An Investigation On The Phase Struture And Electrochemical Properties Of The La-Mg-Ni-based A₂B₇ And A₅B₁₉ Type Hydrogen Storage Electrode Alloys

In this thesis, previous research works on non-AB₅ type RE-based hydrogen storage alloys have been extensively reviewed. On this basis, the La-Mg-Ni based A₂B₇ and A₅B₁₉ -type hydrogen storage electrode alloys were selected as the subject of this study,by means of XRD, SEM and EPMA analysis and the electrochemical test methods, on the basis of of La₃MgNi₁₄, La₄MgNi₁₉-type alloys, the effect of element substitution on alloy structure and electrochemical were investigated systemically.It is hoped to improve the cycle stability by the element substitution and different phase structure.For the La_0.75Mg_0.25Ni_3.5(TiNi₃)_x(x=0.1) alloy, the effect of annealing treatment temperature on structure and electrochemical properties of the alloy were also investigated systematically in this paper. Experiment results showed that all alloys were composed of Ce₂Ni₇-type, Gd₂Co₇-type, LaNi₅-type, TiNi₃-type. the TiNi₃-type was along the grain boundary of main phase in the form of three-dimensional network of the alloy annealed for 1 hour.Incresing annealing time leaded the agglomeration phenomenon for the secondary phase.The appearance of three-dimensional network was useful to the cycle stability and kinetic properties of alloy electrodes. The hydrogen diffusion rate in alloy bulk was mainly controlling factor for the high-rate dischargeability.When x was equal to 0.3, the three-dimensional network phenomenon was abviously ,at the same time ,the alloy had better cycle stability.For the La_0.75Mg_0.25-xTi_xNi_3.5-yMn_y (x=0～0.25,y≥2x),with the Ti and Mn element substitution ,the phase structure and electrochemical properties changed a lot.The original alloy was composed of A₂B₇ type phase (Ce₂Ni₇-type and Gd₂Co₇-type structure).The TiNi₃ phase appeared when Ti was added.When y=0.5,the LaNi₅ phase appeared.Adding excessive Mn was useful to the appearance of LaNi₅ phase. MnNi phase appeared with excessive Ti and Mn .The Ti and Mn substitution had little influence on activated properties,but the maximum discharge capacities decresed abviously.When x=0.25,y=0.5,the maximum discharge capacities of alloy had only 128.32 mAh/g. When y=2x and x≤0.1,the cycle stability had little changed, when x=0.1and y=0.5,the alloy electrode had worse cycle stability, as x = 0.25 and y=2x,the discharge capacities didn't degraation nearly after 100 cycles.The phase structure and the Mg content had important influence on the cycle stability of alloy electrodes.As x and y increseing,the high-rate dischargeability (HRD) of alloy electrodes decresed abviously.It was decresed from 85.13% (x=0) to 13.05%(x=0.25).The hydrogen diffusion rate in alloy bulk was mainly controlling factor for the high-rate dischargeability.Based on La_4-xPr_xMgNi₁₉(x=0～2.0) alloys, Pr content on alloy structure and electrochemical properties were examined systematically. Experiment results showed that the Pr elecment was useful to the formation of La₄MgNi₁₉ phase, the La₄MgNi₁₉ phase abundence incresed from 29.92%(x=0) to 94.31%(x=2.0).At the same time,as the Pr atom radis(0.183nm) is smaller than La atom radis(0.188nm),the cell parameter and cell volume linely decresed with x incresed. As x incresed, the plateau pressure incresed,large Pr content decresed the activated properties. The maximum discharge capacity of the alloy electrodes increaseed first and then decreased with the increase of Pr substitution in the alloys with x=0.4 is the highest (380.14mAh/g). the cyclic stability had been improved with increasing x,the reason was the the pulerization become smaller with incresing x.It was also showed that the A₅B₁₉-type alloy had better cycle stability in La-Mg-Ni based hydrogen alloys. The high-rate dischargeability (HRD) of the alloy electrodes increased first and then decreased with the increase of x. The charage-transfer on the alloy surface was mainly controlling factor for the high-rate dischargeability.