The Influence of Solid Electrolyte Interface (SEI) on Li Dendrites Formation in Li-Metal Battery
Authors: Ramasubramanian Ajaykrishna , Yurkiv Vitaliy , Ragone Marco , Foroozan Tara , Shahbazian-Yassar Reza 
Published: May 2020

The composition, structure and the formation mechanism of the solid-electrolyte interphase (SEI) in lithium-based (e.g., Li-ion and Li-metal) batteries have been widely explored in the literature. However, very little is known about the ions transport through the SEI, specifically through their grain boundaries (GBs) of the inorganic inner layers, and the corresponding mechanism for Li dendrites nucleation. Understanding the underlying ions diffusion processes across the inorganic components of SEI could lead to significant progress, enabling the performance increase and improving the mitigating strategies of dendritic growth and other safety aspects of these batteries. For this reason, in this work, we present the results of a combined ab initio Density Functional Theory (DFT) calculations to study Li diffusion across the SEI and Li nucleation at the Li metal/SEI junction and an extended phase-field model (PFM) for Li migration and electrodeposition that is a step forward toward better understanding of Li dendrites formation and growth.

The present DFT calculations are performed using the VASP (Vienna Ab initio Simulation Package)1 code with the plane wave basis sets and the projector augmented wave (PAW)2 pseudo-potentials in the framework of Perdew-Burke-Ernzerhof sol (PBEsol)3 generalized gradient approximation (GGA)4. The migration barriers and diffusion coefficients are evaluated using the Nudged Elastic Band (NEB) method, as implemented in VASP.

The major GB structures of SEI that are of interest are the ones between LiF/LiF, Li2O/Li2O and the mixed GBs between LiF/Li2O slabs. The energy barriers for the diffusion of Li vary significantly depending upon the structure of these channels, with the general trend being that Li diffusion in the GB is generally faster than in the neighboring crystalline regions within the grain interiors. The diffusion through LiF/Li2O slabs has the lowest barriers whereas GBs of LiF/LiF slabs has the highest. To understand the role of these GBs in dendrite nucleation, the SEI/electrode interface is analyzed for its stability. The energetics from the DFT analyses depend heavily on the grain structures, with (LiF/LiF)/Li grain structures being most stable and (LiF/Li2O)/Li being the least stable. The interfacial energies in the Li/SEI interfaces show that the interface between (LiF/Li2O)/Li has the least critical length (~1.6 Å) for stable dendritic growth and most favorable for the initiation of crack in the SEI.

To quantify the influence of the SEI on the evolution of Li electrodeposits at the continuum scale, we performed the PFM calculations. The PFM is developed employing MOOSE framework5, and the SEI model includes the typical GB structures within 20 nm thickness of the SEI grains. The data from the DFT as described above is used for Li diffusion through the SEI and for mechanical properties of GB and grains. The results from PFM reveal that the anisotropic nature of Li presence at the metal/SEI interface leads to uneven electrodeposition at the Li metal surface. Also, the results allow for monitoring the stress field evolution and its influence on Li filament structure. The present results also show that significant stress is observed at the root of the Li electrodeposits and the triple junction between the GBs.

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