ON THE ORIGIN OF THE 2ND LOW-VOLTAGE PLATEAU IN SECONDARY ALKALINE BATTERIES WITH NICKEL-HYDROXIDE POSITIVE ELECTRODES

The nickel oxyhydroxide electrode (NOE) that acts as the positive elec trode in Ni-based rechargeable alkaline batteries was studied. A surve y of the influence of crystal-chemistry factors (nature and ratio of t he various nickel hydroxide phases), cycling parameters (charge/discha rge rates, charge/discharge cutoff voltages, and percentage of overcha rge), and technological parameters (nature of the current collector, a ctive material morphology, and type of additives) on the appearance of the second voltage plateau was performed. Direct experimental evidenc e shows that the appearance of the second plateau is directly linked t o the amount of the gamma-phase present in the nickel oxyhydroxide ele ctrode prior to its discharge. The occasional appearance of this phase in the electrode results from a poor active material/current collecto r interface related to the electrode-forming technology, or to seconda ry reactions that can lead to a physical disconnection of the active m aterial upon cycling. Based on these findings, the presence of this ga mma-phase accounts for the ohmic drop that has led to previous specula tion of a barrier layer as the origin of the second plateau. Finally, tentative recommendations to eliminate or mitigate the appearance of t he second plateau phenomena are given.