Study of the aging of LaNi3.55Mn0.4Al0.3(Co1−xFex)0.75 (0⩽x⩽1) compounds in Ni-MH batteries by SEM and magnetic measurements

The study of LaNi3.55Mn0.4Al0.3(Co1−xFex)0.75 (0⩽x⩽1) alloys as material for negative electrodes in Ni-MH batteries has shown that the electrochemical cycle life is strongly dependent on the amount of substituted iron. The samples have been characterized before and after 2 to 50 electrochemical cycles by scanning electron microscopy (SEM) and magnetization measurements in order to follow the decrepitation and the decomposition of the alloys. The bulk magnetic properties of the alloy show an evolution from a spin glass behaviour dominated by antiferromagnetic interactions towards a ferromagnetic behaviour as the Fe content increases. After electrochemical cycling, the alloys are partially decomposed into La hydroxide and small metallic and oxidized transition metal particles. A correlation has been established between the loss of electrochemical capacity and the alloy decomposition which reaches 45% after 50 cycles for x=1, whereas it remains limited to 10-15% for x=0 and 0.47. A model combining both SEM and magnetic results has been used to estimate the average thickness of the corrosion layer.