XRD Study On Microstructure Of Positive And Negative Active Materials In Ni-MH Battery

Nickel-metal-hydride (Ni-MH) battery is widely used in electric tools and hybrid electric vehicles due to its high specific energy and power, long cycle life, reasonable price and environmental friendship. The current research mainly focuses on composition, preparation method and macro-performance characterization of the positive and negative active substance, the key materials to the batteries. With the depending of the knowledge, the microstructure of the positive and negative active substance has been paid a lot of attentions. However, the relevant studies are not sufficient enough and the relationship between the microstrutural parameters and the related properties of the Ni-MH batteries is rarely investigated.Based on the relevant modern theory and method of characterizing and analyzing the microstructure of the materials, the wide-angle X-ray diffraction method was used to separate the diffraction peak broadening-effect contributions of the micro grain, the crystallite and the fault. The least-square method was introduced for separating the two-fold broadening effect of the crystallite and the fault along with the three-fold broadening effect of the crystallite, the micro strain and the fault. The relevant calculation program for computing the crystallite shape and size, the micro strain and the fault probability of the positive active substanceβ-Ni(OH)₂ of the Ni-MH batteries and analyzing the crystallite size and the micro strain of the hydrogen storage alloys AB5 as the negative active substance were also put forward and established.With the aforementioned method the cycle number at 20℃and 60℃and the effect of the charge-discharge regime on the microstructure of the positive and negative active substances were investigated. The results show the irreversible change of the microstructure of the positive and negative active substances, the corrosion of the hydrogen storage alloys and the consumption of the electrolyte are the main causes for the capacity decay of the batteries during cycling. According to the study of the cyclic voltammetry, the exchange current density, the charge transfer coefficient and the protonic diffusion coefficient ofβ-Ni(OH)₂, the variation of the electrochemical performance and the kinetic parameters ofβ-Ni(OH)₂ is consistent with that of its micro-structural parameters.By the above method the effects of various positive additives Lu₂O₃,Y₂O₃,CaF₂,Ca(OH)₂ and CaCO₃ on the capacities, the discharge rates, the charge efficiencies at high temperature and the cyclic lives of the type AAA Ni-MH batteries were investigated. The micro-structural variations ofβ-Ni(OH)₂ and the hydrogen storage alloys AB5 after cycling were also analyzed. According to the obtained conclusion, an effective additive, which can improve the cycling performances of the batteries, was found and the possible mechanism was also proposed. The additive CaF₂ inhibits the refinement of theβ-Ni(OH)₂ grain, decreases the micro strain before and after cycling and reduces the variation of the total fault probability. It also suppresses the micro strain of the hydrogen storage alloys, the refinement of grains and the formation of the corrosive resultant rare earths hydroxide. Due to this low price and the significant effects, CaF₂ can be prospecting applied to the Ni-MH batteries.