New Iron (III) Hydroxyl-Phosphate with Rod-packing Structure as Intercalation Materials
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New Iron (III) Hydroxyl-Phosphate with Rod-packing Structure as Intercalation Materials Yanning Song, Peter Y. Zavalij and M. Stanley Whittingham Department of Chemistry and Institute for Materials Research, State University of New York at Binghamton, Binghamton, NY 13902-6016 ABSTRACT A new iron hydroxyl-phosphate, H2Fe14/3(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 Li2Fe14/3(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. INTRODUCTION Iron phosphate materials are pervasive in nature. They have long been used as catalyst in the synthesis of organic acids and in steel and glass industry. Recently, they are used as cathode materials in lithium batteries. They are advantageous because of the low-cost, environmentfriendly properties. With a NASICON-related-3D framework, Li3Fe2(PO4)3 [1,2] can be electrochemically inserted 1.8 lithium atoms per formula unit at 0.05 mA/cm2 (~116 mAh/g). And when the current density is increased to 1.0 mA/cm2, the capacity fades to 60 mAh/g. The more successful material is LiFePO4 with the olivine structure [3,4]. About 0.6 lithium atoms (~110 mAh/g) can be inserted into the structure at a current density of 2 mA/g. The excellent reversibility is due to the striking similarity of the charged and discharged materials. However, the capacity fades quickly also at higher current density due to the low diffusion rate of ions and electrons [5]. FePO4 is also investigated as a candidate [6,7]. Although the amorphous phase shows practical capacity, the possibility of commercialization has been limited by the low density and low voltage. Here we report our study on an iron hydroxyl phosphate with “rod-packing” structure. EXPERIMENTAL DETAILS Synthesis The reaction mixture for synthesis contains 0.001mol FeCl3 (J. T. Baker), 0.004 mol H3PO4 (Fisher) and 0.2 mol water. Methylamine (Aldrich) was used to adjust pH. The mixture was loaded in a Teflon-lined stainless steel autoclave and heated at 170 °C for 3 days. The resulting
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