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

摘要

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 10~6 K s~(-1). XRD analysis revealed that over-stoichiometry alloys would segregate TiNi_3 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 TiNi_3, 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.