Effect of halogen dopants on the Li$_{2}$O$_{2}$ properties: Is chloride special?

There is consensus on the fact that one of the main limitations of the Li air batteries (LABs) is the insulating character of Li$_{2}$O$_{2}$ and that it becomes crucial to explore new conduction paths. Recent studies indicate that doping with chloride increases the ion conductivity of Li$_{2}$O$_{2}$, although to a much lesser extent than expected if chloride is assumed as a donor dopant [Gerbig et al., Adv. Mater.,2013, 25, 3129]. Subsequently, it has been shown that the addition of lithium chloride, LiCl, to the battery electrolyte increases its discharge capacity while this effect is not observed with other halogens [Matsuda et al., J. Phys. Chem. C, 2016, 120, 13360]. This fact was attributed to an increase in the conductivity of Cl-doped Li$_{2}$O$_{2}$, but still the responsible mechanism is not clear. In this work, we have performed first principle calculations to study the effect of the different halogens (F, Cl, Br, I) as substitutional defects in the electronic and transport properties of Li$_{2}$O$_{2}$. We have calculated the formation energies of the different defects and impurities and we analysed how they affect the activation barriers and diffusion coefficients . We have demonstrated that the chloride does not behave like a donor dopant, thus explaining the meager increase of the ionic conductivity experimentally observed neither it promotes the polaron formation and its mobility. We have also found that chloride does not present any special behaviour among the halogen series. Our results reveal that all the studied configurations associated with the halogen defects do not derive in metallic states nor extra polarons that would increase considerably the electronic conductivity. This is mainly due to the ionic characteristics of the Li$_{2}$O$_{2}$ crystal and the capability of the oxygen dimers to adapt its valence rather than to the nature of the dopant itself.