[adrian_b]

Not really. In a fuel cell the reaction products are discarded (the reactants cannot be discarded, as they are needed for the reaction to take place).

In a metal-air battery, air from the atmosphere is taken into the battery and the oxygen from it becomes bound to the metal, in a metal oxide.

So unlike for a fuel cell, where the vehicle becomes lighter after the fuel is consumed and the reaction products are discarded, a metal-air battery becomes heavier when the metal fuel is spent, because the reaction product is stored inside the battery.

The metal-air battery becomes lighter again when it is charged and the oxygen stored inside it is released into the atmosphere.

A lithium-air battery can have a much better energy per mass than any other kind of lithium battery, but it cannot reach the energy per mass of hydrocarbons.

The reason is that for hydrocarbons the mass that counts is just the mass of the hydrocarbons, while for lithium-air batteries the mass that counts is not the mass of lithium, but the mass of the lithium oxide, i.e. the mass of the battery when it is mostly discharged.

A carbon atom from hydrocarbons can provide 6 electrons per atom, while a lithium atom provides only 1 electron per atom, albeit at a voltage more than 3 times greater than carbon atoms. The mass of a lithium atom is half of that of a CH2 group from hydrocarbons, so if the mass of lithium would have been the one that mattered, the ideal energy per mass would have been about the same for hydrocarbons and for lithium. However the additional mass in lithium oxide reduces the ideal energy per mass more than 2 times (when Li2O is the reaction product) or even 3 to 5 times (when peroxide or superoxide of lithium are the reaction products).

[BWStearns]

Interesting. So if it was used for aviation then your takeoff weight would be lower than your landing weight. That'll be counterintuitive.

[thomasmg]

Unless if the spent battery calls are released during the flight, via parachute, to be recharched later.

We currently call spent batteries "empty" but in this case spent = "full" (of oxygen).

[BWStearns]

I imagine the extra complexity, parachute and release system weight, and risk would nix that.

[namibj]

Just use drop pod technology; some jet fighters have AFAIK disposable under-wing fuel tanks.

Only dropping one that self-tests to high enough confidence/quality, and using e.g. GPS or laser guided bomb technology with a parachute and catch net at the landing site:

With a good net you can drop a human in a wind proof ski-suit (cold air!) and with an oxygen bottle from the Armstrong limit where their blood starts to boil, something like 18km, notably without a parachute. So it will be easy to catch a pod that falls sideways on a drouge chute to trade off parachute size (weight!) and net height/give.

[BWStearns]

So all the batteries are on the belly and a door opens up in flight when they get heavy and they get yeeted? Now you need to have sensors and actuators and inspections on all these doors. The cost of the plane just meaningfully increased.

Mechanical systems experience failure, so now your pilots need to train on the procedure for a stuck drop pod door. It'll probably change the flight characteristics and fuel efficiency while stuck open, so any time that happens it is now an emergency. That just added some operational costs for stranded passengers. You also would probably need a whole new cert for the airframe for any changes to your drop pods.

Throw them in little nacelles under the wings? Now slipping it onto the runway gets harder, more pilot training. What's the procedure for a battery pod strike? More training and more procedure and more certification. More redundant safety systems.

Throw them in the wings themselves? Now if the drop doors get stuck you really have a suboptimal flight condition.

I don't doubt that the technical challenges are surmountable, but all these considerations could literally double the cost of operating a plane compared to just accepting a lower MTOW. The military does all this crazy stuff because it's a hard mission requirement and they don't care how much it costs.

[lupusreal]

Totally impractical for virtually any application.

[sroussey]

How would an aluminum-ion battery work out as for as theoretical charge per weight or volume?

I know that is decades out, of course.

[adrian_b]

Per each electron provided in the circuit, the mass of aluminum is 9/7 of lithium and the maximum voltage is around 93% of that of lithium, which results in an energy per mass for aluminum of around 73% of that of lithium, when only the mass of the metal is considered.

In an aluminum-air battery vs. a lithium-air battery, the mass per electron is, in the most favorable case for lithium, of 17 for aluminum vs. 15 for lithium, which results in an energy per mass for aluminum of around 82% of that of lithium. However lithium forms by oxidation not only Li2O, but also peroxide Li2O2 and superoxide LiO2, which may worsen a lot the energy per mass.

In the parent article, they have succeeded to produce mostly Li2O, but even so their batteries have still produced some amounts of peroxide and superoxide during deep discharges.

So the energy per mass for aluminum-air batteries could be up to 80% to 85% of that of lithium-air batteries.

Most other oxidants besides the oxygen from air are heavier, which would reduce the advantage of lithium vs. aluminum (because the oxidant mass would be a greater fraction of the battery mass), so aluminum-ion batteries, if possible, could have an energy per mass very close to that of lithium-ion batteries.

On the other hand, aluminum metal and aluminum oxides are much denser than lithium metal and lithium oxides, so aluminum batteries could have much better energy per volume than lithium batteries. Unfortunately, until now the problems caused by aluminum as a cathode material have not been solved.

[bcrl]

Alcoa and Phinergy demonstrated an aluminium-air battery over a decade ago that enabled 1600km range. The downside is that it had to be swapped out and recycled after it was spent (which explains why Alcoa's interest since it would gain business from the ongoing re-processing of aluminium).

https://eepower.com/news/ev-with-1000-mile-range-unveiled-by... https://www.cbc.ca/news/science/electric-car-with-massive-ra...

[travisporter]

So suppose a car had methane and oxygen onboard like a rocket and held on to its exhaust products, and you were able to reverse the reaction back to methane and oxygen, it would be a battery not a fuel cell.