E-Mobility Engineering 014 l InoBat Auto dossier l In Conversation: Brandon Fisher l Battery monitoring focus l Supercapacitor applications insight l Green-G ecarry digest l Lithium-sulphur batteries insight l Cell-to-pack batteries focus

Skeleton Technologies uses a type of curved graphene in its supercapacitors and hybrid SuperBatteries, as noted above. “The main impact is higher energy density, so less space is needed for energy storage – lower volume and weight while maintaining the same performance,” says Dr Pohlmann. “What we have seen during testing is the ability of graphene to help reduce the ESR of a supercapacitor,” adds Hall. “The challenge of course is finding a balance for a volume application that needs higher power than a commercial supercapacitor can provide while incorporating the high cost of the graphene. Many applications developers say they need higher power or energy, but most are not willing to pay much more for incorporating exotic materials.” Structural supercapacitors As with other low energy density power systems, such as photovoltaic solar panels, finding space for enough supercapacitors in a vehicle is difficult, so integrating them into structural components is an attractive option. Structural supercapacitors are therefore a growing area of academic research, Sleppy says, as these multi-purpose materials offer structural integrity and energy storage capabilities simultaneously. However, their development poses a number of challenges. As Dr Oladeji points out, “Due to high electrode resistance and low ionic conductivity of the non-liquid electrolyte currently in use, structural supercapacitors need years more of development.” Hall is sceptical about the practicality of the technology. “While the idea is great, to bring the cost down you need to be able to manufacture a product at volume for a wide range of applications, but creating a special product for a single application runs counter to those ideas,” he says. There are also efficiency losses when the structure of the product is changed, he adds. Cylindrical cells are wound on machines with carefully controlled tension that is needed to minimise resistance while enabling the roll to enter the casing easily and avoiding short-circuits and breaking. Similarly, prismatic cells need external pressure to achieve their design cycle life. “While that could be built into a structural product, it isn’t very cost- effective. Using the cells inside an existing structure, for example on a door post or in a frame, could work, but access for maintenance would be the issue.” He also points out that there are regulations, in the EU for example, that effectively rule out the structure from being used for energy storage, stipulating that body panels that could come into contact with pedestrians must not be energised. “At Ioxus, we have explored the idea of using capacitors for storage inside body panels and wheel wells, but to date we have not had any real commercial interest in these products.” Dr Pohlmann notes that structural supercapacitors are at a low technology readiness level, and are being built on only a small scale in laboratories. The main problem to solve, he explains, is the need for liquid electrolytes. “Having a liquid component always adds a layer of complexity as well as safety considerations,” he says. “It’s possible to use water as the electrolyte, but that reduces the performance significantly, although it would also reduce the cost. “The question is, do structural supercapacitors make sense? It might be more cost-efficient to just add a larger battery pack or a supercapacitor pack in a vehicle, assuming that the space is available.” In conclusion, a thorough understanding of all the duty cycles of all the systems in a vehicle is crucial in assessing where best to use supercapacitors, and in sizing and matching them to each application. Equally important is a thorough understanding of battery technology to identify the gaps in battery performance that ultracapacitors can fill. Deep insight | Supercapacitor applications Winding machine in Skeleton’s production facility in Germany (Courtesy of Skeleton Technologies) 48 Summer 2022 | E-Mobility Engineering