Covalent organic framework-based Li–S batteries: functional separators promoting Li+ transport and polysulfide trapping

Abstract

Lithium–sulphur batteries (LSBs) prevail as a viable alternative to Li-ion batteries due to their high theoretical specific capacity (1672 mA h g_S⁻¹). However, the formation of soluble polysulfides and their shuttle from the cathode to the anode cause irreversible capacity loss and uncontrolled self-discharge, limiting the performance of commercially available prototypes. In this work, we present a comparative analysis of two Kagome-shaped imine-based covalent organic frameworks (COFs) as functional modifiers for polypropylene (Celgard) separators in LSBs. We demonstrate, by using the KS60@Celgard separator modified with an optimized content of COF with the thienothiophene linker, the realization of LSBs reaching a specific discharge capacity of 850 mA h g_S⁻¹ at C5. The proposed separator has an extraordinarily high Li⁺ diffusion coefficient (D_Li⁺) of 1.6 × 10⁻⁷ cm² s⁻¹ at the first cathodic peak, as well as the lowest S₈ : Li₂S_x content ratio in the ex situ post mortem XPS analysis. These findings demonstrate that the use of separators modified with COFs allows the mitigation of shuttle effect, and is further accompanied by an efficient oxidation of Li₂S_x to S₈ (electrocatalytic effect). The equivalent K60@Celgard, based on a COF carrying a phenyl linker, results in LSBs with a specific discharge capacity of 599 mA h g_S⁻¹. This work highlights the synergistic effect of polysulfide retention, selective Li⁺ sieving and electrocatalytic activity of COF-modified Celgard separators in the development of high-performance LSBs.