44 Focus | Battery technology top of the module. The modules are placed on a rail and connected in parallel with connectors that have four power lines, one charge control line and seven data lines. There is cooling for each module as these are in air. There are two fans that move the air around to minimise hotspots with a typical 3.5 C to 35 C operation, and 7 C to 45 C for charging. The housing is aluminium, stainless steel or titanium, depending on the depth of operation, with aluminium down to 100 m and stainless steel for highreliability projects to 1500 m. The cylinders are also used for underwater power storage, with both wired and wireless inductive charging of UUVs being developed. The highest demand was 1 MWh in a unit measuring 3 m high and 4 m wide with 12 86 kWh battery packs. Metalcage barriers are used to prevent thermal runaway between the modules. For a UUV, the packs can be fully cylindrical or half-round if there is a space limitation. Unlike for aircraft, the weight is not that important for UUVs because of the buoyancy. It is typically 400-600 kg, 1 m to 1.3 m in length, and 416 mm in diameter – consisting of 10 modules with the BMS for power demand for 4-6 kWh. It is possible to move to the larger 21700-format cells, but this would require a change in design as the standard stencils mean there is actually less capacity because of the additional space between cells. Sulfur A 3D version of graphene is key for a new generation of lightweight lithiumsulfur cells. The 3D graphene is tuneable in the production process and can be engineered at the atomic level to bond with other elements. This can optimise thermal and electrical properties, or customise porosity to improve strength and stiffness or decrease weight. The battery chemistry requires no nickel, cobalt or manganese in the cathode, and no graphite in the anode, allowing the lithium-sulfur battery to be fully sourced and manufactured locally in North America or Europe. Lithium-sulfur cells in both cylindrical and pouch formats are being produced using the same equipment and manufacturing processes as traditional lithium ion batteries. These have an energy density of up to 900 Wh/kg, making them suitable for UAV designs. Solid-state batteries There are several competing technologies for solid-state batteries (SSBs) that combine lithium-metal anodes with a solid electrolyte to boost the energy density of cells. This drive to SSBs aims to address the safety issues of cells with liquid electrolytes that can leak and catch fire. Mitigating the safety risks in a battery pack with separators and monitoring adds weight and cost to the system, which is a key consideration for UAVs. Using a higher-capacity cell that is inherently safe with a higher gravimetric energy density, as measured in Wh/kg, and an energy density above 800 Wh/L will drive adoption in airborne systems. However, these cells need to have a high C-Rate for faster charging, so that a UAV can get back in the air, and a longer cycle life than current solid-state batteries so the packs can operate reliably and safely for several years. The main contenders for SSBs are sulfite materials, oxides and polymers. A solid-state polymer electrolyte that works with the silicon anode starts as a liquid, but converts into a solid in the cell, so it doesn’t change the manufacturing method. This was developed for military products that need to be bullet-proof but can be integrated into other products. Maintaining the manufacturing process is key. Changing one step needs to match the throughput of the rest of the factory, and if the equipment is not mature it can take two to three years to catch up. That’s why it will take time for silicon anodes to be integrated into cell production at GWh scales of manufacturing. Nano-composite electrolyte One potential technology for solid-state batteries for aircraft such as UAVs and maritime uncrewed surface craft is a silica-based, nano-composite matrix. The key for the energy density, as measured in Wh/L, is to finesse the thickness of the electrolyte material, and the first versions use a layer that varies from 500-900 μm, or almost 1 mm thick. This is reducing to 30-50 μm and, along with very thin lithium-metal anodes, allows multiple layers to be stacked in a high-capacity cell. One version of the electrolyte is thermally stable above 200 C, up to April/May 2024 | Uncrewed Systems Technology 3D graphene for lithium sulfur cells (Image courtesy of Lyten)
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