Study On Preparation And Electrochemical Properties Of La-Mg-Ni-Based Pr₅Co₁₉-type Hydrogen Storage Alloys

La0.75Mg0.25Ni3.8 alloy was prepared by inductive melting method followed by annealing in different conditions. The existing temperature range of La–Mg–Ni-based Pr5Co19-type phase structure was studied. Meanwhile, La-Mg-Ni based Pr5Co19-type single phase alloy and a series of La-Mg-Ni based two phase alloys with Pr5Co19-type main phase were obtained by Step-wise powder sintering. The characteristics about structure and electrochemical properties of Pr5Co19-type single phase alloy and effects of phase structure on electrochemical properties of Pr5Co19-type alloys were investigated.X-ray diffraction(XRD) analysis and Rietveld refinement show that the as-cast La0.75Mg0.25Ni3.8 alloy has three phases, Pu Ni3, Ce2Ni7 and La Ni5. In the annealed alloy under 1073 K the phase abundance of Ce2Ni7 is 58% and that of La Ni5 is 42%. In this process, Pu Ni3-type phase was converted into Ce2Ni7-type phase. When annealed at 1123 K, Pr5Co19-type phase formed and both Ce2Ni7-type phase and La Ni5 phase abundance decreased with Ce2Ni7-type phase converting into Pr5Co19-type phase. The phase abundance of Pr5Co19-type phase in 1173 K annealed alloy increased compared with that of alloy annealed at 1123 K, but it is the contrary to the Ce2Ni7-type phase. When the annealing temperature increased to 1223 K, La5 Mg Ni24 phase appeared and Ce2Ni7-type phase totally decomposed. Both Pr5Co19-type phase and La Ni5 phase decreased. There are two transformation processes, Ce2Ni7-type phase converting into Pr5Co19-type phase and Pr5Co19-type phase converting into La5 Mg Ni24 phase.The La-Mg-Ni based Pr5Co19-type single phase alloy was prepared starting form two precursors La Ni4.8 and La Mg Ni4 with a mole ratio(x) of 1.2. When 1.2﹤x﹤1.5, the prepared alloys has a main phase of Pr5Co19 and a secondary phase of La Ni5. When 0.75﹤x﹤1.5, the prepared alloys has a main phase of Pr5Co19 and a secondary phase of Ce2Ni7. Electrochemical studies show that the Pr5Co19-type single phase alloy has a ordinary discharge capacity(Cmax) of 338 m Ah g-1 and good cycling stability. Its high rate dischargeability at 1500 m A g-1(HRD1500) was 51.5%. When the content of Ce2Ni7-type phase increases from 0 to 40.6 wt.%, the Cmax enhances from 338 to 388 m Ah g-1 and the S100 is improved from 77.0% to 85.3%. With the existence of 20.7 wt.% La Ni5 phase in the alloy, HRD1500 increases to 56%. It is proved that the presence of Ce2Ni7-type phase in the alloy is more favorable to the maximum discharge capacity and cycling stability while the existence of La Ni5 phase has a beneficial effect on the HRD.