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Li-S Energy’s lithium-sulfur battery cells have successfully completed pressure testing equivalent to depths of 1,000 meters, marking a key step in evaluating the technology for large Uncrewed Underwater Vehicles (UUVs).
Conducted under a collaboration with UK undersea systems specialist MSubs, the evaluation subjected 10 Ah lithium-sulfur cells to repeated pressure cycles of up to 100 bar, simulating dives and returns to the surface. Testing took place in an MSubs pressure vessel commissioned at Li-S Energy’s Battery Testing Center in Geelong during the June 2026 quarter.
Throughout the testing and for 24 hours afterward, cell voltage remained stable, with no mechanical damage, swelling, leakage or mass loss observed. The cells also continued delivering propulsion current under pressure, addressing a key requirement for their potential use in water-filled UUVs operating at ambient pressure.
Dr Steve Rowlands, Chief Technology Officer of Li-S Energy, commented, “This is a result the whole team is proud of. The pressure at 1,000 metres is one of the hardest environments you can ask a battery cell to work in. Our cells did not just survive it, they kept delivering propulsion current through repeated dives and came back to the ‘surface’ undamaged, with no change in mass and a textbook recovery to open-circuit voltage. That removes a key risk for using our cells in water-filled UUVs at ambient pressure, and it reiterates to us that the cell design is fundamentally right for this application.”
According to Li-S Energy, its lithium-sulfur cells offer approximately twice the energy density of lithium-ion technology, alongside lower weight and near-neutral buoyancy. These characteristics could support UUV designs without the need for heavy pressure containment or extensive buoyancy foam, while potentially improving endurance. The cells can also charge and discharge at 5°C, supporting operation at representative subsea temperatures.
Based in Plymouth in the United Kingdom, MSubs designs and manufactures crewed and uncrewed underwater vehicles for defense and commercial customers, with projects for the UK Ministry of Defense, the US Navy and US Special Operations Command. Its work includes the Project CETUS Extra-Large Uncrewed Underwater Vehicle (XLUUV), named XV Excalibur, which was handed over to the Royal Navy in December 2025.
The collaboration between Li-S Energy and MSubs, announced in December 2025, is focused on evaluating lithium-sulfur technology for large-scale propulsion battery systems in MSubs’ extra-large UUV platforms. The successful pressure testing represents the first major phase of that assessment.
Rob Shaw, Engineering Director at MSubs, added, “Pressure tolerance was a major gate in our feasibility assessment, and Li-S Energy’s cells have passed it convincingly. Cells that keep delivering propulsion current through repeated dives to 1,000 metres, and surface with no damage and a stable voltage, are precisely the evidence we needed to see. We are very pleased with the outcome. Pressure tolerance, low weight and near-neutral buoyancy are attractive features in a battery system for high-endurance UUV propulsion, and these results give us real confidence as we continue the qualification process with Li-S Energy.”
The results also have potential implications beyond MSubs’ platforms, with Li-S Energy targeting opportunities across the wider UUV market. Australia, the United Kingdom and the United States are fielding XLUUVs, with Australia committing A$1.7 billion to its Ghost Shark program in 2025. Similar battery requirements apply to commercial UUVs used for subsea inspection, offshore energy operations and seabed surveys.
MSubs has accepted the initial test report, and both companies have agreed on the next stages of evaluation. These include pressure testing at greater simulated depths, charging at depth and assessing performance across subsea thermal operating profiles. Subject to successful results, the companies intend to trial a Li-S Energy battery pack during an actual dive mission aboard an XLUUV in open water.
