The Phase Structures And Electrochemical Properties Of Low-Co And Co-free AB₅-type Hydrogen Storage Alloys

In this thesis, the research and development of AB5-type mischmetal-based hydrogen storage electrode alloys have been reviewed, On this basis, low-Co AB5-type alloys ReNi3.5Coo3Mno3Alo.4Feo.5 - = 0-O.4;MSn, Cr) and Co-free AB5-type alloys ANi^Sno.3 (A=La, Mi.; x4.5-5.2) were chosen as objects of study. By means of XRD, SEM/EDS analysis and electrochernical measurements, the effects of the partial substitution of Fe by Sn and Cr, the stoichiometric ratio and annealing treatment on the phase structures and electrochemical performance of the alloys were studied systeimcally.For the ReNi35Co03Mn0.3A104 Feo.5..^Sn^x4i-0.4 alloys, both of the as-cast and annealed alloys consisted of the LaNi5 main phase and some LaNiSn second phase in Sn-containing alloys ,and the amounts of the second phase increased with the increase of the Sn content x, after annealed 5h at 1000 0C the second phase still remained, but the peaks of LaM5 phase became sharper and narrower. The as-cast alloys showed a typical coarse dendrite structure, but this structure disappeared after annealing. With the increase of x, the width of desorbing-hydrogen plateau narrowed, the available capacity of desorbing-hydrogen decreased. When x⁰.2, the desorbing-hydrogen equilibrium pressure of the alloys increased with the increase of x, but it decreased when x increased further. The as-cast alloys was activated in 2-3 cycles, with the increase of x , the maximum discharge capacity and cycling stability decreased, but the high-rate dischargeability of the Sn-containing alloys was improved. Annealing treatment decreased the activation and hi2h-rate dischargeability of the alloys, but improved the cycling stability. Annealing treatment increased the maximum capacity of the alloys when x 0.2,but decreased for other alloys. It was found that the annealed non-Sn-containing alloys showed the best overall electrochemical properties among all the alloys studied: its maximum discharge capacity Cmax289.7mAhIg. high-rate dischargeability HRD60051%, and the capacity retention reached 92.4% after 250 cycles. Among the Sn-containing alloys, the alloy of x⁰.2 showed better properties: its maximum discharge capacity Cmax=249.3rnAhlg, high-rate dischargeability HRD600-55%, and the capacity retention reached 94.3% after 250 cycles.For the ReNi35Co03Mn03A104 Fe0 s^Cr^x=0-0.4 alloys, both of the as-cast and annealed alloys consisted of the LaNi5 main phase arid some LaNi phase and Cr-rich phase in Cr-containing alloys. After annealed 5h at 10000C,the peaks of LaNi5 phase became sharper and narrower, but the second phases still remained. With the increase of x, the microstructure of the as-cast alloys changed from a typical coarse dendrite to a flake or needle structure, but all the structures disappeared after annealing. A small amount of Cr replacing Fe lowered the desorbing-hydrogen equilibrium pressure, but when x increased further, the value of x almost had no effect on the pressure. With theIIIincrease of x, the width of desorbing-hydrogen plateau narrowed, the slope of plateau increased, and the available capacity of desorbing-hydrogen decreased. After annealing, the P-C-T curves became flatter and broader, then the available capacity of desorbing-hydrogen increased .The as-cast alloys was activated in 1 -2 cycles, a small amount of Cr replacing Fe lowered the cycling stability and the maximum discharge capacity. With the increase of x, the high-rate dischargeability of Cr-containing alloys improved. Annealing treatment decreased the activation and high-rate dischargeability of the alloys, but improved the cycling stability. It was found that among the Cr-containing alloys, the alloy of x0.4 showed better properties: its maximum discharge capacity Cmax=20 1 .9mAh!g, high-rate dischargeability HRD600-3 1.4%, and the capacity retention 8217 reached 85.3% after 217 cycles.For the as-cast ANi^Sno3(ALa,M1;x=4.5-S.2) alloys, it consisted of the LaNi5 phase and an unknowing phase which also showed the CaCu5 structure, in addition, there was LaNi...