New Insights into the Anchoring Mechanism of Polysulfides inside Nanoporous Covalent Organic Frameworks for Lithium-Sulfur Batteries

The application prospects of lithium-sulfur (Li-S) batteries are constrained by many challenges, especially the shuttle effect of lithium polysulfides (Li2Sx). Recently, microporous covalent organic framework (COF) materials have been used to anchor electrodes in Li-S batteries, because of their preferable characteristics, such as self-design ability, suitable pore size, and various active groups. To identify the ideal anchoring materials that can effectively restrain the shuttle of Li2Sx species, the anchoring mechanism between COF materials and Li2Sx species should be investigated in depth. Therefore, we systematically investigated the anchoring mechanism between specific COF nanomaterials (consisting of boron and oxygen atoms and benzene group) and Li2Sx (x = 1, 2, 4, 6, or 8) species on the surface and inside the pore using density functional theory methods with van der Waals interactions. The detailed analysis of the adsorption energy, difference charge density, charge transfer, and atomic density of states can be used to determine that the COF nanomaterials, with the structure of boroxine connecting to benzene groups and boroxine groups not constructed at the corner of the structure, can effectively anchor the Li2Sx series. Accordingly, this study provides the theoretical basis for the molecular-scale design of ideal anchoring materials, which can be useful to improve the performance of the Li-S batteries.