The Structure Stability And Electrochemical Performances Of The Mg-based Amorphous Hydrogen Storage Alloys

Nickel-metal hydride（Ni-MH）batteries show good electrochemical reaction kinetics,wide operating temperature range and safety.However,its relatively low energy density greatly restricts its application.Mg-based alloys have the advantages of high electrochemical capacity（theoretically 999 m Ah/g for Mg₂Ni H₄）and abundant resource,the utilization of Mg-based alloy electrodes would lead to a huge jump in the energy density of Ni-MH batteries.Therefore,Mg-based alloys is a promising anode material for Ni-MH batteries.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,including surface modification,composite with other materials,alloying with various metals,electrolyte modification and etc.Unfortunately,the progress in improving the cyclic electrochemical performances of Mg-based alloy has been rather limited until now,and it still can not satisfy the requirements for anode materials of commercial Ni-MH battery.Based on the previous work,the present research focus on reveal the capacity fading mechanism of Mg-based alloy electrodes and to explore a new strategy to improve the electrochemical properties of Mg-based alloy electrodes.In addition,the effect of using new electrolytes,tetramethylammonium hydroxide（TMAH）and high concentration LiOH+Li₂SO₄,respectively,on the electrochemical performances of Ni-MH batteries was also investigated,which provides new ideas for improving the cycle life and energy density of Ni-MH batteries.Although high electrochemical capacity was achieved in amorphous Mg-Ni alloys prepared by the ball milling method,the capacity decays rapidly during the charge/discharge cycle.To re-examine the origin of the electrochemical capacity and the mechanism for capacity decay in ball-milled Mg-Ni based alloys,the electrochemical hydrogen storage properties of Mg₂Ni,nanocrystalline Mg₂Ni and amorphous Mg_0.5Ni_0.5alloys were comparatively studied together with their microstructural evolution during cycling.The results indicate that it is the amorphous Mg Ni phase instead of Mg₂Ni phase contributes to electrochemical discharge capacity,because the Mg₂Ni H₄hardly released hydrogen under electrochemical conditions.Then,it has been found that the hydrogenation-induced crystallization is a dominating reason for the electrochemical capacity decay in the milled amorphous Mg-Ni anode,in addition to the electrochemical corrosion.Both hydrogenation-induced crystallization rate and corrosion rate of the milled amorphous Mg-Ni anode are directly proportional to the charge input level.When the charge input is high,the amorphous Mg Ni phase easily crystallized to the nanocrystalline Mg₂Ni H₄,and Mg is easily corroded,which cause rapid decay of the capacity of the amorphous Mg-Ni anode.On the contrary,the crystallization of amorphous phase and corrosion of Mg are not serious at lower charge input,and therefore,amorphous Mg-Ni alloy electrodes show better cycle performance.Ti addition can improve the cycle performance of amorphous Mg-Ni alloy electrodes.To reveal the mechanism for that,the electrochemical properties of Mg_0.50Ni_0.50,Mg_0.45Ti_0.05Ni_0.50and Mg_0.40Ti_0.10Ni_0.50alloys are studied and compared systematically in combination with their microstructural evolution during cycling.The results indicate that Ti addition enhances the stability of the amorphous phase in the alloys,and thus improves cyclic performance of the alloy electrodes by increasing resistance to hydrogenation-induced crystallization.This is different to the mechanism proposed by previous studies that Ti addition does not substantially improve the corrosion resistance of the alloy electrodes.In addition,the Ti addition results in the formation of nanocrystalline Ti Ni phase inside Mg-Ni amorphous matrix,and thus,the alloy electrodes show better electrochemical reaction kinetics and significantly improved high-rate dischargeability（HRD）.At the discharge current density of300 m A/g,the HRD values of Mg_0.50Ni_0.50,Mg_0.45Ti_0.05Ni_0.50and Mg_0.40Ti_0.10Ni_0.50alloys are41.7%,83.7%and 90.3%,respectively.At higher discharge current density of 1200 m A/g,the HRD values are 26.7%,34.8%and 46.2%,respectively.Ti addition enhances the stability of the amorphous phase in milled Mg-Ni alloys,but not substantially improve its corrosion resistance.Thus,the electrode still suffers from serious capacity decay.To decrease the corrosion rate of the Mg-based alloy electrodes,the present work demonstrates a new electrolyte（TMAH）.By using 4.5M TMAH electrolyte,Mg_0.4Ti_0.1Ni_0.5alloy anode shows an improvement cycle stability due to effectively inhibited corrosion of it in comparison to 6M KOH electrolyte.After 100 cycles,the discharge capacity of Mg_0.40Ti_0.10Ni_0.50alloy electrode in 4.5M TMAH electrolyte is 210 m Ah/g,higher than that of in 6M KOH electrolyte（69 m Ah/g）.And the self-discharge of Mg_0.40Ti_0.10Ni_0.50-Ni（OH）₂cell in 4.5M TMAH is much smaller than that in 6M KOH.Moreover,with the addition of Cu（OH）₂to the 4.5M TMAH electrolyte,fine particles of Cu can be coated on the surface of Mg_0.4Ti_0.1Ni_0.5alloy electrode during charging process by electrodeposition,further improve the cycle stability of Mg_0.40Ti_0.10Ni_0.50alloy electrode and increases its electrochemical reaction rate.The discharge capacity of the Mg_0.40Ti_0.10Ni_0.50alloy electrode in 4.5M TMAH+0.01M Cu（OH）₂electrolyte is 313 m Ah/g after 100 cyclesFinally,the present work use high-concentration LiOH+Li₂SO₄solution as the electrolyte for Ni-MH batteries,to increase the discharge voltage of Ni-MH batteries.As the concentration of LiOH increases in LiOH+Li₂SO₄solution,the solubility of Li₂SO₄decreases and the ionic conductivity of the solution increases.The electrochemical stability window of high-concentration LiOH+Li₂SO₄solution is larger than that of 6M KOH and 5M LiOH solutions.The full Ni-MH cell using an AB₅anode and a Ni（OH）₂cathode in the 5M LiOH+0.8M Li₂SO₄electrolyte shows higher discharge voltage than that in 6M KOH electrolyte.It increases from 1.28 V for 6M KOH to 1.34 V for 5M LiOH+0.8M Li₂SO₄electrolyte.Using Mg_0.40Ti_0.10Ni_0.50alloy of higher electrochemical capacity as anode materials and 5M LiOH+0.8M Li₂SO₄as electrolyte,the full cell can delivers an energy density of 169 Wh/kg.In addition,the corrosion rate of Mg_0.40Ti_0.10Ni_0.50alloy electrode in5M LiOH+0.8M Li₂SO₄electrolyte is less than that in 6M KOH electrolyte.Therefore,the use of 5M LiOH+0.8M Li₂SO₄electrolyte not only improves the energy density of Ni-MH batteries,but also improves its cycle life.