Modeling Growth of the Solid Electrolyte Interphase

Kurzfassung

Lithium ion batteries have been used for decades in numerous applications. In the near future, they are expected to be the most promising candidate for electric vehicles. This requires increase in cycle life from 3-4 years up to 10-15 years. This means the cycle life has to be tripled or even quadrupled for electric vehicles to benefit from lithium ion batteries. Graphite is the most common negative electrode material used in lithium ion batteries due to its low voltage vs. Li/Li$^{+}$ and its mechanical stability. However, when a lithium ion battery is operated with a graphite negative electrode, a passivating film called Solid Electrolyte Interphase (SEI) forms. The SEI is the most crucial contributor to the capacity fade in lithium ion batteries. Despite its significance in terms of capacity fade, the SEI has not been completely understood. Many researchers have investigated formation and morphology of the SEI, but yet no consensus is reached on how it grows and what it is made of. To study how the SEI grows, we use a continuum-scale mathematical model and simulate SEI growth. Since the exact chemical composition of the SEI is not known yet, we model a one-component SEI. It has been agreed upon by many researchers that the SEI has a porous structure. Therefore, we model a porous SEI, which is allowed to change as the SEI grows. In the SEI, electrolyte is assumed to diffuse through the SEI pores. Electrons are only conducted in the SEI and lithium ions are assumed to both diffuse and to be conducted in the electrolyte pores. No one has considered movement of lithium ions in the pores yet and this makes our approach unique. Results show that the SEI growth has a $\sqrt{t}$ behaviour with respect to time. The electron conductivity in the SEI and the formation voltage of the SEI have a great impact on both the SEI growth and on the SEI porosity. Bruggemann coefficient of the electron current in the SEI does not affect the SEI growth, but it does affect the SEI porosity. In this work, we study an omni-directional SEI growth with a porosity which changes as SEI grows. To the best of our knowledge, no truly porous SEI growth has been modeled so far. Therefore, it is important to see how the SEI porosity changes with time and how varying parameters affect it. This work should be validated with further experimental studies (e.g. in-situ characterization of the SEI morphology and the electron conductivity of the SEI).