The structure of Ni(OH)2:: From the ideal material to the electrochemically active one

A structural model is proposed to explain the abnormal broadening of the (10l) and (20l) lines in the X-ray diffraction pattern of nickel hydroxide. This model, based on a hypothesis of the presence of stacking faults, allows us to rationalize the empirically established relationship between the presence of such peculiarities in X-ray diffraction patterns and the good electrochemical behavior of the material. Two types of stacking faults, i.e., growth and deformation faults, corresponding to the existence within the hexagonal oxygen packing of one or two face-centered cubic oxygen sequences, respectively, have been identified. The simulation, with the DIFFaX program, of the X-ray diffraction patterns of nickel hydroxide samples has allowed us to determine in a general way the nature and the amount of stacking faults. It is shown that the stability of protons in tetrahedral sites depends on whether they are in the vicinity of a stacking fault or not, and this explains the improvement of both the chargeability of the material and its electronic conductivity in the presence of defects. It is shown as well that stacking faults in the electrochemically active material lead to a more facile transition to the gamma phase during overcharge in concentrated electrolyte. (C) 1999 The Electrochemical Society. S0013-4651(98)08-048-3. All rights reserved.