Air travel is near the top of the environmental agenda. Electric and hybrid aircraft could well be the future, as aerospace companies, airlines and start-ups work to reduce the industry’s dependence on oil. However, with at least one major UK airline aiming to operate electric aircraft within this decade, are battery technologies ready to fulfil this ambition?
We need more powerful, lighter batteries
Although engineers in most applications would look for a battery with maximum power output per unit of mass or volume, high energy density/specific energy is particularly critical in aircraft where weight and space are severely restricted. A battery with a sub-optimal energy density will require compromises in aircraft speed, range or passenger/cargo capacity.
The jet fuel used by most commercial airliners has a specific energy of around 45 MJ/kg. In comparison, the energy density of lithium-ion batteries is typically under 0.7 MJ/kg. In practical terms, this means that over 1,500 tonnes of lithium-ion batteries would be required to achieve the same power output as the fuel in a typical short-haul jet — more than 14 times the maximum take-off weight of the whole plane.
Physical improvements such as the use of advanced materials and aerodynamic improvements can’t solve the problem alone. More powerful and lighter batteries are needed.
The candidates to power future air travel
There are three types of battery that could help us make the jump to commercial electric aircraft.
Lithium-sulfur batteries offer improved energy density when compared to conventional lithium-ion batteries. Lithium and sulfur are both relatively light elements, which allows for lighter cell assemblies. Lithium-sulfur batteries have been used successfully in unmanned solar-powered aircraft such as the Airbus Zephyr.
One potential drawback is that they can suffer from rapid loss in capacity over multiple cycles due to changes in electrode volume. A recent international patent application PCT/AU2019/051239 is directed towards the use of binders within the cathode to improve stability, which holds hope for the future.
2. Solid state batteries
Solid state batteries have several advantages over conventional wet cell batteries. Importantly, they can make use of lithium metal anodes to improve energy density, without the need for harmful or flammable liquid electrolytes.
Lithium-sulfur batteries are also promising candidates in solid-state battery technology. Prototype cells from OXIS Energy have achieved a specific energy of around 1.7 MJ/kg, with significant further improvements expected soon. This technology forms the basis of several patents and patent applications including EP2824739, and is already being applied to prototype light aircraft.
Recent research has shown that the development of solid-state lithium-sulfur batteries with a specific energy in excess of 9 MJ/kg is possible, if a complex hydride lithium superionic conductor solid electrolyte is used.
3. Metal-air batteries
Metal-air batteries have a metal anode with an ambient air cathode and an electrolyte (typically a sodium/potassium hydroxide or sodium chloride solution). The metal anode is oxidised to a metal hydroxide by oxygen in the air.
The specific energy of metal-air batteries can be much higher than lithium-ion — a lithium-air battery has a theoretical maximum specific energy of 46 MJ/kg, comparable with jet fuel. However, technical issues, such as the need for flammable organic electrolytes, prevent widespread use.
Aluminium-air batteries have high specific energy (theoretically up to 29 MJ/kg), can be used with aqueous electrolytes and use an abundant, inexpensive and environmentally-friendly metal. Despite a high energy capacity, the current output of aluminium-air batteries is restricted by sluggish reaction rates. This problem can be overcome with structured electrocatalysts such as noble metal nanomaterials.
Recharging aluminium-air batteries is also problematic, though removable and replaceable battery packs may be a more practical solution. This technology is showing promise for business and regional aircraft — Israeli company Eviation is reportedly considering an aluminium-air battery for its next generation Alice electric business aircraft. There have also been several modelling studies done that look into the application of aluminium-air batteries for regional airliners.
Up and away
With several promising battery technologies from a range of established companies and start-ups, it seems likely that the next few years will see many more electric aircraft take to the skies. In order to be commercially successful, companies and research institutions must ensure that their innovations are protected to prevent others unfairly benefitting from research investment. Patents have a wide application when it comes to battery technologies and can protect novel chemical processes, battery materials and methods of manufacture and use.