Thought Leadership from Mewburn Ellis: Aviation battery systems and energy development
Words by Thomas Lonsdale, Patent Attorney and Helen Eastmond, Patent Technical Assistant, Mewburn Ellis
Batteries are the most efficient way to power vehicles using green energy, so there is currently great interest in battery-powered aircraft for short-distance flights. Norway, for example, has committed to electrifying all short-haul flights by 2040.
A key company working on electrifying flight is Rolls-Royce, who have developed the world’s fastest all-electric propeller aircraft. This small plane has achieved a speed of over 560 kph with a battery containing 6,480 cells, weighing 700 kg and described as ‘the most power- dense propulsion battery pack ever assembled in aerospace’. eVTOL aircraft envisaged for shorter flights/urban mobility often make use of proprietary battery systems.
One company working in this area is Wisk, who have developed a driverless eVTOL, with a range of 144 km and a speed of 225 kph. With six propellors mounted on each wing, once aloft it flies like a small plane. A central issue for battery powered aviation is that batteries are heavy, meaning they only store a fraction of the energy per unit weight vs kerosene.
Typically, small aircraft operate with an energy density of 250–270 Wh/kg; however, experts predict energy densities of 350- 400 Wh/kg will be needed for the electric aviation industry to really take-off, with still higher figures required to make long journeys practical.
Which Battery is Best?
Several different types of battery are currently being explored for battery-powered aviation. Lithium-ion batteries are the most widely used battery, e.g. the Tesla Model 3 uses a Li-ion battery with an energy density of 260 Wh/kg. A number of companies are working to use these in the sky.
Joby Aviation has designed a five-seat air taxi that can travel 240 km powered by Li-ion cells that are already used in electric cars. They hope to launch their eVTOLs in New York in 2025.
Li-ion batteries traditionally have graphite and transition metal oxide electrodes, contributing significantly to their weight. Recently, the energy density of Li-ion cells has been improved with alternative electrode solutions, such as Amprius’ siliconanode battery which was marketed with a record 500 Wh/kg this year. Fellow manufacturer Ionblox have also patented silicon-anode cells which are being used in eVTOLs.
Another battery with big potential is solid-state batteries (SSBs). Using a solid electrolyte avoids the safety issues, such as swelling and flammability, seen with Li-ion batteries; as well as having the potential to achieve higher energy densities.
NASA is developing a sulfur and selenium SSB which has achieved 500 Wh/kg in prototype and can operate in temperatures twice that of Li-ion batteries. Further work is required before SSBs move from basic research to commercial reality, but developments in imaging and in situ characterisation techniques are pushing these batteries forward.
The Faraday Institution expects SSBs may make up over 10% of the global market share for aircraft by 2040. Lithium-air batteries could also find application in passenger aviation. Researchers at Japan’s National Institute for Materials Science have developed Li-air batteries with an energy density of more than 500 Wh/kg, which are also able to undergo repeated charging and discharging cycles.
While Li-ion batteries dominate for now, the aviation industry is often quick to take up new technology and in the longer term the industry is likely to shift to alternatives such as SSBs or Li-air batteries as they develop, unlocking higher energy densities and offering other advantages beyond the limits of Li-ion batteries.
Companies such as Ionblox who have protected their intellectual property in this area are set to benefit as eVTOLs start flying but there is still space for innovation as the industry turns to future technologies.


From L‑R: Helen Eastmond, Patent Technical Assistant, and Thomas Lonsdale, Patent Attorney.