Lithium superoxide

Lithium superoxide (LiO2) is an inorganic compound which has only been isolated in matrix isolation experiments at 15-40 K.[1] It is an unstable free radical that has been analyzed using infrared (IR), Raman, electronic, electron spin resonance, soft X-ray spectroscopies, and a variety of theoretical methods.[1]

Lithium superoxide
Identifiers
Properties
LiO2
Molar mass 38.94 g/mol
Density g/cm3, solid
Melting point <25 °C (decomposes)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Structure

Experimental studies indicate that the LiO2 molecule contains highly ionic bonds.[2] Eighteen different values were attained using six isotopic species. This indicated that the force constant between the two oxygen atoms corresponds with the constant found for the O2− ion. Studies indicate that there is little to no covalent character in the LiO2 molecule.

The bond length for the O-O bond was determined to be 1.34 Å. Using a simple crystal structure optimization, the Li-O bond was calculated to be approximately 2.10 Å.[3] Lithium superoxide is extremely reactive because of the odd electron present in the π* molecular orbital.[4]

There have been quite a few studies regarding the clusters formed by LiO2 molecules. The most common dimer has been found to be the cage isomer. Second to it is the singlet bypyramidal structure. Studies have also been done on the chair complex and the planar ring, but these two are less favorable, though not necessarily impossible.[1]

Reactions

In a lithium-air battery, when there is a one electron reduction during discharge, lithium superoxide is formed as seen in the following reaction:[5]

Li+ + e− + O2 → LiO2

This product will then react and proceed to form lithium peroxide, Li2O2:

2LiO2 → Li2O2 + O2

The mechanism for this last reaction has not been confirmed and chemists are having difficulties developing a theory of what may be happening. Another significant challenge of these batteries is finding an ideal solvent in which to perform these reactions; ether- and amide-based solvents are currently used, but these compounds readily react with oxygen and decompose.[6] A suitable solvent would need to be able to resist autoxidation to enable a long life cycle for the battery.

Presence of the Compound

The predominant use of lithium superoxide is in rechargeable lithium batteries. As portrayed in the reactions above, this lithium compound is a major component as an intermediate, an area where there is much research to be done. Researchers have much anticipation for the potential energy that may be provided by these batteries—-some say it is comparable to the internal combustion engine.[5] One study claims that alkali superoxides affect the function of the alkyl metals in the atmosphere as well. The alkali metals are found predominantly in the mesosphere and the superoxides are found just below this where the metal reacts with the excess oxygen.[7] Rarely are superoxides stable for any significant amount of time as they exist merely as transition states. In January 2016, researchers from Argonne National Laboratory, US, claimed that crystalline LiO2 could be stabilized in a Li-O2 battery by using a suitable graphene-based cathode.[8]

See also

References
  1. Bryantsev, V.S.; Blanco, M.; Faglioni, F. Stability of Lithium Superoxide LiO2 in the Gas Phase: Computational Study of Dimerization and Disproportionation Reactions. J. Phys. Chem. A, 2010, 114 (31), 8165–816.
  2. Andrews, L. Infrared Spectrum, Structure, Vibrational Potential Function, and Bonding in the Lithium Superoxide Molecule LiO2. J. Phys. Chem. 1969, 50, 4288.
  3. Lau, K.C.; Curtiss, L.A. Density Functional Investigation of the Thermodynamic Stability of Lithium Oxide Bulk Crystalline Structures of Oxygen Pressure. J. Phys. Chem. 2011, 115 (47), 23625-23633.
  4. Lindsay, D.M.; Garland, D.A. ESR Spectra of Matrix-Isolated LiO2. J. Phys. Chem. 1987, 91(24), 6158-6161.
  5. Das, U.; Lau, K.C.; Redfern, P.C.; Curtiss, L.A. Structure and Stability of Lithium Superoxide Clusters and Relevance to Li—O2 Batteries. J. Phys. Chem., 2014, 5 (5), 813-819.
  6. Bryantsev, V.S.; Faglioni, F. Predicting Autoxidation Stability of Ether- and Amide-Based Electrolyte Solvents for Li–Air Batteries. J. Phys. Chem. A. 2012, 116 (26), 7128–7138.
  7. Plane, J.M.C.; Rajasekhar, B.; Bartolotti, L. Theoretical and Experimental Determination of the Lithium and Sodium Superoxide Bond Dissociation Energies. J. Phys. Chem. 1989, 93, 3141-3145.
  8. Lu, Jun (2016). "A lithium - oxygen battery based on lithium superoxide". Nature. 529 (7586): 377–381. doi:10.1038/nature16484. PMID 26751057. S2CID 4452883.


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