On the misuse of the crystal structure model of the Ni electrode material

Ni-Cd pocket plate single cells have been float-charged at 1.40 and 1.42 V at room temperature and 40 degreesC for a time period of 2-3 years. Subsequ ent ex situ X-ray diffraction measurements were performed on the charged po sitive active Ni electrode material. The appearance of the diffraction pattern of one of the charged electrode m aterials (conventionally named nickel oxy-hydroxide-beta -NiOOH) shows rese mblance to the pattern of discharged nickel hydroxide (beta -Ni(OH)(2)). In situ X-ray diffraction, electrochemical and transmission electron microsco py measurements were used to further investigate this charged phase. Our results clearly show the difficulties to interpret the data from X-ray powder diffraction measurements in order to study and characterise aged pos itive active material, especially in the charged state. The use of structur e models from well crystalline, often synthetically manufactured, powder sa mples in order to describe amorphous material must be questioned. There are numerous parameters that may affect the charge/discharge transformation an d the structural appearance of positive active material. Transmission electron microscopy (TEM) measurements indicate structural dif ferences of the sample subjected to Rietveld refinement on X-ray diffractio n data compared to earlier published structure. (C) 2001 Elsevier Science B .V. All rights reserved.