19 July 2012
Robert F Service
Researchers have long had high hopes for lithium-air batteries, a device that has the potential to store 10 times more energy than the best lithium-ion batteries on the market today. But so far, lithium-air batteries have been unstable, falling apart after a few charges. Now researchers report that they’ve made the first stable lithium-air batteries. If the batteries can leap other hurdles needed to make them practical, they may one day give electric cars a driving range similar to today’s gas guzzlers.
For lithium-air batteries to operate, several different components all need to work together. As they discharge, lithium atoms at a lithium metal electrode called the anode are stripped of electrons, turning them into mobile lithium ions. These ions then float through a conductive solution, or electrolyte, to a second electrode, called the cathode, where they combine with electrons in the cathode as well as oxygen atoms from the air to generate lithium oxide. When the batteries are plugged into an electrical outlet, the added voltage drives the reaction in reverse, recharging the battery. For the cycle to work, however, the electrodes and electrolytes must be stable.
But that hasn’t been the case in early versions of these cells. The carbon used to make the cathodes and the different electrolytes researchers have tried so far undergo unwanted side reactions, falling apart and quickly causing the battery to fail after just a few charge and discharge cycles.
So for their current work, researchers led by Peter Bruce, a chemist at the University of St Andrews in the United Kingdom, opted to swap out both of the previous offenders. They replaced the conventional carbon-based cathode material with one made from inert gold nanoparticles that they hoped would be more stable. They also replaced the electrolyte—previously made from compounds called polycarbonates or polyethers—with one made from a common conductive solvent abbreviated DMSO that previous studies had shown may be less prone to react at the cathode. The new combo worked. As the team reports online today in Science, the new batteries were stable for 100 charge and discharge cycles with only a 5% loss of power.
“The results are very encouraging in showing that it’s not all hopeless,” to try to make lithium-air batteries, says Linda Nazar, a chemist at the University of Waterloo in Canada. But Nazar and others are quick to add that the new lithium-air batteries aren’t yet ready for commercialization. For starters, Nazar says, gold is too heavy and too expensive to serve as the only cathode material in a practical cell. And over time, DMSO can react with lithium metal at the anode causing the electrolyte to break down. So even though the new results are heartening for the field, considerable work still lies ahead to make lithium-air batteries a real world technology.