Defect and dopant properties of the alpha- and beta-polymorphs of the Li3FeF6 lithium battery material

Li3FeF6 has attracted recent interest as a promising positive electrode for rechargeable lithium batteries. The defect chemistry and cation doping behaviour of the alpha- and beta-polymorphs of Li3FeF6 have been investigated by advanced atomistic modelling techniques. Our simulations show good reproduction of both monoclinic (alpha) and orthorhombic (beta) experimental structures, which are related to the cryolite crystal structure. The most favourable defect types are found to be the Li Frenkel and off-stoichiometry (Li-excess) disorder, suggesting that interstitial lithium is possible in the cryolite structure, and is important in rationalizing the lithium intercalation chemistry. Monovalent dopant substitution for Li and divalent substitution for Fe are energetically favourable, and could be a synthesis strategy to optimise the electrochemical behaviour of Li3FeF6.