The structure of has been studied with numerous techniques including x-ray diffraction, electron microscopy, neutron powder diffraction, and EXAFS. The solid consists of layers of monovalent lithium cations that lie between extended anionic sheets of cobalt and oxygen atoms, arranged as edge-sharing octahedra, with two faces parallel to the sheet plane. The cobalt atoms are formally in the trivalent oxidation state and are sandwiched between two layers of oxygen atoms. In each layer, the atoms are arranged in a regular triangular lattice. The lattices are offset so that the lithium atoms are farthest from the cobalt atoms, and the structure repeats in the direction perpendicular to the planes every three cobalt layers. The point group symmetry is in Hermann-Mauguin notation, signifying a unit cell with threefold improper rotational symmetry and a mirror plane. The threefold rotational axis is termed improper because the triangles of oxygen are anti-aligned.
Preparation
Fully reduced lithium cobalt oxide can be prepared by heating a stoichiometric mixture of lithium carbonate and cobalt oxide or metallic cobalt at 600–800°C, then annealing the product at 900°C for many hours, all under an oxygen atmosphere. Nanometer-size particles more suitable for cathode use can also be obtained by calcination of hydrated cobalt oxalate β-·2, in the form of rod-like crystals about 8 μm long and 0.4 μm wide, with lithium hydroxide, up to 750–900°C. A third method uses lithium acetate, cobalt acetate, and citric acid in equal molar amounts, in water solution. Heating at 80°C turns the mixture into a viscous transparent gel. The dried gel is then ground and heated gradually to 550°C.
The usefulness of lithium cobalt oxide as an intercalation electrode was discovered in 1980 by an Oxford Universityresearch group led by John B. Goodenough and Tokyo University's Koichi Mizushima. The compound is now used as the cathode in some rechargeable lithium-ion batteries, with particle sizes ranging from nanometers to micrometers. During charging, the cobalt is partially oxidized to the +4 state, with some lithium ions moving to the electrolyte, resulting in a range of compounds with 0 < x < 1. Batteries produced with cathodes have very stable capacities, but have lower capacities and power than those with cathodes based on nickel-cobalt-aluminum oxides. Issues with thermal stability are better for cathodes than other nickel-rich chemistries although not significantly. This makes batteries susceptible to thermal runaway in cases of abuse such as high temperature operation or overcharging. At elevated temperatures, decomposition generates oxygen, which then reacts with the organic electrolyte of the cell. This is a safety concern due to the magnitude of this highly exothermic reaction, which can spread to adjacent cells or ignite nearby combustible material. In general, this is seen for many lithium ion battery cathodes.
