Lithium-ion battery electrodes are generally made up of porous, thin films that are structurally heterogeneous on multiple length scales due to the manufacturing process. This in turn causes spatial variability in the electrical properties of the film. This work reports development of a low-cost, flexible probe for measurement of film conductivity and contact resistance to the current collector. When mounted on a moveable stage, the probe can quickly produce maps of electrode electrical properties at sub-millimeter resolution. Bulk conductivity and contact resistance are determined by inverting a 2D model of the experiment. The method is validated using a conductive silicone rubber placed on top of plain and corroded steel with significant contact resistance variation. Measurements on commercial quality electrode films, two cathodes and one anode, show statistically significant macro-variations in film properties perpendicular to the rolling direction and micro-variations throughout the film, variations that will impact electrode performance. The flexible-surface probe can be a significant tool to determine and minimize sources of variation in electrode manufacturing.
Electrical conductivity is a key metric in the performance of lithium-ion batteries. However, accurately measuring the electronic conductivity nondestructively is difficult. We have developed a micro-four-line probe that has been used to make this measurement and to demonstrate the heterogeneity of electrical conductivity of lithium-ion electrodes on a mm scale [1]. Such heterogeneity or non-uniformity can cause the lithium ion battery to degrade unevenly overtime, which is undesirable. The design of the micro-four-line probe has been improved into a flexible-four-line probe. This flexible measurement device is able to conform to the surface of the electrode, which improves the contact and the resulting electronic conductivity measurement. The flexible probe has been used to take accurate measurements of electronic conductivity and the contact resistance between the film and the current collector. The microstructure of lithium-ion electrodes is another important characteristic that determines the performance of lithium-ion cells, including electronic and ionic conductivity of electrodes. Calendering, an important step in the manufacturing process, affects the microstructure as the film is compressed to a uniform thickness. Prior to calendering there are more substantial variations in the thickness of the electrode film, which constitutes surface roughness, as illustrated in the included thickness profile (figure). The effect of calendering on the electronic heterogeneity of electrode films will be reported. The flexible probe will be used to measure the variation in electronic conductivity across the electrodes. A thickness analysis of the electrode films before and after calendering will complement the electrical measurements. This information increases our understanding of electrode manufacturing processes to allow for future optimization of these processes. References: Vogel et al., Electrochimica Acta 297, 820 (2019). Figure 1
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