Or in the slightly changed words of Asia: What Happens in the 'Heat of the Moment' with Lithium Ion Batteries?
Lithium Batteries and Heat Managemet - More Heat Than Light?
Compared to other frequently used batteries, lithium ion batteries offer high energy and power density, have a long service life and are made from low-pollution materials. As such, lithium batteries are increasingly used in all areas of everyday life, primarily as a self-contained power supply independent from the mains and as buffer batteries for electronic devices. Lithium batteries are also being used more and more in the automotive industry, with the small vehicle segment currently showing phenomenal growth for applications such as hybrid drivetrains and high-voltage electric drivetrains, etc. The suitability of an electromechanical energy storage device for use inside electric vehicles depends on a variety of technical, economic and ecological factors. The United States Advanced Battery Consortium (USABC) has identified safety and service life, in addition to energy density, as key indicators for setting battery development goals and measuring any progress made. Additionally, there are also specifications for battery systems laid down in the automotive industry: cost-effective solutions with high power and energy density are required, which noticeably restrict the usable volumetric and gravimetric overhead available for the battery design and thermal management.
Fraunhofer's Battery Thermal Management Solution
The study “Spatial and Temporal Temperature Homogenization in an Automotive Lithium-Ion Pouch Cell Battery Module” [Gepp, Lorentz, März, Geffray & Guyon, 2017] recently published by the Fraunhofer Institute for Integrated Systems and Device Technology IISB (Erlangen) specifically looked at this problem. “There is a clearly identifiable trend: the energy density of lithium-ion cells will continually increase, whereby the range of battery electric vehicles will be improved in the near future. However, when the amount of energy increases, so too do the risks. By implication that means that safety aspects will become increasingly important and will have to be taken into consideration a lot more in the design process. And that is where, for us, thermal materials became particularly interesting in order to provide an economically feasible alternative to expensive cooling management solutions.” explains Markus Gepp, who spearheaded the study. The interesting aspect of his approach is the optimisation of thermal management, by integrating new materials, and thus also the considerable reduction in size and volume. The module structure of the presented BTM (battery thermal management) design is described in the figure to the left which reflects a sectional view from above.
What’s more, a pyrolytic graphite sheet (PGS) was used as the heat distributor, which is a new ultra-light graphite sheet developed by Panasonic, the thermal conductivity of which is five times higher than that of copper. The pyrolytic graphite sheets are stuck directly on to and fully flush with the cell surfaces and bound with PCM and metal profiles using flexible and thermally conductive adhesives. The aluminium profiles are designed as shaped inlays for the cast plastic frame and provide mechanical stability and a thermal interface with the cooling plates. Commercial fluid cooling plates are bound with compressible gap fillers in order to compensate for geometric and production-related tolerances whereby air gaps can be eliminated. All cells are connected in series by means of ultrasonic welding. The advantage of this design is that the manufacturing process is economically cost-effective even for small quantities. The figure on the right shows the assembled prototype. When it came to implementation, the decision to incorporate Panasonic’s PGS was made swiftly, which Gepp explains as follows: “We initially began to work with a pouch cell module with fixed graphite cooling plates. However, PGS then caught our attention on account of its low material density of 70µm, flexibility/bending cycle stability and high conductivity of 1,000 W/m.K. In addition, the sheet could be directly attached to the pouch cell without any further procedures, thus ensuring low thermal contact resistance. Also, the problem of the volume increase of the cells, which occurs both within a charging cycle and over service life, could be compensated for by the flexible sheets through corresponding relief folds in combination with foams.” In a nutshell - these are the benefits PGS brought to the BTM design:
- The degree of spatial homogenisation, which can be defined based on the maximum measured temperature difference between the cells.
- The degree of temporal homogenisation, which is determined through the maximum thermal resistance and therefore is in reference to the maximum temperature increase/dissipating power loss relationship.
- The overhead, which is measured and stated in terms of weight and volume and therefore makes the mechanical design quantifiable and comparable independently of the cells.
See for Yourself, Which Advantages PGS Provides You With for Your Thermal Management Solution
Panasonic sat down with Markus to discuss the overall background as well as outlook of his project:
Mr. Gepp, Just A Word, Please
A: Lithium-Ion batteries have temperature dependent properties, such as useable capacity, internal electrical resistance or ageing rate. For example, the ageing rate is considerable accelerated at the upper and lower specified temperature limits which results in a, with regard to lifetime, preferred temperature range. Also temperature differences between the single cells in the battery system cause different ageing rates, reducing the system lifetime and requiring electrical voltage balancing. These aspects are addressed by the approach of temperature homogenization, both temporal, to keep the system in an optimal temperature range, and spatial to reduce inner temperature differences.
Q: What are the causes of thermal runway propagation?
A: A thermal runaway concern most Li-ion chemistries and occurs if thermal, electrical, mechanical or ageing impacts damage a cell while safety mechanisms on cell or system level fail. Heat spreads to adjacent cells, causing a thermal runaway of the latter itself. By thermally separating the cells within a module, the thermal runaway propagation is limited.
Q: How do you expect the future of electric drive batteries to look like?
A: The energy density of the cells will continue to increase, improving the range of BEVs. As the amount of energy increases, the risks of potential hazards rise
and safety aspects will have crucial importance. In order to cope with this challenge, safety and functional features will be integrated on module or cell
level (Smart Cell). Also, the focus will be set on effective extraction of the dissipated heat out of the system, enabling higher current rates and ultra-fast
charging. This can be achieved with integrated functionalized thermal materials.
Q: Electromobility as a cross-industry development will not only have an influence on the automotive industry, it will also lead to convergence of the automobile, ICT, and energy sectors. Do you see a benefit within your study from which also other industries can benefit from and why?
A: While the investigated aspect of reduced gravimetric and volumetric overhead of the battery system referred to the cells cumulated weight and volume is mainly of interest for automotive applications, the challenges in thermal management also are relevant to other sectors. The approach of spatial and temporal temperature homogenization in battery systems by means of the thermal material solutions including benefits in service life and in an effective heat extraction is transferable to other battery system applications.
Q: Thermal Management comes in many different shapes and variations, active systems, passive systems (material solutions), thermal capacitors etc.. what do you see as a benefit of thermal management solutions based on material solutions?
A: In this investigation, material solutions improved the thermal coupling between the single battery cells and the module thermal interface to an active liquid cooling system. The thermal resistance of this heat path mainly influences the maximum temperature at a certain dissipated heat of the cells. To give an example, a reduction of the thermal resistance was achieved by the direct and large contact area between the cells and the high thermal conductive PGS heat spreader.
Q: Which other materials did you look into when you were doing your research and for which purpose in the set up did you consider them? Did you check other artificial graphite-sheets? Can you elaborate on why you choose PGS your?
A: In earlier projects, we also tested a pouch cell module with solid graphite cooling sheets. In comparison, the advantage of thin and flexible PGS with high bending cycle stability, together with appropriate designed relief folds, is
the compensation capability of geometrical cell swelling due to the intercalation of Li ion in the graphite electrode. This ensures a direct and low resistance contact over the state-of-charge range and over the lifetime.
Q: You used PCM and PGS in your approach – can you elaborate a little bit on each materials benefits and disadvantage?
A: The integration of Phase-Change-Materials in battery systems enables potential for spatial temperature homogenization, thermal peak-shaving and preconditioning. On the other hand, accompanied adverse effects are thermal inertia, heat accumulation and the additional gravimetric and volumetric overhead. In this approach, the negative aspects were compensated by the use of otherwise wasted design space around the cell edges, by thermally connecting PCM in parallel to the heat path and by an improved PCM thermal conductivity due to additives. Depending on boundary conditions, an optimum between weight overhead of PCM and the positive impact on thermal management has to be found.
Below You Will Find Another Example of the Heat Management Capabilities of PGS:
For further reference please compare: Panasonic
Please also compare: Farnell