Prussian Blue with Different Synthesis Temperature as Cathode for Sodium-Ion Batteries

A global priority is the development of low-cost, efficient storage of off-peak electric power and of electrical energy generated by energy sources other than fossil fuels. Rechargeable batteries are expected to replace the current technology utilizing their advantages of higher efficiencies and the possibility of simple installation [1 -2 ] . Na is readily available worldwide and therefore much less costly than Li. Therefore, there is a huge incentive to develop a rechargeable, low-cost Na-ion battery (SIB) of high energy density capable of discharge/charge at a high rate. A great number of researchers pay attention to Prussian blue(PB) and its analogues as sodium iron electrode material. Owing to the existence of Fe(CN) 6 vacancies occupied by coordinating H 2 O in the Prussian Blue framework, the electrochemical performance of PB for sodium iron battery is far from ideal situation. Here, we focus on the investigation of the electrochemical behavior of PB compounds prepared by simple precipitation method at different solution temperature. According to XRD analysis (Fig1), all samples showed sharp and strong peaks of a face-centered cubic structure. The calculated d-spacing of PB-a (0℃), PB-b (20℃) PB-c (60℃), and PB-d (80℃) were 10.22 Å, 10.25 Å, 10.32 Å and 10.38 Å. The galvanostatic profiles of PB-a, PB-b PB-c, and PB-d in the first cycle showed initial coulomb efficiency was 78.8%，75%，70.95% and 60.84% when measured at a rate of 100 mA g -1 . After the second cycle, the charge/discharge efficiency increased to 98.5%, and then remained steady at prolonged cycling. The PB-a, PB-b electrode delivered a capacity of 106 mAh g -1 , whereas PB-c and PB-d delivered a capacity of 96 mAh g -1 (100 mA g -1 ). It was obvious that the PB-a electrode displayed better cycling performance with capacity retention of 60.8 % after 400 cycles. PB-d electrode showed much faster capacity decrease, the discharge capacity remains only 40 mAh g -1 after 400cycles, which was 41% of the initial discharge capacity. (Fig2.) In addition, With increasing charge–discharge rate, there was a large difference in discharge capacity among the different electrode. The discharge capacity of PB-a , PB-b, PB-c, and PB-d electrode at 1 C rate could reach 117 mAh g -1 ,116 mAh g -1 , 105 mAh g -1 and 107 mAh g -1 ; Even when the current was elevated to a high rate of 8C, PB-a exhibited a good rate capability with 83.5 mAhg -1 , corresponding to 66 mAh g -1 of PB-d. In summary, the cathode material prepared in low temperature exhibited a better cycle life and rate capability, which provided a new insight into the industry material for applications of electric energy storage on a large scale. Acknowledgements This work was supported by the Major State Basic Research Development Program of China (2014CB239703), the National Natural Science Foundation of China (21573147, 21506123 and 21336003) References 1. B Dunn, H Kamath, J M. Tarascon, Science, 2011, 334(6058): 928-935. 2. Z G Yang, J L,Zhang, M C WKintner-Meyer, et al. Cheml revs, 2011, 111(5): 3577-3613. Fig. 1 XRD patterns of PBs synthesized at different temperatures Fig. 2 Cyclic performances of PBs (synthesized at different temperatures) between 2.0V~4.0V at a current density of 100 mA g -1 Figure 1