Predicting SEI Morphology from Transport and Reaction Kinetics

Kurzfassung

Lithium-ion batteries are the technology of choice for a broad range of applications due to their good performance and long-term stability. The structure and composition of the solid electrolyte interphase (SEI) is the key for the long time stability of low-voltage anodes such as graphite in combination with high performance liquid electrolytes. Direct contact between these phases results in continuous reduction of electrolyte solvent and salt. Thus, during the first charge of a pristine graphite electrode, a protective layer of connected solid SEI particles is formed which separates electrode and electrolyte, reducing the rate of electrolyte reduction considerably. However, the reaction is never absolutely suppressed and long-term SEI growth remains the biggest contributor to capacity fade in lithium-ion batteries. Our model describes the long-term evolution of a porous SEI [1,2], considering the interplay of structure, reaction kinetics and transport mechanisms. It extends the approach of previous models [3,4] which describe SEI thickness evolution using only a single rate-limiting transport mechanism. We consider all potentially rate-limiting transport mechanisms, as e.g. electron conduction, solvent diffusion and lithium interstitial diffusion. The emergence of a spatially structured SEI as well as details of the structure is the consequence of coupling these transport mechanism with different SEI formation reactions. The first model considers a single representative SEI formation reaction and predicts continuous growth of a porous SEI. Electron conduction drives film growth and thickness evolves with the square-root of time. We can show that the emerging structure is not specific for electron conduction but could also be observed with diffusion of neutral lithium interstitials through the SEI as main transport mechanism. The predicted porosity of the layer can influence the rate limiting transport mechanism, as we show in parameters studies. Adding a second SEI formation reaction leads to the formation of a dual-layer SEI. The properties of the additional layer depend on the type of the second reaction, i.e., solvent reduction or reduction of SEI compounds. Furthermore, we predict an equilibrium relation between the thickness of the inner layer and the total SEI thickness. Different SEI profiles generated in our model are studied with impedance simulations of graphite anodes. The impact of the rate limiting transport mechanism on possible inhomogeneities of the SEI thickness on the scale of the microstructure of the electrode is demonstrated in structure resolved 3D cell simulations. All predictions above are observable with suitable experimental techniques, such as neutron reflectometry, and can be used for model validation. [1] Single, F., Horstmann, B., & Latz, A. (2016). Phys. Chem. Chem. Phys., 18, 17810–17814. doi:10.1039/C6CP02816K [2] Single, F., Horstmann, B., & Latz, A. (2016). Paper #2 arxiv [3] Pinson, M. B., & Bazant, M. Z. (2012). Journal of the Electrochemical Society, 160(2), A243– A250. doi:10.1149/2.044302jes [4] Christensen, J., & Newman, J. (2004). Journal of the Electrochemical Society, 151(11), A1977. doi:10.1149/1.1804812