Physical limitation on energy quantity stored in a battery?

[pa5tabear]

I keep hearing about new and more efficient batteries.

Is there a hard limit on the amount of energy that could be extracted from a stored chemical energy source?

Obviously the absolute maximum amount of energy would be given by E = mc^2, but I think that would be far beyond any realistic limit. Is this true?

 

[Borek]

One way of looking at the problem: in chemical batteries you need two half cells for a full redox process to take place. It almost always means several atoms per each electron exchanged. In the best case I can think of - hydrogen/oxygen fuel cell - it is a single atom of hydrogen and half an atom of oxygen, so about 7 amu per electron.

So far it is about charge, not about amount of energy, if you are looking at the amount of energy solely, you can browse tables of standard enthalpies for chemical reactions.

 

[pa5tabear]

One way of looking at the problem: in chemical batteries you need two half cells for a full redox process to take place. It almost always means several atoms per each electron exchanged. In the best case I can think of - hydrogen/oxygen fuel cell - it is a single atom of hydrogen and half an atom of oxygen, so about 7 amu per electron.

So far it is about charge, not about amount of energy, if you are looking at the amount of energy solely, you can browse tables of standard enthalpies for chemical reactions.

Thanks! I hadn't thought of it in this simple a manner.

Do you know what the current most efficient energy storage methods are (in terms of mass efficiency)?

 

[Borek]

Lithium-ion batteries.

 

[pa5tabear]

Lithium-ion batteries.

Do you know why Lithium is the metal of choice?

I'd guess that since it's the smallest alkali metal, it can provide greater energy density than, say, a Sodium-ion battery. Also I'm assuming it's abundant enought that supply is not a significant roadblock.

Are there potential "better" choices?

 

[Borek]

I'd guess that since it's the smallest alkali metal, it can provide greater energy density than, say, a Sodium-ion battery. Also I'm assuming it's abundant enought that supply is not a significant roadblock.

I would guess you are right about the first part. When it comes to abundance and availability - yes, there is a lot if lithium on Earth, but it is dispersed, with not many minerals being a commercially viable sources. So the situation is not as good as we would like it to be.

Are there potential "better" choices?

None that I am aware of, but then I don't follow the developments in the battery electrochemistry, so it doesn't mean much.

 

[Topher925]

The theoretical limit of batteries is more or less determined by the first and second laws of thermodynamics. That is, the relationship between the change in gibbs free energy for each half cell reaction and the molecular weight of the active species involved in the reaction.

To understand these relationships more simply, just look at the periodic table. In a nutshell, you want your negative electrode to be an element farthest to the left (for greatest activity) and the positive material to be an element farthest from the right (except for noble gases). And you want both materials to as closest to the top (least density). To see this more clearly, look at a Standard Reaction Table (below). You can see that the greatest cell potential (and therefore energy) comes from Lithium and Fluorine. This is about as a good of a battery as you're ever going to get. But obviously, building a battery like this is a huge engineering challenge. Not only is Li metal hard to contain and work with but F is obviously gaseous and tends to react with everything.

But there's a lot more to obtaining high energy density than just active materials. Packaging, electrolytes, separators, and containment, are all factors which greatly influence a battery's energy density. As for today's technology, most people are looking at using Li-Air and Zn-Air batteries for achieving energy densities of up to ~400Wh/kg. In a nutshell, with this design you only carry the anode of the battery and let your cathode be oxygen in the air so theoretically you can cut your battery weight in half.

But as Borek stated, the "holy grail" of electrochemical energy density for practical purposes is really hydrogen fuel cells. You will never see a battery that is capable of providing the energy density of a PEMFC system for suitably large enough systems (i.e. consumer vehicles). There's a lot of companies out there that have claimed they can, including supercap makers, but so far no one has been able to demonstrate it.