Insights into the Anchoring of Polysulfides and Catalytic Performance by Metal Phthalocyanine Covalent Organic Frameworks as the Cathode in Lithium-Sulfur Batteries

The outstanding advantages of lithium-sulfur (Li-S) batteries have made them a potential energy storage device. However, shuttle effect is one of the main problems that restrict the commercial application of Li-S batteries. Apart from it, accelerating the dissociation of Li2S to LiS and Li+ is also very important to achieve high Coulombic efficiency. Recently, it has been found that metal atoms exhibit strong interactions with lithium polysulfides (Li2Sx), which can be used as single-atom catalysts by embedding in the matrix. Covalent organic framework (COF) materials have porous structures and large surface area, which can anchor Li2Sx by introducing active sites. In this study, we constructed metal phthalocyanine COFs (MPc-COFs, M = Ti, V, Mn, Cu, and Zn) as the cathode, which combined the advantages of metal atoms and COFs. The adsorption and catalysis performance of Li2Sx species were investigated by MPc-COF cathode using density functional theory. The results show that the strong adsorption capacity of TiPc-COF and VPc-COF and the formation of axial complexes make them unfavorable for Li-S batteries. MnPc-COFs show excellent conductivity and sulfur fixation capacity, which also have a lower energy barrier in the catalytic oxidation of Li2S. This work has a guiding significance in the design of catalytic electrodes for Li-S batteries.