Research On Prussian Blue Analogues As Cathode Materials For Sodium Ion Batteries

Due to limited storage in world and negative impact on environment of coal and oil, the need of clean energy is increasingly high for persistent power supply. In order to solve discontinuous and instable problems of novel energy(such as solar energy, wind energy and tidal power), energy storage systems(ESSs) can be used. Among a variety of batteries, lithium ion batteries(LIBs) with a superior performance in various electrochemical aspects are often viewed as energy storage device. However, limited resource and high price of lithium salts restrict its application in EEs. Sodium ion batteries(SIBs) have received considerable attention from the community owing to their high performance and potential applications in ESSs. Because of abundant reserves and higher redox potentials of sodium, the sodium-ion batteries are low-cost, environmentally friendly and safe. The biggest challenge for developing sodium ion battery is the bad performance of cathode materials. Cathode materials of SIBs have been widespread studied for accelerating the commercialization of SIBs, which is the critical aspect of high rate capability and cycling stability in a whole cell. Na-rich type Prussian blue and its analogues can provide three-dimensional framework structure and large enough channels for Na+ ions insertion/extraction. This material firstly reported by Goodenough et.al, and then was further investigated even for aqueous sodium ion batteries.However, many negetive influence factors lead to the poor electrochemical performance of PBAs. Such as the presence of a large number of [Fe(CN)6] vacancies in material, which may cause the structure collapse during the process of Na+ ions insertion/extraction. In addition, the presence of coordinating water can also destroy the performance of material. In the final analysis, the main problem comes from the material itself due to different transition metal elements have their own shortcomings.Different transition metal elements have beem used for synthetizing Prussian blue analogues, such as Mn HCF, Co HCF, Ni HCF, Cu HCF and Zn HCF. Their initial discharge capacity can reach 117.8, 123.9, 62.8, 78.0 and 49.2 m Ah g-1. After 50 cycles, the electrodes can retain 67.1%, 70.5%, 91.7%, 70.0% and 86.4% of initial capacity, respectively. Besides, the coulombic efficiencies of Mn HCF and Co HCF are very poor, the other materials both can remain at 100%. Except for Zn HCF, the other materials both belong to Fm3 m space group. These Prussian blue analogues have a large range of voltage platforms between 2.7-3.7V, among them, the redox voltage of [Fe(CN)6]3-/4- corresponds to the 3.0-3.3V.Herein, we report a new class of PBAs structure by simply replacing the transition metal ion with Ti3+, and the newly compound was named as sodium titanium hexacyanoferrate(NTH). The XRD pattern of sample reveals a well-defined crystal structure of face-centered cubic structure with Fm3 m space group. The calculated lattice constants of NTH-60 is a=b=c=9.29 ?,α=β=γ=90°. The color of the precipitate changed from green to blue with the temperature increasing because the Ti3+ ions occurred hydrolysis reaction at high temperature. Different colors illustrate the end-products with different chemical composition and electrochemical performance. The initial discharge capacity of NTH-60 can deliver over 90 m Ah g-1, and then remained at 65 m Ah g-1 after 50 cycles. By comparison, the initial capacity of NTH-80 is 10 m A h g-1 lower than that of NTH-60. The NTH-60 electrode shows better rate performance than NTH-80. In addition, a novel sulfones electrolyte(TMS: PTSI=95:5) with good SEI formation is first applied for sodium ion battery, which efficiently improves the specific capacity and cycling stability of the cell.In order to promote the capacity and cycling stability of electrode material, we successfully synthesized Na-rich ternary compounds Na2NixCo1–xFe(CN)6(PBNxC1–x) as cathode materials of SIBs by partially substituting electrochemically inert Ni sites for Co redox sites. There is an equilibrium point between initial discharge capacity and cycling stability. The best ratio of Ni and Co is 4:6. The lattice parameters of PBN, PBC, and PBN0.4C0.6 were calculated to be 10.40 ?, 10.24 ?, and 10.35 ?, respectively. The increased lattice parameters were attributed to the slightly larger ionic radius of Ni2+ than that of Co2+. However, it is not very serious to destroy the crystal structure of the material. Ni can play a role as the framework support, and Co2+/3+ couple and Fe(CN)64–/3– couple provide active sites. Due to rational optimization of composition, PBN0.4C0.6 delivers an initial discharge capacity of 91.8 m Ah g–1 with a high Coulombic efficiency of 100%. After 100 cycles at 50 m A g–1, this material exhibits a cycling stability with capacity retention of 85.9%. The rate performance of this material also has been greatly improved to 68.5 m A h g–1 at 800 m A g–1.