Visualization of Inhomogeneuous Reactivity on Battery Material Using Scanning Electrochemical Cell Microscopy

To understand the metal oxide coating effect on battery performance, the following two techniques are required: 1) constructing a flat thin-film electrode surface to realize a well-defined interface and 2) analyzing the electrode/electrolyte interface reaction with nanoscale resolution. We previously studied flat LiCoO2 thin-film electrodes using in situ surface-sensitive X-ray absorption spectroscopy (XAS) and reported that Co reduction at the LiCoO2 surface resulting from electrolyte contact caused the initial degradation. We also showed that the ZrO2 layer successfully prevented physical contact between LiCoO2 and the electrolyte. And it confirmed that a thicker ZrO2 layer (above 2 nm) increased the diffusion resistance of the lithium ions in the ZrO2 layer. However, since XAS lacks in-plane resolution and provides only averaged information, it is impossible to analyze the ZrO2 morphology in detail. Recently, Taguchi et al. investigated a thin Li-Zr-layer (ca. 2 nm) on a LiCoO2 composite electrode by transmission electron microscopy (TEM). They suggested that this thin layer could improve the durability. However, it is difficult to analyze the electrochemical properties using TEM. To evaluate the intrinsic mechanism of the metal oxide coating effect, it is necessary to develop a novel in-situ method that can analyze the surface morphology with high spatial resolution and simultaneously determine the local electrochemical properties. Scanning electrochemical microscopy (SECM) is a powerful technique for linking the surface morphology of a sample to its electrochemical properties. For the battery materials, the SECM feedback mode is effective in monitoring solid electrolyte interphase formation. To directly and quantitatively investigate spatially resolved ionic processes, mercury-capped platinum ultramicroelectrodes were developed and employed for Li+ imaging based on Li stripping. However, it is difficult to visualize the Li+ flux in battery materials at the sub-micrometer scale by SECM. Scanning electrochemical cell microscopy (SECCM), which uses a nanopipette as a probe and forms a local electrochemical cell, is effective in characterizing surface reactivity. We recently applied SECCM for visualization of electrochemical activities on a lithium-ion battery cathode material at sub-micrometer resolution. The SECCM was applied to collect or provide Li in specified area confined by the nanopipette. Further, it collection visualized the electrochemical properties by scanning the nanopipette as an image. There are some strong advantages in SECCM for battery material research such as its high spatial resolution, small capacitive current, and isolated electrochemical cell. In this report, we applied SECCM to characterize a ZrO2-coated LiCoO2 thin-film electrode prepared by pulsed laser deposition. Local cyclic voltammetry (CV) and galvanostatic charge/discharge were performed to characterize the cycle durability and rate performance of ZrO2-coated LiCoO2 thin-film electrodes and to reveal the relationship between the ZrO2 morphology and thickness.