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 Cs_9−xK_xFe₇(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 Cs_9−xK_xFe₇(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 Li_9+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.