Structures, Electrochemical Properties And The Mechanism Of Improving The Cycling Stability Of The Ti-V-Fe-Based Hydrogen Storage Alloys

Ti-V-based hydrogen storage electrode alloys have attracted much attention because of their high discharge capacity.Whereas,the high price and the poor cycling stability and high rate dischargeability of this type of alloys prevent them from being used as the negative materials of the MH/Ni batteries.In this thesis,based on an overall review of the research and development of the Ti-based C14 type Laves phase hydrogen storage electrode alloys,the V-based solid solution type hydrogen storage electrode alloys and Ti-V-based hydrogen storage electrode alloys,a series of Fe additional Ti-V-based hydrogen storage electrode alloys were developed and studied on the purpose of lowing the price and improving the overall electrochemical properties of this type of alloys.By means of XRD,SEM,EDS,TEM, XPS,AES and AFM analyses and electrochemical test methods including the galvanostatic charge-discharge,Electrochemical impedance spectroscopy(EIS),linear polarization,anodic polarization and potentialstatic discharge etc,the microstructures and electrochemical properties of the Ti-V-Fe-based alloys were studied systematically.Furthermore,the mechanism of improving the cycling stability and the pulverization behaviors of alloy particles of the Ti-V-Fe-based alloys were also investigated.The Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_1.25 alloy was selected as the pristine alloy,and effects of Fe substitution for Cr for different amounts on the microstructures and electrochemical properties of the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8-xNi_1.25Fe_x(x=0.0～0.8) alloys were studied systematically.The results show that all of the alloys are consisted of two phases,the interpenetrating hexagonal C14 Laves phase and the dendritic V-based solid solution phase with a BCC structure.With the increase of Fe substitution content,the unit cell volumes of the two phases decrease,and the content of the C14 type Laves phase increases,accordingly that of the V-based solid solution phase decreases.The grain size of V-based phase decreases firstly,and then increases with the increase of Fe content.Proper amount of substitution of Fe for Cr can improve the maximum discharge capacity(C_max),the cycling stability(C₂₀₀/C_max) and the high rate dischargeability(HRD) of the alloy electrodes.The cycling stability and high rate dischargeability of the alloys with the amount of V-based phase being a little more than that of the C14 phase is optimal.Among the alloys studied,the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25Fe_0.2 alloy electrode shows a relatively good overall electrochemical properties of C_max being 339.1 mAh/g,C₂₀₀/C_max being 75.1%and HRD₆₀₀ being 63.8%. Effects of different cooling rates(water cooling,air cooling and furnace cooling) of the annealing treatment(1173K×8h) on the microstructures and electrochemical properties of the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25Fe_0.2 alloy were investigated.It is found that the alloys performed by annealing with the different cooling rates are still consisted of an interpenetrating C14 Laves phase and a dendritic V-based solid solution phase as the as-cast.The unit cell volumes of the both phases increase slightly after the annealing treatments.However,the relative abundance of the V-based phase increases with decreasing the cooling rate,and higher cooling rate(water cooling) slightly refines the crystal grain of the V-based phase,while lower cooling rate,especially the furnace cooling coarsens the grain size of the V-based phase.The cycling stability and the high rate dischargeability(HRD) of the alloy electrodes are greatly improved after annealing treatment,despite the cooling rate.The C₂₀₀/C_max of the alloy electrode treated by water cooling reaches 84.2%,which is higher than that of the others including the as-cast alloy electrode(73.3%).The HRD₈₀₀ of the alloy electrode treated by furnace cooling shows the highest value,which is 62.1%,much higher than that of the as-cast, 46.5%.But the maximum discharge capacity(C_max) and the activation properties(N_α) of the alloy electrodes decrease after annealing treatment.The effects of Co substitution for Ni on the microstructures and electrochemical properties of Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25-xCo_xFe_0.2(x=0.00～0.25) alloys were investigated systematically.The main phases of all of the alloys are still C14 type Laves phase in a three-dimensional network and a V-based solid solution phase with a dendritic structure.The unit cell volumes of the two phases slightly increase and the relative abundance of the two phases slightly varies with the increase of Co concentration.The grain of the V-based phase is obviously refined after Co substitution.With a proper substitution of Co for Ni(x=0.10),the cycling durability(C₂₀₀/C_max) of the alloy electrodes is improved,which is mainly due to the suppression of both the pulverization of the alloy particles and the dissolution of the main hydrogen absorbing elements(V and Ti) into the KOH solution during charge/discharge cycling.The cycling stability of the alloy electrode slightly decreases with the increase of x from 0.10 to 0.25.The highest value of C₂₀₀/C_max of the alloy electrode is obtained when x=0.1,which is 79.8%.However,the maximum discharge capacity(C_max) and the high rate dischargeability(HRD) gradually decrease with increasing the Co amount.The studies of the partial substitution of the rare earth metals(Y,La,Ce,Pr and Nd) for Ti on the microstructures and electrochemical properties of the Ti_0.8-xM_xZr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25Fe_0.2(x=0,0.1,M=Y,La,Ce,Pr and Nd) and Ti_0.8-xY_xZr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25Fe_0.2(x=0～0.25) alloys are performed.The phase composition, structure and fraction of all the alloys after substitution don't change obviously,but the grain size of the V-based phase decreases.The unit cell volumes of the two main phases remain almost unchanged with the substitution of the rare earth elements for Ti except for Y,which causes a slight increase.A phase with an approximative composition of M₂Ni₃(M=La,Ce,Pr, Nd) occurs for the alloys with the corresponding substitution of La,Ce,Pr,Nd,which obviously improves the activation property(N_α) of the alloy electrodes.The maximum discharge capacity(C_max) and the high rate dischargeability(HRD₆₀₀) of the alloy electrodes are improved,but the cycling stability(C₂₀₀/C_max) slightly decreases with the substitution of the rare metals for Ti.Especially,with the substitution of small amount of Y(x=0.05),the alloy displays an improved overall electrochemical properties,of which the C_max,the C₂₀₀/C_max and the HRD₆₀₀ are 346.5 mAh/g,76.1%and 73.2%,respectively.Three Ti-V-Fe-based alloys with good cycling stability,the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25Fe_0.2 anneal treated with water cooling,the as-cast alloys of Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.15Co_0.1Fe_0.2 and Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.4Ni_1.25Fe_0.4,and one alloy with poor cycling stability,the as-cast Ti_0.8Zr_0.2V_2.7Mn_0.5Ni_1.25Fe_0.8 alloy were selected representatively to study the mechanism of improving the cycling stability of the Ti-V-Fe-based hydrogen storage electrode alloys.The results show that lowering the unit cell volume expansion of the two phases,increasing the corrosion and oxidation resistance and anti-pulverization ability during charge/discharge cycling can all effectively increase the cycling stability of the alloy electrodes.Moreover,the alloy electrodes with good cycling stability can effectively prevent the decrease of the electrochemical reaction rate during charge/discharge cycling.The pulverization behaviors of the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25Fe_0.2 alloy particles during charge/discharge cycling were further studied.It is found that,for the alloys with different particle size(＜300 mesh),the dissolution rate exponents of the main hydrogen absorbing elements(V and Ti) are almost same,but the dissolution rate constants are different. The dissolution rate constant of the alloy particles less than 700 mesh is three times larger than that of the alloy particles between 300 and 400 mesh.During the 10 to 20 charge/discharge cycles,the electrode alloys with smaller size show a higher value of HRD₆₀₀,indicating a better kinetic property.However,with the cycling,after about 50 cycles, due to the severer pulverization of the larger particles,the electrode alloys with larger size show a better kinetic property.The observation of the interface of the two phases of the alloy under TEM shows that the interface is an incoherent interface which has high interface energy and causes the microcracks of the alloy particle firstly form on the phase boundary during cycling.With the charge/discharge cycling,the microcracks propagate to the C14 phase. Finally,the C14 phase cracks and collapses.This can be attributed to the low fracture toughness of the C14 phase.Besides,the spalling extent of the C14 phase is much more serious than that of the V-based phase.From the above studies,it can be concluded that the decrease of the grain-boundary segregation,the increase of the bonding strength of the phase interface,and the proper reduction of the C14 phase content can increase the anti-pulverization ability of the alloy particles during charge/discharge cycling.