OXIS Energy Ltd.’s lithium-sulfur (Li-S) battery cell technology might well end up on Bye Aerospace’s future aircraft and air-taxi or “urban air mobility” (UAM) designs. The Oxford-based battery developer and the Denver-based electric aircraft company are launching the initial phase of a high-voltage Li-S advanced battery pack development program that could result in significantly higher desity energy cells for aerospace applications.
In contrast with conventional lithium-ion cells, sulfur is a natural cathode partner for metallic lithium and the chemical processes include dissolution from the anode surface during discharge and reverse lithium plating to the anode while charging. What this means is, Li-S provides a theoretical specific energy in excess of 2700 watt hours per kilogram – a specific energy nearly 5 times higher than that of Li-ion.
The Li-S battery design also safer than current Li-ion batteries, due to the ceramic lithium sulfide passivation layer and a non-flammable electrolyte. The OXIS’ Li-S cells can withstand extreme abuse situations such as bullet and nail penetrations with no adverse reaction and performance remains unchanged in high pressure environments, up to 660 bar. Li-S batteries are capable of 100 percent depth-of-discharge and can use all of their stored energy, as compared to Li-ion cells 80 percent depth-of-discharge.
“Aviation is one of OXIS’ target markets, and in the first instance, Regional Rapid Air Taxi Transportation. A key measure of OXIS’ suitability is to be able to consistently produce cells in excess of 400 watt hours per kilogram, which are already undergoing evaluation,” says Huw Hampson-Jones, OXIS CEO.
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“OXIS expects to achieve 500 watt hours per kilogram by early 2020. Our Li-S cells and battery systems are ideally suited for aviation. They are over 50 percent lighter than the current Li-ion cell and battery systems, with the winning formula of a high energy cell at the power required. The use of the same cell format across batteries will also help our customers to minimize cost and improve serviceability,” adds Hampson-Jones.
In ground tests in Europe, OXIS has already shown that Li-S has demonstrated a significant improvement in simulated flight duration compared with lithium nickel manganese cobalt oxide (NMC); it will now undertake a further series of tests which will provide a robust indication of the OXIS Li-S performance.
OXIS’ Li-S cells also require no self-maintenance and maintain their properties without maintenance recharges, unlike Li-ion cells. Additionally, the cathode sulfur used in OXIS cells is a byproduct of the oil industry, which results in a lowered environmental impact when compared to extracting expensive heavy metals, like cobalt, out of the ground. This aligns with Bye Aerospace’s goals of reduced cost, clean, electric aviation.
Phase one of the project with Bye Aerospace will begin in September 2019 and applications on Bye Aerospace’s four-seat eFlyer 4 and future designs will be considered; however, current two-seat eFlyer 2 designs and agreements will not be altered. Over the next 18 months, the aim is to move towards the production of commercial cells and battery systems in order to meet the volume roll-out criteria set by Bye Aerospace.
“New Li-S battery cells from Oxis have the potential to greatly enhance the quality, cost, and performance of eFlyer 4 and our other future aircraft projects. Our collaboration with OXIS is separate from the current eFlyer 2 agreements and intentions we have with confirmed and our future supply chain partners,” says George Bye, CEO of Bye Aerospace. “Bye Aerospace is working with Oxis on the Li-S battery cell characteristics to significantly improve our eFlyer 4 and future air taxi aircraft designs' performance. The average age of the global market for turboprop fleets is 28 years. OXIS has the potential to provide a Li-S battery cell that is truly a game-changer."
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William Kucinski is content editor at SAE International, Aerospace Products Group in Warrendale, Pa. Previously, he worked as a writer at the NASA Safety Center in Cleveland, Ohio and was responsible for writing the agency’s System Failure Case Studies. His interests include literally anything that has to do with space, past and present military aircraft, and propulsion technology.
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