One of the big issues confronting underwater craft has been the power source. Cables (literally) only get you so far, and established lithium ion batteries have been bumping up against certain issues: not just the expense but also power-per-kilo limitations which impact range.
Enter UK cell developer OXIS: the company has been investigating the potential of lithium sulphur chemistries and just two years ago it created its largest lithium sulphur cell, achieving 325Wh/kg on the test bench. In addition OXIS has achieved an increase in cell capacity to a 25Ah cell – a world first and a twelve fold improvement in 18 months.
The next goal is an energy density in excess of 400Wh/kg by the end of 2016 and over 500Wh/kg by the end of 2018, far above the ability of present battery technology. In fact the theoretical energy density of Li-S is actually five times that of of li-ion (a useful number for comparison if not practice).
Despite this, Li-S is also much safer than li-ion: it can withstand short-circuits, overcharging, nail penetration and even bullets without exploding or catching fire. However, it is the resistance to pressure which is perhaps of most use to AUVs explained Tom Cleaver of OXIS: “The electrodes in li-ion batteries are a little like a sponge, the pores in the electrodes can get crushed shut under pressure, stopping the lithium moving in and out of them. Current results indicate that Li-S performance doesn’t suffer in the same way under pressure because the reaction takes place in the electrolyte – like a giant soup.”
This, he said, means that the battery could be placed externally on the AUV rather than sheltering inside the main body as present tech demands. It makes it much more attractive to designers, explained Mr Cleaver, because as at present the battery cannot be taken out of the expensive pressure housing, recharging takes up significant time. “Using a pressure tolerant battery will allow the users to ‘hot swap’ them – freeing up the AUVs to do much more work.”
However, while the cells have shown promising results when exposed to conditions that replicate 6,000m depths, Mr Cleaver pointed out the overall battery management and architecture likewise needs to be pressure tolerant and so a complete unit is being developed by the Marine Autonomous Systems (MAS) consortium. Funded by Innovate UK to the tune of £1.1m it’s led by battery pack manufacturer Steatite and includes OXIS, underwater vehicle designers and manufacturers MSubs with scientific expertise from the National Oceanography Centre (NOC).
Although Mr Cleaver admitted “like most R&D it’s a number of iterative steps”, once production rises and economies of scale start kicking in, lithium sulfur should allow a much more cost-effective battery array to be manufactured: sulfur is, after all, cheaper than many other present alternatives.
By Stevie Knight