Modeling of lithium ion transport at the LixMn2O4/LiF interface

Abstract

LiF-rich cathode-electrolyte interface (CEI) is of great interest for the development of high-performance Li-ion batteries. However, understanding ion transport through such interface is far from complete, which hinders further improvement of battery performance. To address this issue, we investigated lithium ion transport through LiF layer on LiMn₂O₄ cathode using density functional theory (DFT). For the vacancy diffusion mechanism, the migration barriers of lithium ion at the LiMn₂O₄/LiF interface were calculated. It was found that the barriers in LiF do not increase compared to the bulk when approaching the cathode surface. Using the double layer model developed for a solid-solid electrochemical interface and based on the Poisson–Fermi–Dirac equation, the concentration distributions of dominant charge defects at the Li_xMn₂O₄/LiF interface was obtained for the composition range 0 ≤ x ≤ 1. Our results indicate an extremely low concentration of lithium vacancies in LiF near the cathode surface, making lithium intercalation hardly possible through the perfect single-crystal LiF which should result in poor rate performance. This suggests that the polycrystalline structure of LiF-rich CEI with multiple grain boundaries and inorganic components, as well as the presence of diffusion-enhancing impurities, are critical to ensure rapid ion transport.