Modern life would be quite different without decent batteries. Can you imagine powering your laptop on something like a standard automobile battery? It simply doesn't bear thinking about. Although we may make fun of battery engineers for claiming that three hours of real-world usage is the equivalent of being unplugged for an entire working day, they really have worked miracles. Unfortunately, even though they may want to think they can, even battery engineers can't bypass the laws of physics.
The performance of the current generation of lithium ion batteries is about to hit a wall, and if we want batteries with higher energy densities, an entirely new system will have to be developed. Among the many possible candidates, lithium-air batteries look very promising. When lithium oxidizes, it releases a lot of energy—so much so that, like sodium, it catches on fire. Lithium is also very light and reasonably abundant, making it the perfect element.
Except it's very hard to make a lithium-air battery that lasts. One of the big issues is unwanted side reactions. The battery contains the lithium, which we are going to repeatedly burn, an electrolyte for transporting charge between electrodes and electrode material. As we oxidize the lithium, a lot of energy is freed up, and not all of it gets extracted to do work. Instead, some of it goes into powering side reactions.
In many lithium-air cells, one of the major side reactions is the creation of lithium carbonate instead of lithium oxide, with the carbon coming from the electrodes or the electrolyte. Once the carbonate forms, it is stable. On recharging, the lithium oxide is reduced back to lithium metal, but the lithium carbonate remains—on each cycle, a small amount of lithium gets irretrievably lost.
To put this situation in perspective, my current laptop is about two years old. I use it nearly every day, and assuming the charge lasts one day (it doesn't), we're talking about 700 discharge-recharge cycles. If even 0.1 percent of the lithium was lost on each cycle, I would only have 50 percent of my battery capacity available today. In reality, most lithium-air batteries show a decay of a few percent per cycle thanks to this side reaction, so this is probably one of the most pressing problems to be solved if lithium-air is ever to leave the lab.
Given these problems, I was quite happy to stumble across a Nature Materials article on a lithium air cell that seems to be reporting a significant improvement. The researchers, from University of St. Andrews in Scotland, managed to achieve this through two changes. In earlier work, they showed that changing both the electrolyte and the electrode could produce an improved battery. First, they changed the electrolyte from one unpronounceable organic chemical to another slightly more pronounceable one (tetraethyleneglycol dimethylether, or TEGDME, to dimethyl sulphoxide, otherwise known as DMSO).
To obtain a stable electrode material, the researchers used nanoporous gold. But gold is expensive and heavy. If you're going to go to all the trouble of using lithium (the lightest metal in the periodic table) in your battery, you don't want to lose the big weight advantage by using gold (one of the heaviest metals) in its electrodes. So this paper was about finding a replacement for gold.
In the current generation of batteries, carbon is a common electrode material, but because lithium likes to form a carbonate, it's not a good choice here. Instead, the researchers found that titanium carbide made a good, stable electrode. In comparison tests, a battery based on titanium carbide electrodes and a DMSO electrolyte outperformed earlier versions by a large amount. After 100 cycles, the lithium air battery retained more than 98 percent of its original capacity, while the control sample lasted just 25 cycles, even when cycled at a lower current density and total capacity.
The key to these results appears to be a couple of factors. First, titanium carbide is very stable while still being able to transport charge. This makes it rather like the gold it replaces (gold is famous for being inert). Secondly, DMSO has much less carbon per molecule. DMSO has only two carbon atoms per molecule, compared to 10 for the TEGDME; even if it does begin to break down, it can't form as much lithium carbonate.
There is also no carbon dioxide produced during discharge. Using a TEGDME electrolyte, carbon dioxide is produced as the electrolyte breaks down and carbonates are formed. However, DMSO does not seem to break down, indicating that the residual carbonates are likely produced by reactions with the electrode.
Overall, the researchers claim a 40-fold reduction in side reactions, which is impressive. Even better, titanium carbide can be formed into electrodes with relative ease compared to gold. Still, 100 cycles is not 1,000 cycles. Using the data from the paper, I expect that after 1,000 cycles, the amount of lithium carbonate will increase to around two percent. I think this number is likely to be acceptable. The key questions are whether these results survive the optimization of the electrode and whether the resulting battery will meet real-life capacity requirements.
Nature Materials, 2013, DOI: 10.1038/NMAT3737