An Atomistic View of the Lithiation/Delithiation Behavior of Carbon Nanotube-Confined Sulfur Cathode for Lithium-Sulfur Batteries

Lithium-sulfur batteries possessing high theoretical energy density have been extensively scrutinized, particularly for solving the problem of poor cycle retention caused by polysulfide dissolution and active mass loss of the sulfur cathode. Encapsulation of sulfur in porous carbon materials has been demonstrated as a promising strategy for overcoming these challenges, but the atomic-level details of its dynamic processes have not been fully understood yet. Herein, we used reactive molecular dynamics simulations to investigate the structural evolution, chemical process, and diffusion behavior of the sulfur confined in carbon nanotube during lithiation and delithiation. Our simulations show distinctive spatial distribution of the material comprising a relatively large amount of lithium bounded near the free surface due to complex interactions, which can retard the formation of the electrochemically inactive Li2S layer during lithiation. This also results in a low lithium fraction remaining in the cathode and improved structural reversibility during the reverse process. Furthermore, the unique structural evolution of the material inside carbon nanotube results in the voltage profile distinguishable from the unconfined systems. Our simulations also provide an atomistic view of restricted migration of the sulfur molecules trapped inside the pore, which could be the origin of cycle retention enhancement.