The Study On Hydrogen-induced Structural Change And Electrochemical Hydrogen Storage Performances Of The Mg-based Amorphous Alloys

Mg-based alloys served as negative electrode of Ni-MH battery have the advantages of high electrochemical capacity(theoretically 999 and 646m Ah/g for Mg₂Ni H₄ and Mg Ni,respectively)and abundant resource.However,Mg-based alloy anodes suffer very serious capacity decay in charge/discharge cycle,which is previously ascribed to the corrosion of Mg in alkali electrolyte.In the past decades,numerous studies have been conducted to improve the cyclic performance of Mg-based alloy electrodes by various methods of preventing the oxidization/corrosion of Mg.Although some progress has been made,it still cannot meet the commercial requirements of Ni-MH battery negative electrodes.In this paper,Mg Ni-based multi-component amorphous alloys were prepared by high-energy ball milling.The stability of the amorphous structure and cycle life were enhanced by means of alloying.XRD,DSC,TG and other methods are used to characterize the structural changes of amorphous alloys during charging and discharging.The main conclusions are as follows:Since the In element has a high solid solubility in the Mg lattice,the effect of partial substitution of In for Mg on the amorphous structure and electrochemical properties of Mg Ni was first investigated.The DSC results show that compared with Mg₅₀Ni₅₀ amorphous alloy,the crystallization temperature of Mg_47.5In_2.5Ni₅₀and Mg₄₅In₅Ni₅₀ amorphous alloys increased from 301?to 367 and 382?,respectively,indicating that their thermal stability is improved.The reason is due to the introduction of In that hinders the relative movement and rearrangement of Mg and Ni atoms to form crystalline phases.The Mg_47.5In_2.5Ni₅₀ alloy electrode has the most excellent overall electrochemical performance and the initial discharge capacity is 432m Ah/g.Compared with the Mg₅₀Ni₅₀ alloy electrode,the capacity retention rate after 30 cycles increases from 11.2%to 38.8%.The addition of In plays an important role in alleviating hydrogen-induced crystallization during the charge/discharge cycles,so the electrochemical cycle life of the amorphous alloy is greatly improved.The effect of Al,Cu,Ti,Sc partially replacing Mg and Ni in Mg₅₀Ni₅₀ alloy on the stability of amorphous structure and cycle stability was studied.And the improving effect on the stability of electrochemical cycling is in the following order:Al>Cu>Ti>Sc.The addition of Cu and Ti improves the thermal stability of the amorphous phase as well as the corrosion resistance of the alloy.The Mg₄₅Al₅Ni₄₅Ti_2.5Cu_2.5alloy electrode has the best overall performance with the maximum discharge capacity of 395.4m Ah/g.The capacity retention rate for 30^th cycle is 50.4%.And the maximum capacity of Mg₄₅Al₅Ni₄₅Ti₅ is 422.1m Ah/g with a capacity retention of 45.7%over 30 cycles.The Mg₄₅Al₅Ni₄₅Ti₅ amorphous alloy was selected to investigate the influence of different charging capacities on the amorphous structure and cycle stability.Under low charging capacity condition of 200m Ah/g,corrosive oxidation occurs at the first cycle and a stable corrosion layer forms on the surface of the alloy particles,resulting in a higher discharge capacity than the charging capacity.The alloy electrode does not decay significantly after 105 cycles,and there are no significant hydrogen-induced volume expansion and powder agglomerate crushing during the cycle.The electrolyte has little effect on the corrosion of the active material,but hydrogen-induced crystallization still occurs slowly within the amorphous alloy particles,leading to a cumulative loss of active material.When the charging capacity increases,the internal hydrogen-induced crystallization process accelerates,the discharging capacity is lower than the charging capacity and the number of stable cycles decreases significantly.When the charging capacity is 500m Ah/g,the hydrogen-induced crystallization intensifies and the charging capacity is higher than the maximum hydrogen storage capacity of the amorphous alloy,resulting in hydrogen evolution reaction during charging and the escape of hydrogen bubbles to break the powder particles,which accelerates the corrosion and oxidation of the alloy and the rapid decay of the discharge capacity of the amorphous alloy.Therefore,further improvement of the structural stability is the key to improving electrochemical hydrogen storage performance of Mg-based amorphous alloy.