Preparation And Electrochemical Performance Of Prussian Blue Based Cathodes For Sodium Ion Batteries

Energy storage and consumption has pushed forward the development of sodium ion batteries (SIBs), an alternative of lithium ion batteries (LIBs), which share similar electrochemical behaviors as LIBs. Since sodium possesses more earth reserves and lower usage cost as compared to lithium, developing high performance SIBs will exert profound effects on the applications of large-scale energy storage systems and smart electric grids in future. Currently, it still stands the beginning step for the research of SIBs, while the electrode materials keep as the bottleneck which restricts the advance of SIBs. The exploration of high performance electrode materials is vital for the practical applications of SIBs. A new family of sodium transition metal cyanides, such as Prussian blue (PB) and its analogues, are particularly attractive because of its unique 3D tunnel structure that may facilitate the Na+ mobility. The thesis mainly focuses on the synthesis, modification and electrochemical characterization of this kind of high performance cathode materials.Chapter 1 includes a general introduction about the working principle and development status of SIBs. After making a generalized summarization for the anode materials, a particular summary on the research status of cathode materials of SIBs is provided.In chapter 2, the experimental reagents, equipments and methods used in this dissertation are introduced, followed by a detailed procedure on assembling a 2025 coin cell, the material characterization methods and electrochemical technology for SIBs.In chapter 3, the FeHCF with the face-centered cubic structure of typical PB was synthesized by a facile co-precipitation method. The electrochemical performance of FeHCF can be greatly improved through the incorporation of nickel ions into FeHCF structure. The effect of nickel content on the structure, morphology and cycle performance was investigated in detail. The most satisfactory results are obtained when the proportion of nickel is 20 at%. Through the incorporation of nickel ions into PB structure, the low-spin Fe2+/Fe3+ couple is sufficiently activated for sodium storage, which leads to higher capacity contribution at larger potential and more stable redox energy. Moreover, FeNiHCF(20%) is better crystallized with less water content compared with single metal hexacyanoferrate of FeHCF. As a result, the FeNiHCF(20%) cathode exhibits synergistic advantages of high capacity, excellent cycling stability, superior rate performance and good Coulombic efficiency. A high discharge capacity of 106 mAh g-1, a Coulombic efficiency of～97% and an excellent capacity retention of 96% after 100 cycles are simultaneously achieved at a current density of 10 mA g-1. In addition, the rate capability of the FeNiHCF is also greatly enhanced with the capacity of 71 mAh g-1 and approaching 100% capacity retention at a high current density of 500 mA g-1.In chapter 4, in-situ composited PB with Ketjen Black (PB@C) cathode material were prepared by single iron-source method. PB particles disperse into the high electric conductive chains of KB particles which offer channels for electrons’motion and ensure the conductivity of the composite electrode. What’s more, compared to the ex-situ composited PB electrode, PB particles in the PB@C composited electrode have a much smaller size, which shortens the pathway of sodium ions in the materials and provides large surface area for the electrochemical reactions. Thus, excellent cycling stability in long cycles and perfect rate performance were obtained in the PB@C composited electrode. At a high current density of 2 A g-1, the maximum specific discharge capacity is 100 mAh g-1, and a retention rate of 90% was achieved even after 2000 cycles. When tested at different current densities for the rate performance, a capacity of 77.5 mAh g-1 can be achieved even at a ultrahigh current density of 9 A g-1. Furthermore, the full cell with the PB@C as the cathode and hard carbon as the anode can exhibit 120 mAh g-1 at a large current density of 100 mA g-1.Finally, in chapter 5, an overview of the dissertation is summarized. Some prospects and suggestions on the possible future research are presented.