Issue 55 Uncrewed Systems Technology Apr/May 2024 Sellafield’s UAV equipment l Applied EV Blanc Robot l Battery tech l Robotican’s Goshawk l UGVs l UAVHE RW1 rotary l Roboat UVD l Autopilots l Arkeocean UVD l UMEX 2024 l CycloTech UVD

41 One method is to produce a full-silicon anode without graphite and binders, and this is being used by all three main HAPS developers. This started in 2009, to develop the structure that supports and adapts to the expansion of the silicon using nanowires without using other materials, thereby improving energy density. This uses a 3D structure, where expansion happens at nano level, so the nanowires move around and can expand without pushing out the cell. The nanowires, typically 10 μm long, are grown from the foil on the current collector directly, like a carpet, with the fine wires on both sides, using a hightemperature process. This produces an electrode that resembles a coated electrode, only thinner and more flexible. Another advantage of the silicon nanowire is that it is grown vertically on the foil, so wires of varying lengths can be produced for different loadings, avoiding the limits on power loading that comes from graphite and carbon/silicon anodes. Most of the HAPS projects are using the anodes in pouch cells of varying sizes to fit into different parts of the airframe, but the anodes can also be used with cylindrical cells. These have a slow charge and discharge requirement that is suitable for high-energy designs at 500 Wh/kg with a 0.5 C charging rate, moving to 1 C. For higher charging rates of 2 C, the power density is lower, at 450 Wh/kg. With full-silicon anodes at 450 Wh/kg and 1300 Wh/L, this is almost double the current technology using standard NMC commercial materials. The smaller cells measure 5 cm2 with a capacity of up to 5 Ah, while the mid-sized cells that are starting to be used in UAVs have a capacity of 10-20 Ah. Larger projects for self-driving cars and eVTOLs for urban mobility are looking for cells with a capacity of over 50 Ah. The NCMA cathodes use nickel, cobalt, manganese and aluminium oxide, with increasing proportions of nickel to reduce the need for cobalt. However, these NCMA cathodes are not yet fully integrated into commercial cells and would give a 5-10% boost to 500 Wh/kg. This can go to 550 Wh/kg with the current materials as a ceiling. New cathode materials in development will raise the energy density to 700-800 Wh/ kg with lithium-ion electrolytes by 2030. Higher than that, the 1000 Wh/kg goal requires a lithium metal anode. Stress-testing Researchers have stress-tested a set of custom lithium-ion cells at very high charging rates up to 60 C to see how performance changes. There is demand for more high powerdriven studies to assess the extremely fast charge and discharge needs for advanced air mobility applications. United States Advanced Battery Consortium (USABC) guidelines for testing batteries for EV applications indicate peak power testing at 5 C discharge current for a pulse duration of 30 s. Depending on the type of eVTOL system, loading can range from 200 N/m2 up to 1000 N/m2, which needs a discharge rate ranging from 10 C to 60 C, with peak power required at both the beginning of the discharge cycle (low depth of discharge) and at the end of discharge (high depth). In specific power terms, these values can potentially exceed 1000 kW/kg, depending on the mass of the payload, mission profile and aircraft design. To test this higher level of power requirement, specially designed cells containing a fast charging and discharging electrolyte were discharged at 15 times the battery’s optimal capacity – the total amount of energy it could store – for 45 seconds. This process simulated the rapid, high-power discharge required during vertical take-off. After the initial discharge pulse, the cells were further drained at a more normal discharge rate and then recharged. The cells are charged at a nominal 1 C-Rate until a full state-of-charge (SoC) is achieved with a 4.2 V cut-off. At the beginning of discharge, a current Battery technology | Focus A pouch cell with silicon anodes (Image courtesy of Amprius) In specific power terms, these values can potentially exceed 1000 kW/kg, depending on the mass of the payload, mission profile and aircraft design Uncrewed Systems Technology | April/May 2024