The key to longer-range electric cars, smartphones that last several days without charging, and long-term storage of renewable energy that could be used on demand by the grid may be the lithium-oxygen ― also known as lithium-air ― battery. These batteries have the potential to store ten times more power than currently used lithium-ion versions. A new breakthrough in lithium-oxygen battery technology reported on 23 August in Science has brought the technology a step closer to reality (1). According to a briefing in Science, these lithium-oxygen batteries “could one day underpin a green energy grid, storing excess wind and solar power and delivering it on demand.”
Lithium-oxygen batteries work in a similar way to their lithium-ion counterparts. Essentially, the batteries alternate between charge and discharge cycles, during which lithium ions flow through two charge-storing electrodes separated by an organic liquid electrolyte. The main hurdle in the development of lithium-oxygen batteries is that during the discharge phase, oxygen is converted into superoxide and then lithium peroxide. These highly reactive compounds corrode the battery cell from within and in the process, the organic electrolyte is consumed by the superoxide, which therefore greatly limits their rechargeability. However, new findings show that running lithium-oxygen batteries at high temperatures― as well as a few other tweaks― can overcome the challenges currently holding back the technology.
For a long time, researchers have been attempting to design electrolytes capable of withstanding the harsh chemical exposure generated by the lithium-ion reaction, but with little success. Then, the first breakthrough was made by a team of researchers two years ago (2). They proposed an alternative electrolyte ― composed of a combination of salts that turn into a liquid when heated ― and although the novel “molten salt” was capable of withstanding the highly corrosive environment, the battery’s carbon-based cathode still remained an issue.
The latest breakthrough made by researchers at the University of Waterloo, led by Prof Linda Nazar, has taken this one step further. The researchers found that by increasing the operating temperature of the battery to 150 ˚C and utilizing Nickel oxide ― instead of the usual carbon cathodes ― supported by a stainless steel mesh and molten salt as the electrolyte, they could increase in the energy per unit of mass by more than 50 percent and achieve around three times the number of charging cycles ― approximately 150 cycles. The increase in temperature leads to the production of more stable reaction products resulting in a highly reversible lithium-oxygen battery with Coulombic efficiency close to 100 percent.
In its current form, the lithium-oxygen battery is not yet suitable for practical use but the technology may not be as far off as once thought. These new findings could lead to more usable designs in the future.
(1) Xia, C., Kwok, C. Y., and Nazar, L. F. A high-energy-density lithium-oxygen battery based on a reversible four-electron conversion to lithium oxide. Science (2018). DOI: 10.1126/science.aas9343
(2) Giordani, V. et al. A. Molten Salt Lithium-Oxygen Battery. Journal of the American Chemical Society (2018). DOI: 10.1021/jacs.5b11744