Structural and Chemical Synergistic Encapsulation of Polysulfides Enables Ultralong-Life Lithium-Sulfur Batteries

Abstract

The fast depletion of fossil fuels and deterioration of environment have led to increasing demand for renewable energies and efficient energy storage technologies. Lithium-sulfur (Li-S) batteries have been regarded as one of the most promising high-energy power sources in broad applications ranging from electric vehicles to large-scale grid energy storage. Li-S batteries deliver a theoretical energy of 2600 W h kg-1 that is an order of magnitude higher than that of the current lithium-ion batteries (LIBs), and utilize naturally abundant sulfur as the cathode material which significantly reduces the cost. The major challenge for the practical implementation of Li-S batteries resides in the dramatic capacity decay. The intermediate lithium polysulfides (Li x S n , 3≤n≤8) formed during cycling dissolve in the liquid electrolyte, migrate through the separator, and deposit on Li metal anode, causing “shuttle effect”. In addtion, the utilization of the active material is strongly hampered by the intrinsic insulating of S and its discharge product Li2S, leading to a low capacity and poor rate capability. Furthermore, the S cathode suffers from the volume variation (~80%) during lithiation/de-lithiation, causing the loss of electrical contact and the structure instability. Herein, we demonstrate an innovative strategy to efficiently entrap Li x S n from synergistic effect of structural restriction and chemical encapsulation using metal oxide-decorated hollow sulfur spheres. The significance of this strategy lies in that we purposely design a material architecture with both structural and chemical encapsulation effect, and that the material architecture provides a prolonged cycling stability. MnO2 is selected as a model and the MnO2 nanosheets-decorated hollow S spheres (hollow S-MnO2) nanocomposites are achieved through a facile synthesis. The nanocomposites with unique structure possess several features favoring highly stable S electrodes: i) the hollow spheres with inner void space not only alleviate the volume expansion of S on lithiation but also structurally restrict soluble Li x S n within the spherical structure; ii) the decorated MnO2 nanosheets with large surface area efficiently and chemically minimize the polysulfides dissolution by forming strong bonding; iii) the small dimensions of hollow S-MnO2 nanocomposites facilitate both ion and electron transport, leading to a better utilization of the S. This design presents a new strategy to prevent loss of polysulfides by structural and chemical dual-encapsulation, and can be expanded to other metal oxides or metal hydroxides. The unique material architecture enables high-performance S cathodes with high capacity, high sulfur loading and extremely low capacity decay of only 0.028% per cycle over 1500 cycles at 0.5 C-rate. Figure 1