Ionic Conduction in Cubic Na₃TiP₃O₉N, a Secondary Na-Ion Battery Cathode with Extremely Low Volume Change

Abstract

It is demonstrated that Na ions are mobile at room temperature in the nitridophosphate compound Na 3 TiP 3 O 9 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 3 TiP 3 O 9 N 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 Na 3 TiP 3 O 9 N 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 Na 3 TiP 3 O 9 N and desodiated Na 2 TiP 3 O 9 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 3 TiP 3 O 9 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.

Improved Electrochemical Performance of NaVOPO4Positive Electrodes at Elevated Temperature in an Ionic Liquid Electrolyte

“…This is attributed to kinetic limitations, due to the high resistance in the particles, that hinder Na + access to their centers. 53 Other causes, such as the concentration polarization, may also account for the increased overpotential at the end of these curves. 54 The GITT curves at 363 K clearly show that the overpotential is suppressed in the full range of voltage as compared to that at 298 K, reflecting a lower kinetic barrier for Na + insertion into this material.…”

Recent Progress in Electrode Materials for Sodium‐Ion Batteries

“…[ 224 ] Approximately 60 mA h g −1 was delivered from the electrode with an average voltage of 2.7 V vs Na + /Na. Experimental and calculation studies indicate that the electrode operation involves a small volume change of 0.5%.…”

Structural and Na-ion conduction characteristics of Na3PSxSe4−x

“…Replacing the liquid electrolyte with an inflammable solid state electrolyte can, in principle, eliminate the safety concerns, offering a promising path forward to extend the application of batteries to devices where safety is of uttermost importance. The development of fast Na-ion conductors as solid state electrolytes is a key enabler for an all-solid-state Na-ion battery technology, which can be less expensive than its Li counterpart due to the wider availability of Na resources and broader choice of Naintercalation cathodes [1][2][3][4][5][6] . However, few Na-ion solid state conductors with conductivity approaching that of liquid electrolytes currently exist [7][8][9][10] .…”