Investigation on the characteristics of La_(0.7)Mg_(0.3)Ni_(2.65)Mn_(0.1)Co_(0.75+x) (x = 0.00--0.85) metal hydride electrode alloys for Ni/MH batteries. Part II: Electrochemical performances

摘要

In this paper, the electrochemical performances of the La_(0.7)Mg_(0.3)Ni_(2.65)Mn_(0.1)Co_(0.75+x) (x = 0.00--0.85) alloy electrodes were studied. The results show that, with increasing amount of Co addition, the maximum discharge capacities of the alloys decrease monotonously due to the relative change of the phase abundance of the (La, Mg)Ni_3 phase and the LaNi_5 phase coexisting in the alloys and the increase of the equilibrium pressure for hydrogen desorption. Moreover, the cycling stability of the alloy electrodes was noticeably improved with increasing Co addition because of the relatively lower value of V_H and the formation of protective oxides (hydroxides). The high rate dischargeability of the alloy electrodes increases first and then decreases with increasing amount of Co addition, and the alloy electrode with x = 0.30 exhibits the best electrochemical kinetics. In addition, the electrochemical kinetic parameters study, including the charge-transfer resistance at the surface R_(ct), the polarization resistance R_p, the exchange current density I_0, the limiting current density I_L and the diffusion coefficient of hydrogen in the alloys D, also confirm this result.