Enhanced Transformation Kinetics of Polysulfides Enabled by Synergistic Catalysis of Functional Graphitic Carbon Nitride for High-Performance Li-S Batteries

Abstract

The introduction of an electrocatalyst to accelerate the kinetics of lithium polysulfides (LiPSs) reduction/oxidation is beneficial to enhance the capacity of sulfur cathode and inhibit the shuttling effect of LiPSs. However, current electrocatalysts mainly focus on the metal-based active sites to reduce the reaction barriers, and there remains a great challenge in developing light-weighted metal-free catalysts. In this work, 1D graphitic carbon nitride nanorods (g-C₃N₄-NRs) with carboxyl (─COOH) and acylamide (─CONH₂) functional groups are designed as metal-free electrocatalysts for lithium-sulfur batteries to accelerate the transport of Li⁺ and the conversion of LiPSs. The density functional theory (DFT) calculations prove that the existence of ─COOH group realizes the adsorption of LiPSs and accelerates the transport of Li⁺, while the ─CONH₂ groups reduce the reaction energy barrier of S₈ to Li₂S. In addition, in situ UV–vis and Li₂S nucleation/dissociation experiments also verify that g-C₃N₄-NRs achieve rapid adsorption and transformation of LiPSs under the synergistic action of ─COOH and ─CONH₂ functional groups. Consequently, the sulfur cathode based on the g-C₃N₄-NRs-PP separator remains at a specific capacity of 700.3 mAh g⁻¹ after 70 cycles at 0.2 C, at 0 °C. This work provides a new strategy for breaking through the bottleneck of metal-free catalysts for high-performance lithium-sulfur batteries.