The Role of Semilabile Oxygen Atoms for Intercalation Chemistry of the Metal-Ion Battery Polyanion Cathodes

Using
the orthorhombic layered Na2FePO4F
cathode material as a model system we identify the bonding of the
alkali metal cations to the semilabile oxygen atoms as an important
factor affecting electrochemical activity of alkali cations in polyanion
structures. The semilabile oxygens, bonded to the P and alkali cations,
but not included into the FeO4F2 octahedra,
experience severe undercoordination upon alkali cation deintercalation,
causing an energy penalty for removing the alkali cations located
in the proximity of such semilabile oxygens. Desodiation of Na2FePO4F proceeds through a two-phase mechanism in
the Na-ion cell with a formation of an intermediate monoclinic Na1.55FePO4F phase with coupled Na/vacancy and Fe2+/Fe3+ charge ordering at 50% state of charge.
In contrast, desodiation of Na2FePO4F in the
Li-ion cell demonstrates a sloping charge profile suggesting a solid
solution mechanism without formation of a charge-ordered intermediate
phase. A combination of a comprehensive crystallographic study and
extensive DFT-based calculations demonstrates that the difference
in electrochemical behavior of the alkali cation positions is largely
related to the different number of the nearest neighbor semilabile
oxygen atoms, influencing their desodiation potential and accessibility
for the Na/Li chemical exchange, triggering coupled alkali cation–vacancy
ordering and Fe2+/Fe3+ charge ordering, as well
as switching between the “solid solution” and “two-phase”
charging mechanistic regimes.