The Impact of Calendering on the Electronic Conductivity Heterogeneity of Lithium-Ion Electrode Films

Abstract

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

A Model for Investigating Sources of Li-Ion Battery Electrode Heterogeneity: Part I. Electrode Drying and Calendering Processes

“…Such heterogeneity has been observed for multiple electrodes from multiple manufacturers and so appears to be intrinsic to the coating operations used in current electrode manufacturing. 53 Furthermore, a contributing factor is likely interparticle interactions that tend to create some aggregation or demixing of CBD from other phases which are not simply caused by poor slurry mixing or other easily fixed manufacturing defects. While our simulations are not conducted on mm lateral length scales, particle interactions can produce a similar result on the throughplane density variations as observed in Fig.…”