Investigation Of High Performance Hydrogen Storage Alloys For Nickel-Metal Hydride Batteries

With the development of society and economy,the consumption of fossil energy is increasing,and the problems of energy crisis and environmental pollution are becoming more and more serious.In order to solve these challenges,it is urgent to explore and develop new energy technologies,which are sustainable and environmentally friendly.As an important representative of new energy technologies,electric vehicles are of great strategic significance for expanding energy sources and improving environmental pollution.However,the popularization and promotion of electric vehicles are restricted by multiple factors,such as high cost,poor low temperature performance,short mileage and long charging time,etc.The energy storage and conversion devices of electric vehicles include lithium-ion batteries,nickel-metal hydride batteries,fuel cells,etc.Among them,the nickel-metal hydride batteries,as a mature secondary battery technology,have high safety,excellent low temperature performance,good assembly performance,abuse-resistant,high recyclable value,environmental friendliness and other advantages,which have been widely used in new energy vehicles,power tools,consumer electronics,emergency devices,military equipment and other fields.However,it is in a disadvantageous position in the market competition with lithium-ion batteries due to their lower specific energy.It is necessary to improve the cycle life and discharge capacity in order to improve their market competitiveness.As the negative electrode material for nickel-metal hydride battery,hydrogen storage alloy is the key factor restricting its performance.However,the cycle life of commercial hydrogen storage alloys(MmNi_3.55Co_0.75Mn_0.4Al_0.3)is only about 500 cycles,which seriously limits the usage cost reduction of Ni-MH battery.In this paper,the dominant factors affecting the cycling life of hydrogen storage alloys are determined by theoretical analysis and the design of related experiments.The mechanism about the corrosion process and capacity deterioration of hydrogen storage alloys is clarified.Ultra-long cycle life alloys with 1407 and 2415 cycles have been developed.However,the discharge capacity decreases with the increase of cycling life for above hydrogen storage alloys.To solve this problem,the design principles for enhancing the discharge capacity and cycling life of hydrogen storage alloys are proposed through theoretical analysis.Mg can accurately substitute the position of Ni inside the alloy by adjusting the stoichiometric ratio and Mg content,so as to achieve the purpose of enhancing discharge capacity and cycling life.Around the above,this paper can be extended into the following three parts:1.Research on hydrogen storage alloys with long cycle life and its influencing factors.Based on the principle of electronegativity,La with lower electronegativity is partially substituted by Y with higher electronegativity to improve the corrosion resistance of hydrogen storage alloys.It is found that the cycle life is inversely related to its corrosion degree by electrochemical testing and SEM observation of alloy surfaces after cycling.However,there is no obvious pulverization phenomenon after cycling,which strongly supports that corrosion is the dominant factor leading to the capacity degradation of hydrogen storage alloys,rather than pulverization.The above experimental results also correct the traditional cognition that the pulverization is the key factor to the cycling life of hydrogenstoragealloys.Amongthem,thehalf-cellcyclelifeof La_0.55Ce_0.3Y_0.15Ni_3.7Co_0.75Mn_0.3Al_0.35.35 alloy is up to 1407 cycles.It is estimated that the usage cost of the Ni-MH battery based on this alloy is only 1/3 that of the lithium-ion battery.2.Research on the hydrogen storage alloy with ultra-long cycle life and its capacity deterioration mechanism.The mechanism of corrosion process is analyzed according to the huge difference in electronegativity between the A-side and B-side elements,and points out that A-side and B-side elements corrode in the first and second stages.It plays a critical role in the second stage for enhancing alloy's anti-corrosion ability due to the higher corrosion resistance of B-side elements.Based on the non-stoichiometric characteristics of the AB₅-type hydrogen storage alloy,the substitution of La with Ni is proposed to increase the atomic coordination number of Ni in the second stage and enhance its corrosion resistance.The simulation results of density functional theory?DFT?also prove that Ni has better corrosion resistance than La,and Ni with higher coordination number has better corrosion resistance than that with lower coordination number.The experimental results show that the cycle life of alloy La_0.73Ce_0.17Y_0.1Ni_3.75Co_1.0Mn_0.3Al_0.35 with the largest Ni-substitution is up to 2415 cycles.It is estimated that the usage cost of the Ni-MH battery based on this alloy is only 1/5 that of the lithium-ion battery.3.Research on the hydrogen storage alloy with high capacity and long cycle life.The discharging capacities of above-mentioned hydrogen storage alloys decrease while enhancing their cycle lives.To solve this problem,on the basis of theoretical analysis,it is proposed that the design principles of improving the discharge capacity and cycle life of hydrogen storage alloys are to reduce average elemental electronegativity in tetrahedral and octahedral interstice and make the Ni atoms on their surface maintain high coordination numbers in the second stage.In this chapter,the structural positions of Ni inside the CaCu5-type hydrogen storage alloy are accurately substituted by Mg,which reduce their electronegativity and make Ni maintain high surface coordination number in the second stage,so that the hydrogen storage alloy has both high discharge capacity and long cycling life.The as-designed alloy La_0.62Mg_0.08Ce_0.2Y_0.1Ni_3.25Co_0.75Mn_0.2Al_0.3 has a discharge capacity of 326.7 mAh g^-1 and a cycling life of 928 cycles.It is estimated that the usage cost of the Ni-MH battery based on this alloy is only 60%that of the lithium-ion battery,which is expected to be used in Ni-MH batteries for the replacement of disposable dry batteries in the retail fields.Meanwhile,it is the first to point out that Mg can not only substitute the structural position of La but also that of Ni in the CaCu₅-type hydrogen storage alloy.The stoichiometric ratio and the amount of Mg are the key factors to decide which structural position is occupied by Mg.