Defects on Li₂S@graphene cathode improves the performance of lithium-sulfur battery, A theoretical study

Lithium-sulfur (Li−S) batteries are promising next generation large-scale electrical energy storage. One of our authors constructed a novel Li₂S@graphene cathode material that exhibits outstanding electrochemical performance (Nat. Energy 2017, 2, 17090) in Li−S batteries, but the underlying mechanism remains unexplored. Herein we performed systematical theoretical study to address the mechanisms. First-principle calculation shows that the defects on graphene directly contributes to the superior electrochemical performance of the Li−S battery in three aspects. First, defects on graphene facilitate the binding of Li₂S, thus lowering the decomposition barrier of Li₂S during the first charge cycle, leading to the lowered activation voltage in experiment. Second, the followed Li ion diffusion is promoted by defects. Third, in the following discharge/charge cycles S-doped graphene is formed, which improves the conversion of S₈ to Li₂S during discharge process as well as inhibits the “shuttle effect”. Moreover, if vacancy defects remain on graphene, the above two advantages still hold, supporting the high rate performance observed in experiment. This work provides a theoretical understanding of the improved electrochemical performance of Li₂S@graphene as cathode in Li−S battery from the perspective of defects on graphene, and helps the rational design of potential 2D materials as cathode for practical Li−S batteries.