How the Effective Thermal Conductivity of Lithium-Ion Cell Electrodes Is Impacted By the Compression during Calendering

Abstract

During the design process of lithium-ion batteries the simultaneous optimization of the mechanical, electrical and thermal properties of the components and their interfaces is crucial. As these are strongly influenced by the production steps, a thorough understanding of their impact on said properties is key to reaching optimized material properties of the cells. While many researchers have already investigated the impact of the calendering process on the electrical and mechanical behavior [1][2][3][4], this study covers the alteration of the thermal material properties due to calendering.In this context, different material coatings consisting of two graphite anodes with different starting thicknesses and two cathodes with NMC622 and NMC811 as active materials were investigated. The electrodes were calendered with different forces to reach a broad spectrum of porosities and layer thicknesses. For each material the specific heat capacity and density of the solid phase were measured with differential scanning calorimetry and gas pycnometry, respectively. The thermal diffusivity was measured with laser flash analysis for different compression rates for each active material. With the derived values, the effective thermal conductivity of the different electrodes was determined and interesting indications were found.Considering those results, the values of the effective thermal conductivity of one-sided coated electrodes, plotted vs. the compression rate do not show a steadily increasing slope as suggested by results of common models for their dependence on a decreasing porosity [5][6]. Instead, the thermal conductivity stays at an almost constant value or even decreases at small to medium compression rates before it increases again for a strong compression. This observation indicates that more complex changes and effects happen within the particulate coating layer and at its interfaces to the substrate during the process of calendering than just a change in the porosity. As an example: In a mechanical context, similar variations of the adhesion strength between coating and current collector have been observed [1][2]. It may thus be assumed that the calendering process affects the thermal connection between coating and substrate. The contribution will discuss this and more potentially happening effects and according modifications and extensions of existing models for the thermal conductivity of electrode stacks.