Core–Shell-Structured Sulfur Cathode: Ultrathin δ-MnO₂ Nanosheets as the Catalytic Conversion Shell for Lithium Polysulfides in High Sulfur Content Lithium–Sulfur Batteries

Abstract

Owing to their low cost and high theoretical energy density, lithium-sulfur (Li-S) batteries are highly promising as a contender for the post-lithium-ion battery era. However, the intrinsic low reversible conversion ability of lithium polysulfides to sulfur/Li₂S during charging/discharging seriously hinders the sulfur utilization, resulting in poor cycling life of batteries. Herein, we report an improvement of core-shell-structured sulfur nanospheres@ultrathin δ-MnO₂ nanosheet electrode materials prepared by a simple precipitation reaction method, in which the ultrathin δ-MnO₂ nanosheets as a catalytic layer can promote the chemical adsorption of lithium polysulfides and their conversion rates to sulfur/L_i2S. Using a combination of the UV-visible adsorption spectra and first-principles calculation, the results indicate that the Mn-O coordination center on the surface of the MnO₂ structure plays an efficient catalytic role in the conversion reaction of lithium polysulfides to insoluble S/Li₂S. The sulfur nanospheres@ultrathin δ-MnO₂ nanosheet composite with a high S mass ratio of 82 wt % reveals a high specific capacity of 846 mA h g^-1 at 1 C rate and good cycling stability. Moreover, the areal capacity of the electrode with a high sulfur loading mass of 10 mg cm^-2 is 5.2 mA h cm^-2, approaching the practical application standard at a current density of 1 mA cm^-2.