Preparation And Properties Of Mg-based Hydrogen Storage Alloys With Layered Structure

In this paper,Mg/Ni hydrogen storage alloy with layered structure was successfully prepared by solid-phase diffusion reaction method in vacuum tube furnace at 550?under the protection of argon after sintering for 4 hours.The phase composition,distribution and microstructure morphology of the new alloy were analyzed by X-ray diffraction(XRD)?scanning electron microscope(SEM)and energy dispersive X-ray spectroscopy(EDX).The charge and discharge performance,electrochemical performance were tested by NEWARE battery tester and CHI660E electrochemical workstation.In addition,Mg_2-xNi Al_x(x=0.1,0.2,0.3),Mg₂Ni_1-xCu_x(x=0.1,0.2,0.3,0.4,0.5),Mg_2-xNi In_x(x=0.05,0.1,0.15,0.2,0.25)alloys were prepared by element substitution method,and the effects of different components on the microstructure and properties of the alloys were studied.The results show that the layered structure composed of Mg Ni₂ phase,?-Mg phase and Mg₂Ni hydrogen storage phase was formed by solid phase diffusion method between magnesium ingot and nickel powder.The thickness of the inner layer was about 600?m and mainly composed of?-Mg phase;The thickness of the middle layer was about 350?m and mainly composed of Mg₂Ni phase and a small amount of?-Mg phase;The thickness of the outermost layer is about 80?m and mainly composed of Mg Ni₂ phase;In addition,there was a small amount of Mg O phase in the alloy.With the addition of Al element,Mg₃Al Ni₂ phase was formed in the alloy.The more Al element was added,the more new phases were formed,and the structure was compact and evenly distributed.The Mg Ni₂phase disappeared after adding Cu and In elements.When the amount of Cu is small,the phase composition of the inner layer and the middle layer didn't change,but the phase of the outermost layer changed to Mg₂Ni phase and Ni phase.Mg₁₀Ni₉Cu₁₁ phase was formed in the surface layer when the content of Cu is more than 0.3,and the strip structure appeared in the alloy.When the content of In is less than 0.15,the structure of inner layer and middle layer had little change,and the outermost phase is mainly composed of Mg₂Ni phase and Ni phase.When the In content exceeds 0.15,Mg₃In phase was formed in the inner layer.The results of charge-discharge and electrochemical test show that the substitution of Al element is beneficial to the improvement of the discharge capacity and the maximum discharge capacity 49.9 m Ah/g was obtained when the Al addition is 0.3.However it had poor activation properties that the number of cycles needed to reach the maximum discharge capacity increased from 1 to 40 times.In addition,the discharge capacity of Mg_1.7Ni Al_0.3 alloy was 120.4 m Ah/g at 333 K.The maximum discharge capacity decreased with the addition of Cu element,but increased slightly when the content is 0.3.The activation performance was great that only need one charge-discharge cycle to reach the maximum discharge capacity.The cycle stability improved significantly and the capacity retention rate after 50 charge-discharge cycles still maintain 77.93%when the addition amount is 0.5.The discharge capacity was increased to varying degrees after the addition of In element.When the addition amount was 0.25,the discharge capacity was the largest,the activation performance was better,and the cycling performance was also improved.The kinetic experiments show that element substitution can improved high-rate discharge performance,increased exchange current density,and reduced charge transfer resistance.