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41 Battery technology | Focus sulphide passivation layer and the non- flammable electrolyte, to provide a cell that is so rugged that it can have a nail driven through it and still operate safely. This has made it a viable technology for passenger transport. The cells can be cycled about 1500 times before they reach 80% of the initial charge, also called Beginning of Life (BoL). In the next two years, cell makers expect this to reach 2500 cycles before the capacity falls to 80% BoL. The chemistry remains safe at over 60 C and even up to 150 C, which surpasses most lithium-ion chemistries and reduces the cost of running such cells in automotive applications as they don’t need cooling systems. Versions of the cells working at -80 C are also being developed for use in the Antarctic. The cells are also tolerant of the high pressures experienced by unmanned underwater vehicle (UUV) systems. They have been tested to 660 bar, equivalent to a depth of 6600 m, without any reduction in performance, something that is not possible with lithium-ion cells. That depth marks the lowest level of the continuous ocean floor (the abyssal plain), with only deep ocean trenches beyond this level, and those are just 2% of the ocean depths. The cells also provide neutral buoyancy which means that much of the buoyancy foam required on a deep dive vehicle can be eliminated, saving space. A 24 V, 600 W battery pack for underwater applications has been created from 24 cells in a 12S2P arrangement, with a specialist battery management system on the side of the pack. The cells (rated at 300 Wh/kg in standard conditions) achieved 289 Wh/kg at 450 atmospheres of pressure and 4 C. These cells have been combined in a battery to power a UUV called Sperre Subfighter 7500, which is being tested at Nottoden, Norway. This low-temperature capability and ruggedness also makes lithium-sulphur cells suitable for space applications. Every kilo of battery weight saved equates to a launch cost saving of more than $20,000. As lithium-Ion batteries can weigh several hundred kilos, the savings from deploying lithium-sulphur would amount to several million dollars. The technology is now being evaluated by NASA. Magnesium Magnesium can also be used for batteries that are intrinsically safer and have twice the typical capacity of lithium- ion batteries. A cathode made from titanium disulphide can be expanded to allow in whole magnesium chloride molecules, rather than having to use energy to break the molecular bonds. This creates a cell with a higher energy capacity. The technique produces batteries with a capacity of 400 mAh/g, up from 100 mAh/g for earlier magnesium batteries. However, the voltage of the resulting battery is low, at about 1 V, compared to 3-4 V for lithium batteries. Another approach is to improve the electrolyte, and a new one for magnesium batteries has emerged from research into hydrogen fuel cells. Here, researchers took a material that was only used in hydrogen storage and made it practical and competitive for magnesium battery chemistry. Instead of using a chloride-based electrolyte, which can be corrosive, boron ions are used to produce a simple, halogen-free magnesium salt that is compatible with the magnesium metal and is more stable than other electrolytes. Aluminium Aluminium is another option for providing twice the charge capacity of lithium-ion batteries. One approach to the use of aluminium is as the anode. A foil material made from aluminium and tin is a quarter of the thickness and half the weight of the graphite and copper anodes used in lithium-ion batteries. The eutectic metal alloy is mechanically rolled into nano-structured metal foils. This family of anodes, called interdigitated eutectic alloy, is also simpler to produce than the anodes in lithium cells, using only two simple steps and making the battery production process cheaper. The initial test cells have an energy density of 250 mAh/g for more than 150 cycles, a big improvement on a graphite- copper composite anode, and the technology can be used for ions such as sodium or magnesium as well as lithium. However there is another, very different approach. Aluminium-air cells have a theoretical energy density of 8100 Wh/kg but come with some major drawbacks. One of these is a toxic aluminium oxide residue in the cell that comes from Unmanned Systems Technology | April/May 2018 A prototype 30 Ah sodium-ion battery for electric vehicles (Courtesy of Faradion)