【作者】 李露；

【导师】 陈立新；

【作者基本信息】 浙江大学 ， 材料学， 2007， 硕士

【摘要】 本文在对国内外V-Ti-Ni基双相贮氢电极合金的研究进展进行全面综述的基础上，首先采用XRD、SEM、EDS以及电化学性能测试方法，比较系统地研究了V_2.1TiNi_0.4Zr_x（x=0～0.08）合金的相结构与电化学性能；然后利用XRD全谱拟合、SEM/EDS面分布、XPS及AES等分析测试方法，分析研究了此类合金的充放电循环容量衰退机制；在此基础上，通过分别添加Co、Cr、Cu、Ta、Nb元素对V_2.1TiNi_0.4Zr_0.06进行多元合金化改性，对比研究了上述合金元素对合金相结构及电化学性能的影响规律，力求进一步提高此类合金的综合电化学性能。通过对V_2.1TiNi_0.4Zr_x（x=0～0.08）合金相结构及电化学性能的研究表明：所有合金均由体心立方结构的V基固溶体主相和第二相组成，第二相沿主相晶界形成三维网状分布；其中，当Zr含量x≤0.02时合金的第二相为体心立方结构的TiNi基相，当Zr含量x＞0.02时，第二相为六方结构的C14型Laves相，且主相和第二相的晶胞体积均随着x的增加而增大。与V_2.1TiNi_0.4相比，添加Zr元素可提高合金的活化性能和合金最大放电容量（除x=0.02时容量稍有下降外），明显改善合金的高倍率放电性能，但是会降低循环稳定性。在所研究的合金中，V_2.1TiNi_0.4Zr_0.06合金的综合电化学性能相对较佳，经过2次充放电循环活化达到最大放电容量468.5mAh/g，在300mA/g放电条件下的高倍率放电性能为70.13％，但经过30次充放电循环后的容量保持率S₃₀仅为22.34％，有待于进一步研究改善。在上述研究基础上，选择V_2.1TiNi_0.4Zr_0.06合金为研究对象，系统研究了此类合金电极的循环容量衰退机制。通过对不同充放电循环次数后合金电极的电化学性能和微结构研究表明：在最初的5次充放电循环中，合金表面形成了V₂O₅、TiO₂和Ni（OH）₂氧化产物，但表面的V以溶出为主而Ti主要被氧化；第二相中V元素的腐蚀溶出导致该相逐渐破坏。随后的充放电循环过程中，次表层的V溶出明显，且Ti在碱液中发生溶出，同时有少量NiO生成，致使合金表面生成厚度约2μm的氧化层；该氧化层减缓了内层V和Ti元素的溶出，使电极表面的电化学反应阻抗增加，因此合金的容量衰减速度减慢。经过30次充放电循环后，合金中的C14型Laves第二相已逐渐消失，并有四方结构和单斜结构的V₂H基相、立方结构的VH₂基相以及TiO₂新相生成。分析认为，充放电循环时合金元素（主要是V与Ti）的腐蚀溶出引起双相结构稳定性变差，尤其是C14型Laves催化相的逐渐消失，加上氧化层不断增厚，是导致合金循环放电容量衰减的主要原因。为提高V_2.1TiNi_0.4Zr_0.06合金的循环稳定性，分别添加Co，Cr，Cu，Ta，Nb五种元素对V_2.1TiNi_0.4Zr_0.06进行合金化改性。研究表明，所有V_2.1TiNi_0.4Zr_0.06M_0.152（M=Co，Cr，Cu）合金均由V基固溶体相和C14型Laves相组成，第二相沿主相晶界形成三维网状分布；且大部分Cr分布在主相中，而Co和Cu主要分布在第二相中。添加Co，Cr，和Cu会减小合金主相和第二相的晶胞体积，降低V_2.1TiNi_0.4Zr_0.06合金的活化性能和最大放电容量，但能有效抑制V与Ti元素的腐蚀溶出，提高合金电极的循环稳定性。同时，Cr还能改善合金的高倍率放电性能，但Co与Cu会使高倍率放电能力降低。相比之下，V_2.1TiNi_0.4Zr_0.06Cr_0.152合金具有相对较佳的综合电化学性能：第4次循环达到最大放电容量397.13mAh/g，30次充放电循环容量保持率为77.96％，在400mA/g电流放电条件下的高倍率放电性能为62.80％。对V_2.1TiNi_0.4Zr_0.06M_0.037（M=Ta、Nb）合金研究发现，所有合金的主相都是V基固溶体，第二相是C14型Laves相，第二相沿主相晶界形成三维网状分布，且Ta和Nb主要分布在主相中。添加Ta和Nb使合金的最大放电容量降低，但由于抑制了V、Ti元素的腐蚀溶出，使得合金的循环稳定性和高倍率放电性能得到改善。比较而言，V_2.1TiNi_0.4Zr_0.06Ta_0.037合金具有相对较好的综合电化学性能：合金在第2次循环达到最大放电容量411.74mAh/g，具有较高的循环稳定性(S₃₀=54.83％)，在400mA/g电流放电条件下的高倍率放电能力为52.36％。更多还原

【Abstract】 In the thesis, previous research and development of V-Ti-Ni based hydrogen storage electrode alloys with dual-phases have been reviewed. On this basis, the microstructure and electrochemical performance of V_2.1TiNi_0.4Zr_x （x=0～0.08） alloys were investigated systematically by means of XRD, SEM, EDS, ICP analyses and electrochemical measurements. Furthermore, The mechanism for cycling capacity degradation of such alloys was analyzed by means of XRD-Rietveld analysis, XPS and AES investigations. Whereafter, the influence of added elements （Co, Cr, Cu, Ta, Nb） on the phase structure and electrochemical properties were investigated for improving the overall electrochemical performance of V_2.1TiNi_0.4Zr_0.06 alloy.The study on the microstructure and electrochemical properties of V_2.1TiNi_0.4Zr_x（x= 0～0.08） alloys shows that all alloys consist of a V-based solid solution main phase with a bcc structure and a secondary phase with a three-dimensional network structure, and the secondary phase precipitates along the grain boundaries of the main phase. For the alloy with x≤<0.02, the secondary phase is the TiNi-based phase. However, the secondary phase in the alloy changes into the C14-type Laves phase with an hcp structure as x>0.02. Moreover, both the unit cell of the main phase and secondary phase expand with the increase of Zr content. The electrochemical measurements reveal that the activation behavior and the maximum discharge capacities except for x=0.02 of the Zr-added alloys are better than those of the V_2.1TiNi_0.4 alloy. As Zr content increases, the high-rate dischargeability is improved significantly, but the cycling stability is degraded gradually. Among the alloys studied, V_2.1TiNi_0.4Zr_0.06 alloy has better overall electrochemical properties than others. This alloy is fully activated at the second cycle and reaches the highest discharge capacity of 468.5mAh/g. Its high-rate dischargeability at the discharge current of 300mA/g is 70.13%. However, its capacity retention after 30 charging/discharging cycles is only 22.24%.Based on the above work, the mechanism for cycling capacity degradation of V_2.1TiNi_0.4Zr_0.06 alloy was investigated by analyzing the relation between its microstructure and electrochemical performance after a certain cycles. It is found that the corrosion and dissolution of vanadium, which leads to the destruction of the secondary phase and the oxidation on the surface of alloy, are induced during the initial five charge/discharge cycles. When the cycling goes on, a little amount of NiO is generated onthe surface, and titanium element begins to dissolve into the KOH electrolyte. The corrosion and dissolution of large vanadium in the subsurface layer of alloy generates a thick oxide layer （about 2μm） which slower the dissolution of vanadium and titanium. The result brings the deterioration of the maximum discharge capacity with a slower reaction rate. Nevertheless, the thick oxide layer degrades electrochemical reaction rate of the electrode surface and namely heightening electrochemical reaction resistance. After 30 cycles, the C14 Laves phase disappears gradually, however, V₂H phase with tetragonal or monoclinic lattice, VH₂ phase with cubic lattice and a TiO₂ new phase appear. The oxide products on the surface of the alloy are composed of V₂O₅, TiO₂ and Ni （OH） 2. Therefore, the stability of the dual-phase structure becomes increasingly bad due to the corrosion and dissolution of vanadium and titanium, especially the C14 Laves secondary phase in possession of electrocatalytic activity gradually disappears, and the oxide layer becomes increasingly thick. Both two factors chiefly induce the deterioration of the discharge capacity.In order to improve the cycling stability of V_2.1TiNi_0.4Zr_0.06 alloy, Co, Cr, Cu, Ta and Nb were adopted as an alloying element. The study on the V_2.1TiNi_0.4Zr_0.06M_0.152（M=Co, Cr, Cu） alloys shows that each alloy has a V-based solid solution main phase with a bcc structure and a secondary phase with a three-dimensional network structure, where Cr predominantly exists in the main phase, and Co or Cu is mainly distributed in the secondary phase. The addition of Co, Cr or Cu leads to a unit cell contraction of both main and secondary phase, a difficult activation and a lower maximum discharge capacities comparing with V2.1TiNio.4Zro.06 alloy. However, as a result of the effective restraint in the corrosion and dissolution of vanadium and titanium by adding Co, Cr or Cu into V_2.1TiNi_0.4Zr_0.06 alloy, the cycling stability is improved. Moreover, Cr enlarges the reaction rate of surface and the high-rate dischargeability. The V_2.1TiNi_0.4Zr_0.04Cr_0.152 alloy obtains better overall electrochemical properties, especially in the cycling stability, namely a maximum discharge capacity of 397.13mAh/g at the fourth cycle, a higher cycling stability S₃₀ of 77.96%, and a high-rate dischargeability HRD₄₀₀ of 62.80% at the discharge current of 400mA/g. The investigation on V_2.1TiNi_0.4Zr_0.06M_0.037（M=Ta、Nb） alloys shows that all alloys possess the similar three-dimensional network structure which is formed by a V-based solid solution main phase and a C14 type Laves secondary phase,and Ta and Nb are distributed predominantly into the main phase. After adding Ta and Nb, the unit cell of the main phase expands and that of secondary phase contract. The Ta-contained or Nb-contained alloy restricts the dissolution of vanadium and titanium into the KOH electrolyte, thus the cycling stability is improved, but the maximum discharge capacity decreases without change of activation behavior. Nb and Ta also enhance the high-rate dischargeability. Among the alloys studied above, V2.1TiNio.4Zro.06Tao.037 alloy shows a maximum discharge capacity of 411.74mAh/g at the second cycle, a higher capacity retention S₃₀ of 54.83%, and a high-rate dischargeability HRD₄₀₀ of 52.36% at the discharge current of 400mA/g.更多还原