A recent experiment [ Jin , E. ; Science 2017 , 357 , 673 - 676 ] shows that the conductivity of a two-dimensional (2D) sp2-carbon-hybridized π-conjugated covalent organic framework (sp2-c-COF) can be enhanced by as much as 12 orders of magnitude after iodine oxidation processing. To understand the physical mechanism underlying such a huge increase in the conductivity, we perform multiscale computations and find that the high conductivity of the iodine-oxidized 2D COF can be attributed to both hole transfer and ion transfer within the 2D COF. The computed dominant charge distribution corresponding to the valence band maximum (VBM) suggests that the delocalized π electrons occur mostly at the active reaction sites. The computed low ionization energy at the active reaction sites further supports that the 2D COF tends to lose electrons during iodine oxidation and to yield cationic COF and anionic triiodide I3-. Complementary classical molecular dynamics simulation shows a relatively high anion conductivity of 13.63 × 10-2 S m-1, consistent with the high conductivity measured from the experiment (7.1 × 10-2 S m-1). Meanwhile, we find that the cations in 2D COF can also induce a shift of the Fermi level to cross the valence band, thereby enhancing the hole mobility to 86.75 cm2 V-1 s-1. For proposing a potential application of the highly conductive iodine-oxidized 2D sp2-c-COF, we design a prototypical model of the 2D spirally wound lithium-ion battery and find that it exhibits enhanced stability than a typical electrolyte material.
Keywords: COF; charge mobility; conductivity; ion transfer; mechanism.