Junji Akimoto, Yoshito Gotoh, Yoshinao Oosawa

National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
E-mail: akimoto@nimc.go.jp

Keywords: crystal structure, X-ray diffraction, lithium cobalt oxide, single crystal growth

Lithium cobalt oxide LiCoO₂ has the trigonal ordered rocksalt structure and is used industrially as the cathode material of lithium-ion rechargeable batteries.  Since the electrochemical deintercalation was first reported on LiCoO₂ by Mizushima et al. [1], the structural, physical and electrochemical properties for LiCoO₂ and Li_xCoO₂ (0<x<1) have been widely investigated using the powder samples [2-4].  To clarify the crystal symmetry, structure and the anisotropic nature of physical properties of the cathode materials such as LiCoO₂ and LiMn₂O₄, sizable, well-characterized single crystal specimens have highly been desired.  In the present study, we report the growth and the X-ray structure analysis of the first single crystal of LiCoO₂.

LiCoO₂ single crystals were grown by a flux method of the slow cooling from 1173 K in a sealed gold tube.  X-ray Laue and precession photographs confirmed the trigonal symmetry and the previously reported lattice parameters.  We could not find any superstructures in this study.  The lattice parameters, determined using a four-circle diffractometer, are a = 0.28161(5) nm, c = 1.40536(5) nm, and c/a = 4.990.  These values agree quite well with the reported powder data for LiCoO₂; e.g. a = 0.28179(1) nm, c = 1.40597(8) nm, and c/a = 4.99 [4].  The crystal structure has been refined to the conventional values R = 3.1% and wR = 3.5% for 217 independent observed reflections.  The refined oxygen parameter z is 0.23951(15), which is in good agreement with the previous result of z = 0.2396(6) [2] using powder neutron diffraction data.  The bond distances and angles are essentially consistent with the previous results but with the high accuracy.  The short O-O contact distance of 0.2614(2) nm in CoO₆ octahedron is confirmed by the present single-crystal X-ray structure refinement.

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3. S. Kikkawa, S. Miyazaki and M. Koizumi, J. Solid State Chem. 62 (1986) 35.
4. R.J. Gummow, M.M. Thackeray, W.I.F. David and S. Hull, Mat. Res. Bull. 27 (1992) 327.