A Study of a Sodium Doped Prussian Blue Cathode Coupled with Sodium Rich and Deficient Transition Metal Oxide Anode

1. Introduction – Sodium batteries are being studied to overcome the drawbacks of Li ion based batteries viz. cost, poor cycle life of Li etc. [1]. Layered compounds are finding applications as anodes and cathodes as these are able to intercalate the large sized Na+ ion better [2]. Prussian blue and layered transition metal oxides are the compounds of choice to facilitate easy intercalation, increase cell performance and provide higher specific capacities [3, 4]. The TiO2 based anodic system is analyzed from different aspects to obtain the optimum system in this case.

2. Fabrication procedures – Fabrication process for the prussian blue cathode involves dissolution of 3 m mol of Na4Fe(CN)6.10H2O in 40 ml of 0.1 M conc. HCl solution followed by vigorous stirring at 500 rpm for 5 minutes. Once this is done, the resulting solution is dried at 80 °C for 20 hours.
For the anode, the crystalline TiO2 anatase and the amorphous TiO2 rutile powder are used as obtained. Na2Ti3O7 is obtained by heating 1g of TiO2 and 0.36 g of NaOH at 750 °C for 20 hours, after they were mechanically milled for 4 hours.

3. Experimental steps and results – The various characterization procedures are as follows:

(i) Analysis of the phase of the TiO2 and Na2Ti3O7 anode.

Figure 1 shows the XRD data of TiO2 (blue, bottom) and Na2Ti3O7 anodes (orange, top). The main peaks are seen and no impurities peaks are noticed, especially in the Na2Ti3O7 sample which indicate phase formation is complete.

(ii) Analysis of the Prussian blue cathode before and after the cell cycling to show the degradation of the cathode.

The graph in figure 2 represents the XRD data of the pristine Na-PB cathode (bottom, in blue) and the cathode after 10 cycles of operation (top, in orange). The post cycling XRD graph shows the presence of the peaks in the same position indicating very little structural damage.

(iii) Cell cycling is performed in a 2 electrode set-up within in an aqueous electrolyte in a voltage range 0 V to 0.7 V.

Figure 3 gives the specific capacity with respect to the voltage for the 1st, 2nd, 5th and 10th cycles respectively. The open circuit voltages can be seen to be approximately around slightly higher than 0.55 V and a maximum specific capacity of 76.52 mAh/g. It is also seen that the graphs are very close to one another which indicate very little capacity fading.

(vii) Comparison of the stability behavior of different TiO2 anodes.

Figure 4 shows the stability characteristic of the different TiO2 based anode systems. As can be seen, the Na2Ti3O7 system is the most stable whereas the amorphous system demonstrates the least stability.

4. Conclusion & future work – Sodium rich prussian blue system has been tried out as a positive electrode versus a transition metal oxide e.g. TiO2 as the negative electrode. The TiO2 system has been tried from different aspects (crystalline, amorphous, heat treated and heat treated) and the imporrtant structural aspects and cell performances have been discussed.
Future work would involve analysis if the corrosion mechanism at the electrode-electrolyte interface and study of the Prussian blue cathode in greater detail.

References:
[1] M.S. Islam, C.A.J. Fisher, Chemical Society Reviews, 43 (2014) 185-204.
[2] D. Kim, E. Lee, M. Slater, W. Lu, S. Rood, C.S. Johnson, Electrochemistry Communications, 18 (2012) 66-69.
[3] A.A. Karyakin, Electroanalysis, 13 (2001) 813-819.
[4] Y. Yue, A.J. Binder, B. Guo, Z. Zhang, Z.-A. Qiao, C. Tian, S. Dai, Angewandte Communications, 53 (2014) 3134-3137.