As weird as it sounds, airplanes of the future might be heavier at landing than at takeoff — because they will actually gain weight during flight.
Photo: Matsuda, Ono, Yamaguchi, Uosaki. Criteria for evaluating lithium-air batteries in academia to correctly predict their practical performance in industry
Lithium-air (Li-air) is a battery technology that, when pushed to its limits, might give jet engines a run for their money.
This article barely skims the surface of the topic, and being new to it, I probably made some important omissions or outright errors. But I think it’s worth knowing that such technology exists.
Jet fuel (kerosene) has an energy density of 12,000 Wh/kg. Sure, this is true only when not counting the mass of oxygen needed to burn that fuel; but that way of counting energy density is justified, as jet engines get their oxygen from the air, for free — it does not need to be stored onboard and does not contribute to aircraft weight.
Can lithium-ion batteries compete with that? No. Lithium-air batteries, however, can get close enough.
Not close enough to match that 12,000 Wh/kg figure — just close enough to become a viable replacement in many aviation applications.
Lithium-air batteries are not another kind of lithium-ion batteries (and they are also a different thing than rechargeable lithium metal batteries, which are nearing large-scale production).
Lithium-air batteries need air to operate.
Here’s how they work. When the battery is discharging, lithium (inside the battery) reacts with oxygen (grabbed from the air), forming lithium oxides (which remain inside the battery). Energy is released. And the neat thing is, that energy is released not as heat, but as electrical energy — so you can use it to efficiently power whatever you want; some kind of electric motor, for example.
And when you are charging the battery, you are putting energy into lithium oxides, in order to split them back into lithium and oxygen.
At this point, someone might say: oooh, now I understand why it’s so much better than current batteries — it’s not a battery at all! It needs to consume something from its surroundings to actually work, right? Well, that sounds like a characteristic of a fuel cell (or an internal combustion engine; or a biological organism), and not of a battery cell.
Well, consider this: a lithium-air battery is still a device that stores electrical energy. It charges and discharges like a battery.
But you must let is grab oxygen from the air as it discharges; and then let it blow off oxygen when it charges again.
And the battery gets heavier — significantly heavier — as it discharges, as lithium turns into heavier lithium oxides. So an electric aircraft using such a battery would take off light, but land heavy — the exact opposite of how jet planes operate today.
That 12,000 Wh/kg figure for the energy density of jet fuel should be used with caution. A jet engine can only convert some percentage of it into thrust; certainly not 100% of it. But how much exactly?
Let’s assume that jet engine efficiency is 37.5%. That efficiency is the result of multiplication of two things: (thermal efficiency) ⋅ (propulsive efficiency).
And let’s assume an electric aircraft engine can achieve 84% efficiency.
Quick calculations seem to show that a battery with an energy density of 5360 Wh/kg would be enough to replace jet fuel (because 12,000 Wh/kg * 37.5% / 84% = 5360 Wh/kg). But that’s not quite true. Jet fuel still has some advantage over a battery of such an energy density. Why?
Well, these values are for when the weight of the fuel (or battery) is at its highest.
In a passenger jet, that’s obviously at the beginning of the flight, before you start burning fuel. Then, over the course of the flight, fuel weight decreases. At the end of the flight, most of the fuel is used up — it’s no longer weighing down the plane. As a result, the average fuel weight during the flight is much lower than the highest (initial) fuel weight.
Li-air batteries will also change their weight as they get bloated up on oxygen during the flight. But no matter how well-optimized they are, their weight will not go from “high” to “almost zero” (like fuel). Their weight will only go from “medium-low” to “high”. As a result, the average battery weight during the flight will not be so dramatically lower than the highest weight.
In short: Li-air batteries suffer from higher average weight than jet fuel — even when their weights at their highest are exactly the same.
And let’s pile up more reasons why airplanes running on batteries are a bad idea. Cycle life, for example. Suppose an airliner flies just two segments a day, for 15 years, and the battery needs to be recharged before each flight. That is about 11,000 cycles, something that would be too much even for current automotive Li-ion batteries. In practice, an aircraft would probably go through multiple battery packs over the course of its service life (so it becomes important that materials from old batteries can be reused in manufacturing new ones).
Sounds discouraging? Well, electric airplanes are something that exists today, even with the current Li-ion battery technology. Which is incomparably worse than Li-air.
The Pipistrel Velis Electro is a good example. It’s not a prototype, but an actual electric aircraft being sold by an established manufacturer. Yes, it is certified, at least in Europe. Yes, the batteries carry enough energy not only for the actual flight phase, but also the required reserves. By the way, the manufacturer was recently bought by the company which owns Cessna.
Clearly, there is huge gap between the energy density sufficient for a light, short-range aircraft — it can’t be much more than 200 Wh/kg in the Pipistrel — and that required to replace a jet airliner.
And there are many interesting applications in between — which might benefit from Li-air technology before larger planes start using it. Example: what if you want an electric personal aircraft with a range of let’s say 800 km (500 miles) that you can charge in your garage? Notice I said aircraft, not airplane; it could be a hoverbike, it could be some sort of a closed-cockpit VTOL. Who knows, maybe when such contraptions become popular, there will be less demand for airline travel.
But let’s skip to airliners. Suppose you have a lithium-air battery that is capable of 2000 Wh/kg (in the discharged, heavy state). Almost three times as heavy as the amount of jet fuel containing the same usable amount of energy. Still, there are many short-haul routes on which that wouldn’t be a problem.
What are the theoretical limits of lithium-air batteries? It depends on the reaction used. If it’s lithium + oxygen → lithium oxide (Li₂O), the upper limit of achievable energy density is about 5200 Wh/kg. If it’s lithium + oxygen → lithium peroxide (Li₂O₂), then it’s about 3500 Wh/kg.
That’s for the fully discharged state, when the battery is at its heaviest.
Another possible reaction, used in aqueous Li-air batteries, is lithium + oxygen + water → hydrated lithium hydroxide (LiOH ⋅ H₂O). In which case the limit is — different sources give different values — 1910 Wh/kg or 2200 Wh/kg.
Instead of giving an overview of what’s going on in the field of Li-air batteries (and there is a lot going on), I’m going to focus on one research team — led by Mohammad Asadi from Illinois Tech.
What I’m describing here is not exactly news — it’s a paper which was published in 2020. They made a Li-air battery that can:
• maintain 70%-80% round-trip efficiency after 800 cycles of charging-discharging at 1C (which means that fully charging the battery takes 1 hour each time)
• operate in ambient air (no need to remove CO₂ or moisture)
It’s not that the battery maintains 70%-80% of its capacity after 800 cycles (because it could be easily misread like that). It’s the round-trip efficiency of the battery is at 70-something percent: when discharging, it gives back 70%-80% of the energy that was required to charge it.
Well, that is still a drawback compared to Li-ion, which typically offers a round-trip efficiency of over 90%. But compared to previous results for Li-air, these results look great. They look commercializable.
It’s time for Li-air batteries to get out of the lab and into the real world.
This article has been edited since first published.