SELF-DISCHARGE CHARACTERISTICS OF SEALED NI-MH BATTERIES USING ZR1-XTIXV0.8NI1.6 ANODES

The self-discharge characteristics of nickel-metal hydride (Ni-MH) batteries, using Zr1-xTixV0.8Ni1.6 (x=0.0, 0.2, 0.3, 0.4) anodes, have been investigated in a modified sealed cell. To measure the self-discharge characteristics of the anode and cathode separately, a modified sealed cell was employed, in which two more electrodes (Pt, counterelectrode; Hg/HgO, reference electrode) were installed. In air at 1 atm at 30 degrees C, the charge retention for both electrodes was measured and it was found that the capacity loss of the anode was greater than that of the cathode by a factor of three after 15 days of open-circuit storage. From the results of X-ray diffraction and thermal desorption spectroscopy, it was found that the self-discharge of the anode can be attributed to the evolution of hydrogen from the MH electrode due to the difference between the hydrogen partial pressure in the sealed cell and the equilibrium hydrogen pressure of the MH electrode. The equilibrium hydrogen pressure of Zr1-xTixV0.8Ni1.6 was less than 1 atm at 30 OC, and the charge retention of the ZrV0.8Ni1.6 alloy, which had the lowest equilibrium hydrogen pressure, was better than that of the other alloys. The influence of the inner gas on the charge retention was observed by applying different gases in the sealed cell. In an Ar or air atmosphere at 1 atm, the charge retention was less than 30% after 20 days. The charge retention was considerably improved by applying hydrogen gas at a pressure higher than the equilibrium hydrogen pressure of the MH electrode. At 1 atm of hydrogen pressure, the charge retention was well above 90%.