Self-discharge behaviour of sealed Ni-MH batteries using MmNi(3.3+x)Co(0.7)Al(1.0-x) anodes

The thermodynamic and kinetic factors controlling self-discharge behaviour of the Ni-MmNi(3.3+x)Co(0.7)Al(1.0-x) (x = 0.0, 0.2) sealed cells have been investigated. A modified sealed cell is employed to measure the charge retention of the anode and cathode separately in the Ni-MmNi(3.3+x)Co(0.7)Al(1.0-x) (x = 0.0, 0.2) sealed cells. It is found that the charge retention of the MmNi(3.3+x)Co(0.7)Al(1.0-x) electrode is the same as the total charge retention and is much smaller than the Ni cathode in air at 1 atm and 30 degrees C. From the results of hydrogen thermal desorption spectra with respect to open-circuit time, it is confirmed that the cause of the self-discharge for the MmNi(3.3+x)Co(0.7)Al(1.0-x) anode is due to the desorption of hydrogen from the metal hydride electrode owing to the pressure difference between the hydrogen partial pressure in the sealed cell and the equilibrium hydrogen pressure of the metal hydride alloy. Also, it is found that the self-discharge behaviour of Ni-MH cells is not correlated with the type or structure of the MH anode. As a diffusion barrier for hydrogen, MmNi(3.3)Co(0.7)Al(1.0) alloy is encapsulated with Cu (which has lower hydrogen diffusivity than MH alloy); however the charge retention of the Ni-MmNi(3.3)Co(0.7)Al(1.0) sealed cell is not improved because hydrogen is desorbed through cracks rather than the Cu-coated layer. The charge retention of the Ni-MmNi(3.3)Co(0.7)Al(1.0) sealed cell is greatly improved by applying 1 atm of hydrogen gas. This pressure is higher than the equilibrium hydrogen pressure of the metal hydride and decreases the driving force for hydrogen desorption from the MH electrode.