Lithium superoxide is an unstable inorganic salt with formula Li O₂. A

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compound, it can be produced at low temperature in matrix isolation experiments, or in certain nonpolar, non-protic solvents. Lithium superoxide is also a transient species during the reduction of oxygen in a lithium–air galvanic cell, and serves as a main constraint on possible solvents for such a battery. For this reason, it has been investigated thoroughly using a variety of methods, both theoretical and spectroscopic.

Structure

The LiO₂ molecule is a misnomer: the bonds between lithium and oxygen are highly ionic, with almost complete electron-transfer. The force constant between the two oxygen atoms matches the constants measured for the superoxide anion (O₂⁻) in other contexts. 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. 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.

Production and reactions

Lithium superoxide is extremely reactive because of the odd number of electrons present in the π* molecular orbital of the superoxide anion. Matrix isolation techniques can produce pure samples of the compound, but they are only stable at 15-40 K. At higher (but still cryogenic) temperatures, lithium superoxide can be produced by ozonating lithium peroxide () in

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: Li2O2(f12) + 2O3(g) -> 2LiO2(f12) + 2O2(g)The resulting product is only stable up to . Alternatively, lithium electride dissolved in anhydrous ammonia will reduce

oxygen gas Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as w ...

to yield the same product: i+e^](am) + O2(g) -> i+O2^](am)Lithium superoxide is, however, only metastable in ammonia, gradually oxidizing the solvent to water and nitrogen gas: 2O2- + 2NH3 -> N2 + 2H2O + 2OH- Unlike other known decompositions of , this reaction bypasses lithium peroxide.

Occurrence

Like other superoxides, lithium superoxide is the product of a one-electron reduction of an

oxygen molecule There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others are: * ...

. It thus appears whenever oxygen is mixed with single-electron redox catalysts, such as ''p''-benzoquinone.

In batteries

Lithium superoxide also appears at the cathode of a lithium-air galvanic cell during discharge, as in the following reaction: :Li⁺ + e⁻ + O₂ → LiO₂ This product typically then reacts and proceed to form lithium peroxide, Li₂O₂ :2LiO₂ → Li₂O₂ + O₂ The mechanism for this last reaction has not been confirmed and developing a complete theory of the oxygen reduction process remains a theoretical challenge . Indeed, recent work suggests that LiO₂ can be stabilized via a suitable cathode made of graphene with iridium nanoparticles. A significant challenge when investigating these batteries is finding an ideal solvent in which to perform these reactions; current candidates are ether- and amide-based, but these compounds readily react with the superoxide and decompose. Nevertheless, lithium-air cells remain the focus of intense research, because of their large

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—comparable to the internal combustion engine.

In the atmosphere

Lithium superoxide can also form for extended periods of time in low-density, high-energy environments, such as the upper atmosphere. The

mesosphere The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...

contains a persistent layer of

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

cations

ablated Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft material for a ...

from meteors. For sodium and potassium, many of the ions bond to form particles of the corresponding superoxide. It is currently unclear whether lithium should react analogously.For arguments claiming (or assuming) similarity, see: * * For an argument that the different photoionization rate of lithium should produce a dissimilar equilibrium, see: *

References