A cross between lithium-ion and lithium-air batteries may lead to a new type of long-lasting energy storage device with high capacity (Nat. Energy 2016, DOI: 10.1038/nenergy.2016.111). The advance could help electric vehicles become more common and affordable.
Lithium-air batteries look great on paper. In theory, they can pack roughly 10 times as much energy per weight as conventional lithium-ion batteries such as the ones that power cell phones. But the “air breathing” devices haven’t made it out of the lab yet because they fail quickly. These batteries are also inefficient electrically in that the voltage required to charge the battery is higher than that obtained while using the battery.
In air-breathing batteries, the oxygen needed for electrochemical reactions comes from the air and reacts with lithium inside the battery, forming lithium oxides, mainly lithium peroxide (Li2O2). That compound is one of the keys to Li-air batteries’ weight advantage: It has a much lower molecular weight than the transition-metal lithium oxides, such as LiCoO2, used in Li-ion batteries.
But the reaction forming Li2O2 is not fully reversible. As the oxide accumulates, it clogs the air-breathing electrode’s pores, making it difficult to get oxygen quickly and reversibly from outside the battery to the electrode surface.
So MIT’s Ju Li, together with Jun Lu and colleagues at Argonne National Laboratory and Lu Qi of Peking University, came up with a battery system that takes advantage of Li2O2 but bypasses the problems of Li-air devices. They made a material consisting of nanosized patches of lithium oxide (Li2O) dispersed in a catalytic cobalt oxide matrix and used the material as the positive electrode in a sealed (non-air-breathing) Li-ion battery.
On the basis of electrochemical and spectroscopy measurements, the team showed that during battery operation Li2O is converted reversibly to Li2O2 and LiO2. The battery boasts an energy storage capacity that approaches Li-air’s theoretical value and is nearly three times as high as that of Li-ion batteries. The capacity fell by only 1.8% after 130 charging cycles. In addition, the battery charges more efficiently than typical Li-air batteries.
There is a worldwide search for batteries that can surpass Li-ion in terms of energy storage, says the University of Liverpool’s Laurence J. Hardwick, who wrote a commentary in the same issue of Nature Energy. The new study adds “a promising high-energy storage battery system into the mix.” He adds that the detailed spectroscopy investigation reported here will greatly facilitate future development of this system.