Feasibility of solid-state batteries for electric airplanes
Solid-state batteries have the potential to revolutionise the electromobility market. Recent innovations have suggested that these batteries can offer 2.5 times the energy density of existing liquid electrolyte lithium-ion batteries whilst being lighter and safer with little degradation in performance from rapid charging. It is expected that within the next two decades, these types of batteries will have a significant impact on all transportation sectors.
Whilst solid-state batteries are likely to find favour in the light, medium, and heavy-duty land transportation markets (assuming solid-state battery technology can reach maturity), whether they might be applicable for powered flights remains difficult to predict. Electrification of the aviation industry is paramount for slowing climate change. At the time of writing, the aviation industry is responsible for 2.4% of all global CO2 emissions. In addition, when combined with other gases and water vapour trails, the industry is thought to be responsible for around 5% of global warming.
To mitigate this impact in the short term, the only viable options are more efficient engines, sustainable fuels (those that are carbon neutral) and carbon offsetting. The first option is commendable but still relies on fossil fuels. The second option is more promising and might offer a more favourable solution but is unlikely to replace traditional fossil fuels anytime soon, although companies like Rolls Royce have already conducted successful testing of sustainably fuelled flights. In the long run however, a hybrid of options including hydrogen, batteries, and sustainable fuels will likely be utilised rather than a single standalone option.
Challenges of using solid-state batteries for powering flights
Replacing liquid fuels with solid-state batteries for powered flight, in particular medium to long haul, is very difficult. Aircraft gas turbine engines are efficient; >55% during high power phases of flight and approximately 40% during cruising. This means that approximately half of the total chemical energy within the fuel (43MJ/kg) is used to develop thrust – approximately 21.5MJ/kg. To put this into context, the most energy dense solid-state batteries have an energy density of approximately 1.8MJ/kg – almost 24 times lower compared to the peak energy density of jet fuel, or 12 times lower when assuming half of the liquid chemical energy is wasted through gas turbine inefficiencies (heat, exhaust, friction etc). Fundamentally, this large difference in energy density has a detrimental impact on aircraft weight and hence its range, if battery technology alone is adopted.
Refuelling is another major milestone that must be overcome. Currently, it takes approximately 30 minutes to completely fill the 16 metric tonne fuel tank of an Airbus A320, resulting in turnaround times of under an hour for some airlines. Rapid charging could be carried out for batteries but is fundamentally slower than replenishment of liquid fuel. Furthermore, although solid-state batteries are less prone to degradation through rapid charging than liquid-electrolyte batteries, hitting these batteries with such high power often 3 or 4 times per day depending on airline carrier schedules, will have some effect on battery longevity.
Discharge rate is another factor that impedes solid-state battery implementation. During take-off and landing, battery discharge requirements are significant – up to 4 or 5 times higher than that during steady cruise at an altitude. This causes extra strain on the battery that can often be repeated several times a day in some cases. In addition, electric airplanes generate more heat and have more end-of-life requirements, necessitating new thermal management tactics, power-fade capabilities, and capabilities to overcome battery pack failure mechanisms.
Industry research and developments
A European project that is advancing the electrification of aviation industry is IMOTHEP – Investigation and Maturation of Technologies for Hybrid Electric Propulsion. The project has taken a holistic approach based upon hybrid-electric propulsion to reduce commercial aircraft emissions and covers the regional aircraft segment for roughly 50 passengers. With improvements to aviation grade solid-state battery energy density (which is currently being investigated as a part of this project), realisation of small ‘hopper’ type aeroplanes utilising energy dense solid-state battery packs would be possible.
A new lithium-air solid-state battery developed by Japan’s National Institute for Materials Science (NIMS) and backed by SoftBank, is said to have a record-breaking energy density of more than 500Wh/kg, nearly double that of the Tesla Model 3’s 260Wh/kg lithium-ion Panasonic battery. This is seen as an important breakthrough in battery technology since engineers regard 500Wh/kg as the threshold for regional electric passenger aircrafts to become a reality.
NASA, USA, is developing a solid-state battery for electrified flights called SABERS – Solid-State Architecture Batteries for Enhanced Rechargeability and Safety. Researchers at NASA have been able to almost double the typically low discharge rates of solid-state batteries making them more suited for powered flights. The SABERS idea proposes a solid-state battery design created with a sulphur-selenium cathode combined with a lithium-metal anode. The combination of sulphur and selenium provides a balanced energy-to-power density ratio that may be customised to the application by adjusting the sulphur-to-selenium stoichiometric ratios. This cathode will be created by using NASA’s unique holey graphene technology as an electrode scaffold that is highly conductive and ultra-lightweight. A solid-state electrolyte will be utilised to replace the highly flammable liquid organic electrolytes currently used in lithium-ion batteries with a safe, non-flammable alternative. To enable dense battery packaging, the solid-state lithium-sulphur-selenium cells will be constructed with a serial stacking arrangement.
QuantumScape Corp’s new solid-state lithium-metal battery, which is being developed primarily for electric automobiles, could be powerful enough to be used in electric aeroplanes, according to the Silicon Valley corporation. QuantumScape, backed mostly by Volkswagen AG, has spent the last ten years developing a novel form of lithium-ion battery with a solid-state separator and a lithium-metal anode. It is hoped that this technology will reach some level of maturity in the next 5-10 years which could result in the proliferation of electrified aircrafts.
‘The Spirit of Innovation’ is an all-electric aircraft engineered and manufactured by Rolls Royce. This single seat aircraft was powered by a 400kW electric powertrain with the most energy dense battery pack ever assembled in aerospace. Completed as a demonstrator as part of the Accelerating the Electrification of Flight (ACCEL) project, this aircraft was flown at 623kmph – the world’s fastest all electric ‘vehicle’. Although only a demonstrator, the technologies that were utilised as part of this project will likely influence the broader electrified aircraft market.
The electrification of aircrafts though the adoption of solid-state batteries offers the promise of sustainable, efficient, and close-to-silent air travel when compared to current fossil-fuelled aircrafts. However, there are numerous technological barriers that stand in the way of that goal—most significantly the fact that the current generation of solid-state batteries have significantly lower energy density as compared to liquid fuels. This is fundamentally why battery technology must be improved if long-haul, and to a certain extent, short and medium haul electrified flights are to be made possible. In the future, it is possible that hybrid systems involving a combination of solid-state batteries, hydrogen, and sustainable liquid fuels might be implemented. For example, liquid fuels or hydrogen might be used during power-intense manoeuvres such as take-off and landing and solid-state batteries might be used during high altitude cruising which is less energy intense.