  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.
