the cell cooling coefficient (CCC). The CCC is a constant for a given cell and thermal management method (e.g. surface cooling or tab cooling), and is measured in W.K-1 (the reciprocal of thermal resistance). The CCC define a cell's ability to reject heat energy. The CCC is defined as the temperature gradient across a cell in idealized operating conditions, normalized against the heat generation rate in the same conditions. This allows for the comparison of geometrically dissimilar cells and can be used to optimize the thermal performance of lithium-ion cells.Experimental results presented in the paper demonstrate that increasing the thickness of the tabs in a typical lithium-ion pouch cell by 34% can yield a tab cooling performance enhancement of 20%, with a specific energy reduction of just 0.7%. Numerical methods predict that for a large format automotive pouch cell, increasing the tab thickness from 0.2 mm (negative) and 0.45 mm (positive) to 2 mm will lead to a 90% increase in the CCC for tab cooling (see in the attached figure). This yields a 51% reduction in temperature gradient across the cell in any instance of operation. As with all geometric optimisation, our models highlight a law of diminishing returns and we discuss the optimal tab thickness for various operations and applications.The authors also consider the use of the CCC metric for edge cooling in pouch cells and cylindrical cells. They find that tabs used to connect jelly rolls to the external can of cylindrical cells create a considerable thermal bottleneck. Redesign of this tab is well documented through Tesla’s proposed tab-less 4680.The use of thermal models at cell-level alongside existing battery models is critical for the development of future lithium-ion cells. The authors demonstrate that optimisation of thermal performance is entirely achievable within the numerical domain, reducing the requirement for expensive and timely prototyping phases during any product development process.By designing cells to have better thermal performance, it is possible to reduce the temperature gradient required between the hottest and coolest points in the thermal management system. This, in turn, reduces the power required by the thermal management system, allowing for more power to be directed towards extending the driving range of electric vehicles. Reduced temperature gradients at the battery pack level will also extend battery pack lifetime, reducing the lifetime cost to end-users and lessening the strain on raw material supply chains and battery recycling technology.The use of the CCC metric offers a promising approach to optimizing the thermal performance of lithium-ion cells. By addressing thermal management challenges, it is possible to reduce the parasitic load of thermal management systems, improve the lifetime and cost-effectiveness of battery packs, and lessen range anxiety for EV users. Figure 1