Elucidation of the Na2/3FePO4 and Li2/3FePO4 Intermediate Superstructure Revealing a Pseudouniform Ordering in 2D

Based on TEM, synchrotron X-ray diffraction, DFT calculations, and Mössbauer spectroscopy, a unified understanding of the Na and Li intercalation process in FePO4 is proposed. The key to this lies in solving the highly sought-after intermediate A2/3FePO4 (A = Na, Li) superstructures that are characterized by alkali ions as well as FeII/FeIII charge orderings in a monoclinic three-fold supercell. Formation energies and electrochemical potential calculations confirm that Na2/3FePO4 and Li2/3FePO4 are stable and metastable, respectively, and that they yield insertion potentials in fair agreement with experimental values. The 2/3 Na­(Li) and 1/3 vacancy sublattice of the intermediate phases forms a dense (101̅)Pnma plane in which the atom/vacancy ordering is very similar to that predicted for the most uniform distribution of 1/3 of vacancies in a 2D square lattice. Structural analysis strongly suggests that the key role of this dense plane is to constrain the intercalation in the diffusion channels to operate by cooperative filling of (bc)Pnma. From a practical point of view, this generalized mechanism highlights the fact that an interesting strategy for obtaining high-rate FePO4 materials would consist in designing grains with an enhanced (101) surface area, thereby offering potential for substantial improvements with respect to the performance of rechargeable Li and Na batteries.