Kinetics and electrochemical characteristics of Mg₂NiH ₄-x wt.% MmNi_3.8Co_0.75Mn_0.4Al _0.2 (x = 5, 10, 20, 40) composites for Ni-MH battery

The structure, kinetics and electrochemical characteristics of Mg ₂NiH₄-x wt.% MmNi_3.8Co_0.75Mn _0.4Al_0.2 (x = 5, 10, 20, 40) composites prepared by mechanical milling have been investigated in this paper. XRD results indicate that the as-milled Mg₂NiH₄ shows nanocrystalline or amorphous-like structure, and it does not react with MmNi_3.8Co _0.75Mn_0.4Al_0.2 during mechanical milling. As the amount of MmNi_3.8Co_0.75Mn_0.4Al _0.2 increases, the maximum discharge capacity decreases initially from 508 mAh/g (x = 5) to 440 mAh/g (x = 10) and then increases to 509 mAh/g (x = 40). Meanwhile, the capacity retention (R₁₀) increases from 12.8% (x = 5) to 23.4% (x = 40), and the corrosion potential of electrode (E _corr) increases from -0.930 V to -0.884 V (vs. Hg/HgO). Especially, the more MmNi_3.8Co_0.75Mn_0.4Al_0.2 content the composite contains, the higher high rate dischargeability (HRD) the electrode exhibits, which could be attributed to the catalytic reaction and reduction of the Mg₂NiH₄ grain size brought by MmNi _3.8Co_0.75Mn_0.4Al_0.2. The improvement in electrode kinetics has been depicted from the bulk hydrogen diffusion coefficient (D), the exchange current density (I₀) and the charge transfer resistance (R_ct) on the alloy surface.