A New Insight 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 (Li2 Sx ). Recently, microporous covalent organic framework (COF) materials have been used to anchor-electrodes in lithium-sulfur (Li-S) batteries, because of its preferable characteristics, such as self-design ability, suitable pore size and various active groups. In order to identify the ideal anchoring materials that can effectively restrain the shuttle of Li2 Sx species, the anchoring mechanism between COF materials and Li2 Sx species should be investigated in depth. Therefore, we systematically investigated the anchoring mechanism between specific COF nanomaterials (consist of Boron, Oxygen atoms and Benzene group) and Li2 Sx (x=1, 2, 4, 6, 8) species on the surface and inside the pore using the 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 Li2 Sx 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.