Study On Hydrogen Storage Structure And Hydrogen Storage Properties Of The A₅B₁₉ Type La-Mg-Ni-Al-M Alloys

Nickel-metal hydride（Ni-MH）rechargeable batteries have been widely studied and applied due to their advantages of environmental protection,safety,resistance to overcharge/discharge,high power,and good low/high temperature performance,among them,the La-Mg-Ni system hydrogen storage alloy as the negative electrode shows good application prospects,and the high rate dischargeability and cycling performance of the alloy are enhanced with the increase of[AB₅]subunit ratio.Therefore,in this paper,two La-Y-Mg-Ni-Al hydrogen storage alloys with different Y and Mg contents were prepared based on A₅B₁₉ type alloy.Mn and Fe elements were respectively added on the B side to replace Ni elements based on the alloy with better overall performance,and the effects of Mn and Fe elements on the structure and hydrogen storage performance were investigated to further optimize the electrochemical performance of the alloy and to provide guidance directions for future industrial production.La0.72Y0.13Mg0.15Ni3.70Al0.15 and La0.72Y0.18Mg0.10Ni3.70Al0.15 alloys were prepared by induction melting method and transformed into A₅B₁₉ type phase at 980°C and950°C,respectively.The cell volume became larger when the Y content was higher.The electrochemical properties measurement results showed that the La_0.72Y_0.13-Mg_0.15Ni_3.70Al_0.15 alloy exhibited better overall electrochemical performance.The La_0.72Y_0.13Mg_0.15Ni_3.70Al_0.15 alloy had a high discharge capacity of 382.5 m Ah g^-1 after activation,as well as excellent high rate dischargeability,especially at 5C(1500 m A g^-1)discharge current density,which delivered a discharge capacity of 235.2 m Ah g^-1,while the La_0.72Y_0.18Mg_0.10Ni_3.70Al_0.15 alloy only gived 126.2 m Ah g^-1.During electrochemical cycling,the decay of La_0.72Y_0.13Mg_0.15Ni_3.70Al_0.15 alloy was slow,while the decay rate of La_0.72Y_0.18Mg_0.10Ni_3.70Al_0.15 alloy was fast and then slow.The two alloys have the same discharge capacity after 300 cycles,which is due to the continuous oxidation of the alloy oxide film prevented by the large amount of Y element.Therefore,the decay rate of the later cycle becomes slower.Based on the experimental results in the previous chapter,the La_0.72Y_0.13Mg_0.15-Ni_3.70Al_0.15 alloy was used as the basis,and Mn and Fe elements were used to partially replace the B-side element Ni in the alloy.La_0.72Y_0.13Mg_0.15Ni_3.65Al_0.15Mn_0.05 and La_0.72Y_0.13Mg_0.15Ni_3.65Al_0.15Fe_0.05 alloys were prepared by induction melting method and heat-treated at 950℃and 930℃,respectively.It was found that the cell volume of both alloys increased after the substitution of Ni element.The partial replacement of Ni by Fe element greatly improved the gas-solid hydrogen storage performance of La_0.72Y_0.13Mg_0.15Ni_3.70Al_0.15 alloy,and its hydrogen storage capacity increased from1.23 wt.%to 1.44 wt.%,and the hydrogen absorption/discharge plateau became flat and wide,and the absolute value of enthalpy change decreased.The hydrogen absorption curve became closer to the hydrogen release curve,and the time to reach the maximum hydrogen storage became shorter.The partial replacement of Fe elements also improved cycling stability,with a capacity retention rate of 62.4%after500 cycles.The partial replacement of Ni by Mn element deteriorated most of the properties of the alloy,but it will optimize the low-temperature performance,and the discharge capacity was increased from 4.4 m Ah g^-1 to 75.0 m Ah g^-1 at-40°C.