Global warming together with a possible energy crisis gives us problems for transport. One of the alleged solutions is battery powered cars. This gives three potential problems. One of these is how to generate sufficient electricity to power the batteries, but I shall leave that for the moment. The other two relate to chemistry. A battery (or fuel cell) has two basic electrodes: a cathode, which is positive, and an anode, which is negative. The difference in potential between these is the voltage, and is usually taken as the voltage at half discharge. The potential is caused by the ability to supply electrons to the anode while taking them at the cathode. At each there is a chemical oxidation/reduction reaction going on. The anode is most easily answered by oxidising a metal. Thus if we oxidise lithium we get Li ➝ Li+ + e-. The electron disappears off to the circuit. We need something to accept an electron at the cathode, and that is Co+++, which gets reduced to the more stable Co++. (Everything is a bit more complicated – I am just trying to highlight the problem.) Now superficially the cobalt could be replaced by a variety of elements, but the problem is the cobalt is embedded in a matrix. Most other ions have a substantial volume change of the ions, and if they are embedded in some cathode matrix, the stresses lead it to fall to bits. Cobalt seems to give the least stress, hence will give the batteries a longer life. So we have a problem of sorts: not enough easily accessible lithium, and not enough cobalt. There are also problems that can reduce the voltage or current, including side reactions and polarization.
In a fuel cell we can partly get over that. We need something at the cathode that will convert an input gas into an anion by accepting an electron, thus oxygen and water forms hydroxide. At the anode we need something that “burns”, i.e. goes to a higher valence state and gives up an electron. In my ebook “Red Gold”, a science friction story relating to the first attempt at permanent settlement of Mars, a portable power source was necessary. With no hidden oil fields on Mars, and no oxygen in the air to burn it if there were, I resorted to the fuel cell. The fuel cell chemistry I chose for Mars was to oxidize aluminium, which generates three electrons, and to reduce chlorine. The reason for these was that the settlement on Mars needed to make things from Martian resources, and the most available resource was the regolith, which is powdered rock. This was torn apart by nuclear fusion power, and the elements separated by magnetohydrodynamics, similar to what happens in a mass spectrometer. The net result is you get piles of elements. I chose aluminium because it has three electrons and hence more power capacity, and I chose chlorine because it is a liquid at Martian temperatures so no pressure vessel was required. Also, while oxygen might produce a slightly higher voltage, oxygen forms a protective coating on aluminium, and that stops that reaction.
An aluminium battery would have aluminium at the anode, and might have something in the electrolyte that could deposit more aluminium on it. Thus during a charge, you might get, if chlorine is the oxidiser,
4(Al2Cl7)- + 3e- → Al + 7(AlCl4)-
which deposits aluminium on the anode. During discharge the opposite happens and you burn aluminium off. Notice here the chlorine is actually tied up in chemical complexes and the battery has no free chlorine. Here, the electrolyte is aluminium chloride (Al2Cl6). For the fuel cell, we would be converting the gas to a complex at the cathode. That is not very practical on Earth, but the enclosed battery would be fine.
The main advantage of aluminium is that it gets rid of the supply problem. Aluminium is extremely common on Earth, as the continents are essentially made of aluminosilicates. The cathode can be simple carbon. A battery with this technology was proposed in 2015 (Nature 520: 325 – 328) that used graphite cathodes. It was claimed to manage 7,500 cycles without capacity decay, which looks good, but so far nobody seems to be taking this up.
Now, for an oddity. For discharge, we need to supply (AlCl4)- to the anode as it effectively supplies chlorine. Rather than have complicated chemistry at the cathode we can have an excess of AlCl4– from the start, and during charging, store it in the cathode structure. During discharge it is released. So now we need something to store it in. The graphite used for lithium-ion batteries comes to mind, but here is an oddity: you get twice the specific capacity, twice the cell efficiency and a 25% increase in voltage by using human hair! Next time you go to the hair dresser, note that in the long term that might be valuable. Of course, before we get too excited, we still need such batteries to be constructed and tested because so far we have no idea how such hair stands up to repeated cycles.
What we do not know about such batteries is how much dead weight has to be carried around and how small they can be made for a given charge. The point about cars is that eventually the critical point is how far will it go on one charge, how long does it take to safely recharge, how much volume of the vehicle does it take, and is it flammable? The advantage of the aluminium chloride system described above is that there are probably no side reactions, and a fire is somewhat unlikely. The materials are cheap. So, the question is, why hasn’t more been done on this system? My guess is that the current manufacturers know that lithium is working, so why change? The fact that eventually they will have to does not bother them. The accountants in charge think beyond the next quarter is long-term. Next year can look after itself. Except we know that when the problem strikes, it takes years to solve it. We should get prepared, but our economic system does not encourage that.