Surface Cooling Causes Accelerated Degradation Compared to Tab Cooling for Lithium-Ion Pouch Cells

Abstract

One of the biggest causes of degradation in lithium-ion batteries is elevated temperature. In this study we explored the effects of cell surface cooling and cell tab cooling, reproducing two typical cooling systems that are used in real-world battery packs. For new cells using slow-rate standardized testing, very little difference in capacity was seen. However, at higher rates, discharging the cell in just 10 minutes, surface cooling led to a loss of useable capacity of 9.2% compared to 1.2% for cell tab cooling. After cycling the cells for 1,000 times, surface cooling resulted in a rate of loss of useable capacity under load three times higher than cell tab cooling. We show that this is due to thermal gradients being perpendicular to the layers for surface cooling leading to higher local currents and faster degradation, but in-plane with the layers for tab cooling leading to more homogenous behavior. Understanding how thermal management systems interact with the operation of batteries is therefore critical in extending their performance. For automotive applications where 80% capacity is considered end-of-life, using tab cooling rather than surface cooling would therefore be equivalent to extending the lifetime of a pack by 3 times, or reducing the lifetime cost by 66%. Due to their high energy and power densities, Lithium-ion batteries are a very important component of electric vehicles, and their use has increased dramatically in recent years as the uptake of hybrid and electric vehicles has increased.1-3 One of the major challenges of using lithium-ion batteries in hybrid and electric vehicles is thermal management, 4 which is important in order to manage degradation at an acceptable rate whilst maximizing the performance of the batteries and reducing the risk of thermal runaway. 5-7Many studies have shown that increased temperature leads to an increase in the rate of degradation, 4,[8][9][10][11] therefore the effectiveness of a thermal management system is vital in order to maximize the lifetime performance of the pack. The design of a good thermal management system for a battery pack should consider both the overall temperature of the pack and both intra-cell and internal thermal gradients. Poor design of thermal management systems could be a major contributing factor in increasing degradation rates to unacceptable levels. 12There are many different techniques that can be used to thermally manage batteries in hybrid and electric vehicles. It is possible to use either air or liquid as the cooling medium, and both of these can be used in either a direct (with cooling medium in contact with the cell) or indirect way. In addition to this, different areas of the cell can be cooled, namely the surfaces of the cell or the cell tabs. 13,14 In general, systems employing air as the cooling medium are considered to be simpler and cheaper to implement, although the performance is limited especially in applications where there is a high heat generation rate or if the batteries are being operated in a high ambient tempe...

Large format lithium ion pouch cell full thermal characterisation for improved electric vehicle thermal management

“…cycles and surface cooling resulted in a loss of usable capacity three times higher than cell tab cooling [26]. This effect is likely to be accentuated in larger pouch cells, where we have shown larger thermal gradients are present for high C rate applications such as in the automotive industry [54].…”

“…Consequently, manufacturers dedicate a large amount of resources to develop battery packs with corresponding thermal management systems, which aim to maintain each cell in identical thermal operating conditions [26]. These thermal management systems require accurate temperature sensing and comprehensive instrumentation of the battery system.…”

Simulation and Measurement of the Current Density Distribution in Lithium-Ion Batteries by a Multi-Tab Cell Approach

“…Also non-properly adapted thermal conditioning can have a crucial impact on the performance of larger cells. [10][11][12] Modeling of internal distributions of potential and temperature along the electrodes is quite challenging, since even to calculate only a few cycles, a lot of computational resources are required for fully resolved models. In literature, there are many examples for spatially resolved multi-dimensional modeling approaches, 8,9,[13][14][15][16] which aim at representing the cell's internal behavior in terms of potential, current density, state of charge (SOC) and temperature distribution.…”

The Effect of Current Inhomogeneity on the Performance and Degradation of Li-S Batteries

“…As a result, for conventional lithium-ion cells, surface cooling can lead to faster degradation than tab cooling at high cycling rates. 14 The effect of thermal gradients and the induced current inhomogeneity have not been explored for Li-S cells, but they are expected to affect battery behavior in real applications, where temperature difference is present across a battery pack as well as among the layers of a single cell. Here the effect of thermal gradients on the performance and cycle life of Li-S batteries is studied using bespoke single-layer Li-S cells, with iso-thermal boundary conditions maintained by watercooled Peltier elements.…”