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The Structure and Electrochemical Properties of AB5-type Hydrogen Storage Alloys Doped with Y/Mg

Author: LiZuo
Tutor: DuYuLei
School: Nanjing University of Technology and Engineering
Course: Materials Science
Keywords: Hydrogen storage electrode alloys Phase structure High temperature Electrochemical properties First-principles calculation
CLC: TG139.7
Type: Master's thesis
Year: 2009
Downloads: 50
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Abstract


The high-power nickel/metal-hydride（Ni/MH） batteries,which can be used on electric vehicles,have attracted a lot of attention in recent years.However,the working temperature of the batteries is so high that the electrochemical properties of the commercialized AB5 type hydrogen storage alloys used as the negative electrode in the batteries will deteriorate seriously.Therefore,how to improve the high temperature electrochemical properties has actual significance in developing the high-power type Ni/MH battery.Thus,in this thesis,the effects of Y or Mg substitution for La in AB5 type hydrogen storage alloys were studied.In addition to the experimental work,the electronic structures of LaNi5 and La0.875Y0.125Ni5 were also studied by first-principles calculations.The main contents and conclusions are as following:（1） It is found that the La1-XYXNi3.55Mn0.4Al0.3Co0.75（x=0,0.05,0.1,0.2） alloys show single-phase CaCu5-type structure after La is partially substituted by Y.However,increasing the content of Y leads to the reduction in the lattice parameters and cell volume.When x equals 0.1,both the as-cast and as-annealed alloys were found to exhibit the best overall electrochemical properties including the maximum discharge capacity（as-annealed: 336.3mAh/g at 298 K,313.4mAh/g at 333 K;as-cast:301.2mAh/g at 298 K,263.2 mAh/g at 333 K）,cycling stability（as-annealed:92.15%after nearly 100 cycles） and high-rate discharge ability at 298 and 333 K.（2） It is found that the La1-xMgxNi3.55Mn0.4Al0.3Co0.75（x=0,0.05,0.1,0.2） alloys show multiple-phases,consisting ofLaNi5,MgNi2 and LaNi3 after La was partially substituted by Mg.When x equals 0.05,the as-cast alloy showes the best overall electrochemical properties including the maximum discharge capacity（306.4mAh/g at 298 K;247.3mAh/g at 333 K） and cycling stability（82.9%after 97 cycles at 298 K;89%after 97 cycles ）.After annealing, although the cycling stability of alloys（x=0.05,0.1,0.2） was improved,the discharge capacity deduced heavily not only at 298 K but also at high temperature which was due to the changes of phase abundance and the unsuitable annealing methodes.（3） The first-principle calculations on the electronic structures of LaNi5 and La0.875Y0.125Ni5 were studied by using the WIEN2K code.It is obviously that the lattice parameters and cell volume reduced and the Fermi energy dropped slightly after La was partially substituted by Y.Also,the densities of states at Fermi level of La0.875Y0.125Ni5 is lower than that of LaNi5,indicating that the structure of La0.875Y0.125Ni5 is much more stable. The band structure also showes metallic character for the doped alloy.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > The alloy learn with a variety of properties of alloys > Other special nature of the alloy > Hydrogen storage alloy
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