Effect of Higher Cobalt on Super Lattice Structure and Electrochemical Properties of (La,Mg)₅ (Ni,Co)₁₉ Hydrogen Storage Alloys

Abstract

A series of (La 0.85 Mg 0.15 ) 5 Ni 19-x Co x (x = 3.25∼11.25) alloys were melted by using intermediate alloys in induction melting furnace, and then annealed at 1173K for 8h. The super lattice structure and electrochemical properties of the alloys were systematically investigated. It is found that the x value of 5.25 or 7.25 can help to form super lattice (La, Mg) 5 Ni 19 phase (>92wt%), and when x exceeds 7.25, there appears (La,Mg) 2 Ni 7 phase in alloys. Corresponding to the change of phase structure, the electrochemical properties of the alloys showed the same trend, for example, the alloy with x = 5.25 have good maximum discharge capacity (C max = 394.44mAh/g), capacity retention rate (S 100 = 82.36%) and high-rate dischargeability (HRD 900 = 92.27%). However, further increase of x obviously decreases those properties. The further testing results illustrated that a higher Co content in the alloy can improve its anti-pulverization and anti-corrosion capacity, but decrease the exchange current density and the diffusion coefficient of hydrogen in alloys. This leads to a higher cyclic life and a lower HRD of the alloys with a higher Co concentration.