Ym. Sun et al., STUDIES ON THE PROPERTIES AND CHARACTERISTICS OF THE FLUORINATED AB(5) HYDROGEN-ABSORBING ELECTRODE ALLOYS, Journal of alloys and compounds, 253, 1997, pp. 520-524
Fluoride layers have been known to contribute to the selective permeab
ility of hydrogen molecules in the gas-solid reaction and provide a pr
otective barrier against impurities such as water vapor, carbon monoxi
de, air, and others. One of the important roles of the fluorination pr
ocesses is the removal of the oxide layer from the surface which consi
derably improves the initial charge/discharge characteristics. However
, it has also been found that during fluorination, metallic Ni on the
surface dissolves rapidly into the aqueous F- and HF2- ion containing
solution (here after, denotes F-solution) and as the result, the elect
ric conductivity is significantly reduced in the fluoride layer. This
paper is aimed at investigating the effects of fluorination conditions
on the electrochemical properties and characteristics of the AB(5) el
ectrodes for Ni-metal hydride batteries. Special care has been taken t
o control fluoride formation and its thickness on the particle surface
. Ni2+ ions were added to the F-solution in order to protect Ni from b
eing dissolved and lost in the vicinity of the particle surface. The t
hickness of the fluoride layer depends largely upon the specific surfa
ce area to be treated as a function of time. The thickness of the fluo
ride layer and the state of metallic Ni near the surface region was in
vestigated by EPMA and ICPS and they were correlated with the initial
discharge capacity, charge/discharge cycle life, and durability, again
st a 6 M KOH electrolyte solution.