Polymer Encapsulated Sulfur Nano-Composite for Long Cycle-Life Lithium Sulfur Battery

The lithium-sulfur (LiS) battery is of great interest owing to its remarkable theoretical capacity of 1672 mAh g-1 and specific energy of 2600 Wh kg-1. The low operating voltage of the LiS battery (~2.0 V) provides safety advantages over high-voltage cathode materials, and elemental sulfur is environmentally friendly and inexpensive. However, well-known issues in Li-S batteries include the loss of electrical contact, low electronic conductivity of sulfur, and shuttling of polysulfides during cycling. The shuttle effect is known to cause the low utilization of the sulfur, degradation of cycle-life and, low Coulombic efficiency in LiS batteries. Using conductive materials to encapsulate sulfur nano-particles to improve the electronic conductivity of sulfur and limit the polysulfide dissolution is one of the striking strategies for the development of LiS batteries of long cycle-life and high Coulombic efficiency (above 95%). Most polymer-encapsulated nanocomposites reported thus far require low-temperature treatment as well as long synthesis time, and have short cycle life barely exceeding 100 charge/discharge cycles. Here, we demonstrate that by controlling the pore size of the encapsulating polymer membrane, we can achieve significant improvement in the cyclability and Coulombic efficiency of an Li-S battery. Polyaniline is used as the encapsulating polymer membrane since it reacts with sulfur at high temperature to form a cross-linked structure, which is known to significantly improve the cycling performance. Morphology of the polymer may be controlled by intercalation of metal oxide nano-particles (e.g., MnO2, ZrO2, TiO2) into the pores of the porous polyaniline (PAN) as the encapsulating polymer. To accommodate the large volumetric expansion of sulfur during lithiation, sufficient empty space can be provided by heat treatment of the sulfur/PAN nano-composite. Up until now, the sulfur/PAN nano-composites production has required low- temperature treatment of reactants for 24-48 hours under nitrogen or argon atmosphere. As a unique advantage, we provide a simplified short-time (3 hours) synthesis method at room temperature under air atmosphere. Acknowledgment: This work is supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.