Polyimide electrode materials for Li-ion batteries via dispersion-corrected density functional theory

Abstract

Compared with organic electrode materials that are used for lithium ion batteries that are constructed using small organic molecules, polymer electrode materials have a better cycling stability, which may be due to their stable long chain structure, and currently, the mechanism of energy storage has not been thoroughly elucidated. In this study, dispersion-corrected density functional theory (DFT-D2) was used to explore the charge/discharge process of polyimide, which is an organic polymer electrode material that could be used in Li-ion batteries. The calculated potentials of PI-1 (polyimide-1) and PI-2 (polyimide-2) are 2.03 and 2.07 V, respectively, and they significantly agree with experimental values, which implies that DFT-D2 is a powerful method to investigate polymer electrodes for use in lithium ion batteries. The calculated potential of pyromellitimide (DPI) is 1.79 V, and DPI is a novel electrode material that has not been reported to date. For each of the three polyimides, lithium ions do not diffuse along the polymer chains but diffuse in the vertical direction, and the migration barriers of PI-1, PI-2, and DPI are 0.47, 0.84, and 0.088 eV, respectively; thus, they have good ionic conductivities (beyond PI-2). Although the calculated band gaps of the three polyimides are all approximately 1.0 eV, the effective electron (or hole) masses are too large, which may limit their electronic conductivities and rate performances. The calculated results show that polyimides are potential Li-ion electrode materials, and this theoretical method could be applied to design novel polymer electrode materials.