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)101 in the 1 M LiNO3 solution under hydrothermal conditions at 200 °C. The fully Li+-exchanged sample Li9Fe7(PO4)102 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 °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. 1 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 → Fe0). 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.