Invited Presentation: Electron Tunneling Based SEI Formation Model

The formation of the Solid Electrolyte Interphase (SEI) has recently attracted much interest, especially after the successful introduction of Li-ion batteries in many applications. It has been considered as one of the predominant factors for the capacity loss in aged Li-ion batteries. A lot of modeling research have been conducted during the last decades to describe the SEI formation [1-4].

SEI is considered as a non-conductive thin film covering the anode, preventing solvent co-intercalating from the electrolyte [1]. It has been analyzed that the structure of SEI is quite complex and basically consists of two different layers: a compact inner layer, mainly composed of inorganic salts and a porous outer layer, predominantly consisting of organic Li salts [5]. Two SEI growth mechanisms have been proposed in the literature. The first one is based on the assumption that electron tunneling through the inner SEI-layer is rate limiting [1,2], while the second considers solvent diffusion to be rate limiting [3,4].

In this contribution a new mathematical model is presented, which is based on the assumption that electron tunneling through the inner SEI-layer is rate limiting and that solvent reduction takes place at the interface of the compact inner- and porous outer-SEI layer, inducing the growth of both layers at this interface as is schematically shown in Fig. 1. The tunneling probability through the inner-SEI can be represented by

P=P₀exp(-4πl_in(2m(U-E(SoC)))½/h),

where  P₀ is a constant, l_in the thickness of the inner SEI-layer [m],  U the energy barrier for electrons to tunnel through l_in[eV], E(SoC) the Fermi level of electrons in the electrode at State-of-Charge (SoC) [eV] and h is the Planck constant. The inner SEI-layer plays a crucial role in protecting the electrode and controlling the SEI formation.

The graphite electrode potential influences the Fermi level and hence the electron tunneling probability. Volumetric changes induced by Li-(de)intercalation has also a signi-ficant influence on the degradation process. The difference of the SEI-formation rate between cycling and storage is considered to be induced by these volume changes. 

All simulations are validated by experimental data. Fig. 2 shows the capacity loss of commercial batteries upon storage and cycling under various conditions. The upper three curves show the capacity loss at storage at various SoC, the bottom curve shows the cycling performance at 0.1C. The symbols represent experimental data and the lines corresponds to the simulations. Degradation is much slower at low SoC storage, while it is significantly enhanced at higher SoC. Degradation is faster when the cell is cycled due to the volume expansion. Fig. 3 shows the calculated changes of thickness of the inner SEI-layer upon storage and cycling. It is almost constant at lower SoC, while it is growing at higher SoC. This indicates that the inner SEI-layer is predominantly formed at high SoC.

References

[1] E. Peled, J. Electrochem. Soc., 126(1979) 2047.

[2] M. Broussely, S. Herreyre, P. Biensan, P. Kasztejna, K. Nechev, R.J. Staniewicz, J. Power Sources 97(2001) 13.

[3] Y. Xie, J. Li, C. Yuan, J. Power Sources, 248(2014) 172.

[4] H.J. Ploehn, P. Ramadass, R.E. White, J. Electrochem. Soc., 151(2004) A456. 

[5] D. Aurbach, Y. Ein-Ely, and A. Zaban, J. Electrochem. Soc., 141 (1994) L1.