The Investigations Of Structure And Electrical Properties In Layered Cobaltite Oxides At High Pressure

Due to its high voltage,high energy density,excellent cycling stability and environmental friendly,lithium cobaltite oxides（LiCoO₂）have widely served as the cathode material for rechargeable lithium-ion batteries（LIBs）in mobile,laptop and electric vehicle field.Cathode material is the key to determine the performance of lithium-ion battery.High energy density,long cycle life and high safety cathode material has now become the focus of research and development of energy material.Lithium cobaltite oxides as one of the earliest,most basic and broadest cathode materials,still has important research value.It is important to study the relationship between its structure and electrical properties for the study and development of more efficient cathode materials.In this work,we report the effect of pressure on the structural and electronic properties of the semiconductor Li_xCoO₂ with x = 0.90,by a variety of experimental techniques,including synchrotron X-ray diffraction and electric resistivity.In addition,the change of structure,band structure and the behavior of electrons and holes were calculated by the first-principles calculations.The results were compared with the experimental results.The results of the study are as follows:First,we studied the crystal structure of Li_0.9CoO₂ under high pressures synchrotron X-ray diffraction.The structure of Li_0.9CoO₂ was very stable and no structural phase transition was observed in the range of 19.8 GPa.The basal lattice parameter a shrinks by 2.7%at 19.8 GPa,whereas c drops 5.2%,exhibiting large anisotropic compression.Upon compression,the Co-O bond length decreased and the interactions between the central Co atom and the O ligands enhanced further,inducing larger crystal field splitting in the CoO₆ octahedron.Secondly,because lithium cobaltite oxides is an electrode material,the study of its electronic property is very important.We measured the low temperature dependence of the electrical resistivity p（P,T）of Li_0.9CoO₂ at different pressures up to 15.9 GPa.With increasing pressure to 15.9 GPa,the sample Li_0.9CoO₂ still exhibited a semiconductor behavior in the measured temperature range with an electrical resistivity（p）increase of 3 orders of magnitude in the low-temperature region.We found,contrary to the results reported on related systems.This is quite unusual because in all other correlated oxides pressure always leads to an increase of the 3d bandwidth and thereby to a transition to a more conduction state.Thirdly,the first-principle calculation of Li_0.9CoO₂ showed that the variation of the crystal parameters was consistent with the experimental results under high pressures.And the conduction band of Li_0.9CoO₂ obviously moved towards the high energy and the bandgap increased with the pressure increasing.From the deformation charge density,we found the hole became more localized.Both the hole localization and band gap opening induced by high pressure made the electrical conductivity worse.Finally,we concluded that the pressure-induced Co-O bond length shrinkage in the CoO₆ octahedron enhanced the crystal field splitting,which lead to band gap opening and the decreasing Co-Co distance caused the t2g bands to overlap and the electron holes to localize.