Their dominance is down to several reasons – they are lightweight; they tend to hold their charge better than other batteries and they do not suffer from so-called “memory effect”, where batteries hold less and less charge if they are not drained and then recharged completely. Critically, they also have a high energy density - a measure of how much punch a battery packs.
Over the last 20 years, researchers and manufacturers have managed to continually squeeze more and more energy out of these packs, doubling their performance. This has been achieved by clever engineering, subtly tweaking the structures inside the battery to make them more efficient, or adding new materials to boost their performance. And the process carries on today, with materials such as silicon receiving a lot of interest as a possible replacement – and improvement - for the graphite anodes in lithium-ion batteries.
Silicon is attractive because it is cheap, abundant and well understood. But more importantly, by weight, it can store ten times more lithium ions than graphite, which means that it could theoretically allow a 10-fold increase in performance. However, to be useful, researchers must overcome a problem. While graphite anodes hold their shape when they soaks up lithium ions, silicon swells, causing silicon particles to become separated, rapidly reducing the performance of the battery.
To try to get around this, Dr Gao Liu, of the Lawrence Berkeley National Laboratory in Berkeley, California, is developing rubbery conductive binders that stick to the silicon particles within the anode, stretching and shrinking as the battery is charged and discharged. He worked with theoretical chemists to identify suitable materials for these restraints, finally settling on a conductive plastic. Initial results suggest his new anode could produce lithium-ion batteries with 25 to 30% greater capacity and longer life span than those on the market today.
"Our latest tests have shown our material works well and is really stable," says Dr Liu. "It keeps its conductivity, structure and capacity well even after more than 1,000 charging cycles." He is collaborating with commercial partners including the multinational 3M and says he is currently taking phone calls "almost every day" from consumer electronics companies.
But he will have to act quickly. US companies Amprius and Nanosys, are already developing lithium-ion batteries with anodes containing silicon nanowires to get around the swelling issue, whilst electronics giant Panasonic is due to launch lithium-ion laptop batteries with silicon alloy anodes, which it says boosts capacity by 30%. The firm has not announced a mobile phone battery but industry watchers suggest one may be in development.
But, despite the commercial interest, these advances will soon reach a bottleneck. At best, these incremental improvements will performance, researchers say.
“The lithium ion research field is 30 years old now and commercialisation is over 20 years old,” says Prof Gerbrand Ceder of the Massachusetts Institute of Technology (MIT) in US. “It's hard as a scientist to say something isn't possible but when you look at the candidates it doesn't look as if we're going to get dramatically better capacity which is why we are starting to see a lot of research on totally different chemistries and totally different technologies."
Short circuit
One example is lithium-air batteries, inspired by the zinc-air packs first used in hearing aids in the 1960s that get their power by reacting zinc with oxygen in the air. Professor Peter Bruce, of the University of St Andrews, Scotland, has been working on the idea since 2007, with anodes made of lightweight porous carbon. Oxygen from the air enters the porous carbon, reacts with the lithium ions in the electrolyte and electrons in the external circuit to form solid lithium oxide. Recharging causes the lithium compound to decompose, releasing the lithium ions and releasing the oxygen.
