Insight into the intrinsic mechanism of improving electrochemical performance via constructing the preferred crystal orientation in lithium cobalt dioxide

Abstract Surface properties of cathode materials play important roles in the transport of lithium-ions/electrons and the formation of surface passivation layer. Optimizing the exposed crystal facets of cathode materials can promote the diffusion of lithium-ions and enhance cathode surface stability, which may ultimately dominate cathode’s performance and stability in lithium-ion batteries. Here, polycrystalline LiCoO2 (LCO) thin films with (0003) and {10 1 - 1} preferred orientations were prepared as the well-defined model electrodes. In situ Current-Sensing Atomic Force Microscopy (CSAFM) was employed to investigate the lithium de-intercalation and electronic conductivity evolution of the (0003) and {10 1 - 1} facts in organic electrolyte at the nanoscale. It was found that the lithium deintercalation following a “Li-rich core model” in the LCO grains, and the LCO grains with (0003) crystal face show less conductivity than those with {10 1 - 1} faces. Moreover, X-ray Photoelectron Spectroscopy characterization of the charged electrode surface indicates that a denser surface passivation layer is formed on {10 1 - 1} than that on (0003) crystal faces. This is caused by the lower adsorption energy of decomposition molecule on {10 1 - 1} crystal faces and higher work function (due to the surface atomic structure) for {10 1 - 1} crystal faces, as confirmed by Density Functional Theory (DFT) and Kelvin probe force microscopy (KPFM) results. In addition, electrochemical measurements confirm that the thin film electrodes with {10 1 - 1} preferred orientation not only show smaller electrode polarization, but also more readily form a stable surface passivation layer compared with the (0003) preferred orientation. This work highlights the importance of cathode conductivity, and suggests that the LCO {10 1 - 1} facet atomic structure may thermodynamically promote the physical/chemical adsorption and decomposition of electrolyte.