One of the trade offs is that engines are actually ridiculously heavy. Compact, extreme high power electric motors are starting to be commercialised. But also, fuel burns so you lose weight as you’re flying whereas batteries stay the same.

Electric aviation is interesting but as someone who knows a bit about the industry, biofuels make more sense here.

Structural batteries were supposed to be the solution where the density wasn't so important. I don't really have a good understanding of the ration of fuel weight to structural weight in existing aircraft though.

casing is around 25% of the mass of a cylindrical cell, with the rest being actual battery bits that can't have any stresses applied. is 25% weight saving that significant?

The idea is that you build the aircraft frame out of the battery.

If you can get it to work (a big if) you get to subtract the weight of the old frame before you make calculate the pseudo-density,

I understand, but that's my point. Currently the case is 25% of the battery. Only the case can be removed. If you move the fragile guts of the battery into the frame (in no way reducing their mass or the frame's), you're effectively turning that 25% case to 0%. So, the most you can save is 25% battery weight.

That doesn't really nudge the power density needle all that much, especially when you consider that you don't throw batteries out the back as their energy is depleted, as you do with fuels.

Jet engine and wing efficiency have increased enormously over the last 50 years.

With diminishing results.

Turbofans and supercritical airfoils are done to the point of engine manufacturers looking to propfans and alternative materials (carbon fibre) to eke out further efficiencies.

Although carbon fibre has significant down sides.

I have patented the idea of replacing the nitrogen in the cabin air with helium. I'm waiting for the money to roll in!

Looks like I picked the wrong week to quit sniffing glue.