| Ferrate(Ⅵ) has high redox potential, three-electron transfer capacity, and innocuous products. Therefore, the batteries using ferrate(Ⅵ) as cathode exhibit corresponding advantages, such as high voltage, high capacity, and environment friendly feature. The studies on ferrate(Ⅵ) electrode materials are of great theoretical significance and practical value, which have attracted extensive attention of many researchers. In this thesis, we systematically investigate the synthesis, structure, and electrochemical properties of ferrate(Ⅵ), aimed on the current difficulties such as difficult preparation of high purity ferrate(Ⅵ), low stability in aqueous even alkali solution, ambiguous discharge mechanism.In this thesis, K2FeO4 (＞97%) and BaFeO4 (＞95%) powder were prepared by using a hypochlorite oxidation and a replacement reaction, respectively. The structural properties were characterized by XRD, SEM, FTIR, TG, and XPS techniques. The results indicate that both K2FeO4 and BaFeO4 have a orthorhombic structure (Dn, Pnma) with lattice parameters a = 7.690(3) (?), b = 5.850(2)(?), c = 10.329(4)(?),and a = 9.12175 (?), b = 5.46235 (?), c = 7.32645 (?), respectively. Their electron binding energies of Fe2p are 712.10 and 712.4 eV. The thermal decomposition of the dry K2FeO4 and BaFeO4 powder takes place at above 130 and 160℃with the same Fe(Ⅲ) oxide products. These thermodynamic data provide the basis for synthesis, storage, and characterization of ferrate(Ⅵ).A single crystal K2FeO4 was grown by the solvent evaporation method at 25℃, and the crystal structure of the K2FeO4 was characterized by the charge-coupled device (CCD) technique. The electrochemical performance of ferrate(Ⅵ) Li-ion secondary battery with K2FeO4 cathode was investigated. It is demonstrated from CCD data that the FeO42- has a regular tetrahedron structure with Td point-group symmetry. The average tetrahedral [Fe - O] bond length, corrected for libration effects, is 1.647(4)(?) and the bond angle of [O-Fe-O] is 109.15°. The molecular packing in the unit cell of single crystalline K2FeO4 indicates that one-dimensional channels exist in the direction of a and b axes of the cell, where the radius of channels is about 0.93(?). These wide channels are beneficial for Li ion (radius = 0.76 (?)) intercalation and deintercalation in the K2FeO4 cathode. The results demonstrate that the initial discharge process of the K2FeO4 cathode can be described as two different processes: an anisotropic one-Li ion intercalation process after discharge to 1.0 V, followed by an isotropic two-Li ion intercalation process after further deep discharge to 0.5 V. The K2FeO4 cathode exhibits a higher discharge capacity of about 400 mAh/g and relative good electrochemical cycle ability during the initial 20 cycles. The crystallinity of the K2FeO4 cathode decreases significantly during 50 cycles, indicating that the decay of the cycle performance of the ferrate(Ⅵ) Li-ion secondary battery is mainly caused by the decrease of crystallinity of the K2FeO4 cathode.The electrochemical performance of the ferrate(Ⅵ) primary alkaline battery was investigated. The test batteries included: K2FeO4 and BaFeO4 as cathode; Zn, MH and TiB2 as anode; KOH solution as electrolyte. The K2FeO4/Zn primary alkaline batteries exhibit the advantage in discharge specific capacity, which reached 521.3 mAh/g (cut-off voltage 1.0 V) in 9 M KOH at a rate of 0.4 C. The results show that the stability of the ferrate(Ⅵ) cathode was seriously affected by the hydrogen, generated by the reaction between the Zn and MH anode and KOH electrolyte during the discharge process. The capacity loss and the significant self-discharge are mainly caused by the dissolution and decomposition of K2FeO4 cathode in KOH electrolyte. The K2FeO4/TiB2 primary alkaline batteries indicate the advantage of high capacity battery, which can reach 223.9 mAh/g above 1.0 V, as calculated with the weight of both cathode and anode. The actual discharge specific capacity is close to the theoretical specific capacity of the primary alkaline MnO2/Zn battery (223.9 mAh/g). Comparing to the K2FeO4 cathode, the alkaline BaFeO4 batteries indicate a less discharge specific capacity and a lower discharge voltage. The results show that it can be attributed to the large internal resistance of BaFeO4 cathode and the insoluble barium salt, generated during the discharge process on the surface cathode.The PVA/PAA-KOH-H2O composite alkaline solid polymer electrolyte was prepared by the solution casting technology and was used as both separator and electrolyte in the solid alkaline ferrate(Ⅵ) battery, which included K2FeO4 cathode and Zn anode. The ionic conductivity and the electrochemical window of the PVA/PAA-KOH-H2O membrane is about 3.5×10-2 S/cm and 3.4 V at room temperature, indicating that it could be used as a separator and electrolyte for the solid alkaline K2FeO4-Zn battery. The electrochemical analyses show that the discharge specific capacity above 1.0 V of the solid alkaline K2FeO4-Zn batteries reaches 220 mAh/g at a rate of 0.4 C. Many additives were used to enhance the electrochemical performance of the solid alkaline K2FeO4-Zn battery. The discharge process of K2FeO4 cathode was changed by 5% KMnO4 additive, indicating good electrocatalysis characteristics for K2FeO4 cathode. The discharge efficiency of the solid alkaline K2FeO4-Zn battery increased by 34% (to 92.9%) and by 26% (to 84.7%), with 5% NaBiO3 and 5% SrTiO3 additive, respectively. The two additives effectively inhibit the capacity loss caused by the dissolution of K2FeO4 cathode, and decrease the electrochemical polarization of K2FeO4 cathode during the discharge process, due to the decrease of the internal resistance and enhancement of the mass-transfer and electron-transfer process.The electrochemical reduction mechanism of FeO42- ion was investigated in alkali solution. Linear sweep voltammetry and sampled-current voltammetry were carried out to calculate the electrochemical reduction with total three-electron reaction of FeO42- ion. The results demonstrate that the total irreversible cathodic reactions of FeO42- include two step reactions at potential region of 0.55 - 0.48 V and 0.45 - 0.28 V, respectively. The first one is the rate-controlling step with single electron transfer, and the other one is a two-electron reduction. Therefore, the electrochemical reduction mechanism of FeO42- in 9 M KOH can be described as FeO42-→FeO3-→FeO2-, in which the intermediate state of Fe (Ⅴ) is generated from ferrate (Ⅵ).
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