The microstructure determines transport properties in lithium-ion battery electrodes. There is generally a tradeoff between electronic and ionic transport when adjusting the microstructure. One way of adjusting the microstructure is through calendering, where the electrode is compressed following drying. Understanding how calendering affects not only the average but also the local electronic and ionic transport provides additional insight when developing battery electrodes and engineering better batteries. If a correlation exists between the two properties, an optimal porosity that maximizes both ionic and electronic transport could be determined.

In order to better understand the influence of microstructure on these transport properties, we tested a series of commercial-grade electrodes including NMC cathodes, graphite anodes, and a graphite-silicon anode. The local electronic conductivity of the electrodes was found using a micro-flexible-surface probe previously developed by our research group [1]. Likewise, the local ionic conductivity was found using an aperture probe previously developed by our research group [2]. All electrodes were obtained from Argonne National Laboratory in calendered and un-calendered states.

Through testing various electrodes before and after calendering, we found that not every electrode experienced an increase in electronic conductivity after calendering, and that in general heterogeneity of the electronic conductivity decreased after calendering. The local ionic resistance, as indicated by MacMullin number, was found to increase after calendering, as expected. Figure 1 illustrates the local ionic and electronic transport results for one cathode. Ionic transport was found to be almost solely influenced by porosity. However, electronic transport was found to be influenced by a variety of factors including the nature, distribution, and connectivity of conductive materials.

[1] Vogel et al., J. Electrochem. Soc. 168, 100504 (2021).

[2] Liu et al., ECS Meeting Abstracts 2021, 444.