Development of magnesium based negative electrode materials for nickel metal hydride batteries

Eyövge, Cavit
Negative electrode materials of the nickel metal hydride (NiMH) batteries generally based on AB5 or similar compounds that make use of rare earth elements. The high cost of these elements makes it necessary to look for other alternatives that are more readily available and of low cost. It is also desirable to aim for materials that would have discharge capacity higher than roughly 350 mAh/g, which is typical of AB5 compounds. Magnesium-based hydrogen storage alloys have attracted considerable attention as an alternative due to their high hydrogen storage capacities. For instance, if Mg alloyed with nickel in the form of Mg2Ni alloy could be hydrided and dehydrided reversibly at room temperature, could yield discharge capacity in excess of 1000 mAh/g. Currently, there are two difficulties with Mg-based alloys. The first is their high stability where reversible hydrogenation occurs at temperatures well above room temperature. Nanostructuring via ball milling or similar methods yielded alloys of low stability. The Mg50Ni50 composition is of particular interest in this respect. The second difficulty is the durability of Mg alloys in the alkaline environment. Mg-rich alloys are subject to corrosion in alkaline environments resulting in a fast decay of discharge capacity. The current work deals with Mg50Ni50 composition and aims to develop an alloy with an improved electrochemical performance and durability as a negative electrode material. Mg alloy in this study was synthesized via ball milling of elemental powders; Mg and Ni mixed in equal atomic proportions. This resulted in a mixed structure composed of amorphous Mg50Ni50 and nanocrystalline Mg2Ni. The powder synthesized yielded a discharge capacity of 329 mAh/g in the first cycle; but, was subject to a fast capacity decay down to 50 mAh/g after 20 cycles, caused probably by corrosion of the active metal in an alkaline environment. So as to obtain a more durable active material, an attempt was made to protect the electrode by surface coating. The electrode was surface coated with nafion, which is known to be permeable H+ but repulsive to OH-. This not only reduced the degradation rate of the electrode but also resulted in a significant increase in the discharge capacity of the Mg50Ni50. With a 2.80 µm thick nafion coating, the electrode yielded a discharge capacity of 489 mAh/g, an increase by a factor 1.5. This capacity was reduced to 338 mAh/g after 20 cycles. The beneficial effect of nafion coating was further checked with A2B7 alloy, where Mg content is low and therefore of better durability. This has shown that the capacity increase does occur also with A2B7 alloy. The results are discussed with regard to the possible origin of capacity increase in both alloys. 
Citation Formats
C. Eyövge, “Development of magnesium based negative electrode materials for nickel metal hydride batteries,” M.S. - Master of Science, Middle East Technical University, 2017.