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

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.