Influences of stoichiometric ratio B/A on structures and electrochemical behaviors of Lao.75Mg0.25Ni3.5Mx （M=Ni, Co; x=0-0.6） hydrogen storage alloys

In order to investigate the influences of the stoichiometric ratio ofB/A （A： gross A-site elements, B： gross B-site elements） and the substitution of Co for Ni on the structures and electrochemical performances of the AB3.5-4.1-type electrode alloys, the La-Mg-Ni-Co system La0.75Mg0.25Ni3.5Mx （M=Ni, Co; x= 0, 0.2, 0.4, 0.6） alloys were prepared by induction melting in a helium atmosphere. The structures and electrochemical performances of the alloys were systemically measured. The results show that the structures and electrochemical performances of the alloys are closely relevant to the B/A ratio. All the alloys exhibit a multiphase structure, including two major phases, （La, Mg）2Ni7 and LaNis, and a residual phase LaNi2, and with rising ratio B/A, the （La,Mg）2Ni7 phase decreases and the LaNi5 phase increases significantly. When ratio B/A-3.7, the alloys obtain the maximum discharge capacities. The high rate discharge（HRD） capability of the alloy （M=Ni） monotonously rises with growing B/A ratio, but that of the alloy （M=Co） first mounts up then declines. The cycle stability of the alloy （M-Co） monotonously increases with rising B/A ratio, but it first decreases slightly then increases for the alloy （M=Ni）. The discharge potential of the alloy （M=Ni） declines with increasing B/A ratio （x〉0.2）, but for the alloy （M-Co）, the result is contrary. The substitution of Co for Ni significantly ameliorates the electrochemical performances. For a fixed ratio B/A=3.7, the Co substitution enhances the discharge capacity from 365.7 to 401.8 mA.h/g, the capacity retention ratio （S100） after 100 charging-discharging cycles from 50.32% to 53.26% and the HRD from 88.65% to 90.69%.