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

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) over bar1} 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) over bar1} 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) over bar1} faces. Moreover, X-ray Photoelectron Spectroscopy characterization of the charged electrode surface indicates that a denser surface passivation layer is formed on {10 (1) over bar1} than that on (0003) crystal faces. This is caused by the lower adsorption energy of decomposition molecule on {10 (1) over bar1} crystal faces and higher work function (due to the surface atomic structure) for {10 (1) over bar1} 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) over bar1} 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) over bar1} facet atomic structure may thermodynamically promote the physical/chemical adsorption and decomposition of electrolyte.