Sodium intercalation in the phosphosulfate cathode NaFe2(PO4)(SO4)2

The compound NaFe2(PO4)(SO4)(2) is successfully synthesized via a solid state reaction route and its crystal structure is determined using powder X-ray diffraction data. NaFe2(PO4)(SO4)(2) phase is also characterized by cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy. NaFe2(PO4)(SO4)(2) crystallizes with the well-known NASICON-type structure. SAED and HRTEM experiments confirm the structural model, and no ordering between the PO4-3 and SO4-2 polyanions is detected. The electrochemical tests indicate that NaFe2(PO4)(SO4)(2) is a 3 V sodium intercalating cathode. The electrical conductivity is relatively low (2.2 x 10(-6) Scm(-1) at 200 degrees C) and the obtained activation energy is similar to 0.60eV. The GITT experiments indicate that the diffusivity values are in the range of 10(-11)-10(-12)cm(2)/s within the measured sodium concentrations.