At first the title might sound counter intuitive, but metal alloy corrosion during cycling influences the cycle life of NiMH batteries in two detrimental ways. Firstly, it consumes the electrolyte drying out the battery resulting in increasing internal resistivity. Secondly, the corrosion produces hydrogen and when the hydrogen is absorbed in the MH electrode, it shifts the electrode balancing. This rapidly consumes the overcharge reserve in the battery leading to increased internal oxygen pressures in the end of the charging cycle, which in turn makes the battery to vent through the safety valve, accelerating the drying out of electrolyte. This is aggravated by the shift of the Ni-electrode balance to higher pressures in the PCT curve, as this contaminates the oxygen with hydrogen. The recombination of oxygen is significantly slowed down by the presence of gaseous hydrogen.¹If oxygen could be added externally, it would react with the hydrogen in the metal hydride forming water. This will counter act the drying out by adding water to the electrolyte and at the same time as it restores the electrochemical electrode balance leading to reduced internal pressures when cycling the battery. This was tested in a bipolar NiMH module comprising 10 cells in series supplied by Nilar AB in Sweden. (cf. figure below) A special feature of the design is that it has a common gas space facilitating external addition of gases. By adding an oxygen aliquot every hundreds of cycles, we could eliminate the pressure build up as the battery aged and significantly reduce the increase in internal resistivity over long time cycling.

1) Z. Ye and D. Noréus, J. Power Sources 208, (2012) 232.