Two-Dimensional Conductive Metal-Organic Frameworks as Highly Efficient Electrocatalysts for Lithium-Sulfur Batteries

Lithium-sulfur batteries (LiSBs) which are expected to fulfill the increasing demands of high-density energy storage have been under intensive investigation. However, the development of LiSBs is facing many obstacles, such as the poor electronic conductivity of sulfur, shuttling effects of lithium polysulfides (LiPSs), sluggish Li₂S decomposition, and low discharging/charging efficiency. Suitable electrocatalysts that can solve the above problems are promising in the development of LiSBs. Herein, 13 two-dimensional (2D) metal-organic frameworks (MOFs) of nitrogen-, sulfur-, and oxygen-coordinated transition-metal (TM) atoms (Co, Ni, Cu, and Zn) are selected and constructed to reveal the structure-activity relationship of 2D MOFs in terms of the electrocatalytic performance. Among all the 2D MOFs investigated, Cu₃(HITP)₂, Zn₃(HITP)₂, and Cu₃(C₁₈H₉O₃N₃)₂ offer moderate binding strength to LiPSs, which effectively suppresses Li₂S dissolution and shuttling. Cu₃(HITP)₂ exhibits good electrical conductivity, a low Gibbs free energy barrier, effective electrocatalytic ability for Li₂S decomposition, and a high sulfur loading amount. A descriptor φ is proposed to correlate the binding energies of the 2D MOFs to the coordination environment and the electronegativity of the TM atoms in the LiPSs via an implicit volcano plot. These findings are helpful for understanding the electrocatalytic effect of 2D MOFs in LiSBs and represent a promising approach for the development of future LiSBs.