| Nowadays,the ABs-type rare-earth based hydrogen storage alloy is the most mature anode material,which is applied to MH/Ni secondary batteries,but the cycle life of hydrogen storage alloy is still a problem that restricts its wide application.However,cycle life is a key factor that decides the practical value of the hydrogen storage alloy.In the paper,I mainly study the reasons of capacity decay in hydrogen storage alloy,and the corresponding solutions,such as A-side,B-side composition optimization,the heat treatment measures,to explore the crystal structure and electrochemical properties of hydrogen storage alloys.The contents of PrNd elements with atomic ratio 0,0.1,0.2 replaced the element La respectively.Ensure the A-side component consistency,designed B-side element Co with the atomic ratio 0.75,0.60,0.45 respectively.Analyzed the X-ray diffraction of alloy powders with 50,100 charge-discharge cycles and the fresh alloy powders,which showed the diffraction peak intensity of the fresh alloy powders was stronger and peak type was sharper.On the contrary,the diffraction peak intensity of alloy powders with charge-discharge cycles decreased obviously,and along with a peak width phenomenon,which revealed that the crystalline grains distort and the integrity of the lattice was destroyed.In addition,from the comparison of SEM graphs on the fresh alloy powders and the used alloy powders,we could conclude that the fresh alloy powders had clear and angular outline and somewhat glossiness,and image contrast was obvious,alloy powders had good integrity.The alloy powder surface with 100 cycles appeared a large number of cracks,and had many fine alloy powders.Furthermore,the alloy surface appeared loose.Among the A-side Components varied samples,the electrochemical tests at 1C,2C,3C,4C and 5C discharge current showed that the discharge capacity of the sample without PrNd elements at 1C,2C,3C were higher than those of the other samples.But when the discharge ratio increased to 4C and 5C,its maximum capacity and platform capacity were the lowest.With the increasing of discharge rate,the capacity decay rate of the sample without PrNd increased obviously,from the value of 0.34mAh/g at 1C to 0.92mAh/g at 5C.The minimum increasing of capacity decay rate was the sample with high PrNd content,from the value 0.13mAh/g.cycle at 1C to 0.50mAh/g.cycle at 5C.It revealed that with the increasing of PrNd elements in hydrogen storage alloys,the high-current discharge ability and cyclic stability could be improved.Applied linear polarization method to study the kinetic parameters of the samples with continuous increasing PrNd contents,which got the result that the polarization resistances Rp were 381.83,412.24,490.20m?.g respectively,corresponding exchange currents I0 were 68.38,63.34,53.27mA/g.Obviously,the polarization resistance increases and exchange current decreased with the content of PrNd increasing,and the charge-transfer resistance Rct and exchange current I0 simulated by EIS were in line with the above-mentioned tendency.In addition,the hydrogen diffusion coefficient D decreased,but did not present a certain law.The oxidation reaction rate and reduction reaction rate of alloy electrode showed a trend of decreasing along with the increasing content of PrNd elements in alloys.In the samples with Co content decreasing,the discharge current at 1C,2C,3C,4C and 5C showed that the capacity decay of the sample with high Co content was smallest with the maximum capacity and platform capacity decreasing from 319.3mAh/g and 318.1mAh/g at 1C to 223.1mAh/g and 215.3mAh/g at 5C.The most varying tendency of maximum capacity and platform capacity were the sample,decreasing from 329.7mAh/g and 322.4mAh/g at 1C to 178.1mAh/g and 166.5mAh/g at 5C respectively.The capacity decay rate increased with the Co content decreasing.Therefore,the Co element played an important role on the ability of large-current discharge and cycle stability.Applied linear polarization method to study the kinetic parameters of the samples,which got the result that the polarization resistances Rp were 394.03,325.57,278.10m?.g respectively,corresponding exchange currents I0 were 66.27,80.20,93.89mA/g with Co content decreasing.Thus it could be seen that with the Co content reducing,the polarization resistance decreased,and exchange current increased.The charge-transfer resistance Rct and exchange current I0 simulated by EIS were in line with the above-mentioned tendency.In addition,the oxidation reaction rate,the reduction reaction rate and hydrogen diffusion coefficient of the alloy electrode increased with the Co content to reducing.After heat-treatment,it showed that the maximum capacity and platform capacity of the sample with no PrNd after 850?heat-treatment at 1C,2C,3C were larger than the other two samples heat-treated at 650?,750?,but when the discharge rates were 4C and 5C,the result was opposite.In addition,the capacity decay rate of hydrogen storage alloy heat-treated greatly reduced,which improved the cycle life of alloy electrode effectively.Moreover,after heat-treatment for hydrogen storage alloy,the charge-transfer resistance of alloy electrode increased with the increasing of heat-treatment temperatures,but the exchange current and hydrogen diffusion coefficient appeared a decreasing tendency.In addition,the reaction rate of alloy electrode decreased after heat-treatment,the higher of heat-treatment temperature,the lower of reaction rate.Taken the battery pole pieces into heat-treatment,the results suggested that the higher heat-treatment temperature,the more seriously bonded on the surface of the alloy electrode.Especially when the heat-treatment temperature at 750?,alloy electrode was suffered serious adhesion,and appeared the "clusters" phenomenon,formed lots of closed pore on the surface.The electrochemical performance tests showed that the maximum capacity and the platform capacity of alloy electrodes heat-treated were less than the unheat-treated ones,however,the capacity decay rate of alloy electrodes was reduced after heat-treatment,especially when the heat-treatment temperature at 650 ?,the capacity decay rates at 1C and 2C were the smallest,whose values were 0.32mAh/g.cycle and 0.05mAh/g.cycle respectively. |
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