Hydrogen Storage And Electrochemical Properties Of AB₅ Alloy Influenced By Stoichiometry And Heat Treatment

AB₅-type hydrogen storage alloy has been widely used as anode material for nickel metal hydride battery due to its superior comprehensive properties.With the rapid economic development,the nickel metal hydride battery market require the commercial AB₅ alloys with lower cost,better cycle life and low temperature performance.In this paper,the properties of La0.585Ce0.19Zr0.025Sm0.2Ni4.337Co0.163Mn0.253Al0.350 alloy were optimized by changing the stoichiometric ratio of alloys and annealing process conditions.Firstly,the La0.582Ce0.191Zr0.025Sm0.202（Ni0.849Co0.032Mn0.05Al0.069）5+x（x=0.00,0.10,0.20,0.35 or 0.47）hydrogen storage alloys are synthesized by vacuum induction melting.XRD results show that these alloys contain Ca Cu₅-type La Ni₅ phase and a small amount of Bi F₃-type Ni₂Mn Al phase.There are also lots of XRD refinement,thermodynamics and electrochemical performance tests for these La0.582Ce0.191Zr0.025Sm0.202（Ni0.849Co0.032Mn0.05Al0.069）5+x（x=0.00,0.10,0.20,0.35 or 0.47）series alloy.With the increase of x（x=0.00～0.35）,the La Ni₅ cell volume V decreases and the charge acceptance capacity of the alloy electrode decreases,thus reducing the values of maximum discharge capacity C_max and peak power density P_peak of the alloy electrode.Meanwhile,with the increment of x,the hydride stability of AB_5+x alloy gradually decreases,and the self-discharge performance also becomes worse.In addition,the increasing x of AB_5+x alloy also results in the increment of La Ni₅ cell anisotropy factor c/a,weakens the electrode polarization degree,promotes the cycle stability of the alloy electrode.Moreover,the high rate discharge ability（HRD）jointly controlled by charge transfer and hydrogen diffusion kinetic processes decreases at first and then increases.What’s more,electrochemical tests show that AB_5.20 alloy has excellent comprehensive properties at room temperature（25°C）,but the electrode polarization is the most serious at-20°C and-40°C,the discharge platform potential is the lowest,and the discharge capacity is the least.In order to meet the performance requirements of commercial alloys in the wide temperature range,the electrochemical properties of AB_5.20 alloy with excellent comprehensive properties at room temperature（25°C）were optimized at low temperatures（-20°C and-40°C）by adjusting the annealing temperatures（900°C,1000°C and 1100°C）.The XRD results show that the as-cast and heat-treated AB_5.20 alloys consist of La Ni₅ main phase and Ni₂Mn Al second phase,and the content of Ni₂Mn Al phase decreases gradually with the increase of annealing temperature.When the annealing temperature increases to 1100°C,the Ni₂Mn Al phase disappears.Electrochemical tests show that the activation performance of single-phase alloy is poor,and the synergistic action of two phases in the alloy can optimize the activation performance.With the increasing of annealing temperature,the discharge capacity,self-discharge,peak power density and high rate discharge ability of heat treated AB_5.20 alloy electrode all decline,but the cycle life of the alloy is improved.Among the as-cast alloy and all the heat-treated alloys,the annealed alloy at 900°C has the best discharge and high rate properties at room temperature and low temperature.