Peripheral group-induced spin-state switch of metal macrocyclic molecule for enhanced redox kinetics in lithium-sulfur batteries

Abstract

Lithium−sulfur (Li−S) batteries have become an ideal candidate for the next generation of rechargeable batteries for the high theoretical energy density. However, the sluggish redox kinetics hampered the development of Li−S batteries. It is reported that the conversion kinetics of lithium polysulfides (LiPSs) can be accelerated by introducing catalytic materials. The macrocyclic metal porphyrins have attracted extensive attention due to their excellent chemical stability and structural tunability. Herein, we designed a series of iron porphyrins substituted by the electron-donating and electron-withdrawing groups. According to electrochemical characterization and theoretical calculation, the functional group can switch the spin state of central Fe and modulate their catalytic performance. The methoxy-substituted iron porphyrin (FeTPP-4OMe) shows smaller band gap and facilitates the electron transfer. As a result, Li−S batteries with FeTPP-4OMe delivered a specific capacity of 1,062.0 mA h g−1 and retained a high specific capacity of 771.4 mA h g−1 at 0.5 C after 500 cycles with a capacity fading of 0.055 %. This work provides a guidance for the functional group modification of metal macrocyclic compounds in Li−S batteries.