Lithium-Sulfur Battery Cells Complete 1,000-Metre Subsea Pressure Tests

Li-S Energy Limited has successfully completed pressure testing of its lithium-sulfur battery cells at simulated depths of 1,000 metres, marking a key stage in evaluating the technology for Uncrewed Underwater Vehicle (UUV) applications.

Conducted under a collaboration between Li-S Energy and UK undersea systems specialist MSubs Limited, the programme evaluated 10 Ah lithium-sulfur cells under pressures equivalent to 1,000 metres (100 bar). Testing took place in an MSubs pressure vessel commissioned at Li-S Energy’s Battery Testing Centre in Geelong during the June 2026 quarter. The assessment aimed to determine whether the cells could operate under pressure conditions representative of large UUV missions.

During testing, the cells completed repeated pressure cycles equivalent to descending to 1,000 metres and returning to the surface. No mechanical damage, swelling, leakage or loss of mass was observed, while cell voltage remained stable throughout testing and for 24 hours afterwards. Li-S Energy also reports that its lithium-sulfur cells can charge and discharge at 5°C, supporting their suitability for representative subsea temperatures.

Dr Steve Rowlands, Chief Technology Officer of Li-S Energy, stated, “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.”

MSubs, headquartered in Plymouth, designs and manufactures crewed and uncrewed underwater vehicles for defence and commercial customers. It has developed platforms for the UK Ministry of Defence, US Navy and US Special Operations Command. Its programmes include the Project Cetus Extra-Large Uncrewed Underwater Vehicle (XLUUV), formally named XV Excalibur in May 2025 and handed over to the Royal Navy in December 2025.

The collaboration, announced in December 2025, focuses on evaluating Li-S Energy’s technology for large-scale propulsion battery systems in MSubs’ extra-large UUV platforms. Pressure tolerance represents an important consideration for these applications, where battery weight, energy density and buoyancy can influence vehicle design and endurance.

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. Their demonstrated pressure tolerance also supports the potential for integration into water-filled UUV battery compartments operating at ambient pressure, potentially reducing reliance on heavy pressure containment and extensive buoyancy foam.

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 development comes amid growing investment in extra-large UUV capabilities, with Australia, the United Kingdom and the United States advancing programmes in this area. Australia committed A$1.7 billion to its Ghost Shark programme in 2025, while commercial applications for underwater vehicles include subsea inspection, offshore energy and seabed surveys.

Following MSubs’ acceptance of the test report, the companies have agreed on further assessments covering greater simulated depths, charging and discharging at depth, and thermal operating profiles representative of subsea vehicles. Subject to the outcome of those tests, the collaboration is targeting a trial of a Li-S Energy battery pack during an open-water dive mission involving an XLUUV.