Metal-Porphyrin Frameworks Supported by Carbon Nanotubes: Efficient Polysulfide Electrocatalysts for Lithium-Sulfur Batteries

Abstract

Lithium-sulfide batteries (LSBs) are highly appealing for the next-generation energy storage systems due to their high theoretical energy density and low cost. However, practical applications of LSBs are still largely hindered by the complicated sulfur redox reactions and the fatal polysulfide shuttling. In nature, cytochrome c reductase works as an efficient biocatalyst for oxidative phosphorylation benefiting from the synergistic electron transfer between porphyrins and sulfur clusters. Inspired by this, herein, two porphyrin-based metal-organic frameworks (Hf-H2DPBP and Hf-CoDPBP) supported by carbon nanotubes (CNTs) were demonstrated to be efficient electrocatalysts for accelerating polysulfide redox kinetics when as separator coatings for LSBs. Consequently, the cells exhibited high initial specific capacities of 1345 mAh g-1 (Hf-H2DPBP/CNT) and 1325 mAh g-1 (Hf-CoDPBP/CNT) at 0.1 C, and maintained outstanding stability with capacities of 582 mAh g-1 (Hf-H2DPBP/CNT) and 513 mAh g-1 (Hf-CoDPBP/CNT) after 500 cycles at 2 C with a decay rate of only 0.079% and 0.086% per cycle, respectively. Even at a high sulfur loading up to 6.2 mg/cm2, the cells still demonstrated excellent performances with an initial specific capacity of 911 mAh g-1 (Hf-H2DPBP/CNT) and 952 mAh g-1 (Hf-CoDPBP/CNT) at 0.1 C, and retained the specific capacities of 745 mAh g-1 (Hf-H2DPBP/CNT) and 818 mAh g-1 (Hf-CoDPBP/ CNT) after 60 cycles, respectively. Furthermore, the high and comparable electrochemical performances can be attributed to the remarkable redox kinetics of both free-base and Co porphyrins toward polysulfides, as supported by DFT calculations.