Study on microstructure and electrochemical performance of the MlNi3.55Co0.75-xMn0.4Al0.3(Cu0.75P0.25)x (x=0-0.5) composite alloys

In order to improve the high-rate discharge capability without diminishing the cycle stability, we investigate the substitution of Cu-P for Co on the microstructure and electrochemical properties of the MlNi(3.55)Co(0.75-x)Mn(0.4)Al(0.3)(Cu0.75P0.25)(x) (x = 0-0.5) composite alloys. The results obtained by XRD, SEM and EDS show that the MlNi(3.55)Co(0.75)Mn(0.4)Al(0.3) alloy consists of a single phase, and the substitutional alloys are composed of multiphase structures. The electrochemical studies reveal that the substitutional alloys can be easily activated within one cycle, but the maximum discharge capacity decreases from 296.8 mA h g(-1) (x = 0) to 275 mA h g(-1) (x = 0.5). With the increase of Cu-P content, the capacity decay rate D-100 initially decreases from 1.26 mA h g(-1) cycle(-1) (x = 0) to 0.80 mA h g(-1) cycle(-1) (x = 0.3), and then increases to 1.19 mA h g(-1) cycle(-1) (x = 0.5). The overall high-rate discharge capability is significantly improved with increasing Cu-P content, which is in agreement with the monotonous increase of the exchange current density and the hydrogen diffusion coefficient. The best compromise between excellent high-rate discharge capacity and favorable cycle stability of the series alloys is obtained when x = 0.3. (C) 2013 Elsevier B.V. All rights reserved.