Structure and optical absorption of combustion-synthesized nanocrystalline LiCoO 2
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Nanocrystalline LiCoO2 powders (10–50 nm) were synthesized by a citrate-nitrate combustion process followed by calcination at different temperatures (300–800 °C) in air. Thermogravimetric analyses indicated a sharp combustion at a low temperature of 225 °C, producing fine crystallites. Quantitative phase analyses from the x-ray diffractograms showed that while annealing at 500 °C produced mixed phases of cubic and rhombohedral LiCoO2, annealing at 800 °C resulted in single-phase rhombohedral LiCoO2. Electronic transitions related to the Co 3d bands were investigated by ultraviolet-visible reflectance spectra in absorbance mode and were ascribed to the Co 3d intra-band transition involving t2g and eg orbitals. The d-d transitions underwent a blue shift of about 0.3 eV as the cubic LiCoO2 transformed into the rhombohedral structure with band gap values of about 1.4 and 1.7 eV.
I. INTRODUCTION
In recent years, extensive research has been focused on LiCoO2 for cathode materials in lithium ion secondary batteries because it possesses a high cell voltage of about 4 V, high charge capacity of around 120 mAhg−1, a long cycle life, and a low self discharge.1–3 Understanding the electronic structure and the electronic processes involved in Co 3d bands in LiCoO2 is a very crucial issue as most of its electrochemical properties would be affected by the electronic structure. Depending upon the temperature, LiCoO2 exists in two crystalline forms: a low-temperature (LT) (500 °C) layered rock-salt structure (space group R3¯m), commonly known as LT and HT LiCoO2, respectively, in the literature.4–12 Both structures are based on the same oxide sublattice (a cubic close-packed oxygen network) but differ essentially in the spatial arrangement of the cations. The cubic phase of LiCoO2 is characterized by a random distribution of cobalt and lithium cation layers of about 75% Co and 25% Li (Co-rich layer) alternating with cation layers of about 25% Co and 75% lithium (Li-rich layer) whereas the rhombohedral phase is distinguished by an ideal rocksalt ␣-sodium ferrite (␣–NaFeO2) structure showing alternating layers of Li and Co.12 The layered rock-salt form of LiCoO2 consists of slightly distorted CoO6 octahedra; thus a ligand field is strong a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0157 1162 J. Mater. Res., Vol. 22, No. 5, May 2007 http://journals.cambridge.org Downloaded: 15 Mar 2015
enough to stabilize a Co3+ low-spin ground state, and hence, all six 3d electrons of Co3+ have a low-spin configuration.13,14 Co 3d bands of layered rock-salt LiCoO2 consist of three t2g (valence band) and two eg (conduction band) levels. The crystal structure of Co3O4 represents a normal spinel structure with Co2+ ions occupying tetrahedral sites with lower eg and higher t2g orbitals and Co3+ ions occupying octahedral sites with lower t2g and higher eg orbitals in the ligand field of oxygen ions.4,8 Although, like Co3O4, cubic LiCoO2 has a normal spinel structure, recent studies by Shao-Horn et al.
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