Effects Of Duplex Surface Modification On Electrochemical Properties Of La-Mg-Ni-Based Hydrogen Storage Alloys

La–Mg–Ni-based A₂B₇-type alloys are supposed to be used as high capacity negative material of Ni/MH battery because of their higher capacity and lower cost. However the alloy get oxidized easily in the electrolyte, and the hydrogen pass lattice repeatedly make particles pulverization, moreover accelerate the oxidation and improve the contact resistance, which lead to capacity decay by a large margin. In order to obtain the cycling stability and high catalytic activity of the alloy electrode, in this paper, we treated the alloy powders with the method of electroplating and electroless plating, the effects of metallic layer on the electrochemical and kinetically properties of electrode alloy were also examined.Electroplating Co–Ni treatment was applied to the surface of the La_0.75Mg_0.25Ni_3.48alloy powders, the SEM results showed that the electrodes were plated with a homogeneous Co–Ni film. The alloy coating significantly improved the high rate dischargeability of the alloy electrode, and the HRD value increased to 57.5% at discharge current density 1875 mA/g after the Co–Ni-coating. The exchange current density I₀ and the limiting current density IL also increased for the coated alloy, the kinetic parameters are consistent with the change of high rate dischargeability. Electroplating improved the cycling stability as well, the capacity retention after 100 cycles of the treated alloy could reach 66.9%. The improvement of overall electrode performances was attributed to an enhancement in electro-catalytic activity and conductivity at the alloy surface, owing to the precipitation of the Co–Ni layer.We treated the hydrogen storage alloy with the method of fluorination and F-duplex electroless plating treatment, the Ni-rich layer with high catalytic and the homogeneous Co-Mo film were deposited on the surface of the alloy after fluorination and electroless plating. Two kinds of treatments both have positive effect on the maximum discharge capacity, the high rate dischargeability（HRD） at current density 1500 mAh/g rosed by2.9% and 9.2% for fluorinated and Co-Mo coated alloy electrodes, respectively. The charge transfer rate on the surface of the alloy improved owing to the change of alloy surface condition after treatment, the exchange current density I₀ increased by 97.4mA/g（fluorination） and 1313.6 mA/g（electroless plating）. During the alloy electrode charge/discharge cycles, the nickel-rich layer formed after fluorination got destroyed,whereas the alloy after F-duplex surface treatment were still stable, therefore the alloy electrode after F-duplex surface treatment exhibit best overall electrochemical properties,especially the cycling stability.