STRUCTURAL INVESTIGATIONS ON LI0.65-ZNI1- XRD AND EXAFS STUDIES(ZO2 CATHODE MATERIAL )

During lithium de-intercalation from Li1-zNi1+zO2 (0.05 <z <0.02), a m onoclinic phase with an extended lithium composition range (0.50 < x(L i) < 0.75) had previously been reported. As this material seems to pla y an important role during the electrochemical cycling, its structure has been carefully determined by X-ray diffraction and X-ray absorptio n studies. The Rietveld refinement of Li0.63Ni1.02O2, obtained by elec trochemical de-intercalation from quasi-stoichiometric Li0.98Ni1.02O2 phase, shows that the structural distortion is very weak in comparison to that reported for layered NaNiO2. Extended X-ray Absorption Fine S tructure data, performed on Li0.63Ni1.02O2, are discussed in compariso n with those of Li0.98Ni1.02O2 and Li0.90Ni1.10O2. While the latter tw o materials exhibit a local distortion of the (NiO6)-O-III octahedra, induced by the Jahn-Teller effect of trivalent nickel ions in the low spin stare t(2)(6)e(1), only a part of them are distorted at room temp erature in Li0.63Ni1.02O2. This result shows clearly that the Jahn-Tel ler effect is not the driving force of the monoclinic distortion in Li x-zNi1+zO2 phases. (C) 1998 Elsevier Science B.V. All rights reserved.