Preparation, Structure And Electrochemical Properties Of AB₅/Mg₂Ni And AB₅/CNTs Matrix Composite Hydrogen Storage Alloys

With the rapid development of MH/Ni battery, higher demands for hydrogen storagematerials are given forth, such as green environmental protection, low price, highperformance and so on. At present, the AB₅type rare earth-based hydrogen storage alloysare still the primary negative electrode materials of the Ni/MH batteries in our country.However because of the lower discharge capacity and the higher cost, the AB₅type rareearth-based hydrogen storage alloys couldn’t fit the demands putted forward by modernmarket. In order to make further improvement on the ratio of performance to price of theAB₅type rare earth hydrogen storage alloy, AB₅type rare earth hydrogen storage alloywill be compounded with higher hydrogen storage capacity Mg₂Ni alloy and rich specificsurface area of CNTs, respectively. The AB₅/Mg₂Ni and AB₅/CNTs composite alloys areprepared. The microstructures and electrochemical properties of the composite alloys werestudied, at the same time some significant results are given in this paper.In order to gain the best compaction pressures of the preparation of electrode, theeffects of the compaction pressure on the electrochemical performances of the AB₅/Mg₂Nialloys electrodes were investigated. All things considered, the electrode at compactionpressure of11ton exhibits the best electrochemical properties.In order to further improve electrochemical properties of Mm（NiCoMnAl）₅/5wt.%Mg₂Ni composite alloy, the alloys were treated by annealing, rapid quenching and rapidquenching associated with the crystallization processing. The effects and the mechanism ofeffects of the annealing temperature, quenching rate and the crystallization temperature onthe microstructures and electrochemical properties of the Mm（NiCoMnAl）₅/5wt.%Mg₂Nialloys are systematically investigated, respectively. The effects of annealing temperatureon the microstructures and the electrochemical properties of Mm（NiCoMnAl）₅/5wt.%Mg₂Ni alloy showed that as-cast Mm（NiCoMnAl）₅/5wt.%Mg₂Ni alloys composed of theLaNi5phase and a small amount of Mg₂Ni phase. However, annealed at1023K compositealloys consist of the LaNi5phase and （La,Mg）Ni₃new phases. Electrochemical propertiesindicated that the maximum discharge capacity, discharge potential characteristic and highrate discharge performance of the alloys increased firstly and then decreased with increasing annealing temperature. The annealing temperature at1023K electrode exhibitedthe best maximum discharge capacity, discharge potential characteristic and high ratedischarge performance. This is contributed to the formation of the （La,Mg）Ni₃new phase inthe composite. The effects of rapid quenching rate on the microstructures and theelectrochemical properties of Mm（NiCoMnAl）₅/5wt.%Mg₂Ni alloy showed that thecomposite alloys consisted of LaNi5phase, a small amount of Mg₂Ni phase and the newphase of LaNi₃and La2Ni₃. With increasing quenching rate, the intensity of the diffractionpeaks of the LaNi5and Mg₂Ni phases significantly decrease and disappear. However, theintensity of the diffraction peaks of the LaNi₃phase increased firstly and then decreasedwith increasing quenching rate. Electrochemical properties show that the maximumdischarge capacity and cycle stability of the alloy first increases then decreases withincreasing quenching rate. The quenching alloy at15m/s and20m/s exhibits the bestmaximum discharge capacity and cycle stability, respectively. The effects of crystallizationtemperature on the microstructures and the electrochemical properties ofMm（NiCoMnAl）₅/5wt.%Mg₂Ni alloy showed that the crystallization alloys consist ofLaNi5phase and amount of LaNi₃phase. Electrochemical properties show that, comparedwith the quenching alloys, the maximum discharge capacity and the cycle stability ofcrystallization alloys obviously decrease and increase, respectively.Finally, In order to take full advantage of hollow tubular structure of CNTs forhydrogen adsorption and storage performance, the CNTs as conductive materials weremechanically mixed with the Mm（NiCoMnAl）₅alloy powders. The effects of the millingtime of CNTs conductive additives on the microstructures and electrochemical propertiesof Mm（NiCoMnAl）₅alloy electrode are systematically investigated. The results show thatthe maximum discharge capacity, discharge properties characteristic and cycle stability ofalloy electrode first increase then decrease with increasing ball-milling time of the CNTs.The alloy electrode with CNTs milled for2h exhibits the best the maximum dischargecapacity. Furthermore, the Mm（NiCoMnAl）₅/10wt.%CNTs composite alloy are preparedby ball-milling method. The effects of the milling time on the microstructures andelectrochemical properties of Mm（NiCoMnAl）₅/10wt.%CNTs alloy electrode aresystematically investigated. The XRD results show the composite alloy consist of LaNi5 phase and CNTs phase, all diffraction peaks of the LaNi5phase obviously widen anddiffraction peaks of the CNTs phase disappear. Electrochemical properties show that themaximum discharge capacity and high rate discharge abilities first increase then decreasewith increasing ball-milling time. The composite alloy milled for20min has the bestmaximum discharge capacity and the best high rate discharge performance. However, thecycle stability of the alloys decrease with the increasing milling time.