Thread: Limitations to electrical storage

I was thinking about batteries today and I was wondering what are some of the limiting factors about electrical storage. What is the theoretical max storage a volume could possibly have?

Let's see, capacitance for a simple pair of parallel plates is εA/d where ε is the permittivity, A is the area of the plates, and d is the distance between them. The capacitance per volume would be (εA/d)/(Ad), which is ε/d². What's the limit of that?

The most intriguing capacitors on the horizon combine the capacitor's speed of charge and discharge with the storage density of batteries. These super-capacitors can pack 2.5 kJ into a 1 kg package.

Is there an upper limit as to how much juice one can pack into either a mass or volume.

Originally Posted by mugaliens

The most intriguing capacitors on the horizon combine the capacitor's speed of charge and discharge with the storage density of batteries. These super-capacitors can pack 2.5 kJ into a 1 kg package.

Originally Posted by tom

Is there an upper limit as to how much juice one can pack into either a mass or volume.

I think there is, but it's many, many orders of magnitude above what we've been able to accomplish. We have a long way to go.

ε/d² shows us that we can stuff as much capacitance as we like into a given volume, by making d as small as possible. Two issues, however: d cannot be smaller than molecular dimensions. And, regardless of that, capacitance and energy stoage are not the same thing. The energy storage in a capacitor is proportional to the capicatance times the square of the voltage. So we have to ask also, what is the maximum voltage that is allowable? Unfortunately, maximum voltage tends to vary inversely with d, meaning that maximum energy varies inversley with d². So in other words, the maximum energy is unaffected by d, but is affected by other parameters. And those would be the permittivity and the dielectric strength per unit thickness of the insulator. So what you need to find is an insulator that has a high polarizability as well as a high dielectric strength. Whatever that material might be will determine the maximum electrical energy that can be stored in a unit volume capacitatively. Now when we get into batteries, batteries actually work on different principles. You have two dissimilar metals and an electrolyte. The amount of electricity generated will depend upon how many electrons are released|absorbed per atom of the metals in question, times the difference of electrical potential between them. The electrical potential is determined mainly by the difference between electronegativities of the elements involved. Obviously, there are limits to the possible electrode elements and therefore to the possible electronegativity difference; and there also is a limit to the number of electrons any one atom could give up -- by really pushing the envelope you might get as many as five electrons out of elements such as bismuth. Those are going to be the main limiting factors for the amount of energy storable in a battery. (I have not considered the electrolyte; obviously it too will enter into the picture, as it has to hold the metallic ions when dissolved). I do not know off the top of my head which technology, batteries or capacitors, has the better theoretical outlook.

Excellent points, Atomic-S! The future is unknown

Originally Posted by tom

Is there an upper limit as to how much juice one can pack into either a mass or volume.

I think the 'upper limit' would depend on how much work we would be willing to exert into the project...YMMV....

Well, there are certain realities imposed by nature. These have to do with the inventory of possible elements, and such things as their ionization potential (related to electrical breakdown), valence, atomic radius, etc.

Originally Posted by David E. Eaton Sr.

I think the 'upper limit' would depend on how much work we would be willing to exert into the project...YMMV....

I suppose we could express it as a (small) percentage of the work/energy involved in the automotive/petroleum/racing/busing/trucking/accessory/advertising industry.