Modeling the influence of battery operation on SEI growth

Kurzfassung

Lithium ion batteries are key for today’s mobile devices due to their high specific capacity and their good cycle stability. However, lithium ion batteries still lose capacity over time, because a solid electrolyte interphase (SEI) forms and grows on the anode. Understanding the mechanisms, which cause SEI growth, is important to further extend battery life. The SEI forms at the first charging cycle, when the electrode is drawn below the reduction potential of the electrolyte. This initiates a reaction of electrolyte molecules with lithium ions. The products of this reaction precipitate at the electrode surface and subsequently shield the electrolyte from low potentials. However, the SEI continuously grows thicker in subsequent cycle and consumes lithium ions and thereby battery capacity in the process. Battery operation affects this mechanism in two ways. First, the change of electrochemical environment during battery operation influences the process taking place during storage. Secondly, the anode particles expand, when lithiated. This causes stresses inside the SEI, which will eventually lead to fracture. If the SEI breaks, pristine electrode surface is in contact with the electrolyte, so that new SEI will form. In previous works [1-3], our group analysed the SEI growth during storage. Different mechanisms were compared to extensive storage experiments [4]. Thereby, we found that the SEI growth during storage is caused by the formation of lithium interstitial atoms at the electrode [3,5]. These neutral atoms diffuse through the SEI to the electrolyte, where they react and form new SEI [3]. We extend this model to also account for electrochemical effects during battery operation. The model development is oriented on the recent findings of Attia et al. [6,7]. They showed in their experiments that the SEI grows faster during chargingand slower during discharging. Additionally, they measured the influence of open circuit potential and cycle number. Our SEI growth model for battery operation agrees nicely with the experiments [6] and is in line with our previous storage model [3]. Based on the developed model we can predict critical operating conditions and aid in the design of improved charging and discharging protocols. We will extend this SEI growth model for the indirect mechanical effect. For this, we developed a thermodynamically consistent particle model, which couples chemical and mechanical effects. Based on this model, we can investigate the influence of the arising stresses on the SEI stability and reformation. Literature [1] Horstmann, B., Single, F., & Latz, A. (2018). Review on Multi-Scale Models of SolidElectrolyte Interphase Formation, 13, 1–8. https://doi.org/10.1016/j.coelec.2018.10.013 [2] Single, F., Horstmann, B., & Latz, A. (2017). Revealing SEI Morphology: In-Depth Analysis of a Modeling Approach. Journal of The Electrochemical Society, 164(11), E3132–E3145. https://doi.org/10.1149/2.0121711jes [3] Single, F., Latz, A., & Horstmann, B. (2018). Identifying the Mechanism of Continued Growth of the Solid-Electrolyte Interphase. ChemSusChem, 1–7. https://doi.org/10.1002/cssc.201800077 [4] Keil, P., Schuster, S. F., Wilhelm, J., Travi, J., Hauser, A., Karl, R. C., & Jossen, A. (2016). Calendar Aging of Lithium-Ion Batteries. Journal of The Electrochemical Society, 163(9), A1872–A1880. https://doi.org/10.1149/2.0411609jes [5] Shi, S., Lu, P., Liu, Z., Qi, Y., Hector, L. G., Li, H., & Harris, S. J. (2012). Direct calculation of Li-ion transport in the solid electrolyte interphase. Journal of the American Chemical Society, 134(37), 15476–15487. https://doi.org/10.1021/ja305366r [6] Attia, P. M., Das, S., Harris, S. J., Bazant, M. Z., & Chueh, W. C. (2019). Electrochemical kinetics of sei growth on Carbon Black: Part I. experiments. Journal of the Electrochemical Society, 166(4), E97–E106. https://doi.org/10.1149/2.0231904jes [7] Das, S., Attia, P. M., Chueh, W. C., & Bazant, M. Z. (2019). Electrochemical kinetics of sei growth on carbon black: Part II. Modeling. Journal of the Electrochemical Society, 166(4), E107–E118. https://doi.org/10.1149/2.0241904jes