The classic irony of new technology is that adopters are forced to limit themselves to two of the three things everyone wants: fast, cheap, and good. When the technology is batteries, the implementation becomes even more complex. Cheap and fast (charging) still counts, but “good” can mean different things, such as light weight, small volume or long life, depending on your needs. However, the same trade-offs are involved. If you want really fast charging, you’ll probably have to give up some capacity.
These trade-offs allow research into alternative battery chemistries to continue, despite the bulk of lead-lithium in terms of technology and manufacturing capabilities – there is still hope that some other chemistry can provide a significant drop in price or a significant increase in performance.
A paper is being published today that appears to offer a low price combined with a large increase in some of these measures. The aluminum-sulfur batteries described therein offer low-cost raw materials, competitive size, and higher capacity-to-weight than lithium-ion batteries, with the great advantage of fully charging the cells in well under a minute. One obvious problem now is that it needs 90°C (almost the boiling point of water) to work.
For some time, people have been thinking about aluminum-based batteries, attracted by their high theoretical capacity. While each aluminum atom is slightly heavier than lithium, aluminum atoms and ions are physically smaller because the larger positive charge of the nucleus pulls in the electrons a bit. Also, aluminum will readily donate up to three electrons per atom, which means you can shift a lot of charge for each ion involved.
The big problem was that chemically, aluminum kind of sucks. Many aluminum compounds are very insoluble in water, their oxides are extremely stable, and so on – easily something that should be a minor side reaction can cause a battery to fail after a few charge/discharge cycles. Thus, while the work continued, the high theoretical ability often looked like something that would never be realized in practice.
The key to the new work was the realization that we had already solved one of the big problems in making an aluminum metal electrode – we had just done it in a completely different area. Pure metal electrodes provide a significant increase in simplicity and volume as there is no real chemistry involved and you do not need additional materials to introduce the metal ions. But the metal tends to deposit unevenly on the battery’s electrodes, forming spikes called dendrites that grow until they damage other battery components or shut down the cell completely. So figuring out how to lay down the metal evenly was a big hurdle.
The key realization here is that we already know how to layer aluminum evenly. We do this all the time when we want to electroplate aluminum on another metal.
This is often done using molten aluminum chloride salt. In the molten salt, the aluminum and chlorine ions tend to form long chains of alternating atoms. When aluminum is applied to a surface, it tends to come out of the center of these chains, and the physical mass of the rest of the chain makes it easier on a flat surface.
In molten salt, aluminum ions can also move rapidly from one electrode to another. The big problem is that aluminum chloride only melts at 192°C. But mixing a small amount of sodium chloride and potassium chloride lowered the temperature to 90°C—below the boiling point of water and compatible with a wider range of additional materials.
Sandwich with salt
At the same time, the researchers had two-thirds of the battery. One of the electrodes was lithium metal, and the electrolyte was liquid aluminum chloride. The second electrode remains to be identified. There have been many examples of aluminum being stored as a chemical compound with elements below oxygen in the periodic table, such as sulfur or selenium. For visualization, the team worked with selenium, creating an experimental battery and confirming that it behaved as expected.
Images of the aluminum showed that the surface was somewhat pitted after some charge and discharge cycles, but there were no large or pointed extensions that could damage the battery. The reactions at the selenium electrode appeared to start in the molten salt before finishing at the electrode surface. Overall, the cell demonstrated consistent performance over dozens of cycles and the high capacity-to-weight that aluminum should provide. So the team moved on to creating and testing the elements they were really interested in: aluminum sulfur.
At low discharge rates, the aluminum-sulfur cells had a charge capacity per weight that was three times that of lithium-ion batteries. This figure dropped as the charge/discharge rate increased, but performance remained excellent. When the cell was discharged for two hours and recharged in just six minutes, it still had a charge capacity per weight that was 25 percent higher than that of lithium-ion batteries, and retained about 80 percent of that capacity after 500 cycles – much more than you d see with most lithium chemicals.
If you reduce the charging time by just over a minute, the capacity per weight is roughly equal to that of a lithium-ion battery, with more than 80 percent of that capacity available after 200 cycles. The battery cell could even sustain a full charge in less than 20 seconds, although the capacity per weight was only a little more than half of what you’d get from a lithium-ion.