Comparative structural and electrochemical study of high density spherical and non-spherical Ni(OH)2 as cathode materials for Ni-metal hydride batteries

In this paper we compare the behavior of non-spherical and spherical beta-Ni(OH)(2) as cathode materials for Ni-MH batteries in an attempt to explore the effect of microstructure and surface properties of beta-Ni(OH)(2) on their electrochemical performances. Non-spherical beta-Ni(OH)(2) powders with a high-density are synthesized using a simple polyacrylamide (PAM) assisted two-step drying method. X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), thermogravimetric/differential thermal analysis (TG-DTA), Brunauer-Emmett-Teller (BET) testing, laser particle size analysis, and tap-density testing are used to characterize the physical properties of the synthesized products. Electrochemical characterization, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and a charge/discharge test, is also performed. The results show that the non-spherical beta-Ni(OH)(2) materials exhibit an irregular tabular shape and a dense solid structure, which contains many overlapped sheet nano crystalline grains, and have a high density of structural disorder and a large specific surface area. Compared with the spherical beta-Ni(OH)(2), the non-spherical beta-Ni(OH)(2) materials have an enhanced discharge capacity, higher discharge potential plateau and superior cycle stability. This performance improvement can be attributable to a higher proton diffusion coefficient (4.26 x 10(-9) cm(2) s(-1)), better reaction reversibility, and lower electrochemical impedance of the synthesized material. (C) 2011 Elsevier B.V. All rights reserved.