Compare the Electronic Insulating Abilities of Inorganic Components in Solid Electrolyte Interphase (SEI): A First Principles Study

Solid Electrolyte Interphase (SEI) can be formed by decomposed electrolytes or various coating materials. One of the functionalities of the SEI, especially its inorganic components, is to electrically passivate the electrode surface thus preventing further electrolyte decompositions, which leads to irreversible capacity loss, reduced columbic efficiency, and shortened cycle life in lithium ion batteries. However, due to the compositional and structural complexity of SEI, the electronic insulating mechanism of SEI still remains an open question. In this article, the electron tunneling barrier in SEI covered anodes was computed based on density function theory (DFT) calculations in order to evaluate the electronic insulating abilities of different SEI components. These SEI components include inorganic components commonly found in naturally formed SEI, such as LiF and Li₂CO₃; and other possible coating materials, such as Al₂O₃ and Li₃PO₄. Among them, Al₂O₃ can be further lithiated to LiAlO₂. Therefore both the original coating material and its lithiated compound need to be considered. As the SEI thickness is at nanometer scales, the electron tunneling barriers vary greatly with film thickness, as a result of quantum size effect. This suggests that the electronic insulating mechanism of SEI will change during its growth as its thickness increases. Therefore, there is a difference between the early stage of the SEI formation and the late stage in SEI growth. Overall, by ranking the electron tunneling barriers and computing the electron tunneling probability, the critical thickness of SEI film required to block electron leakage from anode to electrolyte is estimated.