Ionic conduction in cubic Na₃TiP₃O₉N, a secondary Na-ion battery cathode with extremely low volume change

It is demonstrated that Na ions are mobile at room temperature in the nitridophosphate compound Na₃TiP₃O₉N, with a diffusion pathway that is calculated to be fully three-dimensional and isotropic. When used as a cathode in Na-ion batteries, Na₃TiP ₃O₉N has an average voltage of 2.7 V vs Na⁺/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⁺. 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 Na₃TiP ₃O₉N is substantial (4 × 10^-7 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 Na₃TiP₃O₉N and desodiated Na₂TiP₃O₉N, 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 ₃TiP₃O₉N by chemical substitution will result in thermostable solid state electrolytes with isotropic conductivities that can function at temperatures near or just above room temperature.