Cooler batteries needed for a green future

The main requirements for any battery are performance, safety, sustainability and financial viability. While the cost of electric vehicle (EV) battery packs has rapidly decreased in recent years, largely due to economies of scale and more efficient manufacturing processes, EVs are not yet matching the purchase price of internal combustion engine (ICE) vehicles. To facilitate a wider market penetration, EVs need to become more affordable.

Since batteries constitute the bulk of the cost, this will require a lower battery cost of ownership, covering both acquisition and operation costs. The battery industry is thus focussed on strategies to further reduce battery production costs, globally measured in US dollars per kilowatt hour (US$/kWh). The impact of battery lifetime on the cost of ownership is often ignored, which neglects an important growth strategy for the industry.

By prolonging battery lifetime, more value can be extracted from the battery itself, compensating for upfront costs and thus decreasing the overall life cycle cost¹. Understanding why batteries deteriorate and implementing mitigating measures is key, either on the battery materials level or through battery pack engineering. The latter can be achieved through advanced thermal management systems, where evidence demonstrates that more effective thermal management strategies result in lower degradation rates for the lithium-ion cells in operation, and therefore increased battery pack lifetime.

This impacts not only life cycle cost, but also the environmental impact of the battery during its entire life cycle: the environmental footprint incurred during battery production can be counterbalanced by extending the useful life of batteries. Moreover, this increases material resource efficiency and decreases the pressure on the supply chain for critical raw materials, such as lithium and cobalt. This shows the significant contribution of thermal management systems to achieve not only safety, but also cost-efficiency, high performance and sustainability of a battery pack.

Improvements required

However, the current generation of thermal management systems are sub-optimal. ‘Surface cooling’ dominates the EV market, yet published research suggests lithium-ion cell lifetime can be tripled if ‘tab cooling’ is effectively implemented in the battery pack design. The shape and size of lithium-ion cells varies enormously across the market, bringing considerable variation in thermal performance, such as how the cell behaves when internal heat generation raises its temperature.

The battery industry is struggling to optimise thermal management, because there is no standard defining thermal performance. Battery engineers may quote effective thermal conductivities, thermal resistance values or the Biot number. However, these have been borrowed from other engineering sectors and are conceptually flawed, because they do not account for the internal heat generated by the lithium-ion cells themselves. Moreover, these metrics are very difficult to calculate, requiring information that will never be presented on a datasheet, because it is a secret held close by the cell manufacturer. Stakeholders in the industry are trapped, unable to compare different thermal management approaches to find the best fit to their requirements.

Over 2020, major players in the EV market began to normalise thermal management, and there has been marked progress. The Audi e-tron, VW ID.3 and Jaguar I-PACE contain similar thermal management systems, and the same concepts are also found in the Asia market, most notably at BYD with its ‘blade’ cell. Yet this uniformity is an exception

In the battery industry today, it is all about specific energy (sometimes referred to as energy density). Specific energy is used across the tiers of the industry and is universally quoted in Watt hours per kilogram (Wh/kg). Cell manufacturers, such as Panasonic or LG Chem, compete against one another at cell level, to determine who can pack the most energy into the smallest cell. Likewise, pack manufacturers, such as Tesla or VW, compete on pack level energy. A cell’s specific energy is displayed on its datasheet, while an EV pack’s specific energy often grabs headlines at automotive symposiums and in wider media coverage. The widescale recognition of specific energy, and the industry’s subsequent trend towards optimising specific energy above all else, highlights the importance of a simple and universal metric.

One figure

The Cell Cooling Coefficient (CCC) can become the universal metric for thermal performance. The CCC is designed specifically for lithium-ion cells in operation, accounting for unique heat generation traits without requiring any information about the cell, which most manufacturers would not want to divulge. Each cell will have its own CCC, given in units of Watts per Kelvin (W/K), defining the temperature gradient required across a cell, in order to remove a certain amount of heat from it³. CCCs from different cells can always be compared directly to one another; their geometry or chemical composition do not matter. CCCs are a metric for driving competition between different cell manufacturers and a comparative tool that can be used by a battery pack designer to select the most appropriate cell for their application.

You cannot optimise for both high energy and high thermal performance; instead, a compromise must be reached. Active materials—the anode, cathode and electrolyte—are thermally insulating, whereas the current collectors, needed for high power applications with reduced specific energy requirements, are metallic and very thermally conductive. As a result, poor thermal performance is inevitable when the cell manufacturer’s sole focus is on maximising the amount of active material present. CCC analysis in a recent publication quantifies this problem for a specific case⁴. The thermal performance (i.e. the CCC) for a particular cell may be increased by 20 % at the cost of only 0.7 % reduction to specific energy. Cell manufacturers must take steps toward realising these design changes, designing for thermal performance as well as specific energy.

Cells designed for thermal performance will improve EV battery packs. The cells will have a slightly lower specific energy, but this loss will be made up many times over by the battery pack designer, which will no longer need heavy and expensive thermal management systems. It is through this that improved thermal performance at cell level will result in improved pack level specific energy. Over 2020, major players in the EV market began to normalise thermal management, and there has been marked progress. The Audi e-tron, VW ID.3 and Jaguar I-PACE contain similar thermal management systems, and the same concepts are also found in the Asia market, most notably at BYD with its ‘blade’ cell. Yet this uniformity is an exception: there is still a very long way to go in cell and system design before we can consider thermal management to be optimised.

The CCC is designed specifically for lithium-ion cells in operation, accounting for unique heat generation traits without requiring any information about the cell, which most manufacturers would not want to divulge

The CCC is important, because it is the metric which the whole industry can use to assess the level to which thermal performance has been improved at the cell level. As such, uptake of the CCC will contribute towards holistic, system level battery pack design, rather than design for subsystem optimisation which is detrimental for the system.

There is no downside to the CCC; incremental advancements in cell chemistry and drivetrain design will continue to trickle down through the tiers of the battery industry. The EV market will benefit from better packs and EVs that are able to travel further between charges. At the same time, the battery pack will last longer, adding value for the end user, reducing the strain on raw material supply and recycling services and ultimately enhancing the environmental benefits of EVs.

Dr Alastair Hales and Dr Laura Lander are Research Associates at the Faculty of Engineering, Department of Mechanical Engineering, Imperial College London. Further assistance came from Dr Jacqueline Edge, Faraday Institution Project Leader for Multiscale Modelling

References

1. Lander, L. et al. Strategies for cost and carbon footprint reduction of EV lithium-ion batteries. Appl. Energy 289, 116737 (2020)
2. Hunt, I. A., Zhao, Y., Patel, Y. & Offer, J. Surface Cooling Causes Accelerated Degradation Compared to Tab Cooling for Lithium-Ion Pouch Cells. J. Electrochem. Soc. 163, A1846–A1852 (2016)
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. et al. The Cell Cooling Coefficient as a design tool to optimise thermal management of lithium-ion cells in battery packs. eTransportation 6, 100089 (2020)