Improvement in cyclic stability of rare-earth-based hydrogen storage alloys prepared by twin-roller rapid quenching process

A hyper-stoichiometric alloy, La0.65Ce0.28Pr0.02Nd0.05 Ni3.9Co0.7Mn0.34Al0.26 (MlB5.2), was prepared by twin-roller rapid quenching process. Its structure and electrochemical properties were characterized by XRD, SEM and constant current charging/discharge cycles, and were compared with those of a stoichiometric alloy, La0.65Ce0.28Pr0.02Nd0.05 Ni3.8Co0.7Mn0.3Al0.2 (MlB5.0). Both of the alloys have a typical single-phase hexagonal CaCu5-type structure. The cell volume of the hyper-stoichiometric alloy MlB5.2 is the same size as that of the stoichiometric alloy MlB5.0, although its lattice constant c/a is smaller. Better cyclic stability can be reached for MlB5.2 than MlB5.0 as negatives of nickel-metal hydride batteries. This improvement in cyclic stability can be ascribed to the slower charge transfer reaction for MlB5.2 than MlB5.0. There is only 11.1% loss of capacity for MlB5.2 under 2C discharge rate, 640 mA g−1, but 20.2% for MlB5.0. It is found that there is a lower capacity, a higher polarization and larger reaction resistance at the beginning of cycling for MlB5.2 than MlB5.0.