Unmanned Systems Technology 004 | Delair-Tech DT18 | Autopilots | Rotron RT600 | Unmanned surface vehicles | AMRC | Motion control | Batteries

76 Insight | Batteries the motors after losses. New materials developed for the anode can also improve performance. For example, UK company Nexeon has unveiled a silicon anode with a large surface area, which has allowed cells to be manufactured with a current capacity of 3.2 Ah (about 620 Ah/kg) in a 18650-type cell that is 64.9 mm long and has a diameter of 18.3 mm. This compares with 2.5-3.1 Ah for the existing carbon-anode 18650 Li-ion cells that typically weigh around 45 g. Using silicon as the anode has allowed the energy density to break through the capacity ‘ceiling’ for present-day 18650 cells. Nexeon says it has achieved even higher capacities with lower discharge rates, and it is planning to deliver 18650 cells with a capacity of 4 Ah by optimising its silicon-anode materials. However, silicon expands more than carbon when it carries current and gets hot, so the challenge has been to find silicon structures that survive this cycle of expansion and contraction and therefore allow a higher number of charging cycles. Another company, US start-up Amprius, is also using silicon for the anode of a Li-ion cell, but with nano- wires rather than shaped silicon. Its first generation of cells has an energy density of 300 Wh/kg, but the company believes it can increase this to 500 Wh/kg. Reducing costs For some unmanned systems, particularly driverless cars, the key issue is one of cost. The batteries can account for half the cost of a vehicle, so there is a move to use less expensive materials such as sodium for the electrolyte. The present cost for cells is about $320 (£210/ e 288) per kWh but sodium battery developers such as Faradion in the UK believe this can be cut by at least 30%. Faradion’s sodium-ion technology has already shown energy densities of 140 Wh/kg, with potential for densities of up to 480 Wh/kg. The key advantage is that the sodium-based materials can be used in existing cell production lines from the production of the active materials and the electrode processing. The material costs for sodium-ion cells are lower than those for Li-ion because the sodium carbonate used for the electrolyte is only about 10% of the price of the equivalent lithium salt. The cathode and electrolyte costs can make up around half the total cost of the cell, so the cost reduction for the whole cell is substantial. The current collectors in sodium-ion cells can also be fabricated from aluminium rather than the more expensive copper necessary in lithium cells. All this leads to the 30% reduction predicted by Faradion and others. To demonstrate this, Faradion has worked with Williams Advanced Engineering and the University of Oxford on a prototype electric bike that uses 12 Faradion cells in four modules. These modules were designed and manufactured by Williams, and controlled by Faradion’s battery management system. The University of Oxford’s expertise has been used to maximise battery lifetime, and initial test results indicate that, as well as comparable performance, sodium-ion cells offer lifetimes comparable to Li-ion products. These first-generation demonstration cells have not been optimised for cost, but Nexeon is confident that the cost benefits will follow. This is not to rule out the use of lithium- ion technology now and in the future for unmanned vehicles. For example, battery maker Saft has signed a e 1m (about £715,000/$1,118,000) contract with Airbus Defence and Space in the UK to develop, qualify and test a Li-ion battery Autumn 2015 | Unmanned Systems Technology The different technologies for battery anodes and cathodes (Courtesy of Nexeon) These prototype 18650-type lithium-ion cells use silicon microstructures as the anode to improve performance (Courtesy of Nexeon)