Study On The Phase Structure And Electrochemical Properties Of La-Mg-Ni Hydrogen Storage Alloys

As the negative electrode of nickel-hydrogen battery,hydrogen storage alloy has a vital influence on the performance of this type of battery.The discharge capacity of current commercial hydrogen storage alloys(rare-earth AB₅ series alloys)has almost approached its limit.Researchers have carried out a lot of work to explore more suitable new hydrogen storage alloys,among which La-Mg-Ni series hydrogen storage alloys with excellent comprehensive properties Shine.However,the Mg element contained in this series of alloys is easy to burn during the smelting process and is difficult to accurately control.In addition,this series of alloy electrodes are easy to pulverize in the process of repeated absorption and desorption of hydrogen,and are easily corroded in the electrolyte,and the cycle stability is poor.Aiming at the above problems,this thesis uses master alloy to replace Mg element to reduce the influence of Mg element burning loss,and studies methods such as ball milling preparation,annealing treatment and surface treatment of anode copper plating to optimize the comprehensive performance of La-Mg-Ni hydrogen storage alloy.When the ball milling time is changed,the high-rate discharge performance of the alloy electrode is mainly affected by the hydrogen diffusion coefficient D;when the ball milling speed is changed,the high-rate discharge performance of the alloy electrode is mainly affected by the exchange current density I₀.The La₄MgNi₁₉hydrogen storage alloy was prepared by ball milling,and it was found that the alloy contained three phases:La Ni₅,La₂Ni₇ and Pr₅Co₁₉.With the increase of ball milling speed,when the milling parameters are 220 r/min and 10 h,the phase abundance of La₂Ni₇ and Pr₅Co₁₉ phase is the highest,which is 51.74%.At this time,the alloy is fully activated in the fifth cycle,C_max=153.5 m Ah/g,S₆₀=65.02%,HRD₉₀₀ is respectively 87.4%,the overall performance of the alloy electrode is excellent.When the milling parameters are250 r/min and 10 h,the high-rate discharge performance is the best.HRD₉₀₀ is respectively91.98%.At this time,the cycle stability of the alloy electrode is the best,and S₆₀ is 69.56%The diffusion coefficient D of hydrogen has a greater impact on the HRD₉₀₀ of the alloy electrode,and the exchange current density I₀ has a lesser impact on the HRD₉₀₀ of the alloy electrode.The La₄MgNi₁₉ hydrogen storage alloy prepared by the ball milling method was annealed,and it was found that with the extension of the annealing time and the increase of the annealing temperature,the phase abundance of La₂Ni₇ and Pr₅Co₁₉ first increased and then decreased.When annealing at 900?for 8 h,these two phases completely disappeared.At this time,the alloy needs 10 cycles of hydrogen absorption and desorption to be fully activated,C_max=155.3 m Ah/g,S₆₀=57.3%,HRD₃₀₀,HRD₆₀₀,and HRD₉₀₀are 99.64%,97.41%,94.01%,respectively.The La Ni₅ phase tends to expand in the process of repeated hydrogen absorption and desorption,resulting in poor powdering resistance of the alloy.Under the condition of annealing for 8 h,with the increase of annealing temperature,the maximum discharge capacity of the alloy electrode gradually increases,from 122 m Ah/g(800?)to 145.7 m Ah/g(850?)and 155.3 m Ah/g(900?).In addition,experiments show that the high-rate discharge performance of the annealed alloy is excellent,with HRD greater than 90%,and with the increase of annealing temperature or the extension of annealing time,its high-rate discharge efficiency will also increase.The high-rate discharge performance of the alloy electrode is mainly affected by the exchange current density I₀,and the hydrogen diffusion coefficient D has little effect on the high-rate discharge.The La₄MgNi₁₉ hydrogen storage alloy electrode prepared by the ball milling method was copper-plated,and it was found that the overall performance of the alloy was the best when copper-plated at a current density of 2 A/dm² for 0.5 h.At this time,the alloy electrode can be fully activated in only 4 cycles,C_max=138.9 m Ah/g,S₆₀=76.67%,HRD₃₀₀,HRD₆₀₀,HRD₉₀₀ are 99.07%,96.37%,93.85%respectively.The copper layer coated on the surface of the alloy electrode forms a passivation film,which prevents the oxidation and passivation of the alloy surface while inhibiting the corrosion of the battery by the electrolyte,and improves the cycle life of the electrode.The S₆₀ of copper plating for 1 h and 2 h was 74.94%and71.61%,respectively.In addition,the deposition of Cu element on the surface of the alloy electrode improved the conductivity and electrocatalytic activity,and increased the rate of charge transfer on the surface of the alloy electrode.The discharge capacity is improved during high current discharge.The discharge efficiency of the copper-plated alloy electrode during high-rate discharge is above 91%.When the discharge current is 300 m A/g and 600m A/g,the HRD₃₀₀ is greater than 98%,and the HRD₆₀₀ is greater than 94%.The HRD of the alloy electrode after copper plating is jointly determined by the hydrogen diffusion coefficient D and the exchange current density I₀.