Impact of Electronic Properties of Grain Boundaries on the Solid Electrolyte Interphases (SEIs) in Li-ion Batteries

Electron leakage through the solid-state electrolyte interphase (SEI) in Li-ion batteries causes the reduction of the electrolyte and the consumption of Li-ions, decreasing the battery capacity and performance. Given the multicomponents and mosaic structures of SEI, the extended defects such as grain boundaries (GBs) and interfaces in SEI are likely to serve as the electron conduction pathways, as the individual SEI components are wide-bandgap insulators in their single-crystalline forms. In this work, the electronic properties of representative GBs of the main SEI components (LiF, Li2O, and Li2S) on the Li-metal in various electrolytes were investigated via density functional theory (DFT) calculations. It was found that all the GB structures have smaller bandgaps than their corresponding single crystals, with an order of amorphous GBs < Tilt GBs < Twist GBs < single crystals. Some GBs, such as the symmetric Li2S Tilt Sigma 3 (121)/[111] GB and the amorphous LiF GB, showed empty electronic states lower than the standard Li+/Li-0 depositing potential. These GB states can trap electrons from the Li-metal, contributing to Li-dendrite growth and electron leakage through SEI. Structural analysis revealed that more under-coordinated atoms in the GBs led to smaller bandgaps and more excess electron localization in the less dense GB regions. These insights suggested that dense SEI structures such as sharp interfaces and well-ordered GBs are preferred to design a fully electronically passivating SEI.