An electrochemical investigation into the lithium insertion properties of Li xCoO 2

Abstract

The thermodynamic, kinetic and interfacial properties of lithium insertion in Li x CoO 2 have been probed by ac and dc electrochemical techniques. The electrochemical voltage spectroscopy method, which was used to cycle the cells, indicated that a significant coulombic inefficiency was present in the first charge-discharge cycle. The second cycle data, however, were close to 100% coulombic efficiency and demonstrated that around x = 0.64 in Li x CoO 2, lithium could be inserted reversibly. This value is significantly higher than has been previously reported. In agreement with the work carried out by Reimers and Dahn[14]we also find two additional reversible differential capacity features at cell potentials of approximately 4.08 V and 4.20 V vs. Li on charge. The kinetic investigation revealed the average diffusion coefficient for the composite Li x CoO 2 cathode to be in the range 10 −9 cm 2 s −1 indicating the relatively facile reaction kinetics for the lithium insertion reaction. The dc impedance data collected by the current interrupt method were broadly consistent with the ac impedance spectra and showed the presence of two distinct impedance regions dependent on the cell state-of-charge. One region exists below approximately x = 0.6 where the system generates relatively low impedance characteristics. The second region occurs above about x = 0.7 where significantly higher impedance properties are found. We suspect morphological changes at the lithium/electrolyte interface to be responsible for these impedance variations.