Na-Ion storage in iron hydroxide phosphate hydrate through a reversible crystalline-to-amorphous phase transition

Iron(iii) hydroxide phosphate hydrate Fe-1.13(PO4)(OH)(0.39)(H2O)(0.61)is investigated for the first time as a Na-ion battery cathode, which reveals that the material exhibits similar storage capacities for Na- and Li-ions at relatively low current rates (i.e.C/10). Interestingly,operandoX-ray diffraction shows that insertion of Na-ions induces a solid solution transition in the crystalline Fe-1.13(PO4)(OH)(0.39)(H2O)(0.61)end-member simultaneously with a major amorphization. This result adds to the series of observations of phosphate-based materials undergoing order-disorder transitions during Na-ion storage. Fe-1.13(PO4)(OH)(0.39)(H2O)(0.61)is thus ideal for enhancing our knowledge on such phenomena. To this end, using total X-ray scattering with pair distribution function analysis, we show that the amorphous phase is Na-rich NaxFe1.13(PO4)(OH)(0.39)(H2O)(0.61)with the local [FeO6]-[PO4] motif retained but with coherence lengths of onlyca.0.6 nm. Our investigation also reveals that the crystallinity of Fe-1.13(PO4)(OH)(0.39)(H2O)(0.61)is regained upon Na-extraction (battery recharge),i.e.the order-disorder transition is reversible.