posted on 2021-09-01, 12:11authored byJustin
C. Hancock, Kent J. Griffith, Yunyeong Choi, Christopher J. Bartel, Saul H. Lapidus, John T. Vaughey, Gerbrand Ceder, Kenneth R. Poeppelmeier
CaFe2O4-type sodium postspinels (Na-CFs),
with Na+ occupying tunnel sites, are of interest as prospective
battery electrodes. While many compounds of this structure type require
high-pressure synthesis, several compounds are known to form at ambient
pressure. Here we report a large expansion of the known Na-CF phase
space at ambient pressure, having successfully synthesized NaCrTiO4, NaRhTiO4, NaCrSnO4, NaInSnO4, NaMg0.5Ti1.5O4, NaFe0.5Ti1.5O4, NaMg0.5Sn1.5O4, NaMn0.5Sn1.5O4, NaFe0.5Sn1.5O4, NaCo0.5Sn1.5O4, NaNi0.5Sn1.5O4, NaCu0.5Sn1.5O4, NaZn0.5Sn1.5O4, NaCd0.5Sn1.5O4, NaSc1.5Sb0.5O4, Na1.16In1.18Sb0.66O4, and several
solid solutions. In contrast to earlier reports, even cations that
are strongly Jahn–Teller active (e.g., Mn3+ and
Cu2+) can form Na-CFs at ambient pressure when combined
with Sn4+ rather than with the smaller Ti4+.
Order and disorder are probed at the average and local length-scales
with synchrotron powder X-ray diffraction and solid-state NMR spectroscopy.
Strong ordering of framework cations between the two framework sites
is not observed, except in the case of Na1.16In1.18Sb0.66O4. This compound is the first example
of an Na-CF that contains Na+ in both the tunnel and framework
sites, reminiscent of Li-rich spinels. Trends in the thermodynamic
stability of the new compounds are explained on the basis of crystal-chemistry
and density functional theory (DFT). Further DFT calculations examine
the relative stability of the CF versus spinel structures at various
degrees of sodium extraction in the context of electrochemical battery
reactions.