Thiophosphate-Functionalized Carbon Nanotubes as a Multifunctional Cathode Additive for Lithium–Sulfur Batteries

Carbon nanotubes (CNTs) are synthetically versatile and conductively tunable materials, making them attractive for energy storage applications. Building on our previous studies on phosphate-functionalized metal–organic frameworks (MOFs), we synthesized a functionalized CNT material with thiophosphate groups to mitigate polysulfide shuttling in lithium–sulfur batteries, a high-capacity lithium-ion battery alternative. The resulting materials were comprehensively characterized by a suite of spectroscopic techniques, such as X-ray photoelectron, Raman, and solid-state nuclear magnetic resonance spectroscopy, and implemented into sulfur cathodes. Like their MOF analogues, thiophosphate-functionalized CNTs in the sulfur cathode promoted higher average specific capacities (1,128 mAh g–1 S) and capacity retention than the oxidized CNT precursor (989 mAh g–1 S). The improved cyclability of thiophosphate-functionalized CNT cells is attributed to enhanced polysulfide redox via high rates of mass diffusion and sulfur utilization at the cathode. This study demonstrates the translatability of our molecular approach in MOFs for other materials classes and presents new opportunities for advancing lithium–sulfur batteries.