Study On Preparation And Properties Of A₂B₇ Type Storage Alloys For Low Self-discharge MH/Ni Batteries

RE-Mg-Ni of A2B7 type based hydrogen storage alloy of nickel metal hydride battery negative electrode material with high capacity and low self-discharge rate has been widely concerned in academia and industry. However, the development of this Mg-containing alloys is slow due to the magnesium content and phase composition is difficult to guarantee in the process of preparation and its cyclic performance is poor. On the basis of previous research work on RE-Mg-Ni system, the A2B7-type alloy of(LaSm)0.86Mg0.14(NiZrAl)3.42 was studied in the perspective of preparation process. We attempt to prepar MH-Ni cathode-materials with good comprehensive properties by changing the melting conditions and sealing annealing process.The results show that, the higher the power or the longer the time of induction melt, the more content of Mg loss of alloy. When the melting conditions was 7.5kW-3.0min, and the excess value of La、Sm、Mg were 5wt%、10wt%、15wt%, the actual composition of alloy was close to the design. In addition, the alloy by melting was consist of(La,Mg)2Ni7 、(La,Mg)5Ni19 and LaNi5 phase, but the abundance of(La,Mg)2Ni7 phase was not more than 51.0wt%.Also,(La,Sm)0.86Mg0.14(NiZrAl)3.42 alloy ingot was placed in a sealed quartz tube for annealing, and a small amount of Mg was still volatile during heat treatment. Heat treatment does not change the phase structure of alloy. Alloy was heat treated by 950℃×2h or 950℃×10h, the abundance of(La,Mg)2Ni7 phase was more than 60wt%, and the maximum discharge capacity was 334mA·h/g、329mA·h/g, of which the cycle life was 516 times. Added some pure metallic magnesium into the container, when the annealing temperaturethe was below 900℃, the content of Mg was slightly higher than that of before annealing; and the content of Mg with different temperature or time of heat treatment was little change. What’s more, the transition of(La,Mg)5Ni19 into(La,Mg)2Ni7 shifted to a higher temperature; the maximum discharge capacity and the cycle life of the alloy was 334mA·h/g and 509 times.The self-discharge of(LaSm)0.86Mg0.14(NiZrAl)3.42 alloy was mainly reversible, the irreversible self-discharge was not more than 2%. When the annealing temperature was 800℃, the self-discharge rate of alloy electrode reaches 98.7%. Increased the annealing temperature, the self-discharge rate of alloy was obviously improved. The self-discharge performance of the alloy could also be improved by prolonging the annealing time at 950 ℃. The self-discharge performance of alloy which was heat treated by 950℃×10h was the best, and the self-discharge rate was only 23.4%. In addition, the self-discharge performance of alloy electrode was improved only in a certain temperature range when the pure metallic magnesium was added into container. When the annealing temperature was higher than 900℃, after heat treatment, the self-discharge rate of alloy with Mg was higher than that of alloy without Mg.