Ionic Conduction in Cubic Na<sub>3</sub>TiP<sub>3</sub>O<sub>9</sub>N, a Secondary Na-Ion Battery Cathode with Extremely
Low Volume Change
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Abstract
It is demonstrated that Na ions are
mobile at room temperature
in the nitridophosphate compound Na<sub>3</sub>TiP<sub>3</sub>O<sub>9</sub>N, with a diffusion pathway that is calculated to be fully
three-dimensional and isotropic. When used as a cathode in Na-ion
batteries, Na<sub>3</sub>TiP<sub>3</sub>O<sub>9</sub>N has an average
voltage of 2.7 V vs Na<sup>+</sup>/Na and cycles with good reversibility
through a mechanism that appears to be a single solid solution process
without any intermediate plateaus. X-ray and neutron diffraction studies
as well as first-principles calculations indicate that the volume
change that occurs on Na-ion removal is only about 0.5%, a remarkably
small volume change given the large ionic radius of Na<sup>+</sup>. Rietveld refinements indicate that the Na1 site is selectively
depopulated during sodium removal. Furthermore, the refined displacement
parameters support theoretical predictions that the lowest energy
diffusion pathway incorporates the Na1 and Na3 sites while the Na2
site is relatively inaccessible. The measured room temperature ionic
conductivity of Na<sub>3</sub>TiP<sub>3</sub>O<sub>9</sub>N is substantial
(4 × 10<sup>–7</sup> S/cm), though both the strong temperature
dependence of Na-ion thermal parameters and the observed activation
energy of 0.54 eV suggest that much higher ionic conductivities can
be achieved with minimal heating. Excellent thermal stability is observed
for both pristine Na<sub>3</sub>TiP<sub>3</sub>O<sub>9</sub>N and
desodiated Na<sub>2</sub>TiP<sub>3</sub>O<sub>9</sub>N, suggesting
that this phase can serve as a safe Na-ion battery electrode. Moreover,
it is expected that further optimization of the general cubic framework
of Na<sub>3</sub>TiP<sub>3</sub>O<sub>9</sub>N by chemical substitution
will result in thermostable solid state electrolytes with isotropic
conductivities that can function at temperatures near or just above
room temperature