Carbon Nitride-Based Hollow Sphere As Polysulfides-Shuttle Restricting Host for Lithium Sulfur Battery

Abstract

Lithium sulfur battery is a promising candidate to replace the currently lithium ion battery due to the high theoretical specific capacity of sulfur cathode (1675 mA h g−1) and low cost. However, the development of sulfur cathode is hindered by the diffusion of highly soluble polysulfide intermediates formed during cycling and the poor electrical conductivity of sulfur, Li2S2 and Li2S. These issues lead to low Coulombic efficiency, fast capacity fading and low sulfur utilization. A variety of materials have been employed as sulfur matrixes in attempt to address the above mentioned issues, such as carbon materials, polymers and metal oxides. Recently, graphitic carbon nitride (g-C3N4), which possesses high nitrogen content, has been used as sulfur host. The abundant N atoms in g-C3N4 could retard diffusion of lithium polysulfides into electrolyte via the formation of Li-N chemical interaction between g-C3N4 and lithium polysulfides. However, g-C3N4 could hardly improve the sulfur utilization especially at high current density due to its intrinsically poor electrical conductivity (10−12-10−13 S cm−1). Herein, we successfully designed and synthesized a hollow carbon nitride-based sphere (HCNx) material with enhanced conductivity (10−2-10−3 S cm−1) via polymerization of ethylenediamine and carbon tetrachloride on silica spheres template and used it as sulfur host. The HCNx with nitrogen-enriched structure could not only retard diffusion of lithium polysulfides via the Li–N chemical interaction but also physically confine lithium polysulfides by its hollow structure and mesoporous shells. Furthermore, the enhanced electrical conductivity of HCNx improves the utilization of sulfur. The S/HCNx cathode exhibits a discharge capacity of 579 mAh g−1 after 500 cycles at 0.5C with a fade rate of 0.076% per cycle. When charging and discharging at 2 C, the S/HCNx cathode still exhibits a high reversible discharge capacity of 658 mAh g−1.