The high theoretical-energy density of lithium-oxygen (Li-O2) batteries and their relatively light weight have made them the Holy Grail of rechargeable battery systems. But long-standing issues with the battery’s chemistry and stability have kept them a purely academic curiosity.
Two of the more serious issues involve the intermediate of the cell chemistry (superoxide, LiO2) and the peroxide product (Li2O2) reacting with the porous carbon cathode, degrading the cell from within. In addition, the superoxide consumes the organic electrolyte in the process, which greatly limits the cycle life.
Science – A high-energy-density lithium-oxygen battery based on a reversible four-electron conversion to lithium oxide [C. Xia, C. Y. Kwok, L. F. Nazar, Science 24 Aug 2018: Vol. 361, Issue 6404, pp. 777-781, DOI: 10.1126/science.aas9343]
Nazar and her colleagues switched the organic electrolyte to a more stable inorganic molten salt and the porous carbon cathode to a bifunctional metal oxide catalyst. Then by operating the battery at 150 C, they found that the more stable product Li2O is formed instead of Li2O2. This results in a highly reversible Li-oxygen battery with coulombic efficiency approaching 100%.
By storing O2 as lithium oxide (Li2O) instead of lithium peroxide (Li2O2), the battery not only maintained excellent charging characteristics, it achieved the maximum four-electron transfer in the system, thereby increasing the theoretical energy storage by 50%.
“By swapping out the electrolyte and the electrode host and raising the temperature, we show the system performs remarkably well,” said Nazar, who is also a University Research Professor in the Department of Chemistry at Waterloo.
Science – Hot lithium-oxygen batteries charge ahead [Shuting Feng, Jaclyn R. Lunger, Jeremiah A. Johnson, Yang Shao-Horn Science 24 Aug 2018: Vol. 361, Issue 6404, pp. 758 DOI: 10.1126/science.aau4792]
The need to increase the energy storage per unit mass or volume and to decrease stored-energy cost from solar and wind has motivated research efforts toward developing alternative battery chemistries. In particular, lithium-oxygen (Li-O2) batteries offer great promise. During discharge, oxygen can be reduced to form either peroxide (Li2O2 in a two-electron pathway) or oxide (Li2O in a four-electron pathway). The estimated energy densities of lithium-oxygen batteries based on peroxide and oxide are two and four times higher than that of lithium-ion batteries, respectively, but degradation of organic electrolytes and of oxygen electrodes (typically made of carbon) by these reactive oxygen species has limited the reversibility of these systems. On page 777 of this issue, Xia et al address these issues by using inorganic components—a molten salt electrolyte and a nickel-based oxide supported by stainless steel mesh for the oxygen electrode—and demonstrate reversible operation for the four-electron–pathway Li-O2 battery at 150°C.
An elevated lithium battery
Batteries based on lithium metal and oxygen could offer energy densities an order of magnitude larger than that of lithium ion cells. But, under normal operation conditions, the lithium oxidizes to form peroxide or superoxide. Xia et al. show that, at increased temperatures, the formation of lithium oxide is favored, through a process in which four electrons are transferred for each oxygen molecule (see the Perspective by Feng et al.). Reversible cycling is achieved through the use of a thermally stable inorganic electrolyte and a bifunctional catalyst for both oxygen reduction and evolution reactions.
Lithium-oxygen (Li-O2) batteries have attracted much attention owing to the high theoretical energy density afforded by the two-electron reduction of O2 to lithium peroxide (Li2O2). We report an inorganic-electrolyte Li-O2 cell that cycles at an elevated temperature via highly reversible four-electron redox to form crystalline lithium oxide (Li2O). It relies on a bifunctional metal oxide host that catalyzes O–O bond cleavage on discharge, yielding a high capacity of 11 milliampere-hours per square centimeter, and O2 evolution on charge with very low overpotential. Online mass spectrometry and chemical quantification confirm that oxidation of Li2O involves transfer of exactly 4 e–/O2. This work shows that Li-O2 electrochemistry is not intrinsically limited once problems of electrolyte, superoxide, and cathode host are overcome and that coulombic efficiency close to 100% can be achieved.