| Na-ion batteries have received particular attention as a promising alternative to their lithium counterparts for large-scale electric energy storage, due to their potential advantage of abundant resources, low cost and environmental friendliness. However, developing suitable host materials for Na insertion reactions still remains a great challenge because Na ions have much larger radius than Li ions. In general, Prussian blue (PB) compounds can be utilized as high capacity and long cycle cathode materials for Na-ion batteries owing to their typical large-open frameworks and abundant Na-insertion sites. Nevertheless, their electrochemical performances reported so far are far from satisfactory because of their structural imperfection and lack of redox centers. This thesis was aimed at exploring the relationship between lattice structure and Na-storage performance of PB materials, optimizing their electrochemical properties by suppressing structural defects and improving crystal integrity, and using them for aqueous and organic electrolyte Na-ion batteries. The main research results and conclusion of this thesis are listed as follows:1. To solve the problem of Na-deficiency in the lattice, we prepared three kinds of Na-rich PB compounds such as Na2NiFe(CN)6, Na2CuFe(CN)6 and Na3Zn2[Fe(CN)6]2 by using Na4Fe(CN)6 as reaction precursors, and investigated their electrochemical Na-insertion behavior in non-aqueous electrolytes. The experimental results demonstrate that Na2NiFe(CN)6 exhibits good Na-storage performance with a reversible capacity of 67 mAh g-1 corresponding to one Na ion insertion per PB unit, a high capacity utilization of 70% at 10 C (1C=60 mA g-1) rate, as well as superior cyclability of 97% capacity retention over 500 cycles at 2C rate. Ex situ XRD tests show that the large-open PB framework can effectively alleviate the lattice strain during the repeated Na insertion/extraction reactions,thus benefiting the long-term cycle stability. Similarly, Na2CuFe(CN)6 and Na3Zn2[Fe(CN)6]2 also demonstrate good Na-storage capability, with a discharge capacity of 80 and 65 mAh g-1 respectively, and impressive capacity retention of>90% over 300 cycles. This work preliminarily revealed the high rate and long cycle ability of Prussian blue frameworks in non-aqueous electrolyte.2. To improve the low capacity utilization and poor cyclability resulted from the Fe(CN)6 vacancies and poor crystalline of PB materials, we fabricated low defect and high crystalline FeFe(CN)6 by a slow precipitation method, and explored its electrochemical performance in non-aqueous electrolyte. Experimental results show that FeFe(CN)6 material contains only 6% Fe(CN)6 vacancies and demonstrate highly reversible 2-Na insertion behavior. This material can realize a high capacity of 136 mAh g"1 by using [Fe(CN)6]3-/[Fe(CN)6]4- and Fe3+/Fe2+ couple, corresponding to 1.5 Na ions transfer per unite and 75% capacity utilization. The cycle performance of this material is also superior, with a capacity retention of 92% over 400 cycles at 2C rate (1C=135 mA g-1) and a coulombic efficiency of -100%. As comparison, the defect rich and poor crystalline Fe4[Fe(CN)6]3 material suffers from low capacity utilization (37%), unstable capacity retention of 73% over 20 cycles, and low coulombic efficiency of 71-98%. This work revealed the relationship between Fe(CN)6 vacancies and Na-storage performance of Prussian blue framework, thus providing a new approach to prepare high capacity and long cycle PB cathode materials.3. On the above-mentioned results, we proposed a facile and generalized method to control the crystallization speed and suppress Fe(CN)6 vacancies of Prussian blue compounds by taking advantage of coordination effects between transition metal ions and complex agents. The experimental results demonstrate that with the help of complex agents, the crystallization speed can be greatly slowed down with the significant inhibition of Fe(CN)6 vacancies and improvement of crystal integrity. Three kinds of Na-rich Prussian blue materials have been synthesized such as Na2CoFe(CN)6, Na2FeFe(CN)6 and Na2MnFe(CN)6. Electrochemical characterizations show that their electrochemical performance has been greatly improved. Na2CoFe(CN)6 delivers a high capacity of 150 mAh g-1, with a coulombic efficiency of?100% and capacity retention of 90% over 200 cycles. Na2FeFe(CN)6 gives a high capacity of 120 mAh g-1 and sustains 90% capacity retention over 300 cycles. Besides, Na2MnFe(CN)6 also has a high capacity of 130 mAh g-1, and maintains a capacity retention of 70% over 70 cycles. This research demonstrate that controlled crystallization method plays an important role in suppressing Fe(CN)6 defects and improving crystal integrity of PB compounds, thus greatly enhancing the 2-Na reaction reversibility of PB frameworks. Besides, this facile method is beneficial for the extended applications of PB materials in energy storage and other fields.4. Taking into account the much safer, greener, and cheaper benefits of aqueous electrolyte, we investigated the Na-storage performance of Na2NiFe(CN)6, Na2CuFe(CN)6, FeFe(CN)6 and Na2CoFe(CN)6 in aqueous Na2SO4 solution, and assembled several high rate and long cycle aqueous Na ion full cells by using NaTi2(PO4)3/C as anode material. The experimental results show that the specific capacity of Na2NiFe(CN)6 in aqueous solution is 59.4 mAh g-1, approaching its value in non-aqueous electrolyte. The Na2NiFe(CN)6-NaTi2(PO4)3 full cell system exhibits superior electrochemical performance, with an average voltage of 1.25 V, an energy density of 42.5 Wh kg-1, a good capacity utilization of 42%at very high rate of 50C (1C=35 mA g-1), and impressive capacity retention of 90% over 1,000 cycles at IOC rate. The reversible capacity of Na2CuFe(CN)6 is 58.5 mAh g-1 and the Na2CuFe(CN)6-NaTi2(PO4)3 full cell delivers an operation voltage of 1.4 V and an energy density of 48 Wh kg-1, along with a capacity utilization of 50% at a high rate of 100C (1C=35 mA g-1) and excellent capacity retention of 87% over 1,000 cycles. FeFe(CN)6 can give a high capacity of?125 mAh g"1, but its Na-deficient nature makes it hard to couple with conventional anode to assemble full cells. Finally, Na2CoFe(CN)6 can realize a high capacity of 128 mAh g-1, and the Na2CoFe(CN)6-NaTi2(PO4)3 full cell has an operation voltage of 1.4 V and an energy density of 67 Wh kg-1. This work shows that Prussian blue materials can demonstrate superior electrochemical Na-storage properties in aqueous electrolyte, and can facilitate the implementation of high rate and long cycle aqueous Na-ion full cells, thus providing new choice and strategy for large scale electric energy storage. |
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