Strong adsorption, catalysis and lithiophilic modulation of carbon nitride for lithium/sulfur battery

Lithium/sulfur (Li/S) batteries have emerged as one of the most promising next-generation energy storage systems with advantages of high theoretical energy density, low cost and environmental friendliness. However, problems regarding to severe shuttle effect of soluble polysulfide, poor electronic/ionic conductor of solid charged/discharged products (S₈ and Li₂S), and fatal swell of volume along with the growth of Li dendrites greatly deteriorate the sulfur utilization and capacity retention during extended charge-discharge cycles. With advantages of high nitrogen content, lithiophilic modulation and tunable charge density and charge transfer, carbon nitride (g-C₃N₄) has played a positive role in restricting the shuttle effects and dendrite formation. This minireview mainly discusses these research achievements of g-C₃N₄ in Li/S batteries, aiming to provide a basic understanding and direct guidance for further research and development of functionalized g-C₃N₄ materials in electrical energy storage. The two-dimensional (2D) structure of g-C₃N₄ with abundant hierarchical pores improves its accommodation capacity for sulfur by effectively confining the lithium polysulfides (LiPSs) into the pores, and provides favorable channels for ion diffusion. The rich nitrogen and carbon defects further offer more active sites for strongly adsorbing LiPSs and bridge electron transfer pathway at atomic scale for catalytic reactions to accelerate redox kinetics of Li/S conversion chemistry. Moreover, the features of lithiophilic wettability, high adsorption energy and densely distributed lithiophilic N of g-C₃N₄ provide a large number of adhesive sites for lithium cation (Li⁺) and disperse the nucleation sites to enable uniform nucleation and deposition of Li on the anode surface and to suppress formation and growth of Li dendrites. Finally, the g-C₃N₄ also effectively regulates the wettability between Li anode and solid inorganic electrolyte, and reduces the crystallinity of solid polymer electrolyte to enhance the Li⁺ migration ability and ionic conductivity.