Simulation-Supported Analysis of Calendering Impacts on the Performance of Lithium-Ion-Batteries
Authors: Lenze Georg , Röder Fridolin , Krewer Ulrike 
Published: February 2020

Lithium-Ion-Batteries (LIB) are promising energy storages for electric vehicles. However, to realize driving ranges equivalent to ones of combustion engine powered vehicles they need increased energy densities. The cells could be improved by targeted optimization of certain electrode manufacturing parameters, such as the calendering strenght [1]. However, calendering does not only affect several geometric parameters of the LIB, like electrode thicknesses and porosities, which can experimentally be measured. Calendering also affects other parameters, such as effective electronic and ionic conductivities and solid-liquid interfacial area [1, 2, 3]. The latter parameters are more complex to investigate, and exclusively experimental or exclusively simulative methods which are usually applied, are not sufficient to understand calendering impacts on battery behaviour. Therefore in this talk we introduce a method where experimental and simulative methods are combined in order to improve the understanding of calendering impact. For the experimental part inhouse made pouchcells were examined by measuring capacity, discharge curves at 1 C and electrochemical impedance spectroscopy. To investigate calendering impacts, these experiments were carried out for cells containing non-calendered NMC cathodes and ones containing cathodes calendered to a degree of 22% compaction of their original thickness, which is a moderate calendering strenght where no electrode damage by compression forces is expected, respectively. Additionally the samples' electronic conductivity was measured. To achieve consistency between experiments and simulation we implemented a pseudo 2D physico-chemical model [4, 5] which enables to simulate discharge curves and Li transport within solid and liquid during discharge process. This model was parameterized [6] and validated to achieve agreement between experimental and simulated discharge curves of pouchcells. Then impacts of the particular parameters affected by calendering (Figure 1 a) were simulated and evaluated stepwisely. With the aid of simulation-supported analysis we show to what extend each of the particular parameters described above contributes to the impact of calendering (Figure 1 b). Geometric parameters, being namely electrode thickness and porosity contribute relatively sparsely, whereas the most significant impact is caused as calendering improves effective ionic and electronic conductivities and solid-liquid interfacial area. Larger electrode thickness and porosity in non-calendered electrodes only cause a slight voltage loss, whereas lower effective ionic and electronic conductivities in non-calendered electrodes not only cause additional voltage losses but also lead to capacity losses due to kinetic limitations of electron and Li ion transport at 1 C discharge. Furthermore the effective solid-liquid interfacial area where electrons and Li ions are exchanged between electrolyte and active material particles appears to be reduced in non-calendered electrodes. Contributions of these effects can be quantified and corresponding cell internal correlations can be analyzed by additionally investigating simulated Li transport within electrolyte and active material. Especially for LIB, where electrodes are complex particle-pore networks with solid and liquid diffusion in addition of binder and carbon black, simulation-supported investigation is shown to yield physically sound correlations between calendering and battery performance. The method presented in this work is also a useful approach to achieve a targeted optimization of LIB manufacturing parameters in order to improve crucial performance quantities, such as the energy density of automotive batteries.

Figure 1: Parameters affected by calendering and their particular impacts on electrochemical performance at 1 C

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Figure 1