Issue 37 Unmanned Systems Technology April/May 2021 Einride next-gen Pod l Battery technology l Dive Technologies AUV-Kit l UGVs insight l Vanguard EFI/ETC vee twins l Icarus Swarms l Transponders l Sonobot 5 l IDEX 2021 report

40 Focus | Battery technology It also replaces the electrolyte, which is sodium potassium fluoride (NaPF 6 ), with customised additives. This provides more than 3000 charge-discharge cycles, giving the battery a 10-year life. Using sodium also allows the use of propylene carbonate, a low-cost solvent with good dielectric properties, which cannot be used with graphene. That in turn allows a much lower operating temperature, down to -40 C, which is a key benefit for unmanned systems operating at slow speeds in harsh environments. Carbon Structured carbon anodes of the type used for supercapacitors are also being adapted for batteries. Using graphene formed into 3D shapes, and a custom electrolyte with lithium foil, promises cells that can charge in 15 seconds and have a lifetime of up to 200,000 cycles. Other materials Aluminium has been used in cell types other than NCA. For example, aluminium-air primary, non-rechargeable cells are used to provide constant but low levels of power for underwater and marine applications. Magnesium is also of increasing interest. One European research project aims to combine all the steps for developing magnesium batteries, from basic research to cell production processes. The goal is to produce batteries with a capacity of 400 mAh/g, up from the 100 mAh/g of earlier versions – twice that of commercial lithium-ion batteries but comparable to silicon battery cells. However, the voltage of the battery remains low, at about 1 V, compared with 3-4 V for lithium batteries. There are also battery cathodes based on titanium fluoride phosphate, which use potassium rather than lithium. A carbon-coated KTiPO 4 F cathode supports voltages of 3.6V, which is high for titanium redox transitions and allows fast charging in minutes for 100 cycles. Battery pack architectures The architecture of the battery pack is also important. Research programmes are looking at different ways to use different types of battery technology for different parts of a mission. A battery technology with a high energy density, such as a solid-state battery, can be used to provide a stable and steady power supply, while a high-power technology such as NMC or a cell with a silicon anode can provide higher instantaneous power when required. However, this requires a different design in the BMS, with electronics that can monitor the requirements and switch in real time between the cells in the pack. That is a complex challenge, over and above monitoring the voltage and current handled by the BMS. That opens up opportunities to redesign the battery cells, for example by using thicker electrodes. It could also open up the use of lithium-air cells, which have a very high capacity but provide power at a low rate. Conclusion Power, energy density, charging rates, safety and lifetime are all competing factors in the design of batteries for unmanned systems. Areas of research include looking at higher voltage outputs. For example, if a cell can deliver power at 5 or 5.5 V then that can reduce the number of cells needed. However, present electrolytes can reach only 4 V before breaking down, so other electrolyte materials are being investigated, such as ionic liquids that support higher voltages but are not as conductive and so don’t provide as much power. Incorporating battery materials into the structure of an unmanned vehicle can increase the battery’s overall energy density but it also increases the complexity of the power and control architectures. It also means using custom cells, increasing the costs. Instead, the move to architectures that combine high energy density cells with high-power ones provides a route towards delivering the energy required for unmanned systems. That requires more r&d though on the fast switching electronics and more complex BMSs. Acknowledgements The author would like to thank Victoria Doherty and Gary Mepsted at Qinetiq, David Branco at Vanguard Power, Doron Myersdorf at StoreDot, Mark Crittenden at OXIS Energy and Jerry Barker at Faradion for their help with researching this article. April/May 2021 | Unmanned Systems Technology A lithium sulphur battery pack provides power in a lightweight package (Courtesy of OXIS Energy)