On the key importance of homogeneity in the electrochemical performance of industrial positive active materials in nickel batteries

A thorough characterisation of three different commercial positive active materials for nickel batteries was carried out both from the crystal chemical and the electrochemical point of view. All the samples had similar physical characteristics and consisted of mixtures of graphite and β-nickel hydroxide with diverse doping levels. Two different electrode technologies (pasted and pocket plate) were used in all cases. Surprisingly, the best capacity values in pocket plate technology (with no extra conducting additives) were supplied by sample A, that is, the one with the lowest doping level. These good performances were attributed to a higher degree of homogeneity in carbon distribution. This hypothesis was confirmed by milling treatments carried out on sample C that did indeed significantly improve its homogeneity and its electrochemical behaviour. In summary, this work shows that homogeneity is clearly an underestimated factor in industrial active materials. Indeed, instead of more expensive dopant addition, low cost milling processes can be used to obtain commercial active materials with improved capacities.