As the world begins to rely ever more on batteries to store and provide power as part of the fight against climate change, scientists are working hard to find ways to make those batteries hold more charge and last longer.
Jasper Woodard, a PhD student at the University of Alberta, is one of those scientists who has been working for years on ways to improve battery life. His focus is on finding a way to replace the carbon used in the anodes of lithium-ion batteries with silicon.
“Silicon offers much higher energy density than carbon,” he says. “So you could use your phone for longer, or drive further, on one charge.”
That is due to the different ways in which the two elements react with lithium. When carbon, usually in the form of graphite, reacts with lithium in the anode, the lithium slots in between the layers of graphite. But with silicon, the two elements form an alloy, which allows silicon to store more atoms of lithium than carbon does, giving the battery a capacity four to five times higher.
The downside of using silicon, however, comes from the same place, says Woodard. As it takes in lithium, the silicon anode expands by up to 300 per cent, and as it expands and contracts over repeated charging cycles, the silicon begins to break apart, react with the liquid electrolyte and quickly loses capacity.
Woodard has been experimenting with a variety of different binders and coatings to keep the silicon from breaking up and to protect it from reacting with the electrolyte. So far, he has tried polyethylene oxides, vinyl ethylene carbonate and various fluorinated compounds as protective coatings, but none have been terribly successful so far – perfluorinated compounds, for example, were pretty good at preventing reactions, but didn’t combine well with the most effective binders. He also discovered another complication – how the binders and coatings are applied to the silicon can be just as important as which ones are used.
“We’re no longer talking which binders to use, and which coatings, but also the degree of coverage on the silicon,” he says. “It just adds another variable.”
Chongyin Yang, who studies battery technology at Dalhousie University, says chemical coatings and binders are just one of the methods scientists are using to try to overcome the problems with silicon anodes. Another solution is deceptively simple: just don’t use the full capacity of the silicon. The energy capacity of silicon is high enough that just using a third, or even a fifth, of the available capacity can still provide a significant improvement in battery life, while minimizing the amount of expansion. “For most uses you just don’t need all of that capacity,” says Yang.
Similarly, you can add a small amount of silicon to traditional graphite anodes – just five to 10 per cent silicon content can boost capacity without the anode breaking down too much. Another strategy is to make the silicon anode porous, so as it expands it just fills in the holes, leaving the surface undisturbed.
“These are all good ideas, but it all boils down to the same problem: cost,” he says.
Graphite is a cheap and easily obtained material, so the anode is usually a tiny proportion of the cost of a battery. Using silicon instead will result in a huge increase in the cost of the anode, and the battery overall, says Yang. “If you need to trade off cost with performance, most will choose low cost.”
Still, both Woodard and Yang expect silicon anode batteries to become common within the next decade or so. Mercedes Benz already plans to use silicon batteries in its electric vehicles starting in 2025. “There are already many companies working on it,” says Yang. “It’s just a matter of which one can combine both low cost and high performance.”