Study On Structure And Electrochemical Properties Of La-Mg-Ni System La₂MgNi_7.5Co_1.5 Hydrogen Storage Alloy

In this paper,the present situation of research of structure characteristic,hydrogenation behavior,electrochemical properties of La-Mg-Ni system hydrogen storage alloy have been reviewed.On this basis,Crystal structure and electrochemical properties of La-Mg-Ni system La₂MgNi_7.5Co(1.5)hydrogen storage alloy electrode was selected as the subject of this study.The structural characterization of La₂MgNi_7.5Co_1.5alloy and its deuteride were investigated by means of X-ray diffraction(XRD),neutron diffraction(ND)and Rietveld method.Moreover,the paper also put forward some new measures to improve electrochemical properties of La₂MgNi_7.5Co_1.5hydrogen storage alloy electrode,Namely, employing partial substitution of Al for Ni to optimize the alloy composition;employing partial Al element substitution for Mg to optimize the alloy composition;utilizing rapid solidified technique to prepare hydrogen storage alloy;depositing copper film on the alloy electrode surface using electrochemical method.The phase structure,microstructure,surface morphology and composition,size of alloy particle and electrochemical properties of hydrogen storage alloy and its hydride electrode have been studied systematically by XRD, EPMA,WDS,XPS,MasterSizer and electrochemical measurement technique.The microstructure of La₂MgNi_7.5Co_1.5alloy and its deuteride were investigated in detail.It can be found that,the Co atom occupies at 18h and 6c positions in PuNi₃ type structure.The distributing of Co atom mainly located at the Caeu₅ structure unit and the boundary between CaCu₅ and Laves phase structure blocks of PuNi₃ type structure.Rietveld analysis for(La₂MgNi_7.5Co_1.5)D₁₂deuteride indicate that the D atom occupy the 36i₂,18h₅, 6c₄ positions within the CaCu₅ structure unit and the 6c₁,18h₃,18h₁ positions within the Laves phase structure unit in PuNi₃ type structure.The uniaxial volume expansion(c/a=0.88,δc/c=15.8%,δV/V =32.8%)of Laves phase structure unit is bigger than that of(c/a=0.77,δic/c=2.4%,δV/V=17.5%)CaCu₅ structure unit in PuNi₃ type structure.It is the key step of the repression of volume expansion of PuNi₃ type structure alloy by decreasing the rate of anisotropic expansion of Laves phase structure unit.The influence of partial Al substitution for Ni on phase structure and electrochemical properties of La₂MgNi_7.5-xCo_1.5Al_x(x=0.0,0.1 and 0.5)hydrogen storage alloy have also been studied systematically in this paper.The result show that small quantity replacement Ni by Al(x=0.1)do not change phase composition of La₂MgNi_7.5Co_1.5alloy,the alloy is still constituted with the LaNi₃ phase andαLa₂Ni₇ phase,but the abundance of LaNi₃ phase decrease and theαLa₂Ni₇ increase in the La₂MgNi_7.4Co_1.5Al_0.1alloy.While the more Al substitution for Ni in La₂MgNi_7.5Co_1.5alloy changed phase composition of the alloy.LaNi₃ phase disappeared,and appear three new phases,LaNi₅ phase,La₅Ni₁₉phase and LaMgNi₄ phase in La₂Mg_1-xAl_xNi_7.5Co_1.5(x=0.3 and 0.5)alloy.The abundance of LaNi₅ phase increase, the abundance ofαLa₂Ni₇ phase decrease,the abundance of La₅Ni₁₉phase decrease slightly and the phase abundance of LaMgNi₄ increase with the increase of Al content.LaNi₅ phase is main phase in La₂MgNi₇Co_1.5Al_0.5alloy.The lattice parameter a and the unit-cell volume V of LaNi₃ phase increase,but the lattice parameter c decrease,the lattice parameters a,c and the unit-cell volumes V ofα_La₂Ni₇ phase,La₅Ni₁₉phase and LaMgNi₄ phase increase with the increase of Al content,but the influence of the change of Al content on cell parameters of LaNi₅ phase is very small.Al element was mainly dispersed in LaNi₅ phase and La₅Ni₁₉ phase of La₂MgNi_7.5-xCo_1.5Al_x alloy,Al element content in LaMgNi₄ phase is very small.Mg element was mainly dispersed in LaMgNi₄ phase,LaNi₃ phase andαLa₂Ni₇ phase,Mg element content in LaNi₅ phase and La₅Ni₁₉phase is very small.The distributing and content in different phase of Mg and Al element present the characteristic of contradict.With the increment of the Al content in the La₂MgNi_7.5-xCo_1.5Al_x alloy,the maximum discharge current capacity decreases continuously.Al substitution Ni is also beneficial to improve the cyclic stability of the alloy electrode.The high rate dischargeability of La₂MgNi_7.5-xCo_1.5Al_x alloy electrode increases firstly then decreases with the increment of Al content.The influence of partial Al substitution for Mg on phase structure and electrochemical properties of La₂Mg_1-xAl_xNi_7.5Co_1.5(x=0.0-0.5)hydrogen storage alloy have been studied systematically in this paper.The result showed that small quantity replacement Mg by Al (x=0.1)didn't change phase composition of La₂MgNi_7.5Co_1.5alloy,The alloy was still constituted with the LaNi₃ phase andαLa₂Ni₇ phase,but the abundance of LaNi₃ phase decreased obviously and theαLa₂Ni₇ increased in the La₂Mg_0.9Al_0.1Ni_7.5Co_1.5alloy.While the more Al substitution in La₂Mg_1-xAl_xNi_7.5Co_1.5alloy changed phase composition of the alloy,LaNi₃ phase disappeared,αLa₂Ni₇ phase still existed,and appeared a new phase LaNi₅ in La₂Mg_1-xAl_xNi_7.5Co_1.5.(x=0.3 and 0.5)alloy.The abundance of LaNi₅ phase increased and the abundance ofαLa₂Ni₇ phase decreased with the increase of Al content.LaNi₅ phase was the main phase in La₂Mg_0.5Al_0.5Ni_7.5Co_1.5alloy.Moreover,the position that Al atom occupied at the position of the Ni atom,not the position of Mg atom in alloy structure.In fact,the Al substitution for Mg changed merely the Al and Mg content in the meantime in La₂Mg_1-xAl_xNi_7.5Co_1.5alloy.The influence of Al substitution for Mg on cell parameters of different phase of the La₂Mg_1-xAl_xNi_7.5Co_1.5alloy was different,the lattice parameter c and the unit-cell volume V of LaNi₃ phase increased,but the lattice parameter a decreased,the lattice parameter a,c and the unit-cell volumes V ofαLa₂Ni₇ phase increased with the increase of Al content,but the influence of the change of Al content on cell parameters of LaNi₅ phase was very small.The influence of Al substitution for Mg on the hydrogen desorption equilibrium pressure of La₂MgNi_7.5Co_1.5alloy was also Very small,in addition, little replacement of Mg by Al lowered scarcely the hydrogen storage capacity and the maximum electrochemical discharge capacity of the alloy,but with the increment of the Al content,the hydrogen storage capacity,the activation property and the electrochemical discharge capacity of the La₂Mg_1-xMg_xNi_7.5Co_1.5alloy decreased continuously.Al substitution for Mg can improve obviously the electrochemical cyclic stability of La₂MgNi_7.5Co_1.5alloy electrode,and is also beneficial to improve the high rate dischargeability of the alloy electrode.After some times electrochemical charge-discharge cycle,LaNi₃ andαLa₂Ni₇ phase in La₂MgNi_7.5Co_1.5and La₂MgNi_7.5Co_1.5alloys still remain PuNi₃-type structure and Ce₂Ni₇ type structure,however,small new phase La(OH)₃, Mg(OH)₂ and Ni appear in alloys.The structure of LaNi₃ phase hydride andαLa₂Ni₇ phase hydride also remain PuNi₃-type structure and Ce₂Ni₇ type structure.The expansion of LaNi₃ cell andαLa₂Ni₇ phase cell were anisotropic in La₂MgNi_7.5Co_1.5hydride,but the rate of anisotropic expansion and volume expansion of the former was larger than the latter.Compared with the La₂MgNi_7.5Co_1.5hydride,the repression of volume expansion of Al substitution for Mg onαLa₂Ni₇ phase in La₂Mg_0.9Al_0.1Ni_7.5Co_1.5alloy is very small,but the expansion of c axis and cell volume V of LaNi₃ phase lower obviously due to Al substitution. The abundance ofαLa₂Ni₇ phase increase,and that of LaNi₃ phase decrease in La₂Mg_0.9Al_0.1Ni_7.5Co_1.5alloy due to Al substitution for Mg,the anti- pulverization property ofαLa₂Ni₇ phase was superior than that of LaNi₃ phase,and the cell volume of LaNi₃ phase was suppressed,so the anti-pulverization and anti-oxidation property of La₂Mg_0.9Al_0.1Ni_7.5Co_1.5is superior to that of La₂MgNi_7.5Co_1.5alloy.That is main reason of improvement of electrochemical cyclic stability of La₂MgNi_7.5Co_1.5alloy due to small Al substitution for Mg(x=0.1).The effect of melt spun on the phase structure and electrochemical properties of La₂Mg_0.9Al_0.1Ni_7.5Co_1.5hydrogen storage alloys was investigated systematically. La₂Mg_0.9Al_0.1Ni_7.5Co_1.5is composed ofαLa₂Ni₇ main phase with Ce₂Ni₇ type structure and a little LaNi₃ phases.With the increase of quenching rate,the content of LaNi₅ phase increase remarkably and the content ofαLa₂Ni₇ phase decrease gradually;a littleβLa₂Ni₇ phase and LaMgNi₄ phase appeared also in the melt spun alloys.The La₂Mg_0.9Ni_7.5Co_1.5Al_0.1alloys prepared by melt spun were fine columnar crystal structure,the melt spun technique has little effect on the activation property,the maximum discharge capacity C_maxdecrease with the increase of quenching rate,The alloy electrodes with a quenching rate 10m/s and 20m/s show a better cyclic stability.Influence of electrochemical depositing Cu film on the electrode surface on the electrochemical properties of La₂MgNi_7.5Co_1.5hydrogen storage alloy has also been studied systematically.The result showed that the thickness and compactness of coating Cu film increase with the increment of charge-discharge cycle.The copper film of the alloy electrode surface can keep alloy particle in the electrode interior from oxidizing availably,but can not improve the anti-pulverization ability of the alloy.Electrochemical coating Cu film on the alloy electrodes surface lower the activation property and high rate dischargeability of the La₂MgNi_7.5Co_1.5alloy,but has no negative effect on the maximum discharge capacity of the alloy electrodc.Moreover,it is effective to improve the cyclic stability of the alloy electrode utilizing electrodeposit Cu film on the electrode surface.After 120 charge-discharge cycles, the cyclic stability of the La₂MgNi_7.5Co_1.5alloy electrode was improved from 33.81%to 71.95%.