To understand the role of structure on the position of the octahedral Fe37Fe2 redox couple in compounds having the same polyanions, four iron phosphates: Li3Fe2(P04)3, LiFeP2O7, Fe4(P207)3, and LiFePO4 were investigated. They vary in structure as well as in the manner in which the octahedral iron atoms are linked to each other. The Fe3t/Fe2 redox couple in the above compounds lies at 2.8, 2.9, 3.1, and 3.5 eV, respectively, below the Fermi level of lithium. The reason for the difference in the position of the redox couples is related to changes in the P-O bond lengths as well as to changes in the crystafline electric field at the iron sites.

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The charge/discharge characteristics of NaTi 2 (PO 4 ) 3 as an anode active material for aqueous sodium-ion battery containing 2 M Na 2 SO 4 aqueous electrolyte were examined. Cyclic voltammograms, galvanostatic discharge/charge and XRD data of the material indicated that sodium can be reversibly intercalated into NASICON-type NaTi 2 (PO 4 ) 3 without serious degradation of the host structure. The best reversible capacity at rate of 2.0 mA cm À2 was 93% of the theoretical capacity of 133 mAh g À1 and the plateau voltage was 2.1 V versus Na/Na þ .

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