Development of functional separators that block the migration of lithium polysulfides (LPSs) is an efficient way to suppress the shuttle effect, a critical issue restricting the practical applications of lithium–sulfur (Li–S) batteries. A metal–organic framework (MOF) has been used as a separator for sieving out the LPSs while allowing the Li⁺ ions to pass through. This study is the first to elucidate the sieving process of the MOF at the atomic level by examining the penetration of Li⁺ ions and LPSs solvated by 1,2-dimethylethane (DME) molecules into the Cu₃(BTC)₂ (BTC = benzene-1,3,5-tricarboxylate) MOF nanosheet. The solvated LPSs, i.e., Li₂S_x(DME)₄ (x = 8, 6, and 4), are too large to enter the pores of the MOF. The partially desolvated LPSs, i.e., Li₂S_x(DME)₂, can enter the pores but get trapped in the pore walls due to the high penetration barriers of 1.73–1.99 eV. However, the partially desolvated Li⁺ ions, i.e., Li⁺(DME)₂, can pass through the MOF layer by overcoming a much lower penetration barrier of 0.66 eV without getting trapped in the pore walls. Moreover, Li⁺(DME)₂ can move rapidly within the MOF layer with a very low diffusion barrier of 0.32 eV. The LPS-blocking ability of the MOF results from (1) the steric repulsion between the LPSs and the pore wall due to the pore size not being larger than the size of the LPSs and (2) the formation of chemical bonds between the LPSs and the pore wall due to the solvation structure of the LPSs incompletely surrounded by solvents. The sieving mechanism of the MOF presented in this study will be helpful not only for designing efficient microporous separators for Li–S batteries but also for understanding the transport across general microporous materials.

中文翻译：

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用于锂硫电池的多硫化物封闭金属有机骨架隔膜的分子筛分机制

开发阻止多硫化锂（LPS）迁移的功能性隔膜是抑制穿梭效应的有效方法，这是限制锂硫（Li-S）电池实际应用的关键问题。金属有机骨架 (MOF) 已被用作筛选出 LPS 的分离器，同时允许 Li ⁺离子通过。本研究首次通过检查 Li ⁺离子和由 1,2-二甲基乙烷 (DME) 分子溶剂化的 LPS渗透到 Cu ₃ (BTC) ₂ (BTC = 苯-1,3,5-三羧酸盐）MOF 纳米片。溶剂化的 LPS，即Li ₂ S _x (DME)₄（ x = 8、6 和 4）太大而无法进入 MOF 的孔隙。部分去溶剂化的 LPS，即Li ₂ S _x (DME) ₂可以进入孔隙，但由于 1.73–1.99 eV 的高渗透势垒而被困在孔隙壁中。然而，部分去溶剂化的 Li ⁺离子，即Li ⁺ (DME) ₂，可以通过克服 0.66 eV 的低得多的穿透势垒而穿过 MOF 层而不会被困在孔壁中。此外，Li ⁺ (DME) ₂可以在 MOF 层内快速移动，扩散势垒非常低，为 0.32 eV。MOF 的 LPS 阻断能力源于 (1) 由于孔径不大于 LPSs 的尺寸，LPSs 与孔壁之间的空间排斥和 (2) LPSs 与孔壁之间形成化学键。由于 LPS 的溶剂化结构不完全被溶剂包围，孔壁。本研究中提出的 MOF 的筛分机制不仅有助于设计用于 Li-S 电池的高效微孔隔膜，而且有助于理解一般微孔材料之间的传输。