Insight on polysulfides harness: COFs for Li-S batteries

Abstract

Nine COF designs have been selected for their porosity, surface area and pore surface chemistry, synthesized or reproduced via a solvothermal method with different degrees of success in terms of crystallinity and porosity. Through various spectroscopy techniques (IR, Raman, XPS), together with imaging tools (SEM, TEM) and system specific analysis (XRD, N2 adsorption) it was possible to characterize the samples in all their aspects and reveal the hidden mechanisms of their nucleation. Three COFs (TAPB-DMTP 100, TAPB-DMTP 50 and DhaTab 100) had high degree of crystallinity and reasonable BET surface areas (1906, 1952, 437 m2 g-1respectively ). IISERP-CON1 was quite amorphous and adsorbed the equivalent of 125 m2 g-1. Nonetheless, as well as TAPB-DMTPs, it became for the first time cathode in sulphur and Li2S batteries. Once the battery fabrication reached its optimum, the maximum reached capacities have been 470 mAh g-1 (B3) and 991 mAh g-1 (B6) and 532 mAh g-1 (B4) in the first half cycle at 0.01C. When run at 0.1C, B3 could deliver 360 mAh g-1 after the first discharge. The fact that IISERP-CON1 delivered the highest capacity must be attributed to the high density of adsorption sites and the strong link in between COF and Li2S that was observed with XPS. In order to reach longer cycling times, it is important that both an ordered porous structure and a rational trapping sites arrangement are achieved in the same COF.