New iron (III) hydroxyl-phosphate with rod-packing structure as intercalation materials

A new iron hydroxyl-phosphate, H₂Fe_14/3(PO₄)₄(OH)₄ has been synthesized under hydrothermal conditions. In this compound, perpendicular chains formed by the face-sharing FeO₆ form rod-packing structure. Only about 60% of the chain sites are occupied by iron atoms; other metals, such as manganese, nickel, zinc, can be incorporated into the chain either by filling in the vacancies and/or replacing some of the iron atoms. Reversible insertion and extraction of lithium into this compound shows it to be an excellent cathode material. At current density of 0.1 mA/cm², 90% of the theoretical capacity (176 mAh/g) can be obtained. The utilization was reduced to about 70% on a ten-fold increase of current density. The electrochemical behavior is attributed to the 3-dimensional rod packing structure, where lithium can move freely even at high current densities inside the 3-dimensional framework without altering the host structure. Two of the protons in the lattice may be exchanged by lithium giving Li₂Fe_14/3(PO₄)₄(OH)₄. These lithium atoms are not removable in electrochemical cycling and similar electrochemical property was found for these two compounds, suggesting an ion-exchange process for the lithiation.