Synthesis, Structure, and Na-Ion Migration in Na₄NiP₂O₇F₂: A Prospective High Voltage Positive Electrode Material for the Na-Ion Battery

In the recent hunt for novel Na-ion cathode hosts, a variety of sodium analogues of classic Li-ion structures have been thoroughly explored. However, Na-ion analogues generally possess modified structures and dissimilar Na-ion energetics compared to their Li-ion analogues due to the large size of Na⁺ (102 pm) vs Li⁺ (76 pm), often resulting in sluggish Na⁺ kinetics. Materials development targeted toward new and different specific host structures possessing optimum properties for Na-ion migration is crucial. Here, we report the first sodium metal fluoropyrophosphate Na-ion host with a three-dimensional frameworkNa₄NiP₂O₇F₂which is predicted to have a high voltage (∼5 V) based on its Ni^2+/3+/4+ redox couple and composition. Structure solution from single crystal diffraction data combined with atomistic simulation computation suggests the presence of low activation energy Na-ion migration pathways (<0.6 eV) in all three dimensions. The particularly low barrier of 0.36 eV calculated for migration along the [010] direction is in full accord with temperature dependent ionic conductivity measurements that yield an experimental value of 0.32 eV. Spacious Na-ion pathways endow the material with good ionic conductivity as determined by ac impedance spectroscopy, and facile exchange of three Na⁺ ions for Li⁺ is observed at slightly elevated temperatures. Furthermore, the polycrystalline material exhibits excellent thermal stability under ambient atmosphere up to 600 °C, crucial for the safe operation of a Na-ion battery