I wonder what the upper limit on battery density is, not from an electrochemical/physics perspective, but on a simple practical level based on collateral damage?
Like, we're already at the point where your cellphone battery isn't more than a few orders of magnitude away from the energy density of a hand grenade, making a tesla catching on fire a minor crisis for a fire department.
At what point do batteries stop getting more dense, to limit the damage when they fail?
Like, we're not going to get to the point where your phone can last a month on a single charge but if you accidentally smash it in a car door, the resulting explosion takes out the whole block.
I think about this when I see sci-fi/fantasy with ancient robots that are still functional.
How do you build a robot that has been running on battery power for a hundred years without that same battery having the destructive power of a tactical nuke?
Especially because so many of these ancient robots are there for the hero to blast with his laser gun.
Like, it doesn't matter HOW exactly that robot is storing the energy in its billion-amp-hour battery, it's gonna be a real bad idea to hit it with a vaporizing ray beam. All that energy has to go somewhere!
Like, there's a reason you don't have an RBMK in your basement, besides the cost.
No government is going to let a private citizen handle something that dangerous if mishandled.
At some point you can't keep making batteries denser, for the same reason you can't step into your local gun shop and purchase a claymore mine.
It's a new version of Larry Niven's Kzinti Lesson ("A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive"):
A chemical battery's efficiency as a bomb is in direction proportion to its capacity as a battery.
That's actually not true, but sadly in an academic way.
Consider a block of ice. I can extract work from it by running a heat engine that moves heat from the environment into the block. So, I have a battery that doesn't really store any internal energy.
For somewhat better energy density, consider a tank of liquid nitrogen. Even simpler than previously, I can just warm it up with heat from the environment and run the nitrogen through a turbine.
In all of these cases the rate of energy release is limited by the rate of heat flow from the environment.
Sadly, I don't think there are any practical batteries of that kind: the best I could come up with would have an absolute upper bound of energy density lower than practically achieved densities of Li-Ion batteries today.
@robryk @foone I can think of a practical example: rather than storing energy by pumping water uphill, you can empty an underwater vessel and then generate energy on demand by allowing it to fill. Not a "battery" in the everyday sense (and not relying on transfer of heat specifically), but still something that's being seriously pursued today. https://hackaday.com/2022/02/02/underwater-tanks-turn-energy-storage-upside-down/