Investigation of Iron Hexacyanoferrate as a high rate cathode for aqueous batteries

Investigation of Iron Hexacyanoferrate as a high rate cathode for aqueous batteries: Sodium-Ion Batteries and lithium-ion Batteries
Authors: Qin Yang, Wei Wang, Hai Li, Juan Zhang, Feiyu Kang and Baohua
Engineering laboratory for next generation Power and Energy storage batteries, Graduate School at Shenzhen, Tsinghua university, Shenzhen, 518055, China. School of materials science and Engineering, Tsinghua University, Beijing, 100084, China. Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong
Journal page number: 8 pages
Introduction: With the increasing needs of our complex lives, batteries are needed to be relatively cheap. When compared to conventional traditional batteries have some limitations (low efficiency, limited depth of discharge). Lithium ion batteries are too expensive, and the lead component is a pollutant. Extensive research and studies have shown that hexacyanoferrate (AxMFe(CN)6.mH2O) has a good rate performance and cycle life, where A represents and Alkali metal and M a transition metal. Besides its non-toxicity and low cost, it’s a suitable material for large scale energy storage. Iron Hexacyanoferrate(FeFe(CN)6), has been considered as a good compound for high power batteries, due to its large reservation, low cost and non-toxicity. Results has shown that hexacyanoferrate always exhibits low efficiency and poor cycle life. This is claimed to be result from the occupied water in its intrinsic crystalline structure, which could hamper the ion storage and induce the lattice distortion or even the destruction of Fe-C N-Fe bridge. FeFe(CN)6 was mainly applied to develop sodium ion batteries instead of lithium ion study, that FeFe(CN)6 exhibits a good stability of Na+ insertion and extraction but has extremely poor performance for Li+ insertion and extraction. FeFe(CN)6 with a crystalline structure has been synthesized and electrochemical performance in aqueous solution of Na+ and LI+ has been investigated.

Experimental: There are two processes of the study of Iron Hexacyanoferrate. Step 1 is the preparation of iron Hexacyanoferrate. The FeFe(CN)6 crystals were synthesized via precipitation. 50mL K3Fe9CN)6 (0,1M) was added slowly to 100mL FeCl3(0,1M), with continuous stirring. The speed of the peristaltic pump was 400?L/min. The mixture was heated to 60 ?C for 6 hours, with continuous stirring. The colour changed from dark brown to dark green. The mixture was then filtered and washed with deionized water and ethanol. FeFe(CN)6 precipitates are dried under 60 ?C overnight in a vacuum. A dark green powder is the obtained. Step 2 is the electrochemical measurements. An electrode was prepared by casting slurry of FeFe(CN)6: Carbon Black: Polyvinylidene fluoride(PVDF) , this has a ratio of 7:2:1 in N-Methyl-2-pyrrolidinone(NMP) on a glassy carbon electrode. It is then dried at 60 ?C overnight in a vacuum. The performance was detected using three electrodes: NaNO3(1M), LiNO3(1M), a platinum counter electrode and Ag/AgCl reference electrode.
Material Characterization: High-resolution transmission electron microscopy (HR-TEM) was employed to view the morphology and a detailed structure of FeFe(CN)6. To obtain a clear lattice the particles with a thickness of 50nm are embedded in epoxy resins for ultrathin sectioning. X-Ray diffraction patterns are also done before and after insertion/extraction of Alkali ions.

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Results and Discussion: A SEM image of FeFe(CN)6 crystals revealed an octahedral structure with the size of ˜500nm. XRD peaks indicate that it is pure faced-centred cubic structure with a lattice parameter of 10,18A. Sharp and Diffraction peaks show the structure regularity. There are large interstitial sites which provide diffusion paths. The insertion/extraction of Na ions and Li ions are investigated via cyclic voltammetry. The cathodic and anodic peaks increased with an increase of the scan rate and the shape of the redox peaks remain in its original shape with little variation, this indicates relatively low polarization. The redox peaks are stable in NaNO3. FeFe(CN)6 CV curves in NaNO3 proved to have better symmetry under high scan rates than FeFe(CN)6 in LiNO3. LiNO3 shows multiple-steps for Li extraction. These Oxidation peaks show that Lithium are in the large open sites and not in the interstitial sites. Li ions and Na ions undergo diffusion to help with further investigation.

Conclusion: Crystalline FeFe(CN)6 was synthesized via co-precipitation. FeFe(CN)6 shows a good reversible redox reaction. Multiple-steps are revealed for the extraction of Li ions. Poor electrochemical performance is due to the large radii of hydrated Li ions and the location of the Li ions. After Li ions insertions an outer petaloid like structure forms and after Na ions are inserted a cubic like structure remains.

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