A Facile Strategy to Improve the Electrochemical Performance of Porous Organic Polymer-Based Lithium-Sulfur Batteries (original) (raw)

Porous organic polymers (POPs), with features of permanent nanopores and designable frameworks, show great promise as sulfur host materials to restrain the shuttling of polysulfides, one of the main obstacles in the development of lithium-sulfur batteries. However, the simple physical entrapment from weak intermolecular interactions via a typical melt-diffusion method results in the diffusive loss of polysulfides that has thus far restricted their potential. Herein, a facile strategy for introducing chemical covalent interactions between POPs and sulfur via the regulation of sulfur infiltration temperature is reported. The results show that increasing the temperature to a suitable value, e.g., 400 C, for a fluorinated triazine-based framework (FCTF), enables chemical bonding between the sulfur and aromatic FCTF backbone. Benefitting from the synergetic chemical and physical confinement effect, the shuttling of polysulfides can be efficiently restrained. As a result, the sample features superior sulfur utilization, high-rate performances, and good cycle stability, as compared with the sample with only physical confinement. The proposed strategy can also be extended to other POPs, such as the boroxine-linked covalent organic framework, by judiciously tailoring the infiltration temperatures. The findings disclose the important role of infiltration temperatures in developing efficient cathode host materials for lithium-sulfur batteries. Due to high specific capacity (1675 mAh g À1) and energy density (2600 W h kg À1), low cost, and natural abundance of elemental sulfur, lithium-sulfur (Li-S) batteries represent a promising energy storage technology and have attracted significant research interests in the past decade. [1-4] However, the practical implementation of Li-S batteries has been prohibited by many problems. One of the major and urgent issues is the shuttle effect of lithium polysulfides, in which the soluble intermediate product generated from redox reactions can dissolve into the electrolyte, causing a diffusive loss of usable electroactive materials and poor cycle stability .[5] Since Nazar and coworkers creatively impregnated sulfur into conductive mesoporous carbon to restrict polysulfides during the charge-discharge process [6] many porous carbons and other porous materials have been used as sulfur host materials to suppress the dissolution of polysulfides. [7-18] Although exciting progress has been made, the rational design of sulfur host materials with tailored structures to efficiently trap polysulfides still has to be further explored. Porous organic polymers (POPs), including covalent organic frameworks (COFs), [19,20] covalent triazine frameworks (CTFs), [21,22] conjugated microporous polymers (CMPs), [23] and hypercrosslinked polymers (HCPs), [24] emerge as a promising family of porous materials with characteristics of low density, high surface area, good stability, and designable structure and function. Therefore, they show great potential in some energy storage systems [25-27] and particularly as host materials for sulfur loading in Li-S batteries. [28,29] First, the existence of abundant nanopores with different pore structures will help suppress the loss of poly-sulfide by physical entrapment within the porous nanodomains of the frameworks. [30] Second, the POP hosts can be rationally synthesized with suitable framework and pore environments, [31-33] offering a wealth of opportunities to design effective sites for the chemical anchoring of polar polysulfides in principle. This is different from the porous carbon hosts that are nonpolar and thus have weak interactions with the polar polysulfide species. To date, several examples employing POPs as sulfur host materials have been reported. [34,45] However, the majority of POPs still show relatively weak interactions with sulfur species, mainly dominated by physical encapsulation/nanoconfine-ment. [34-37] Thus, intensifying the interactions with sulfur