The electrochemical and chemical processes that are involved in Li-O₂ batteries are complex, and depend strongly on electrode materials, electrolytes, interfaces, and cell operating conditions. In non-aqueous Li-O₂ batteries, the main discharge products are commonly known to be lithium peroxide (Li₂O₂), and possibly some other parasitic solid components (e.g. Li₂CO₃, LiOH, Li₂O, etc.). Relative to these solid compounds (i.e. Li₂CO₃, Li₂O, LiOH, Li₂O₂) that are commonly found in discharge products, little is known about the solid LiO₂. To have a basic understanding of lithium superoxide solid, a combined theoretical studies and experimental characterizations are necessary.

Compared to sodium superoxide (NaO₂) and potassium superoxide (KO₂), the superoxide intermediates and lithium superoxide (O₂^-, LiO₂) are commonly known to be metastable and there have been no reports of a Li-O₂ battery based on LiO₂.  With this as a motivation, a systematic studies of lithium superoxide in Li-O₂ battery have been carried out in our laboratory recently [1-4]. Consistent with our previous Density Functional Theory (DFT) predictions, it has been experimentally confirmed that the LiO₂ solid can be stable as a component along with Li₂O₂ discharge product [1] or in pristine crystalline form [2] in the battery.  Consistent with DFT predicted structure [3], these LiO₂ solids are found to be in crystalline form with an orthorhombic phase [shown in the attached figure] that confirmed by high intensity X-ray diffraction and transmission electron microscopy measurements.

 Although LiO₂ is thermodynamically unstable with respect to the disproportionation process, theoretical calculations have indicated that some forms of LiO₂ may have a long lifetime [4]. Based on both DFT and Ab Initio Molecular Dynamics (AIMD) simulation, we found the disproportionation process will be slower at an electrolyte/LiO₂ interface compared to a vacuum/LiO₂ interface [1], and this has been experimentally confirmed that the LiO₂ component or solid can be possibly stable up to days when an electrolyte is left on the surface of the discharged cathode. In addition, the pristine crystalline LiO₂ solid can be electrochemically synthesized and stabilized in a Li-O₂cell environment using a suitable graphene-based cathode with iridium nanoparticles [2] based a novel templating growth mechanism as suggested by theory. In this presentation, the recent developments, studies and findings of this elusive metastable solid compound will be discussed systematically based on our recent efforts.

This work was primarily supported by the US Department of Energy under contract DE-AC02-06CH11357 from the Vehicle Technologies Office, Department of Energy, Office of Energy Efficiency and Renewable Energy.

References:

1. Nano Lett. 15 (2) 1041-1046 (2015).
2. Nature doi: 10.1038/nature 16484 (2016).
3. J. Phys. Chem. C 115 (47) 23625-23633 (2011).
4. J. Phys. Chem. Lett. 5, 813-819 (2014).