Effect Of Ce And Annealing Treatment On Kinetics And Electrochemical Properties Of Low-Co AB₅-Type Alloys For High-Power Ni/Mh Battery

Nickel/Metal hydride (Ni/MH) secondary batteries as negative electrode materials can not meet the practical requirements of high rate dischargeability. For improving discharge performance of rare earth-based AB₅-type alloys at high discharge current density, the effect of Ce and annealing temperature on the phase structure and electrochemical kinetics was investigated systematically in this paper.The hydrogen storage alloys La_0.90-xCexPr_0.05Nd_0.05Ni_3.90Co_0.40Mn_0.40Al_0.30 (x = 0.10, 0.20, _0.30, _0.40) with low Co content and La_0.77Ce_0.19Mg_0.04Ni4.20Mn_0.40 Co_0.30Al_0.30 with low Co and Mg content were prepared by inductive melting and annealing treatment at different temperature. The final composition was confirmed by ICP, and the phase structure of the alloys was characterized by XRD. The electrochemical test methods included galvanostatic charge-discharge, linear polarization, Electrochemical Impedance Spectroscopy (EIS), anodic polarization, potentialstatic step discharge and cyclic voltammogram etc.XRD analyses indicate that La_0.90-xCexPr_0.05Nd_0.05Ni_3.90Co_0.40Mn_0.40Al_0.30 alloys and La_0.77Ce_0.19Mg_0.04Ni4.20Mn_0.40Co_0.30Al_0.30 alloys consist of hexagonal CaCu5-type structure LaNi5 phase.For the La_0.90-xCexPr_0.05Nd_0.05Ni_3.90Co_0.40Mn_0.40Al_0.30 (x = 0.10, 0.20, _0.30, _0.40) alloys, with increase in Ce content, hydride stability and the maximum discharge capacity decreases, high rate dischargeability (HRD) at 1440 mA/g increases from 44.1% (x = 0.10) to 59.9% (x 0.30) and then decreases to 53.9% (x 0.40), the exchange current density I0 increases firstly from 201.4 mA/g (x = 0.10) to 277.9 mA/g (x 0.30) and then decreases to 208.5 mA/g (x = 0.50), the hydrogen diffusion coefficient D increase, the alloy with x 0.30 has higher kinetics and exhibits better electrochemical properties.For the La_0.77Ce_0.19Mg_0.04Ni4.20Mn_0.40Co_0.30Al_0.30 alloys, annealing treatment could improve their electrochemical kinetics. Compare with as-cast alloy, the desorption plateau of annealed alloy electrodes become lower and flatter. When discharge current density is 1440 mA/g, high rate dischargeability (HRD) of as-cast alloy is 77.5%, HRD of annealed alloy at 1223 K is 82.1%. With the annealing temperature raises further, the exchange current density I0 decreases, the hydrogen diffusion coefficient D increases firstly and then decreases. The order of D value is consistent with that of HRD of the alloy value.