Effect of annealing treatment on electrochemical properties of Mm-based hydrogen storage alloys for Ni/MH batteries

Several multi-component Mm-based hydrogen storage alloys with cobalt content from 0.8–1.0 have been prepared. The hydrogen absorption–desorption characteristics in gas–solid reactions and the electrochemical properties as MH electrodes have been investigated. The addition of small amounts of Al effectively lowers the hydrogen equilibrium pressure and improves the cycling stability of the alloys. Electrochemical measurements show that the MmNi_3.4Co_1.0Mn_0.5Al_0.1 alloy exhibits a maximum electrochemical capacity of 322 mA h g⁻¹ with a capacity decay of about 19.5% after 100 cycles. Annealing treatments flatten the plateau region and lower the hydrogen equilibrium pressure, which results in an increase of hydrogen uptake below 1 atm. The increasing trend of hydrogen storage capacity from the as-cast sample to the annealed sample in the gas–solid reactions is in good agreement with the electrochemical results. The electrochemical discharge capacity of the MmNi_3.4Co_1.0Mn_0.5Al_0.1 alloy increases to 334 mA h g⁻¹ and 340 mA h g⁻¹ after annealing for 3 h and 28 h, respectively, from 322 mA h g⁻¹ in the as-cast condition. The electrochemical cycling stability of the annealed samples was also greatly improved. The capacity decay for both annealed samples is about 8.3% and 6.8%, respectively, after 100 charge–discharge cycles. It was suggested that annealing treatments enhance the compositional homogeneity and cause the secondary phase (separated phase) to dissolve in main phase, which result in the improvement of electrochemical cycling stability of the alloy electrodes.