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Study of the expansion of Li-ion cells with Si-based anodes with a combined approach of measurements and finite-element simulations

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Summary:

Si-based anode materials show promising volumetric energy density but suffer from significantly increased expansion upon lithiation compared to conventional graphite. This expansion causes detrimental effects in the cell and drastically reduces the cell’s lifetime. One reason for these effects are large mechanical stresses within the cell stack as a consequence of repeated expansion and contraction.
To understand and quantify these effects, on the one hand, precise measurements of expansion on cell level are necessary. On the other hand, mechanical cell models need to be developed to investigate the distribution of mechanical stresses and strains within the single layers of the cell since these quantities are not directly accessible by measurements. The talk given will present results for both approaches.
On the measurement part, a test-rig has been developed for measuring the thickness change of lab-scale pouch cells for a given current profile. The cells are under a well-defined mechanical pressure which is monitored throughout the experiments.
The investigated cells contain a Si-alloy/graphite anode and a 𝑁𝑀𝐶 622 cathode. Experiments have been conducted at three different mechanical loads (0.08 𝑀𝑃𝑎,0.42 𝑀𝑃𝑎 and 0.84 𝑀𝑃𝑎) and three different temperatures (5 °𝐶, 25 °𝐶 and 45 °𝐶). Results show a typical overall expansion of about 4 % compared to the original cell thickness. The expansion curves show deviations from linear behaviour as well as significant hysteresis between charge and discharge. An interesting phenomenon is, that the cell has its maximum thickness not at 100% SoC but further expands to about 85% SoC during discharge. A series of rate capability tests has been performed to investigate the dependence of cell expansion on current, pressure and temperature. The results indicate that elevated temperature and an applied pressure have beneficial effects on cell performance.
On the simulations part, a meso-mechanical finite-element model for the pouch cell has been set up. The model considers the full layer structure of the cell stack (current collectors, active materials, separator, pouch foil). All components are modelled and parameterised separately (but homogeneously within each layer) with data from tensile and compression tests. The components have been soaked in electrolyte to consider its influence on the mechanical behaviour. For the anode, a thermal expansion model is applied to simulate layer expansion.
Simulations of the cell compression have been performed and analysed for the component layers, showing very high compression for the soft separator. Consequences of cell charging and anode expansion on stresses inside the cell have been investigated for different constraint situations.

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