(312g) Investigating Carbon Microstructure in Composite Battery Electrodes Using Electrical Resistivity

Composite battery electrodes consist of an active material, conductive additive (often carbon black) and polymer binder. Understanding the mechanisms by which electrode microstructure evolves during processing is crucial for the commercialization of novel energy storing devices. The carbon distribution is largely responsible for electrode performance.[1] Despite this, the mechanisms for carbon microstructure formation during electrode slurry processing are not yet understood. This lack of knowledge leads to an inability to predict changes in battery performance with changing electrode processing parameters and difficulty in determining the optimum carbon microstructure for the electrode.

In previous studies by our group, we showed the impact of parameters such as slurry rheology, drying temperature, shear during film formation and free carbon black concentration on battery performance.[1,2] In this study, we build on these results with an in-situ sensor for studying the carbon microstructure formation based on four-point probe resistivity measurements. It is well known that the depth the electric field can penetrate into the sample using a four-point probe increases with increasing probe spacing.[3] We hypothesize that varying the spacing between four-point probes will enable measuring the vertical resistivity profile of a composite electrode film, which in turn can be correlated to its carbon microstructure. Our hypothesis is supported by experimental results showing in-situ sedimentation of carbon black slurries. As the slurry sediments, the overall resistivity of the slurry, measured by widely-spaced probes, decreases due to the loss of connectivity in the carbon network. However, sedimentation causes the concentration of carbon particles to increase at the bottom and decrease at the top of the sample, producing a resistivity that increases with height. Thus, the resistivity measured by closely-spaced probes, which can only measure the resistivity at the bottom of the slurry, actually decreases. These results are corroborated by the rheological characterization of the slurry before and after sedimentation. The impact of electrode processing parameters on the dynamic evolution of the carbon network during coating and drying and its implications on electrode performance will also be discussed.

[1] S. Morelly, N. Alvarez, M. Tang, Short-range contacts govern the performance of industry-relevant battery cathodes, Journal of Power Sources, 2018, 387, 49-56.

[2] R. Saraka, S. Morelly, M. Tang, N. Alvarez, Correlating processing conditions to short- and long-range order in coating and drying lithium ion batteries, ACS Appl. Energy Mater., 2020, 3, 12, 11681-11689.

[3] Hasegawa, S., Shiraki, I., Tanikawa, T., Petersen, C. L., Hansen, T. M., Boggild, P., & Grey, F. Direct measurement of surface-state conductance by microscopic four-point probe method. In J. Phys.: Condens. Matter (2002),(Vol. 14)