Inexpensive, high capacity and fast: the new aluminum battery technology promises everything

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There's a classic irony with new technology, that adopters are forced to limit themselves to two of the three things everyone wants: fast, cheap, and good. When the technology is battery technology, adoption is even more difficult. Cheap and fast (charging) still matters, but "good" can mean different things, like light weight, low volume, or long life, depending on your needs. Yet the same kinds of trade-offs are involved. If you want really fast charging, you'll probably have to give up some capacity.

These compromises allow research into alternative battery chemistries to continue despite lithium's enormous lead in terms of technology and manufacturing capabilities. There is always the hope that some other chemistry could lead to a big price drop or a big increase to some degree. performance.

Today, an article is published which seems to propose a low price combined with a strong increase in several of these measures. The aluminum-sulfur batteries he describes offer low raw materials, competitive size, and more capacity per weight than lithium-ion, with the big advantage of fully charging cells in less than a minute. The only obvious problem it has right now is that it needs to be at 90°C (near the boiling point of water) to work.

People have been thinking about aluminum-based batteries for a while, attracted by their high theoretical capacity. While each aluminum atom is a bit heavier than lithium, aluminum atoms and ions are physically smaller because the higher positive charge of the nucleus pulls the electrons in a bit. Additionally, aluminum will easily give up three electrons per atom, which means you can move a lot of charge for every ion involved.

A big problem is that, chemically, aluminum sucks a bit. Many aluminum compounds are highly insoluble in water, their oxides are extremely stable, etc. It's easy for something that should be a minor side reaction to cripple a battery after a few charge/discharge cycles. So while the work continued, the high theoretical capabilities often seemed like something that would never be realized in practice.

The key to the new work was realizing that we had already solved one of the big problems with making an aluminum metal electrode; we had just done it in a completely different area. Pure metal electrodes offer big improvements in simplicity and bulk because there's no real chemistry involved and you don't need any additional materials to insert the metal ions into them. But metal tends to deposit unevenly on battery electrodes, eventually producing spines called dendrites that grow until they damage other battery components or short out the cell altogether. So figuring out how to deposit the metal evenly was a big hurdle.

A key realization here is that we already know how to deposit aluminum evenly. We do this all the time when we want to galvanize aluminum onto another metal.

This is often done using a molten aluminum chloride salt. In molten salt, aluminum and chlorine ions tend to form long chains of alternating atoms. When aluminum is deposited on a surface, it tends to come out of the center of these chains, and the physical mass of the rest of the chain makes this easier on a flat surface.

In molten salt, aluminum ions can also move rapidly from electrode to electrode. The big problem is that aluminum chloride only melts at 192°C. But mixing a little sodium chloride and potassium chloride brought it down to 90°C - below the point of water boiling and compatible with a wider range of additional materials. /p> salt sandwich

With this, the researchers had two-thirds of a battery. One electrode was metallic aluminum and the electrolyte was liquid aluminum chloride. This leaves a second electrode to identify. Here there were many examples of storing aluminum as a chemical compound with elements below oxygen on the periodic table, such as sulfur or selenium. For imaging purposes, the team worked with selenium, creating an experimental battery cell and confirming that it behaved as expected.

Imaging of aluminum showed that after a certain charge and discharge cycle...