Phase structure and electrochemical characteristics of high-pressure sintered La–Mg–Ni-based hydrogen storage alloys

Abstract

A novel method high-pressure sintering was applied to prepare La0.25Mg0.75Ni3.5 alloy as negative electrode material for nickel/metal hydride battery. The phase structures, electrochemical performance and electrochemical kinetics of the alloys sintered with various pressures have been investigated. When sintered within 1.5–2.5GPa, the alloys have Ce2Ni7-type and Pr5Co19-type main phases and LaNi5 minor phase. Pressurizing promotes the formation of Ce2Ni7-type phase with higher crystalline density. But when the sintering pressure reaches 4GPa, the atomic diffusion is hindered, leading to the rise of LaNi5 phase, appearance of MgNi2 phase, and decrease of Ce2Ni7-type and Pr5Co19-type phases. Electrochemical measurements show that when the sintering pressure changes from 1.5 to 4GPa, the maximum discharge capacity first increases then decreases. The alloy electrode sintered at 2GPa shows superior high rate dischargeability and the gentlest capacity decrease with increasing discharge current density. Furthermore, kinetic study demonstrates that the reaction of alloy electrodes is controlled by charge-transfer step. Cycling stability is deteriorated as the sintering pressure increases due to higher expansion ratio of the cell volume and denser structures of the alloy.