Porous Electrodes in Lithium-Ion Batteries: Heterogeneity and Performance

Abstract

The presence of inhomogeneity in the porous electrodes of lithium-ion cells can significantly affect the active material utilization, energy/power density, and ageing dynamics of the cell, among others [1-3]. Inhomogeneity might be defined as a non-zero directional gradient of the physical and/or chemical formulation in a porous electrode. In Li-ion cells, such inhomogeneity is caused by an uneven spatial distribution of the active material, carbon, binder, and electrolyte, or porous electrode micro/nanoscale properties such as the contact resistances, porosity and tortuosity. The existing literature suggests that the heterogeneity could inadvertently be introduced to the electrode with improper combination of design parameters and manufacturing steps, e.g. slurry formulation, thickness, mixing and drying [4].Important insights into the local microstructure of the porous electrodes have been provided by FIB-SEM [5] , X-ray tomography [6] or full simulation of electrode microstructure [7]. Little attention has been paid, however, to develop formalisms to quantify the electrode heterogeneity and its correlation to the design parameters, manufacturing steps, and the battery performance.In this study, a group of 36 NMC porous electrodes with different thickness, porosity, and formulation (NMC loading and carbon/binder ratio) were systematically prepared and were characterized for the microstructure and electrochemical performance. We spotlight the sensitivity of the energy and power density of the electrodes, in front of a Li counter electrode, to the microstructural indexes including tortuosity, porosity, and effective conductivities. The experimental results are analysed with the help of a macroscopic battery model to introduce a series of semi-empirical correlations to quantify the state of heterogeneity in the porous electrodes at a given manufacturing recipe.