Exploration\nof Oxyfluoride Frameworks as Na-ion Cathodes

Na-ion batteries (NIBs) are increasingly considered as\na viable\nalternative to Li-ion batteries due to the abundance, low cost, and\nthermal stability of Na-based systems. To improve the practical utilization\nof NIBs in applications, it is important to boost the energy and power\ndensities of the electrodes being used by the discovery of novel candidate\nmaterials. Thus, we explore the chemical space of transition metal-containing\noxyfluorides (TMOFs) that adopt a perovskite structure as possible\nNIB electrodes. Our choice of the perovskite structure is motivated\nby the “large” cationic tunnels that can accommodate\nNa<sup>+</sup>, while the chemistry of TMOFs is motivated by the high\nelectronegativity and inductive effect of F<sup>–</sup>, which\ncan possibly lead to higher voltages. We use density functional theory-based\ncalculations to estimate the ground state polymorphs, average Na (de)intercalation\nvoltages, thermodynamic stabilities, and Na<sup>+</sup> mobility on\ntwo distinct sets of compositions: the F-rich Na<sub><i>x</i></sub>MOF<sub>2</sub> and the O-rich Na<sub>1+<i>x</i></sub>MO<sub>2</sub>F, where <i>x</i> = 0–1 and M = Ti,\nV, Cr, Mn, Fe, Co, or Ni. Upon identifying the ground state polymorphs\nin the charged compositions (i.e., MOF<sub>2</sub> and NaMO<sub>2</sub>F), we show that F-rich perovskites exhibit higher average voltages\ncompared to those of the O-rich perovskites. Also, we find six stable/metastable\nperovskites in the F-rich space, while all the O-rich perovskites\n(except NaTiO<sub>2</sub>F) are unstable. Finally, our Na-ion mobility\ncalculations indicate that TiOF<sub>2</sub>–NaTiOF<sub>2</sub>, VOF<sub>2</sub>–NaVOF<sub>2</sub>, CrOF<sub>2</sub>, and\nNaMnOF<sub>2</sub> can be promising compositions, albeit with challenges\nto be resolved, for experimental exploration as NIB cathodes. These\noxyfluoride compositions can be promising if used primarily in a\nstrained electrode configuration and/or in thin film batteries. Our\ncomputational approach and findings provide insights into developing\npractical NIBs involving fluorine-containing intercalation frameworks.