Anisotropic Surface Effect on Electronic Structures and Electrochemical Properties of LiCoO2

Increasing industrial needs for rechargeable batteries delivering higher power has encouraged advanced research on nanosized electrochemically active compounds. Although nanosized electrode materials are expected to possess much higher power output due to short diffusion length, the nanosize effects on electrodes have not been understood well to date. Here, we report. that nanosized LiCoO2 as suggested as electrode material for Li-ion rechargeable batteries has anisotropic surface properties affecting electronic structures, which is evidenced by electron energy loss spectroscopy (EELS). EELS spectroscopy reveals the reduced valence state of the cobalt near the surface, especially along the stacking direction of CoO2 layers. The observed anisotropic surface property explains the nanosize effect on the electrochemical properties,