Revealing the Structure and Properties of Polycrystalline Components of the Solid Electrolyte Interface

Abstract

The formation, structure and composition of the solid-electrolyte interphase (SEI) in lithium batteries have been extensively explored in the prior literature. It is commonly accepted that the SEI consists of two inorganic layer followed by organic layer. However, very little is known about the actual grain boundary (GB) structure of the inorganic layer and the ions transport through it. The present work seeks to understand the structure and stability of the GBs formed between various inorganic components of the SEI and mechanisms of Li diffusion through them. The density functional theory (DFT) calculations supported by the high-resolution transmission electron microscopy (HRTEM) measurements are employed to understand the SEI GB atomic structures and their chemo-mechanical properties. DFT calculations were performed using the Vienna Ab Initio Simulations Package (VASP) code1 employing the generalized-gradient approximation (GGA) using the PBE (Perdew, Burke, and Ernzerhof)2 functional to account for the exchange-correlation effects. The Nudged Elastic Band (NEB) method3 is used for the Li migration barriers calculations as implemented in VASP. To build the GB structures, the Atomistic Tool Kit (ATK)4 “Interface Builder” tool is used, which allows to analyze all possible interphases between two slabs. Cryogenic TEM (Cryo-TEM) images are recorded on a JEOL ARM200CF TEM, equipped with a Gatan Oneview camera operated at 200 kV. The HRTEM measurements revealed the polycrystalline structure of the SEI inorganic layer consisting mainly of LiF, Li2CO3, Li2O and LiOH components. In addition, different surface orientations (e.g., 111, 200, 002, etc.) of the aforementioned components are identified. Based upon the HRTEM results (cf. Fig. 1 left-hand side), the corresponding computational slabs have been built as shown on the right-hand side in Fig. 1. After the optimization of all structures, Li adatom diffusion inside grain and GB and mechanical properties were calculated. For each GB, several Li adatom positions with multi-atom bonding were identified. The activation energy of Li diffusion varies significantly depending upon the slab’s structure and the number of Li adatoms. The NEB calculations also reveal a non-symmetric transition state for all considered structures, which indicates a complex multi-atom hopping mechanism. The calculated elastic properties show the decrease of Young’s modulus with an increase of Li adatoms. Our combined experimental and modeling findings provide valuable insights into developing mitigation strategies towards a better understanding of the SEI components stability and Li migration through the SEI. Figure 1. Identification of grain and GB of the SEI busing HRTEM (left) and the corresponding built DFT slab (right). Lavender spheres denote Li atoms, red spheres are the oxygen atoms, grey spheres represent carbon atoms and green spheres are F atoms. Blue sphere in the GB denotes Li adatom. References G. Kresse, Phys. Rev. B, 59, 1758–1775 (1999). J. P. Perdew, A. Ruzsinszky, G. I. Csonka, O. A. Vydrov, G. E. Scuseria, L. A. Constantin, X. Zhou, K. Burke, Phys. Rev. Lett., 100, 136406 (2007). G. Henkelman, B. P. Uberuaga, H. Jónsson, and G. Henkelman, J. Chem. Phys, 113 9901 (2020). Synopsys QuantumWise A/S, 2017/2. Figure 1