Phase transitions in LaNi4Co during electrochemical cycling an in situ X-ray diffraction study

Phase and crystal structure changes during the electrochemical hydriding and dehydriding of LaNi4Co were investigated using in situ X-ray diffraction (XRD). A specially designed cell allowed dynamic XRD measurements during five charge-discharge cycles. This enabled the direct observation of the activation process. In addition, crystal lattice information derived from these measurements help to explain the long cycle life of cobalt containing AB(5)-type battery alloys. The formation of an intermediate gamma -phase hydride between the hydrogen solid-solution alpha -phase and the fully hydrided beta -phase was clearly observed during absorption and desorption. The volume expansion in the formation of the gamma and beta hydride phases is highly anisotropic. Lattice expansion in the alpha -phase to gamma -phase transformation occurs mainly in the basal plane, whereas the transition from the gamma -phase to the beta -phase causes a lattice expansion in the c axis direction. It is believed that the two-step phase transition in this Co-substituted alloy generates less internal stress than the single-step volume expansion of the archetype LaNi5 compound. This reduces the stress-induced pulverization that occurs during electrochemical cycling. Consequently, the metal hydride electrode maintains larger particle sizes and, thus, smaller surface areas subject to corrosion by the electrolyte, which is the principle cause of capacity loss. (C) 2001 The Electrochemical Society. All rights reserved.