Iron Cyanides As a Cathode Material for Lithium-Ion Batteries

Lithium ion battery is one of the most popular power sources for clean and efficient electrochemical energy storage and conversion. There are increasing interests for both the cathode and anode, in accordance with the development of portable electronic equipments, and automotive industry. As one of the potential alternatives, organic cathode¹has advantages of low cost, environmentally friendly, easy fabrication compared to  the inorganic cathodes. However, the low voltage plateau and poor cycle performance restrict its applications.

         Iron cyanide and its analogs² have been pursued as attractive materials for fundamental research and large scale industrial applications since its electrochemical activity was first reported in 1978. These zeolite-like compounds have perfectly 3D polymeric frameworks due to the asymmetric CºN anion. Its large open spaces can not only accommodate transition metal ions but also some small molecules. Redox reaction can also take place there thanks to the iron valence state for charge balance. These characteristics exactly offer the tunnels and spaces for the transport and storage of lithium ions.

Fe₄[Fe(CN)₆]_3³ is the foretype of prussian blue, its theoretical specific capacity is about 100 mAh/g with one charge transport of  Fe²⁺/Fe³⁺. Here we synthesize this compound by a simple one-step precipitation method. The cell shows an initial specific capacity of 95.1 mAh/g and about 75% of its original capacity is retained after 50 cycles. In order to increase the specific capacity, Fe²⁺  was replaced with Fe³⁺ to provide more redox active sites. The as-prepared FeFe(CN)₆ has a regular cubic structure with the particle size of 50 nm. Two redox peaks at 2.88V/3.19V and 3.81V/3.91V are observed by cyclic voltammetry, corresponding to the oxidation/reduction of the low spin Fe²⁺/Fe³⁺ adjacent to the carbon and the high-spin of Fe²⁺/Fe³⁺bonding to the nitrogen, respectively. Galvanostatic charge/discharge cycling shows that its initial discharge capacity is 137.3 mAh/g, and the capacity retention is 60% after 100 cycles. In addition, its rate performance is very excellent.

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3.             N. Imanishi, T. Morikawa, J. Kondo, R. Yamane, Y. Takeda, O. Yamamoto, H. Sakaebe and M. Tabuchi, J Power Sources, 1999, 81, 530-534.