Electrochemical capacity of Ni mass when subjected to various conditions, and the relation to changes in the nickel hydroxide phase and crystallite size

Abstract: The electrochemical capacity of the nickel mass in the positive electrode when subjected to various conditions, and the relation to changes in the nickel hydroxide phases and crystallite size was studied in this thesis work. The various conditions being: two different electrolytes, KOH and KOH/LiOH, different temperature during charging and different charge times. The nickel mass was made into monopockets to subject it to these conditions and register the capacity of the mass in a computer, which was connected to the charging station. From the monopocket analysis it could be seen that for most samples the capacity increased with increasing charge time. When the monopocket tests were done, the testing electrodes were cut open to analyse them in an X-ray diffractometer (XRD) to see what nickel hydroxide phases were present in the samples. The analyses showed that the electrolyte with LiOH added to it was more prone to form γ-phase and generated smaller crystallite sizes than the electrolyte without this addition. When comparing the monopocket results and the results from the XRD, it could be seen that the samples with KOH as electrolyte that had lower capacity than expected also had larger amounts of γ-phase. This implies that there was some loss of contact in the active material due to the swelling connected with the γ-phase. When cycling the monopockets further than the standard three times, it was found that the samples that used pure KOH as electrolyte still had γ-phase residues left even after the twentieth cycle whilst the samples with LiOH added to the electrolyte had no γ-phase left. The temperature testing gave a bit inconclusive results but the electrolyte with LiOH additions was clearly superior during these tests, which was no surprise as it has been found to improve the performance of the positive electrode at higher temperatures.