Improving diffusion kinetics and phase stability of LiCoO2 via surface modification at elevated voltage

Abstract

To achieve higher energy density, the lithium cobalt oxide (LiCoO2) cathode, which owns an absolute advantage on theoretical and volumetric energy density, is selected to obtain more capacity by lifting the upper cut-off voltage. However, the structure of LiCoO2, especially at elevated temperature, is unstable in the deeply delithiated state. Here, we explore a superior Li+ conductive Li1+xAlxTi2-x(PO4)3 (LATP) coating coupling with trace Ti-Al co-doping on the surface of LiCoO2 (C-LCO). The obtained C-LCO exhibits high capacity retentions at both 30 °C (87.5% after 50 cycles) and 50 °C (88.8% after 40 cycles) at a high cut-off voltage of 4.5 V. Furthermore, the C-LCO shows outstanding rate capability which displays a high discharge capacity of 150 mA h g−1 at up to 5 C. Various analysis techniques are used to understand the mechanism of the excellent electrochemical performance of C-LCO. The critical attribute for the decreased voltage polarization and superior rate capability is the improved lithium ion diffusion kinetics, which is revealed by cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT). Furthermore, in situ X-ray diffraction (XRD) measurement is performed and the results indicate that the surface modification successfully stabilize phase structure and help to reach better reversibility of LiCoO2 cycled to 4.5 V.