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

He does not expect the combination to be used on a large scale in high- voltage applications, however. “Not every EV will have supercapacitors and batteries paired, but the combination is relevant to high-performance vehicles,” he says. While all vehicles could benefit from battery-supercapacitor hybrid energy storage systems, those used for last- mile deliveries where there is a lot of starting and stopping stand to benefit the most, argues Sleppy. “That is especially true in applications where space is limited, and the battery operates at high C-rates when accelerating or going up a hill,” he says. Hall concurs, but adds that by incorporating a ‘sailing’ mode, in which the motor/engine is switched off while coasting and by capturing brake energy, long-haul vehicles would also see a significant increase in both miles per charge and battery life. “The best on-road fit are large trucks and construction equipment,” says McMillen. “Large vehicles use huge amounts of energy to get moving.” Dr Pohlmann emphasises the value of integrating supercapacitors directly into the powertrain of performance and sports vehicles in particular, as well as motorsport machinery, where the company is working with push-to- pass and kinetic energy recuperation systems. Luxury vehicles also benefit, he adds, where much of the value is in providing braking energy recuperation and improved acceleration. “For integration into 12 V and other low-voltage board nets, the application is less niche,” he says. “We are sure there will be plenty of supercapacitors on the road by 2030.” Integration issues One of the main obstacles to supercapacitor integration into EV powertrains where substantial amounts of energy are required, comes from their principal disadvantage of low energy density in Wh/litre. This makes them relatively large, so finding the physical space to add enough of them to make a meaningful difference to the performance of the energy storage system can be difficult, says Sleppy. However, technological progress is easing this problem, Hall notes, citing power electronics that have become smaller and cheaper, allowing DC-DC converters to transmit more power. “Due to its much lower resistance, the power will come from the ultracapacitor before the battery when power demand is high,” he says. “But good power electronics are needed to manage the power and energy for long life, and safe, reliable operations of both the battery and capacitor.” To achieve its promise, McMillen says, supercapacitors need separate control and switching in the circuit so that they can provide the power to overcome inertia to get a heavy vehicle moving, and to get the battery to jump back in for the sustained loads. “The main challenge is to find out if the EV can actually benefit from supercapacitors, followed by convincing the OEMs to integrate not one but two novel technologies in their vehicles,” Dr Pohlmann notes. “This is connected to the willingness of any OEM to share vehicle performance data. As only specific combinations of supercapacitors and batteries bring strong benefits – as in smaller batteries combined with high- power powertrains – the application needs to be carefully evaluated before reaching a decision.” Also, supercapacitor technology is still somewhat unfamiliar to the automotive industry, leading to higher integration thresholds. “Here, only constant interaction can improve the situation,” Dr Pohlmann adds. Graphene potential As in many areas of technology, the use of graphene is expected to bring major performance benefits to supercapacitors. “Graphene is a class of material with very high electrical conductivity and a lot of surface area,” says Capacitech’s CTO Dr Isaiah Oladeji. “Successful adoption will lead to lower internal resistance and high capacitance.” Graphene is already part of the recipe in many supercapacitor products, remarks McMillen, but he cautions that much more research is needed into its use in higher concentrations. Graphene’s structure is inherently two-dimensional, which prevents it from holding on to ions to store energy, Hall points out. That is because graphene sheets tend to stick together face to face. A lot of r&d effort is therefore going into making graphene sheets curve, to prevent that from happening and allowing the formation of ‘mesopores’ (pores larger than 2 nm across) that can be wetted by environmentally benign ionic liquids. One group of Chinese researchers, for example, achieves this by introducing oxygen atoms through synthesis of graphene oxide. SkelStart module made up of eight, 3200 F SCA3200 cylindrical-format Skelcap ultracapacitor cells (Courtesy of Skeleton Technologies) Summer 2022 | E-Mobility Engineering 47 Deep insight | Supercapacitor applications