A Bit About Batteries

powering the engines of change

Batteries are the technology that will change the 21st century.

I'm a physicist, not a chemist, so take all of this with a grain of salt. Battery technology has fascinated me for a long time, but about half of it looks like magic and a great deal of the rest looks like nonsense because chemists think in ways that are alien to my mindset. So do mathematicians. Biologists, oddly, seem to think in ways that are quite similar to physicist's mode of thought.

Physicists and biologists tend to think in terms of fairly direct causes, even when talking about fuzzy subjects like quantum mechanics, where there are events without causes that we keep boxed away from everything else and mostly ignore.

Chemists, on the other hand, talk about stuff moving with no mention of the cause of motion. I'm sure it's so obvious to them that they never think to explain it, but it certainly isn't obvious to me.

In any case, the basic idea of a battery is to take three layers made of different materials: an anode, an electrolyte, and a cathode. When an external circuit is connected between the anode and cathode, the anode material is ionized by the electrolyte, giving up electrons to the external circuit and creating positive ions (anions) in solution in the electrolyte. These positive ions flow toward the cathode, where they pick up electrons from the external circuit, reacting with the cathode material in the process.

So fundamentally, a battery is a way of getting the anode and cathode materials to react with each other via electrons flowing around an external circuit where they can do useful work, as ions flow through the internal circuit of the electrolyte.

In a lead-acid battery which was the first of the commercially important battery types and which is still important today, the anode (sometimes called the positive plate or positive electrode) is pure lead and the cathode(the negative plate or electrode) is lead oxide. The electrolyte is dilute sulphuric acidic (H2SO4), and the reactions are:

1) At the anode: Pb + SO4-- ==> PbSO4 + 2e-

2) At the cathode: PbO2 + 4H+ + SO4-- + 2e- ==> PbSO4 + 2H2O

So the reaction turns both lead and lead oxide into lead sulphate by way of two electrons passed through the external circuit. The first reaction, at the anode, releases electrons and the second reaction, at the cathode, consumes them. For some reason the electrons can't pass through the electrolyte--this is one of the many areas where chemistry loses me, as the "explanations" of how batteries work are full of statements like "the electrons can't pass through the electrolyte" with no mention of why they can't... I mean, water isn't a great conductor, but it's not a great insulator, either, so what's the deal?

A fully charged lead-acid battery has an anode of pure lead and a cathode of pure lead oxide, and a fully discharged one has both anode and cathode converted to lead sulphate. The reactions that produce this state are highly reversible, however, and so lead-acid batteries are rechargeable. They are in many ways the ideal power storage system, as their longevity as a technology attests.

They are also heavy and bulky, which means they have low power density.

A chemist named John Goodenough is responsible for the most important developments in battery technology in the past forty years, all of which are focused on lithium, which quite happily lies at the opposite end of the periodic table from lead. Lithium ion batteries operate in a somewhat different way from lead-acid batteries: rather than lithium being reacted away at the anode, lithium is stored in the anode and released as the battery is discharged.

Lithium-ion batteries generally have a graphite anode and a complex, lithium-based, cathode. When fully charged the lithium atoms--which are relatively small--are trapped in the interstitial spaces of the graphite matrix that forms the anode. The electrolyte is also a dilute lithium salt, so lithium ions are present throughout, all the time.

During the discharge phase, lithium ions dissolve out of the anode into the electrolyte solution, releasing an electron as their bond to the carbon anode breaks. At the cathode, lithium ions are taken up by a complex metal compound, which can be anything from lithium cobalt oxide to lithium iron phosphate to more exotic compounds.

How all this works is tricky: to use the example of lithium iron phosphate, when fully charged the cathode is pure iron phosphate: the lithium has been entirely leached out. One of the many subtle aspects of lithium battery chemistry is the disparity of size between the small, light, lithium ions and the much larger iron phosphate structure. This difference in size allows lithium to migrate through the iron phosphate during the charge/discharge cycle, and this mobility is critical to the operation of the battery. The same is true of the anode, where the relatively open graphite structure allows free migration of lithium into various intercalary sites that it can bind to.

Because lithium is so light, the energy density of lithium-ion batteries is much higher than lead-acid batteries, but it is by no means the highest.

That honour is reserved for the aluminum-air battery, which has the potential to reach an energy density of something more than half of hydrocarbon fuels.

It's not too much to say that if this can be achieved it will completely remake the world.

Hydrocarbons have an energy density of around 40 MJ/kg. It varies depending on whether we're talking about petrol or diesel, but that's the rough figure.

Aluminum-air batteries should run around 24 MJ/kg, which means we could, with a little adjustment, do almost anything we do with hydrocarbons, with batteries. Fly aircraft. Run ships. Everything.

How long it will take us to reach that nirvana is a matter of some speculation, but the odds are good that it will be achieved in this century. Maybe even this decade. Stay tuned!