Study of Potassium-Rich Prussian Blue and MoO3 Aqueous Secondary Cell with Nanoscale TiO2 Coatings

1. Introduction – Lithium-ion batteries are regarded as the current benchmark for new battery designs. While lithium-ion batteries offer both high energy densities and large cell potentials, they also possess many inherent disadvantages such as flammability and high material costs [1, 2]. Potassium-ion (K+) batteries can be of great environmental and economic importance. Potassium possesses an oxidation/ reduction potential closest to lithium among all alkali metals, only 0.12 V lower than that of lithium [3]. Furthermore, nanoscale coatings of transition metal oxides are known to behave as inhibitors to cell capacity degradation [4].

2. Fabrication procedures – Prussian blue (Fe4[Fe(CN)6]3) cathode is fabricated by dissolution of 4 mmol K4Fe(CN)6•3H2O in 50 ml of 0.1 M HCl solution followed by vigorous mixing at 500 rpm at 80 °C overnight until dry. Resulting powder is ground, washed and separated by centrifugation and at 80 °C overnight. α MoO3 anode is fabricated by titration of 0.7 mmol (NH4)6Mo7O24•4H2O in 10 ml of 5 M (NH4)2CO3 with Nitric acid until a white precipitate is formed. Solution is aged overnight, dried at 80 °C, washed with HCl solution and separated by centrifugation, and then annealed at 500 °C for 20 hrs. Electrolyte of 0.5 M aqueous KPF6 is used. Atomic Layer Deposition technique is used to deposit thin TiO2 films of 5 and 10 nm on the Prussian blue cathode surface.

3. Experimental setup and results –XRD and SEM is done to study the crystallinity and morphology of both pristine and TiO2 coated cathodes. XRD spectrum of Prussian blue cathode and α-MoO3 anode is shown in Figure 1. Cycling performance is studied using a charge/discharge current of 2 mA and 0.5 mA respectively and voltage range of 0 to 1 V. Discharge capacity and capacity retention performance of aqueous cells of pristine, 5 nm and 10 nm TiO2 coated Prussian blue cathodes and α MoO3 anode is shown in Figures 2 and 3 respectively.

4. Conclusions and future work – A decrease in initial discharge capacity and increase in capacity retention is observed for cells utilizing a TiO2 coated cathode. Initial discharge capacities of the 5 nm and 10 nm TiO2 coated cathodes are observed to be less than that of the pristine cathode. After 10 cycles all three cell configurations are observed to approach approximately the same discharge capacity. Both TiO2 coated cathode materials show considerable increase in capacity retention with respect to the pristine cathode. Future work aims to increase cell capacity by post-fabrication modification of anode and cathode. Post-cycling changes in electrode structure and morphology are also of interest.

5. References –
[1] D. Lisbona, T. Snee, Process Safety and Environmental Protection, 89 (2011) 434-442.
[2] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Advanced Functional Materials, 23 (2013) 947-958.
[3] A. Eftekhari, Journal of Power Sources, 126 (2004) 221-228.
[4] X. Li, J. Liu, X. Meng, Y. Tang, M.N. Banis, J. Yang, Y. Hu, R. Li, M. Cai, X. Sun, Journal of Power Sources, 247 (2014) 57-69.