Decades after the lithium-oxygen (Li-O2) battery was first conceived, this design continues to capture the imagination of investigators who anticipate that it might yet improve energy density by at least a full order of magnitude. University of Waterloo chemistry professor Linda Nazar remains among the most dedicated members of this research community, which recently marked some progress to counter the ongoing challenges of turning such batteries into practical commodities.
Late this summer she and her colleagues reported in Science on a reversible, four-electron redox in a Li-O2 cell that points the way to a high capacity electrical storage system with minimal loss of energy. Such results contrast with past attempts over the past two years, which have established a two-electron pathway displaying much smaller voltage output and energy loss upward of 30%.
Nazar suggests earlier efforts to construct such batteries relied on organic electrolytes, which were eventually revealed to be responsible for fundamental problems such as the breakdown of carbon electrodes after just a few dozen cycles. She credits her group’s recent progress to an inorganic electrolyte and a process that invokes lithium nickel-oxide as a bifunctional catalyst promoting oxygen reduction as well as oxygen evolution, along with its in-situ formation on the surface of a nickel mesh electrode.
“It’s a little bit like a fuel cell, where you take hydrogen (the fuel) and oxygen and make water,” explains Nazar. “If you want to take water back to hydrogen and oxygen, you have to do water splitting, which uses an electrolyzer. In this case we’re taking lithium (the fuel) and oxygen and making Li2O. But the catalyst that we have in the electrode is bifunctional — it does both the forward and the reverse reaction and is fully reversible with close to 100% coulombic efficiency.”
Some significant obstacles still stand in the way of turning this approach into the basis of a functional battery. For one thing, a molten salt electrolyte, LiNO3-KNO, reduced the chemical instability of the system, but it must operate at temperatures around 150°C. Nor is the lithium ion conducting membrane fully compatible with this electrolyte. Nazar hints that there may be different ways of getting around these problems, but at this point she is satisfied to have found a way of overcoming the longstanding difficulties with electrolytes, even if that means facing a new set of research hurdles.
“It’s not perfect, but at least we know this oxygen chemistry is reversible if you fix the right problem,” she concludes, adding cautiously: “not to say that the problems that you solve don’t create more problems.”
