ON THE BEHAVIOR OF THE LIXNIO2 SYSTEM - AN ELECTROCHEMICAL AND STRUCTURAL OVERVIEW

Lithium nickel oxide exhibits a departure from stoichiometry (Li1-zNi1 +zO2) consisting in the presence of extra-nickel ions within the lithi um sites. Using optimized experimental synthesis conditions, compositi ons very close to the ideal stoichiometry (z = 0.02) can be obtained. By using the sensitivity of the lithium site isotropic temperature fac tor to the stoichiometry, the amount of extra-nickel ions can be deter mined in a very precise way. The loss of reversibility at the first cy cle is mainly related to the change in the oxidation state of the extr a-nickel ions, which induces a local collapse of the structure and mak es difficult the lithium re-intercalation, A systematic structural stu dy of LixNiO2 phases has been performed by extended X-ray absorption f ine structure (EXAFS) as well as X-ray and electron diffraction. In th e case of the starting Li0.98Ni1.02O2 phase, a local distortion of the NiO6 octahedra, resulting from a dynamic Jahn-Teller effect of low sp in trivalent nickel ions has been evidenced from the EXAFS study. For the partially de-intercalated materials (0.50 < x < 0.75) which crysta llize in the monoclinic system, the EXAFS study shows that the NiO6 oc tahedra are only slightly distorted due to the occurrence of a hopping phenomenon between Ni-IV and Ni-III. Electron diffraction experiments show the existence of a superstructure due to a peculiar lithium-ion ordering. Systematic electrochemical studies have shown that this orde ring is strongly sensititve to the presence of extra-nickel ions. (C) 1997 Elsevier Science S.A.