Study on structure and electrochemical performance of melt-spun non-stoichiometry alloys Ml(NiCoMnTi)5+X

The structure, secondary phases and electrochemical properties of over-stoichiometry alloys Ml(NiCoMnTi)5+X prepared by melt-spinning method were investigated. The microstructure of alloys with different X was a fine columnar structure as solidified at the high cooling rate of about 106 K s−1. XRD analysis revealed that over-stoichiometry alloys would segregate TiNi3 secondary phase as the X⩾0.4 and segregated Ni as the X⩾0.7. XRD analysis also indicated that the cell parameters were changed and the cell volume contracted with the X increased from 0 to 1.0. Electrochemical testing showed that the capacity of alloys Ml(NiCoMnTi)5+X were closely related to the X, and the capacity of alloys was decreased quickly from 310 to 155 mA h g−1 as the X increased from 0 to 1.0. But all of the alloys with different stoichiometry X were activated within 5 charging/discharging cycles. Cycle life of the alloy was greatly prolonged as the X increased. The capacity decay rate kept very small as the X reached above 0.7, which was ascribed to the small cell expansion and pulverization in charging/discharging process due to their small capacity. High rate dischargeability of alloys was obviously improved as the X increased due to the secondary phase, both Ni and/or TiNi3, segregated in the grain boundary to help to conduct the hydrogen atoms. Among the alloys Ml(NiCoMnTi)5+X investigated, Ml(NiCoMnTi)5.1 (X=0.1) had the best comprehensive electrochemical properties with high discharge capacity and longer cycle life and suitable for practical use in Ni/MH batteries.