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

39 Battery technology | Focus used as the anode, and gives an energy density of 400 Wh/kg. However, the power output is lower than for silicon, and there are issues with scaling up production and making the cells affordable. Like silicon, many labs around the world are looking at different combinations of cathodes, anodes and ceramic electrolyte materials, as well as the processes to build the cells. Lithium titanate and similar materials are key for these solid-state batteries. For example, the latest generation uses a titanium niobium oxide anode material that has double the lithium storage capacity by volume of graphite-based anodes. The previous version used a lithium titanium oxide anode with a lifetime of 20,000 cycles. A 20 Ah cell using lithium titanate has an energy density of 89 Wh/kg; a 50 Ah prototype using titanium niobium oxide has double the energy density by volume, but with a 5000-cycle lifetime. A key aspect of this technology is that it supports rapid recharging in cold conditions, at temperatures down to -10 C, in only 10 minutes. The technology has recently been approved for use in marine vessels. Lithium sulphur One battery technology of particular interest for unmanned aircraft is lithium sulphur (LiS). This has a theoretical energy density of 2700 Wh/kg, although the practical upper limit is considered to be around 470 Wh/kg. The cells also have good safety characteristics, as a passivation layer is created where the sulphur coats the lithium and the dendrites erode away, creating a mossy layer as part of the degradation process. That makes the cells robust and still able to operate, even after short-circuit and bullet penetration tests. The cathode is primarily sulphur mixed with carbon and a binder. One of the main drivers behind its development is to reduce the amount of carbon and maximise the amount of sulphur. The anode is lithium metal, which helps give the high energy density. The drawback is the challenge of fast charging, as that takes about 3 hours at the moment. The anode is currently a metal foil, but developments are leading to the use of thin layers of ceramic materials to improve the fast-charging performance. That is necessary for use in unmanned aircraft applications such as air taxis, which would need to recharge quickly. The design of the cells is important. LiS cells can use aluminium foil rather than copper as the contact tabs, but they have to be designed to match the power output of the cell to avoid resistive heating. Most of the expertise in LiS cells goes into the electrolyte, the cathode and the assembly process for applications where weight is critical. As well as aircraft, there is demand for them from unmanned mass transport such as large buses, where slower charging overnight in the depot is less of an issue, as well as unmanned marine systems that spend weeks at sea. Sodium Sodium is another battery material, particularly for large off-road transporters that do not need to accelerate quickly. The current state of the art for a cell with a sodium nickel oxide cathode provides 12 Ah and 150 Wh/kg, which is equivalent to LFP for a similar cell size in volume manufacturing. This is evolving into a cell with 190 Wh/kg by improving the capacity to 30 Ah with the same materials. Both use a solid carbon anode, as graphite cannot be used with sodium. Unmanned Systems Technology | April/May 2021 Silicon nanowires can potentially boost the energy density of lithium-ion cells to more than 1000 Wh/kg but they are a challenge to make (Courtesy of Amprius Technologies) Building a sodium battery cell using existing manufacturing techniques (Courtesy of Faradion)