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

Abstract

Ni–Cd pocket plate single cells have been float-charged at 1.40 and 1.42 V at room temperature and 40°C for a time period of 2–3 years. Subsequent ex situ X-ray diffraction measurements were performed on the charged positive active Ni electrode material. The appearance of the diffraction pattern of one of the charged electrode materials (conventionally named nickel oxy-hydroxide-β-NiOOH) shows resemblance to the pattern of discharged nickel hydroxide (β-Ni(OH) 2). In situ X-ray diffraction, electrochemical and transmission electron microscopy 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 positive active material, especially in the charged state. The use of structure models from well crystalline, often synthetically manufactured, powder samples in order to describe amorphous material must be questioned. There are numerous parameters that may affect the charge/discharge transformation and the structural appearance of positive active material. Transmission electron microscopy (TEM) measurements indicate structural differences of the sample subjected to Rietveld refinement on X-ray diffraction data compared to earlier published structure.