Surface Modification and Discharge Performance of LaNi5-Based Alloy Electrodes

• Main Author: 方慶發
• Other Authors: Perng, T.P.
• Format: Others
• Language: en_US
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/82608149101370132910

碩士 === 國立清華大學 === 材料科學工程研究所 === 85 === In this study, various process conditions were chosen to explore the electrochemical discharge characteristics of the LaNi5-base alloy electrodes. The composition of the test electrodes was LaNi4.25Co0.5V0.1Al0.15. The sample powders were homogenized at 1100℃ for 10 hours before the test. For the electrodes with particle size of 100-325 mesh and various amounts of electroless Ni-P coating, coating with 15wt% resulted in the highest peak capacity 326mAh/g, whereas for the bare alloy, the peak capacity was only 295mAh/g. For the electrodes with particle size of －325 mesh, the peak capacity increased with the amount of Ni-P. This was probably due to an optimum combination of hydrogen permeability and resistivity to corrosion required to get an ideal thickness. The ideal thickness was about 3.3μm. In addition, the effect of pH value on the electroless plating was also examined. It was found that the peak capacity of the alloy with particle size 100-325 mesh coated with 15.1wt% in the solution of pH＝5 increased to 335mAh/g and the charge retention after 50cycles of test was 70.7%. On the other hand, when the same plating condition was performed for particle size of －325mesh, with 19.3wt% coating, the peak capacity was increased to 356mAh/g, which was only slightly lower than the theoretical capacity 361mAh/g. The charge retention after 50cycles of test was 72.1%. When the alloy was plated with 21.4wt.% in the plating solution of pH＝4.52, the peak capacity reduced to 327mAh/g. This was ascribed to the increased content of P in the coating layer due to the decrease of pH value. In general, a certain amount of P in the layer would promote the oxidation resistance. But if the content of P is over a critical value, the electric resistivity will increase and the peak capacity wil decrease. The plating solution with pH＝5 was the best condition. It was also tried to study the as-melted alloy withouit the homogenization treatment. For particle size of 100-325mesh, the peak capacity of the alloy coated with 13wt% was 318mAh/g, higher than the peak capacity 261mAh/g of the homogenized treated alloy coated with 12wt%. In contrast, the peak capacities of the as-melted and annealed alloys of particle size of －325mesh and with about 12-13wt% of coating were 275mAh/g and 356mAh/g, respectively. The as-melted alloy had much lower capacity. The reason was not sure, but could be related to the amount of grain boundaries and the fraction of surface covered with Ni-P. Finally, the alloy coated with Ni-P was subjected to heat treatment in order to get a smoother coating layer and to increase the adhesion of the coating layer to the particles. However, the heat treatment led to crystallization of the amorphous layer, which decreased the oxidation resistance as well as the peak capacity when the cycle time increased.