【作者】 王永龙；

【导师】 吴锋；

【作者基本信息】 南开大学 ， 无机化学， 2009， 博士

【摘要】 高铁酸盐具有高的氧化还原电势、三个电子的转移能力以及无毒无害的产物。因此,高铁电池具有高电压、高容量以及绿色环保等优势。高铁酸盐电极材料的研究具有重要的理论意义和应用价值,已经引起了电池工作者的广泛关注。本论文针对目前高铁酸盐正极材料研究中仍存在的合成困难、稳定性差以及放电机制不明确等诸多问题进行了系统的研究。本论文采用次氯酸盐氧化法和利用置换反应分别制备了纯度大于97%的K₂FeO₄和纯度大于95%的BaFeO₄固体粉末。使用XRD、SEM、FTIR、TG和XPS多种测试技术表征了其结构特征。结果表明,K₂FeO₄和BaFeO₄同属于正交结构,空间群为D_n（Pnma）。晶胞参数分别为a=7.690（3）（?）,b=5.850（2）（?）,c=10.329（4）（?）和a=9.12175（?）,b=5.46235（?）,c=7.32645（?）。Fe_2p电子结合能分别为712.10 eV和712.4 eV。干燥的K₂FeO₄和BaFeO₄粉末分别在130℃和160℃以上发生热分解反应,产物均为Fe（Ⅲ）氧化物。这些热力学数据为高铁酸盐的合成、储存和表征提供了依据。采用溶剂挥发法培养了K₂FeO₄单晶并利用CCD技术研究了K₂FeO₄单晶的通道结构,并探索了以K₂FeO₄为正极材料的锂离子二次电池的电化学性能。CCD结果表明,FeO₄^2-离子具有正四面体结构,对称性为T_d点群。Fe-O共价键键长为1.647（4）（?）,O-Fe-O键角为109.15°。分子晶胞图显示了在K₂FeO₄晶胞的a和b方向上存在着半径为0.93（?）的一维通道结构,这些通道有利于锂离子（半径为0.76（?））在K₂FeO₄中的脱嵌。电化学测试结果表明,K₂FeO₄阴极的首次放电过程可以描述为两个不同的过程,首先是当放电到1.0V时,一个锂离子的各向异性的嵌入过程;另一个是放电到0.5V时,两个锂离子各向同性的嵌入过程。K₂FeO₄阴极展示了高的放电比容量大约为400 mAh/g,以及前20周良好的循环性能（380 mAh/g）。循环过程中对K₂FeO₄的结构变化的研究验证了K₂FeO₄阴极的结晶度的持续降低是高铁锂离子二次电池循环性能衰减的主要原因。论文研究了以K₂FeO₄和BaFeO₄为正极材料,以Zn、MH和TiB₂为负极材料的高铁一次性碱性电池的电化学性能。K₂FeO₄/Zn碱性电池展示了K₂FeO₄阴极优越的容量优势,尤其是以9 M KOH为电解液,在0.4 C的电流密度下,1.0 V以上比容量达到了521.3 mAh/g。研究表明,Zn和MH负极材料在放电过程中产生的H₂严重影响了高铁阴极的稳定性。K₂FeO₄在电解液的溶解和分解反应是K₂FeO₄碱性电池容量损失和自放电性能差的主要原因。K₂FeO₄/TiB₂碱性电池表现了良好的容量优势,1.0 V以上放电比容量达到了220 mAh/g（以正负极活性物质总质量计算）。实际放电容量已接近MnO₂/Zn碱性电池的理论比容量223.9mAh/g（以正负极活性物质总质量计算）。相比较于K₂FeO₄阴极,BaFeO₄碱性电池的放电比容量较小,放电电压平台低。研究结果表明,BaFeO₄较大的内阻和放电过程中产生的不溶钡盐是造成这一现象的主要原因。采用溶液铸膜法制备了PVA/PAA-KOH-H₂O复合碱性固态电解质膜,并以K₂FeO₄为正极材料,Zn为负极材料,组成高铁碱性固态电池。研究结果表明,PVA/PAA-KOH-H₂O碱性固态电解质膜电导率为3.5×10^-2S/cm,电化学窗口为3.4 V。电化学性能研究表明K₂FeO₄-Zn碱性固态电池以0.4 C倍率放电1.0 V以上放电比容量达到220 mAh/g。使用多种添加剂改性碱性固态高铁电池的研究结果表明,5%KMnO₄改变了K₂FeO₄正极的放电过程,在放电过程中起电催化作用。5%NaBiO₃改善了K₂FeO₄正极的放电效率（92.9%）,比未添加时提高34%左右。5%SrTiO₃提高了K₂FeO₄正极的放电效率（84.7%）,比未添加改性时提高26%,放电容量为344.0 mAh/g。NaBiO₃和碱土金属钛酸盐一方面有效抑制K₂FeO₄因溶解造成的容量损失,另一方面减小了放电过程中的电化学极化,有效改善了传质传荷过程,从而提高了K₂FeO₄正极的放电效率。论文研究了高铁酸根在碱性溶液中的电化学还原机理。使用线性扫描法和取样电流伏安法测试并计算了总电子数为3的高铁酸根的电化学还原反应。结果表明,FeO₄^2-离子的电化学还原反应为完全不可逆反应,该反应包含两个分步反应,分别发生在电位范围0.55-0.48 V和0.45-0.28 V。第一步还原反应是单电子的速控步反应,第二个还原反应为两个电子的扩散控制步反应。因此,高铁酸根在9 M KOH中的电化学还原机理可以表述为FeO₄^2-→FeO₃^-→FeO₂^-,其中Fe（Ⅴ）过渡态由Fe（Ⅵ）单电子还原产生。更多还原

【Abstract】 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 （D_n, 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 Fe_2p 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 FeO₄^2- has a regular tetrahedron structure with T_d 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 FeO₄^2- 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 FeO₄^2- ion. The results demonstrate that the total irreversible cathodic reactions of FeO₄^2- 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 FeO₄^2- in 9 M KOH can be described as FeO₄^2-→FeO₃^-→FeO₂^-, in which the intermediate state of Fe （Ⅴ） is generated from ferrate （Ⅵ）.更多还原