Effect Of Mg On Crystal Structures And Electrochemical Cyclic Stability Of La-Ni-based Hydrogen Storage Alloys

To study the effect of Mg on the crystal structure and the electrochemical performances of La–Ni-based alloy,La_0.75Sm_0.25Ni_3.50 and La_0.55Sm_0.25Mg_0.20Ni_3.50.50 alloys consistedofA₂B₇-typesingle-phasestructureandLa_0.75Sm_0.25Ni_3.00.00 and La_0.55Sm_0.20Mg_0.25Ni_3.00.00 alloys consisted of AB₃-type single-phase structure are prepared by inductive melting and powder sintering methods,respectively.The discharge capacity of La_0.55Sm_0.25Mg_0.20Ni_3.50.50 alloy is 364 mAh g^–1 at the first charge/discharge cycle,the activation property and discharge capacity of La_0.55Sm_0.25Mg_0.20Ni_3.50.50 is superior to La_0.75Sm_0.25Ni_3.50.50 alloy.However,the discharge capacity of La_0.75Sm_0.25Ni_3.50 alloy remains unchanged as the charge/discharge cycle numbers increase up to 120,which show better electrochemical cyclic stability.It is found that the expansion of[A₂B₄]subunits volumes of the Mg-containing alloy is bigger than that of the Mg-free alloy,while the expansion of[AB₅]subunits volumes of the two alloys are almost the same,hence the deviation value of[A₂B₄]and[AB₅]subunits volumes and strain value of super stacking structure for the Mg-containing alloy is obviously higher than that of the Mg-free alloy,and Mg-containing alloy is easier to be pulverized during charge/discharge cycles.And the Mg-containing alloy has more apparent cracks of the alloy electrodes at the same cycle numbers from the SEM,which make a superior anti-pulverization of the Mg-free alloy during charge/discharge cycles.Besides,it is observed from the SEM that the presence of Mg hydroxides with a regular hexagon shape make the surface of Mg-containing alloy become loose,and the alloy electrode will be more vulnerable to corrosion.The Tafel curves show that corrosion current density of the Mg-containing alloy is larger than that of the Mg-free alloy,so the Mg-containing alloy is easier to be oxidized.The addition of Mg to some extent increases the discharge capacity,but it decreases the cycling stability because of pulverization and oxidization.Moreover,the charge retention property and high rate discharge ability of La_0.75Sm_0.25Ni_3.50.50 alloy are superior to La_0.55Sm_0.25Mg_0.20Ni_3.50.50 alloy.The research of the La_0.75Sm_0.25Ni_3.00.00 and La_0.55Sm_0.20Mg_0.25Ni_3.00.00 alloys further evidence that the effect of Mg on the crystal structure and the electrochemical performances of La–Ni-based alloy.The effect of Mg on crystal structure and electrochemical properties of AB₃-type alloys are the same as A₂B₇-type alloys.The La_0.55Sm_0.20Mg_0.25Ni_3.00 alloy has superior activation property and discharge capacity,but poorer charge retention property and high rate discharge ability.Besides,the absorption and desorption hydrogen cycling stability of La_0.55Sm_0.20Mg_0.25Ni_3.00 is better than that of La_0.75Sm_0.25Ni_3.00 alloy.