'Middle East Technical University, Faculty of Architecture'
Abstract
Ni/MH battery negative electrodes normally make use of rare earth AB5 compounds. Development of rare earth-free negative electrode materials for Ni/MH batteries is desirable so as to lower the cost, and widen the availability. Although there are a number of alternatives, of these AB2 alloys are particularly attractive, as they offer better capacities than AB5. A major problem in this group of alloys is that they are difficult to activate, i.e. they require a large number of cycles before they could reach their full capacity. The present work concentrates on a Laves phase C14 AB2 alloy, (TiZr)(VNiMnCrSn)2, and aims to develop methods for their rapid activation. The study examines the methods in two groups. In one, the methods aim to increase the surface area of the powders and in the other the methods aim for surface modifications. Pristine alloy had zero initial capacity, which has increased to and saturated at 220 mAh/g after 14 cycles. So as to examine the effect of surface area on activation two methods were used; sieving and ball milling. AB2 powder was sieved into different particle sizes, namely d(0.5)=37.3, 62.7 and 82.5 μm. It was found that with coarse particles, the activation was relatively fast reaching a capacity of 245 mAh/g after 7-8 cycles. The fast activation of coarse particles was attributed to the ease of particle fragmentation which led to the generation of new fresh surfaces. Electrodes with fine powders (e.g. d(0.5) = 37.3 μm) activates later with a lower saturation capacity. The low saturation capacity was attributed to ineffective utilization of the active powder, i.e. a fraction of powders not in contact with the electrode. Ball milling was more effective in improving the activation behaviour of the alloy. Milling of powders leads initially to a decrease in particle size. But with prolonged milling the particles do agglomerate yielding particle sizes similar to the initial one. A saturation capacity of 330 mAh/g was obtained after 5-6 cycles, which is slightly above the expected capacity of the powder based on the PCI curve measured with gas phase storage. The saturation capacity was less with prolonged milling. For surface modifications, the main method used involved hot alkaline treatment. This included treating the AB2 powder in boiling 6M KOH solution for various periods of time before the electrode was prepared. KOH treatment was effective in all cases, as the electrode was fully active after 1-2 cycles. SEM examination of treated alloy has shown that KOH treatment results in the leaching of powders leaving behind a nickel rich surface layer. The saturation capacity has steadily increased with increased duration of the treatment reaching a maximum value of 390 mAh/g after 80 minute-treatment. This capacity is very much higher than the gas phase storage capacity of the alloy expected at 1 atm hydrogen pressure, i.e. 1.2 wt. % H corresponding to 320 mAh/g. This was attributed to the formation of rough surfaces generated by the treatment, as such surfaces could stabilize hydrogen bubbles whereby allowing an increase in local hydrogen evolution pressure. Another method under investigation in the current study was NiO coating of AB2 powders. For this purpose, using sol-gel approach pristine particles were coated with NiO which upon charging in the electrode would be reduced to Ni, thus aiming for the formation of Ni rich surfaces as in KOH treatment. Two routes were employed; one with the use NiCl2, and the other with the use of NiNO3. Of these, with details used in the present work, only the second route gave a capacity, but overall activation performance in all samples were poor. M.S. - Master of Scienc
