Amprius Technologies, Inc., a leader in next-generation lithium-ion battery technology, today announced that its silicon anode batteries powered AALTO’s Zephyr high-altitude pseudo-satellite (HAPS) aircraft during a world-record-setting 67-day flight in the stratosphere, which concluded on April 28, 2025. This uninterrupted flight, conducted at altitudes around 70,000 feet, surpassed Zephyr’s previous endurance record of 64 days and marks the longest continuous aircraft flight ever recorded.

The Zephyr is a solar-powered unmanned aerial vehicle that operates in the stratosphere, charging its batteries during the day via solar panels and relying on battery power overnight. Amprius’ ultra-high-energy-density lithium-ion cells provided the critical energy storage needed to keep the Zephyr flying through multi-night cycles in the thin air of the stratosphere. By enabling reliable overnight power, these silicon anode batteries were a key factor in the mission’s success, allowing the aircraft to maintain continuous operation for over two months. Amprius’ cells are among the only commercially available batteries known that are capable of delivering the required endurance for persistent HAPS missions, a capability that was essential for Zephyr’s day-and-night flight longevity.

This achievement was made possible through the close collaboration between Amprius and AALTO HAPS, a U.K.-based Airbus subsidiary that designs, manufactures, and operates Zephyr. Amprius has been a key supplier of high-performance batteries to AALTO for several years, including for a 2022 Zephyr flight that lasted 64 days. The new 67-day flight builds on that milestone, further proving the reliability of Amprius’ technology and the Zephyr system’s ability to push the limits of aviation endurance.

“We are extremely proud to see our batteries enable this groundbreaking 67-day flight,” said Dr. Kang Sun, CEO of Amprius Technologies. “This accomplishment demonstrates the transformative impact of our silicon anode battery technology in aerospace. By providing the energy needed to keep Zephyr flying through the night, our silicon anode cells made it possible to achieve a new level of endurance for stratospheric aircraft. We are honored to partner with AALTO in pushing the boundaries of what high-altitude platforms can do for both commercial and defense applications.”

“Achieving this 67-day flight is a major milestone for Zephyr and HAPS technology more widely,” said Pierre-Antoine Aubourg, Chief Technical Officer at AALTO. “Amprius’ high-energy batteries provided the reliable power storage we needed to maintain Zephyr’s day and night operations. Their technology was instrumental in enabling us to break our own endurance record, demonstrating that persistent stratospheric missions and capabilities are now a reality. We look forward to continuing our collaboration with Amprius as we focus on our remaining flight program for 2025.”

The record-breaking flight underscores the potential of high-altitude pseudo-satellite platforms for both commercial and defense applications. Operating at roughly 70,000 feet, the Zephyr can fly for months at a time, effectively combining the persistence of a satellite with the flexibility of an unmanned aerial vehicle. This capability opens the door for missions such as continuous border surveillance, secure communications relay, and environmental monitoring that require long-term, persistent coverage. Such ultra-long endurance is especially valuable for defense, where HAPS vehicles like Zephyr can provide persistent ISR (intelligence, surveillance, and reconnaissance) and connectivity over critical areas without interruption.