Ion exchange and electrochemical evaluation of the microporous phosphate Li9Fe7(PO4)10

A new lithium iron(III) phosphate, Li9Fe7(PO4)(10), has been synthesized and is currently under electrochemical evaluation as an anode material for rechargeable lithium-ion battery applications. The sample was prepared via the ion exchange reaction of Cs5K4Fe7(PO4)(10) 1 in the 1 M LiNO3 solution under hydrothermal conditions at 200 degrees C. The fully Li+-exchanged sample Li9Fe7(PO4)(10) 2 cannot yet be synthesized by conventional high-temperature, solid-state methods. The parent compound 1 is a member of the Cs9-xKxFe7(PO4)(10) series that was previously isolated from a high-temperature (750 degrees C) reaction employing the eutectic CsCl/KCl molten salt. The polycrystalline solid 1 was first prepared in a stoichiometric reaction via conventional solid-state method then followed by ion exchange giving rise to 2. Both compounds adopt three-dimensional structures that consist of orthogonally interconnected channels where electropositive ions reside. It has been demonstrated that the Cs9-xKxFe7(PO4)(10) series possesses versatile ion exchange capabilities with all the monovalent alkali metal and silver cations due to its facile pathways for ion transport. I and 2 Were Subject to electrochemical analysis and preliminary results suggest that the latter can be considered as an anode material. Electrochemical results indicate that Li9Fe7(PO4)(10) is reduced below 1 V (vs. Li) to most likely form a Fe(0)/Li3PO4 composite material, which can Subsequently be cycled reversibly at relatively low potential. An initial capacity of 250 mAh/g was measured, which is equivalent to the insertion of thirteen Li atoms per Li9+xFe7(PO4)(10) (x= 13) during the charge/discharge process (Fe2+ + 2e -> Fe-0). Furthermore, 2 shows a lower reduction potential (0.9 V), by approximately 200 mV, and much better electrochemical reversibility than iron(III) phosphate, FePO4, highlighting the value of improving the ionic conductivity of the sample. (c) 2008 Elsevier Ltd. All rights reserved.