Lithium batteries are the reigning champion in a key category of performance: energy density. This density is why they've dominated applications like portable devices and electric vehicles, where size or weight matter. But there are reasonable doubts about how much energy density can be increased using lithium, so researchers have continued to look into alternative chemistries for batteries.

One of the more promising alternatives is aluminum. It's abundant, cheap, and lightweight, and each atom has the potential to liberate up to three electrons during charge/discharge cycles. Lithium can manage only one. But the batteries themselves have been disappointing so far, with performance dropping radically after a few cycles. So it's big news that researchers have now managed to craft an aluminum battery that is stable out to over 7,000 cycles—plus it's flexible and poses no fire risk. But the battery doesn't take advantage of some of aluminum's more appealing properties, so there is clearly more work to be done.

One of lithium's limitations is that on its own, as a pure metal, it makes a lousy battery material. Lithium metal electrodes have a tendency to deform and/or short-circuit over charge/discharge cycles, which is why we use lithium ion batteries—the lithium is complexed with other elements, keeping the metal from causing trouble. That requirement means you need to have a distinct material for the electrodes, which adds to the weight and complexity.

By contrast, the aluminum battery described here simply uses aluminum metal for the anode. In fact, it uses aluminum foil, which provides a high surface area for shuffling charges and contributes to flexibility of the battery. In this regard, the battery does represent a useful advance from lithium.

The electrolyte the researchers used was a solution of aluminum trichloride dissolved in an organic solvent that also contained chlorine. During charge/discharge cycles, electrons were donated to form AlCl 4 - and Al 2 Cl 7 - ions. This chemistry did not take advantage of the three electrons that aluminum has to donate, so it doesn't represent much of an improvement over lithium. In any case, these ions could slip in between layers of the cathode material—a process called intercalation—at which point they could hand over their spare electrons.

As for the cathode, the researchers decided to experiment with various forms of carbon. Graphite itself doesn't work especially well, as its structure tends to get destroyed by the intercalation of ions. So the authors tried a different form of graphite (pyrolytic graphite) that has cross links between the different sheets of carbon in the material. This graphite added structural strength, but it slowed down the process of intercalating ions so much that the charging and discharge rates were limited.

The authors therefore turned to a material that had first been described by a different lab in 2012: graphene foam. Graphene foam is an open, flexible structure of interconnected graphene sheets and tubes. (It's made by creating a metal foam, then catalyzing graphene formation on its surface.) It's rugged and flexible, but it still provides a good surface area for the aluminum chloride ions to interact with.

The graphene foam electrode worked very well. Batteries based on it showed stable performance for over 7,000 charge/discharge cycles and could be charged quite quickly. But the capacity per weight is no better than a typical lead-acid battery, so it's not going to compete with lithium any time soon. On the plus side, it's a bit safer than lithium batteries; the authors drilled a hole through it while it was in operation with no ill effects.

So the results are a bit of a mixed bag. Getting aluminum to work well and have an all-metal electrode are both promising developments, and there are possibly things we can learn from this battery that can be applied to other systems. But the fact that aluminum atoms only transferred a single electron when they transited to the cathode is really not taking full advantage of the whole reason that people think the material would be good for batteries. And that leads to the low power density of these batteries.

All that said, there are battery applications—electrical storage on the grid, for example—where energy density doesn't matter at all. If these batteries could be produced economically, they could still find a place on the market.

Nature, 2015. DOI: 10.1038/nature14340 (About DOIs).