An important way to improve the performance of mass-produced lithium-ion batteries is to develop a better understanding of the manufacturing process’s effects on the electrode structure, and thereby its performance. Drying of the electrode slurry is a key step in that process. The parameters that affect drying electrodes have been mainly studied through ex-situ methods. There are far fewer examples of in-line metrologies of drying electrode properties, with notable exceptions including stress measurement by bending beam and dynamic mechanical analyzer, and ultrasonic measurements revealing the evolution of acoustic reflecting surfaces in the electrode (1). With so few in-line metrologies having been investigated, the possibilities are quite wide. However, it is striking that perhaps the most obvious of in-line metrologies has not been systematically investigated in the literature – that of simple visual observation of the drying surface (specifically with a microscope).

Light microscopy can reveal a surprising amount of detail about the drying electrode. In Figure 1 micrographs of a drying electrode under two different lighting conditions – with ring lighting and with coaxial lighting – are shown. In the coaxial lighting condition, the surface of the drying solvent can be seen. In freshly coated electrode slurry, the surface of the drying solvent is uniform and unremarkable, then holes appear as the solvent evaporates eventually leading to a honeycomb-like structure. In the ring lit images, the active material particles themselves are visible directly, and their movement while drying can be tracked. Here, we will examine how microscopy supported by other techniques which yield averaged properties to complement microscopy’s local measurements, such as film stress, can elucidate the drying electrode more fully.

1. Y. S. Zhang, N. E. Courtier, Z. Zhang, K. Liu, J. J. Bailey, A. M. Boyce, G. Richardson, P. R. Shearing, E. Kendrick and D. J. L. Brett, Advanced Energy Materials, 2102233 (2021).