The Effect Of Mg And Stoichiometric Ratio On Electrochemical Characteristics Of Ml-Mg-Ni-Based Hydrogen Storage Alloys

In order to obtain a new-type R–Mg–Ni-based (R = RE, Ca or Y) AB3 type hydrogen storage alloys with high discharge capacity and good cycling stability, the effect of Mg content and the stoichiometric ratio on the phase structure and electrochemical characteristics of Ml–Mg–Ni-based hydrogen storage alloys were studied under using Ml (Ml stands for lanthanum-rich misch-metal) instead of pure elemental La in this paper.By means of XRD analysis, Ml_1-xMg_xNi_2.75Co_0.55Mn₍0.100Al_0.10(x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30), Ml_0.88Mg_0.12(NiCoMnAl)_x(x = 3.5, 3.6, 3.7, 3.8, 3.9, 4.0) and Ml_1-xMg_x(NiCoMnAl)_y(x = 0.10, 0.15; y = 3.6, 3.7, 3.8) hydrogen storage alloys were consisted of La₂Ni₇ phase and LaNi5 phase, and some slight shifts of LaNi_2.28 phase were observed.According to the electrochemical measurements results it can be seen that the maximum discharge capacity and the high rate dischargeability (HRD1200) of Ml_1-xMg_xNi_2.75Co_0.55Mn_{0.100Al0.10 (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) hydro- gen storage alloys first increased and then decreased with the increase of x. The maximum values of them were reached 378 mAh/g and 65.0% when x is 0.15, respectively. At 233 K, the discharge capacity of alloys increased from 36 mAh/g (x = 0.05) to 210 mAh/g (x = 0.30). However, the cycling stability of alloys decreased with the inscrese of x, and this was mainly caused by corrsion of Mg element in alkaline electrolyte.With the increase of x, the maximum discharge capacity of Ml0.88Mg0.12 (NiCoMnAl)x (x = 3.5, 3.6, 3.7, 3.8, 3.9, 4.0) hydrogen storage alloys decreased from 378 mAh/g (x = 3.5) to 336 mAh/g (x = 4.0). When x is 3.7, the high rate dischargeability (HRD1200), the hydrogen diffusion coefficient (D) and the low temperature dischargeability (LTD243) of Ml0.88Mg0.12(NiCoMnAl)x (x = 3.5, 3.6, 3.7, 3.8, 3.9, 4.0) hydrogen storage alloy electrodes improved remarkably, andwere up to 52.6%, 5.85×10^-11 cm²/s and 92.9%, respectively. The discharge capacity retention (S200) was up to 83.0% when x was 3.8.The composition of Ml1-xMgx(NiCoMnAl)y (x = 0.10, 0.15; y = 3.6, 3.7, 3.8) hydrogen storage alloys were designed by the principle of cross-over experiment. The electrochemical characteristics of alloys were measured by charging/dis- chargeing at galvanstatic current density. It was indicated that Ml0.850Mg0.15 (NiCoMnAl)3.7 alloy electrode showed the better overall electrochemical characteristics. The maximum discharge capacity was up to 377 mAh/g. The high rate dischargeability (HRD1200) and the low temperature dischargeability (LTD243)} can reach 64.7% and 88.2%, repspectively. The discharge capacity retention after 50th, 100th and 200th cycle (S₅₀, S₁₀₀ and S₂₀₀) was up to 90.4%, 84.3% and 73.3%, repspectively.According to the mentioned results, it can be seen that the discharge capacity, the high rate dischargeability, the low temperature dischargeability and the cycling stability of Ml–Mg–Ni-based multiphase hydrogen storage alloys were improved remarkably by changing the content of Mg and the stoichio- metric ratio.