Search for carbon additives in Co-free Ni(OH)2 cathodes for NiMH batteries

Abstract

In this study, carbon black and plasma black are used as conductive additives solely or in combination to explore the possibility of replacing cobalt based compounds as additive in NiMH cathodes. Two production methods, namely; ball milling and dispersion methods, are employed to improve the homogeneity and the contact between carbon and the active material. With carbon additives, either singly or in combination the discharge capacity rises steadily with cycling but this behavior changes suddenly resulting in the capacity decay. As a result, Ni(OH)2 fails to reach its full capacity. The origin of this sudden change was investigated in cathodes using charge profiles, linear sweep voltammetry and ex-situ X-ray diffraction. It is concluded that this loss of capacity built-up was due to the formation of γ-NiOOH as a major phase during charging which is associated with a large volume change and concurrent oxygen evolution reaction leading to particle drop-out and hence the capacity decay.