The Structure And Electrochemical Properties Of AB₅-type Hydrogen Storage Alloys Doped With Y/Mg

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 La_0.875Y_0.125Ni₅ were also studied by first-principles calculations.The main contents and conclusions are as following:(1) It is found that the La_1-XY_XNi_3.55Mn_0.4Al_0.3Co_0.75(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 La_1-xMg_xNi_3.55Mn_0.4Al_0.3Co_0.75(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 La_0.875Y_0.125Ni₅ 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 La_0.875Y_0.125Ni₅ is lower than that of LaNi₅,indicating that the structure of La_0.875Y_0.125Ni₅ is much more stable. The band structure also showes metallic character for the doped alloy.