Divalent Iron Nitridophosphates: A New Class of Cathode Materials for Li-Ion Batteries

L transition metal phosphates, especially olivine LiFePO4, have been found to be promising rechargeable Li-ion battery cathode materials for many applications since they can safely deliver a good energy density utilizing only earth-abundant materials. However, the olivine materials have limitations which include a poor electrical conductivity and one-dimensional (1D) Li-ion diffusion pathways that can be easily blocked. The search for next-generation phosphate materials has broadened to include transition metal fluorophosphates which incorporate a second anion moiety of F− and which have good electrochemical performance and favorable intrinsic properties such as 2D ion diffusion channels and small volume changes during ion removal and insertion. Here we demonstrate the design of a battery cathode material incorporating N3− anions as a distinct structural building block. While the low mass/charge ratio of N3− is not surpassed by any other anionic group used in battery applications, the voltages previously measured for transition metal nitrides and oxynitrides were low and thus only suitable for anode applications. Electrodes with N3− which can deliver suitable voltages for cathode applications have never before been demonstrated. In the present work, it is shown that A2M II 2P3O9N compounds with M II = Fe and A = Li can be utilized as cathodes in rechargeable Li-ion batteries. In this structure type, N3− anions are present in the form of nitridophosphate PO3N tetrahedra that link together MO6 octahedra. The compound Na2Fe2P3O9N (Figure 1) belongs to a class of known compounds with general formula A2M II 2P3O9N which are known to exist for A = Na and for M = Mg, Fe, Mn, and Co. Compounds with A = Li are clearly of interest for battery applications but have not previously been prepared and could not be directly synthesized under conditions that we have tested due to the preferential formation of olivine LiFePO4, although methods for producing Li-containing nitridophosphate compounds through ion exchange will be demonstrated. Powders of Na2Fe2P3O9N were synthesized by reacting stoichiometric amounts of NaPO3, Fe2O3, and (NH4)2HPO4 at 625 °C under flowing NH3 (50 mL/min) in an open-ended quartz boat for 40 h with one intermediate grinding, similar to prior studies. The full crystal structure of Na2Fe2P3O9N was determined from the Rietveld refinement of high resolution synchrotron X-ray diffraction data (Figure 2a) and time-offlight neutron diffraction data (Supporting Information, Figure S1). The expected primitive cubic phase (P213, No. 198) was observed with a lattice parameter of 9.3459(1) A, substantially smaller than the 9.40 A reported originally. The structure of 2:2 Na2Fe2P3O9N shares the same framework of tetrahedral trimeric P3O9N 6− anionic blocks as 3:1 Na3MP3O9N analogues (M III = Ti, V, etc.). Although these two structural variants both maintain charge balance for the anionic groups using four cations that are found at the same crystallographic sites, they differ in their ratio of monovalent to multivalent cations (2:2 or 3:1). The additional divalent Fe cation in Na2Fe2P3O9N occupies an octahedral site that corresponds to the Na2 position in the 3:1 structure type. As a result, the two FeO6 octahedra in Na2Fe2P3O9N share a face, as illustrated in Figure 1. This close proximity of the two Fe ions (∼3.0 A) is expected to strongly influence redox potentials both through electrostatic repulsions and through potentially contradictory bond length preferences for the three shared oxygen ligands that occur when two Fe ions of different valence are found in a single dimer. Full crystallographic parameters (Tables S1−S5) and important bond distances (Table S6) are provided in the Supporting Information.