Unravelling Effects of Microstructural Electrode Architecture on Cell Performance and Aging - a Combined Experimental and Simulation-Based Study on Commercial 21700 Lithium-Ion High-Energy Cells

Abstract

In order to balance energy and power density of Li-ion batteries – and to ensure a long battery life – it is crucial to understand in detail how the composition and the microstructural design of the electrodes affects their capacity and kinetics.In our study we compare different commercial 21700 high-energy cells with very similar capacities (5 Ah) and energy densities (264 Wh/kg, 735 Wh/l) according to the supplier’s data sheets. However, in electrochemical tests, these cells showed significant differences with respect to rate capability and cyclic ageing. Detailed microstructural, chemical as well as electrochemical tests utilizing rebuilt half cells and symmetrical cells were done to identify microstructure-property relationships and the most critical microstructural parameters. In doing so, our study contributes to derive guidelines for optimized microstructural electrode design that combines energy with power and a long battery life.For material and microstructural analyses of fresh and aged cells, we used complementary methods such as optical and scanning electron microscopy, µCT- and FIB/SEM tomography, EDS and XRD to characterize the architecture of the electrodes at different length scales. We combined these results with results from detailed electrochemical studies such as OCV and half-cell tests with rebuilt cells as well as electrochemical impedance spectroscopy. Moreover, the 3D data from FIB/SEM tomography has been used as the input for microstructure-resolved simulations in the framework BEST, which allows for further investigation of the electrode microstructure and its effect on the electrochemical behaviour. In doing so, we gained a comprehensive dataset that allowed us to understand the different electrochemical behaviour of the different cells in the pristine state as well as their different behaviour during cyclic ageing.Some of the main findings are: (i) Different suppliers realize the same nominal capacity and energy densities of their cells by different electrode loadings; we observed differences of about 25 %; (ii) All suppliers strive to highly densify both, cathode and anode. Typical densities are 3.4 – 3.6 g/cm³ for the cathode and 1.5 – 1.6 g/cm³ for the anode. (iii) All investigated cells contain Si-rich phases within the anode, however, the chemical composition and the morphology is quite different and affects the electrochemical behaviour; (iv) We found significant differences in the binder-additive content; microstructure-resolved simulations indicate that this critically affects the electronic conductivity of the electrodes and thus, their electrochemical behaviour; (iv) The rate capability of the cells is significantly affected by the areal capacity of the electrodes (and presumably, their electronic conductivity); (vi) This in turn affects the cyclic aging behaviour of the cells. The investigated cells exhibit significant differences under various cycling protocols; we varied upper and lower cut-off voltage as well as the C-rate to investigate how microstructural features affect aging under different conditions.Within the talk, we will provide detailed insights into the results briefly outlined above and try to put the individual pieces of the puzzle together into an overall picture to elucidate how different microstructural features in combination determine the performance and cycling stability of the cells. Figure 1