Unveiling the sodium intercalation properties in Na1.86□0.14Fe3(PO4)3

Abstract The new compound Na 1.86 □ 0.14 Fe 3 (PO 4 ) 3 was successfully synthesized via hydrothermal synthesis and its crystal structure was determined using powder X-ray diffraction data. Na 1.86 Fe 3 (PO 4 ) 3 was also characterized by operando XRD and Mossbauer spectroscopy, cyclic voltammetry, and galvanostatic cycling. Na 1.86 Fe 3 (PO 4 ) 3 crystallizes with the alluaudite-type structure with the eight coordinated Na1 and Na2 sodium atoms located within the channels. The combination of the Rietveld- and Mossbauer-analyses confirms that the sodium vacancies in the Na1 site are linked to a partial oxidation of Fe 2+ during synthesis. The electrochemical tests indicated that Na 1.86 Fe 3 (PO 4 ) 3 is a 3 V sodium intercalating cathode. At the current densities of 5, 10, and 20 mA g −1 , the material delivers the specific capacities of 109, 97, and 80 mA h g −1 , respectively. After 100 charge and discharge cycles, Na 1.86 Fe 3 (PO 4 ) 3 exhibited good sodium removal and uptake behavior although no optimizations of particle size, morphology, and carbon coating were performed.