Abstract:

Lithium-ion batteries are the most widely used energy storage device owing to their advantages such as high energy density, high cycle life, and low self-discharge rate. Because two-dimensional (2D) materials are commonly used as anode materials, the study of their lithiation behaviors is significant for improving the energy density and cycle life of batteries. Although some spectroscopic methods have been developed for studying the intercalation/deintercalation process of lithium in graphene, a new characterization technique that can directly investigate ion diffusion pathways at a microscale level would be beneficial to provide more detailed information on the mechanism of electrochemical reactions. It is an efficient solution to utilize the high spatial resolution of microscopic characterization to study the microscale electrochemical process. For this purpose, it becomes necessary to develop special specimens and setups that can undergo electrochemical experiments and are also compatible with microscopic characterization techniques. Herein, we developed a new planar micro-battery architecture on a SiO₂-coated silicon substrate that can be used to study the lithiation behaviors of various 2D materials using the micro-Raman mapping technique. In this planar micro-battery, the mechanically exfoliated few-layer graphene was used as the positive electrode, the thermal-evaporated lithium metal was employed as the negative electrode, and the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide with lithium bis(trifluoromethane)sulfonimide was used as the electrolyte. The micro-battery was tested using the galvanostatic discharge method on a probe station in an argon glove box. The selected lab-on-chip solution makes the lithiation of graphene observable under the micro-Raman spectroscope with a high spatial resolution. Raman mapping was successfully performed and graphene G-band signals were observed. Based on the facts that a small amount of lithium intercalation in graphene induces a blueshift of its G-band, and a large amount of lithium intercalation induces the splitting of the G-band into G^- and G⁺, we can correlate the degree of lithiation in graphene with its G-band signals and thus monitor the lithium intercalation process on graphene in the planar micro-battery. The time-dependent lithium distribution in graphene at different discharge stages could be obtained by comparing the G-band Raman mapping images to the corresponding optical micrographs. On the basis of these analyses, it was found that lithium ions diffuse between the layers in graphene and terminate at the graphene fault. These results help us understand the diffusion process of lithium in the graphene electrode during discharge. Moreover, the as-developed micro-battery is compatible with more characterization methodologies, such as optical microscopy, electrical transport, and electron microscopy, providing a broad application platform.

Key words: Planar micro-battery, Graphene, Lithiation, Raman mapping, Fault