Study Of Hydrogen Storage Alloys For High-power MH/Ni Battery

High-power MH/Ni batteries show good promise for hybrid electric vehicles, power tools and new type 42V vehicle power supply systems. It is crucial to develop hydrogen storage materials with superior high-rate discharge performance to meet the requirements of high-power MH/Ni batteries. The research and development of materials with superior high-rate performance have obvious significance in both theoretical and practical application aspects.This work aims to improve the high-rate capability of ABs-type hydrogen storage alloys mainly through adjusting non-stoichiometry, doping with boron, molybdenum, or tungsten, and optimizing the rare earth composition, with the mind to enhance the surface electrochemical activity of the alloys and to facilitate the hydrogen transfer in the alloys. X-ray diffraction (XRD), Pressure-composition-temperature (PCT) curves, Second electron image (SEI), Electron probe X-ray microanalysis (EMPA), Pattern recognition methods and several kinds of electrochemical techniques are used to study systematically the relationship between the electrochemical performances of hydrogen storage alloys and their microstructure, thermodynamic, and kinetic characteristics. Some experimental rules to improve the high-rate capability of hydrogen storage alloys have been acquired. Moreover, the alloys with superior high-rate capability have been developed, which show a prospect in the practical use.The study shows that there is a clear relationship between the high-rate capability of hydrogen storage alloys and their microstructure, thermodynamic, and kinetic characteristics. It was found that the precipitation of secondary phase with high catalytic activity in the alloy and/or the alloy with higher surface area, higher catalytic activity and higher hydrogen diffusion coefficient are favorable to improve the high-rate capability.The results for Ce-rich MmNixCoo.75Mno.4Al0.3 (3.055.0 exhibit better high-rate capability which may be ascribed to the lower stability of metal hydrides. With the increase of B/A ratio, the high-rate capability of alloys becomes even better. Furthermore, the over-stoichiometrical alloys show much better cycle stability.For La-rich MmNi3.55Co0.75Mn0.4Al0.3Mx (M=B, Mo, W; x=0.05, 0.1, 0.2, 0.3) alloys, it is found that doping boron leads to the precipitation of CeCo4B-type secondary phase with high catalytic activity, which increase the specific surface area, catalytic activity and hydrogen transfer coefficient, and thus improve the high-rate capability ofthe alloy. The higher the boron content, the better the high-rate capability of the alloy. However, doping boron decrease the electrochemical capacity of the alloy under lower current density because the CeCo4B-type phase cannot absorb/desorb hydrogen reversibly. The alloy doped with molybdenum shows complex variation of phases from single to double and triple with the increase of molybdenum content. Doping molybdenum increase the specific surface, catalytic activity and hydrogen diffusion coefficient, thus improve the high-rate capability of the alloy. Of all molybdenum-contained alloys, the alloy with x=0.2 shows best high-rate capability. The effect of doping molybdenum on high-rate capability is less than that of boron, however, the former one shows smaller electrochemical capacity loss under lower current densities. For the tungsten-doped alloy, the metallic tungsten is directly precipitated and no other secondary phase is observed. After activation, the alloys doped with tungsten shows higher surface catalytic activity and hydrogen diffusion coefficient. However, the precipitated metallic tungsten enhances the anti-pulverizing ability of the alloys, hence, the alloys show smaller specific surface. As the results, doping tungsten can only slightly improve high-rate capability. Additionally, doping boron, molybdenum, or tungsten all decreases the cycle stability of the alloy with different degree.The alloy doped with boron shows prospects...