Cross Sectional Mapping of Nano-Mechanical Properties of Composite Electrodes for Lithium Ion Batteries Using Bimodal Mode Atomic Force Microscopy

Abstract

Lithium-ion batteries (LIBs) have been widely used for various applications including portable devices and electric vehicles. Improvement of the cyclability and capacity is an important challenge. In the manufacturing process, active materials, conductive additives and binders are mixed in a solvent at a certain ratio and the slurry is casted, dried and pressed on a metallic foil current collector to yield a composite film. The composite films are assembled together with an electrolyte solution and a separator in coin or laminate cells. The packing density and mechanical strength of each material are important parameters for the cell performance because the volume expansion and resulting mechanical stress may cause a destruction of the electrical connections between binders, current collectors and the active materials during the charge/discharge processes. Young's modulus is one of the most important mechanical properties because it is an indicator of the mechanical strain in a material under a certain amount of stress. Nanoindentation has been used to measure the Young's moduli of various materials. However, it has a few restrictions as follows: (i) sample surface has to be destructed in nanoindentation and (ii) it has a limited spatial resolution, typically a few micrometers, due to the size of the disruption. Although recent atomic force microscopy (AFM) techniques allow non-destructive and high-resolution Young's modulus mapping by measuring force curves at each point using relatively weak force of<100 nN, this method is not applicable for stiff materials with modulus larger than 100 GPa because their sample deformation is too small to be detected by the cantilever deflection. Here, a newly developed technique, bimodal AFM that allows nanoscale mechanical mapping for a wide range of Young's modulus from 0.1 to 200 GPa in a non-destructive manner, was applied to LiCoO2-based positive and graphite-based negative composite electrodes before and after the charge-discharge cycles in a coin cell for a demonstration. Reference: H. Sakai, Y. Taniguchi, K. Uosaki, T. Masuda, “Quantitative cross-sectional mapping of nanomechanical properties of composite films for lithium ion batteries using bimodal mode atomic force microscopy”, 2019, 413, 29-33.