Pressure induced abnormal insulating state in LixCoO2 - Cong Xu

SEPTEMBER 8, 2017

New work led by HPSTAR graduate student, Cong Xu found abnormal semiconductor to insulator transition in cathode material, Li0.9CoO2 under pressure. The findings uncovered by the comprehensive high pressure investiations provide deep insights into the complex relationship of the crystal structure and electronic performance in this important cathode material, which may be used for guiding the future lithium metal oxides battery industry for better performane. The discoveries are published by the Journal of Materials Chemistry A (DOI: 10.1039/C7TA06084J).

Lithium cobaltite oxides (LixCoO2) has been widely served as the cathode material for rechargeable lithium ion batteries (LIBs) due to its high voltage, high energy density and excellent cycling stability.

LixCoO2 is a reference compound for a whole family of layered materials obtained by the substitution of other metals for cobalt. In order to improve battery performance, We need to understand the structural and electronic properties of LixCoO2. In this work, the team utilized high-pressure technique, a very effective, clean tool, tuning the crystal and electronic structure of the layered compound Li0.9CoO2.

“Up to ~20 GPa, we didn’t observe any sign of crystal structural change in the sample, While surprisingly, we found its electric transport behavior changed from a semiconducting state to an insulating state”, said Cong, a former graduate student of HPSTAR, led the project while earning his master degree at Dr. Wenge Yang’s group.

“This semiconductor to insulating state transition in Li0.9CoO2 under high pressure is quite unusual. In most correlated oxides, pressure usually broadens the 3d bandwidth and thereby leads to a transition towards a more conducting state. The phenomenon observed in this work is apparently connected with different physical mechanisms”, added Dr. Yang.

Their multiple investigations, including X-ray diffraction (XRD), electric resistivity, UV-vis absorption spectroscopy, and DFT calculations give deep insights into the complex relationship of crystal structure and electronic performance in this important cathode material.

“This quite abnormal pressure effect, a transition from semiconducting state to an insulating state, is a consequence of the enhanced crystal field splitting in distorted CoO6 octahedron with pressure increasing” explained Cong.

“The shrankage of Co-O bond length enhances the crystal field splitting, which leads to band gap opening and the decreasing Co-Co distance caused the electron holes to be localized.”

自钴酸锂（LixCoO2）被用作锂离子电池正极材料以来，由于其工作电压高，充放电结构稳定，循环寿命长，重量轻等特点被大量应用。研发更高性能的正极材料已经成为全球各国研究开发的重点。近期，北京高压科学研究中心的杨文革研究员小组和北京大学深圳研究院的潘峰教授携手合作，通过金刚石对顶砧的实验方法以及理论计算对钴酸锂的高压结构性质，电学性质等进行了一系列的研究，研究发现，在高压下钴酸锂的晶体结构十分稳定，但是其电学输运性质出现半导体到绝缘体转变的现象。他们进一步的讨论了该转变的机制，揭示了钴酸锂的晶体结构和其电子结构之间的联系，为寻找更高效的锂离子电池正极材料提供了重要途径。