Optimization Of Microstructure And Electrochemical Performance Of Co Free AB₅ Type Hydrogen Storage Alloys

Nowadays,AB₅ rare earth hydrogen storage alloys have been widely used in commercial MH-Ni batteries because of their good discharge capacity,stable cycle stability and favorable hydrogen absorption and desorption kinetics.Although existing AB₅ hydrogen storage alloys have been used in production,the production cost of MH-Ni batteries containing precious metal elements such as Co and Ni in the alloys is too expensive,which brings problems to the popularization and use of MH-Ni.In this paper,Co-free AB₅ hydrogen storage alloy was selected as the research object.After adjusting the stoichiometric ratio of La and Ce,Ni in the alloy was replaced by cheap Cu element,and then the selected alloy was modified by adding self-made 3D carbon frameworks and annealing process.La_1-xCe_xNi_4.2-yCu_yMn_0.5Al_0.3?x=0.2-0.5;y=2-4?alloys were prepared by vacuum induction melting.The alloys that have the best properties were modified by self-made 3D graphene addition and annealing process.XRD,BSE,HRTEM and EDS were used to observe and analyze the phase composition and microstructure of the experimental alloys.The discharge capacity,cycle stability and high rate discharge performance of the alloys were tested by battery tester.The exchange current density I₀,charge transfer impedance R_ct,limit current density I_L and hydrogen diffusion coefficient D were tested by electrochemical workstation.La_1-xCe_xNi_4.2-yCu_yMn_0.5Al_0.3?x=0.2-0.5;y=2-4?alloys are composed of CaCu₅ type LaNi₃CuAl main phase and LaCu₂ second phase.With the increase of Ce and Cu substitution,the cell volume of LaNi₃CuAl phase decreases,and the cell axis ratio?c/a?increases,so that the hydrogen absorption and desorption expansion rate of the alloys decreases.The maximum discharge capacity of the alloys decreases with the increase of Ce and Cu substitution,but the cyclic stability increases,the maximum discharge capacity of the alloy is 220 mA/g,S₅₀=55.05%.In conclusion,La_0.7Ce_0.3Ni_2.2Cu₂Mn_0.5Al_0.3.3 alloy has the best electrochemical properties.With the increase of Ce substitution,the dynamic properties of the alloys improve first and then decrease.The increase of Cu substitution will decrease the dynamic properties of the alloys.The charge movement rate on the surface of the alloys is the main factor affecting the dynamic properties of the experimental alloys.The self-made 3D carbon frameworks can be well mixed in the La_0.7Ce_0.3Ni_2.2Cu₂Mn_0.5Al_0.3.3 alloy.However,some Fe elements doped in the self-made 3D carbon frameworks will be introduced into the experimental alloy.With the increase of the amount of self-made 3D carbon frameworks in the alloy,the maximum discharge capacity and cycle stability of the alloy increase first and then decrease,the maximum discharge capacity of the alloy is 208.1 mA/g,S₅₀=81.40%.But the high rate performance of the alloy decreases gradually.For the La_0.7Ce_0.3Ni_2.2Cu₂Mn_0.5Al_0.3+z%self-made 3D carbon frameworks?z=2-10?alloys,the electrochemical and kinetic properties of the alloys are both affected by self-made 3D carbon frameworks and Fe element.With the increase of annealing temperature and the prolongation of annealing time,La_0.7Ce_0.3Ni_2.2Cu₂Mn_0.5Al_0.3.3 alloy gradually transforms from polycrystalline structure to CaCu₅ single crystal structure.The uniformity of composition and structure of the alloys are better,the crystallinity is higher and the internal stress of the alloy is eliminated well.Besides,more Cu in the alloy will enter LaNi₃CuAl phase,which all these factors affect the electrochemical and kinetic properties of the alloy.With the increase of annealing temperature and time,the discharge capacity of La_0.7Ce_0.3Ni_2.2Cu₂Mn_0.5Al_0.3.3 alloy increases first and then decreases,and the cyclic stability of the alloy improves gradually,but the high rate discharge performance of the alloy decreases continuously.When the annealing condition is 1274 K+4 h,the alloy has the best maximum discharge capacity of 228.9mA/g.