The new compound Na-1.86 square Fe-0.14(3)(PO4)(3) was successfully synthesized via hydrothermal synthesis and its crystal structure was determined using powder X-ray diffraction data. Na1.86Fe3(PO4)(3) was also characterized by operando XRD and Mossbauer spectroscopy, cyclic voltammetry, and galvanostatic cycling. Na1.86Fe3(PO4)(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 Fe2+ during synthesis. The electrochemical tests indicated that Na1.86Fe3(PO4)(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, Na1.86Fe3(PO4)(3) exhibited good sodium removal and uptake behavior although no optimizations of particle size, morphology, and carbon coating were performed. (C) 2016 Elsevier B.V. All rights reserved.