ZnO with abundant Zn vacancies assisted by reduced graphene oxides for high-performance lithium‑sulfur batteries

Abstract

Defects engineering in electrode materials is popular in lithium‑sulfur (Li-S) batteries. Generally, the defects can improve the adsorption and catalytic activity for polysulfides of materials. Herein, we synthesize zinc oxide (ZnO) samples with different concentration of Zn vacancies (ZnO-1 h, ZnO-2 h, ZnO-4 h). We find that the Zn vacancies in ZnO render high catalytic activity for fast kinetics of polysulfides, while the more Zn vacancies cause the worse adsorption of ZnO. When these samples combine with reduced oxide graphene (rGO) to load sulfur as sulfur cathodes (noted ZnO-2 h/rGO/S etc.), the ZnO-2h/rGO/S cathode exhibits the optimal performance because ZnO-2 h has the best catalytic activity and modest adsorption. Under electrolyte to sulfur (E/S) of 7.5 μL mgS−1, the ZnO-2 h/rGO/S cathode exhibits a higher initial capacity of 1369.7 mAh g−1 at 0.2C (1C = 1672 mA g−1) and better rate performance than ZnO/rGO/S. The density functional theory calculations reveal the higher conductivity but lower adsorption for polysulfides of ZnO with Zn vacancies (Zn1-xO) than ZnO. Further, the stronger catalytic activity of Zn1-xO can be explained by the increase of S-S length in Li2S4 absorbed on Zn1-xO. This paper exposes that the high catalysis is predominant than the strong adsorption for choosing electrode materials.