The microstructures and electrochemical properties of non-stoichiometric low-Co AB₅ alloys containing small amounts of Mg

Two non-stoichiometric low-Co AB₅ alloys, Ml _0.95Mg_0.05Ni_3.8Co_0.3Mn _0.3Al_0.4 (AB_4.8) and Ml_0.95Mg _0.05Ni_3.6Co_0.3Mn_0.3Al_0.4 (AB_4.6), were designed to reduce the cost of AB₅ alloys. The maximum discharge capacities of the two low-Co alloys can reach 307 mAh/g for AB_4.8 alloy and 298 mAh/g for AB_4.6 alloy, respectively. After 300 cycles, the capacity decay is 23% for AB_4.8 alloy and 19.1% for AB_4.6 alloy. XRD patterns and the microstructure, examined by optic microscope and SEM, show that both of the two low-Co alloys are composed of three phases, LaNi₅ matrix phase, AlMnNi₂ secondary phase and LaNi tertiary phase. For the two low-Co alloys, the microhardness (HV) of the secondary phase are only half of that of the matrix phase, which could resist the crack propagating and make the alloys not amenable to pulverization. The small amount of Mg promotes to form the soft secondary phase, which is believed to be the reason for improving the cycling stability.