Kinetics and electrochemical characteristics of Mg2NiH4-x wt.% MmNi3.8Co0.75Mn0.4Al0.2 (x = 5, 10, 20, 40) composites for Ni-MH battery

The structure, kinetics and electrochemical characteristics of Mg2NiH4-x wt.% MmNi3.8Co0.75Mn0.4Al0.2 (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 MmNi3.8Co0.75Mn0.4Al0.2 during mechanical milling. As the amount of MmNi3.8Co0.75Mn0.4Al0.2 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 (Ecorr) increases from −0.930 V to −0.884 V (vs. Hg/HgO). Especially, the more MmNi3.8Co0.75Mn0.4Al0.2 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 MmNi3.8Co0.75Mn0.4Al0.2. 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 (Rct) on the alloy surface.