Nuclear Instruments and Methods in Physics Research B 97 (1995) 75-77 ELSEVIER XAS study of lithium nickel oxide A. Rougier a, A.V. Chadwick b, C. Delmas a, * zyxwvutsrqponmlkjihgfedcbaZYX Beam Interactions with Materials 8 Atoms zyxwvutsr a Laboratoire de Chimie du Solide du CNRS and Ecole Nationale Supkrieure de Chimie et Physique de Bordeaux. 351, tours de la LibCration, 33405 Talence Cedex, France b Unitrersity Chemical Laboratov. University of Kent at Canterbury, Kent, CT2 7NH, UK zyxwvutsrqponmlkjihgfedcba Abstract LiNiO, crystallizes in the rhombohedral symmetry with the layered a-NaFeOz type structure. On the contrary of the homologous NaNiO, phase, it does not exhibit macroscopic structural distortion due to the Jahn-Teller effect of low spin trivalent nickel ions. The EXAFS study shows that in fact all NiO, octahedra are distorted (two long and four short Ni-0 distances). 1. Introduction Lithium nickelate is one of the most promising positive electrode materials for lithium batteries [1,2]. It exhibits a layered structure made of NiO, slabs built up of edge-shar- ing NiO, octahedra. Magnetic measurements have shown that the trivalent nickel ions are in the low spin state (tze’) and, therefore, a Jahn-Teller distortion of the NiO, octa- hedra is expected [3-51. Nevertheless, such a distortion has never been reported, even at low temperature. In fact, this material exhibits a strong departure from stoichiometry: the true formula is Li,_,Ni, +r02 (0.05 I z I 0.20) with 2 z nickel ions in the divalent state [6,7]. The z extra-nickel ions are situated in the interslab space as illustrated in Fig. 1. The simultaneous presence of trivalent and divalent nickel ions can prevent a distortion of the NiO, octahedra: then, our goal was to synthesize ideal LiNiO,. In a recent paper, we have described several preparation methods leading to various stoichiometries of Li,_,Ni, +=OZ [4]. We have selected “the most 2D” material for the X-ray absorption structure @AS) spectroscopy experiments pre- sented in the following. 2. Experimental Quasi-2D LiNiO, was synthesized from a stoichiomet- ric mixture of Liz0 and NiO heated at 700°C under oxygen for 15 h. The Rietveld refinement of X-ray diffrac- tion data shows that this phase exhibits a high crystallinity. Its composltlon 1s L1,,.9sNi1.0202, leading to a very small amount of divalent nickel ions (22 = 0.04) [4]. * Corresponding author. tel. +33 56846296, fax +33 56846634. XANES and EXAFS spectra were collected on the 7.1 specialized station of the SERC Daresbury Synchrotron Radiation Source (SRS). Ni K-edge data were measured in transmission at RT using a two Si(ll1) crystal monochro- mator. The storage ring was operating at 150-250 mA beam current. The EXAFS oscillations were analyzed with the Daresbury suite of EXAFS programs (EXCALIB, EXBACK and 0EXCURV92). Good quality spectra were obtained to 2! A-‘, leading to resolution in bond distances AR of 0.08 A (AR = 7r/2k). Pure NiO was employed as model compound to determine non-structural parameters such as the phase shifts and backscattering factors. To analyze the EXAFS spectra, the multiple scattering contri- bution was removed. 3. Results and discussion 3.1. Xanes The XANES Ni K-edge spectra of NiO and LiNiO, are shown in Fig. 2. For each material there is a very weak pre-edge feature which is slightly stronger in LiNiO, than in NiO. It is assigned to the 1A -+ 3d transition usually forbidden in a centrosymmetric site by the dipole selection rule. However, in nickel oxide derivatives this transition is allowed by a weak quadrupole transition, and exhibits a low intensity [8,9]. The apparent shift to higher energy of the edge position from NiO to LiNiO, is expected due to the higher Ni oxidation state in lithium nickelate. 3.2. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF EXAFS As the global structure of LiNiO, was known from X-ray diffraction data, we were mainly interested in the surrounding of the nickel ion in order to try to find any 0168-583X/95/$09.50 0 1995 Elsevier Science B.V. Al1 rights zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA reserved SSDI 0168-583X(94)00398-X II. zyxwvutsrqponmlk CERAMICS