| The rare-earth based AB5-type hydrogen storage alloy is one of the most promising candidates in MH/Ni secondary batteries due to their favorable overall electrochemical properties. But the high-rate discharge capability can not meet the requirement of the dynamic power which is rapidly developed in recent years. In order to obtain the low-Co AB5-type hydrogen storage alloy with excellent electrochemical performance, the element substitution method was adopted. The purpose of this work is to improve the high-rate discharge capability without diminishing the cycle life. In this paper, we investigated the influence of the partial substitution of Cu-P for Co on the microstructure and electrochemical performance of the composite alloys MlNi3.55Co0.75-xMn0.4Al0.3(Cu0.75P0.25)x (x= 0,0.1,0.2,0.3,0.4,0.5). Furthermore, the effect of different annealing temperatures and times on the microstructure and electrochemical properties of the MlNi3.55Co0.45Mn0.4Al0.3(Cu0.75P0.25)0.3 alloy was studied.Generally speaking, it is difficult to measure the reacation rates of the hydrogen storage alloy electrode. For this problem, a convenient method was proposed to quantitatively investigate the rate constants of the charging/discharging processes for the metal hydride electrodes based on the energy barrier theory. The transfer coefficients obtained from the slope of the Tafel polarization branch can deduce the changes of the standard Gibbs free energy of the charging/discharging reactions. The Arrhenius equation can be used to calculate the corresponding rate constants of the charging/discharging reactions. The proposed method can be usefully applied as a convenient tool to quantitatively investigate the reaction rates of the MH/Ni secondary batteries and predict kinetics properties for further development of other batteries. Furthmore, the electrochemical impedance spectroscopy was adopted to investigate the hydrogen adsorption process which was neglected in the previous work. The results demonstrated that the hydrogen adsorption process could be the limiting step of the electrode reaction.Based on the above methods, the influence of the Cu-P content on the microstructure and the battery performance was investigated firstly. The results indicated that the MlNi3.55Co0.75Mn0.4Al0.3 alloy consisted of LaNi5 phase, and the substitutional alloys were composed of multiple phase structures:LaNi5 phase, P-rich phase, Mn-rich phase. With the increase of Cu-P content, the abundance of the precipitated phase increased. Meanwhile, the results of the battery performance showed that the maximum discharge capacity decreased from 296.8 mAh/g (x= 0) to 275 mAh/g (x= 0.5); the minimum value of the capacity decay rate was 0.80 mAh/gcycle (x=0.3); the high-rate discharge capability was significantly improved with the increase of Cu-P content. At discharge current density 1500 mA/g, the values of HRD increased from 48.23%(x=0) to 67%(x=0.5).Secondly, the influence of the Cu-P content on the kinetics properties of the substitutional alloys was studied. The rate discharge capability is determined by the kinetics properties of the charging/discharging reactions. However, the previous investigations reported that the kinetics parameters of the charging/discharging reactions existed large differences. Therefore, several electrochemical methods were adopted to investigate the kinetics parameters of the metal hydride electrodes in this paper. The linear polarization curves were employed to study the charge-transfer process. With the increase of Cu-P content, the charge-transfer resistance Rct decreased from 421.9 mΩ·g (x=0) to 173.4 mΩ·g (x= 0.5). and the corresponding exchange current density I0 increased from 61.88 mA/g (x=0) to 150.5 mA/g (x=0.5). The impedance spectroscopy was used to study the hydrogen adsorption process, and the hydrogen transfer resistance Ra remarkably decreased from 149.1 mΩ·g (x= 0) to 7.67 mΩ.g (x 0.5). which proved that the electrocatalytic activity of the alloy electrode was improved as the partial substitution. The galvanostatic intermittent titration technique (GITT). cyclic voltammograms (CV). electrochemical impedance spectroscopy (EIS) methods were applied to characterize the hydrogen diffusion process of the composite alloys. The obtained hydrogen diffusion coefficient increased with increasing x. but the specific values were slightly different, which was mainly attributed to the difference of the experimental principle for the various methods. With the increase of Cu-P content, the side reactions of the metal hydride electrodes were efficiently inhibited, and the rate constants of the oxidation/reduction reactions increased, which demonstrated that the kinetics properties of the charging/discharging processes were improved.Finally, the influence of the annealing treatment on the microstructure and electrochemical properties was investigated. The ingots had vast crystal defects and large lattice stress because of the slow cooling rate, which had negative influence on the cycle stability of the hydrogen storage alloys. In order to overcome these drawbacks, annealing treatment was employed. The MlNi3.55Co0.45Mn0.4Al0.3(Cu0.75P0.25)0.3 alloys were annealed at 1073,1173,1273 and 1373 K respectively, and the treatment time was 8 h. The results indicated that the main phase of all alloys was LaNi5 phase, and the composition of the hydrogen storage alloys gradually spread evenly. When the temperature was 1373 K, some low melting elements formed the precipitated phased. The minimum value of the capacity decay rate was 0.74 mAh/g·cycle at 1273 K. At the discharge current density of 300 and 600 mA/g, the rate discharge capacity decreased with increasing annealing temperature. At the discharge current density of 900,1200, 1500 mA/g, the rate discharge capacity achieved the minimum value at 1273 K Furthermore, the influence of the annealing time on the microstructure and electrochemical performance for MlNi3.55Co0.45Mn0.4Al0.3(Cu0.75P0.25)0.3 alloy at 1273 K was investigated. The annealing time was 2,4,6 and 8 h respectively. The results showed that the main phase was still LaNi5 phase, and the composition of the hydrogen storage alloys gradually spread evenly. The minimum value of the capacity decay rate was 0.56 mAh/g·cycle for 4 h. At the discharge current density of 300 and 600 mA/g, the annealing time had no business with the rate discharge capacity. At the discharge current density of 1200 and 1500 mA/g, the rate discharge capacity decreased with increasing annealing time. |
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