Effect of Charged Polymers on Electrochemical Performance of Lithium Sulfur Batteries Using Ionic Liquids

Recently a lot of research efforts have been devoted to lithium-sulfur batteries as one of the next-generation energy storage devices because of their exceptional theoretical capacities (1675 mAh/g or 2600 mWh/g). However, migration of soluble polysulfide species through the liquid electrolyte from a positive electrode to a negative electrode, also known as the shuttle effect, often limits the performance of lithium-sulfur batteries, resulting in irreversible cycles and capacity degradation that hinder practical applications. In this study, we employed a positively charged polymer as a binder material that can attract negatively charged polysulfide species generated during the discharge cycle of the device operation to inhibit the shuttle effect and achieve stable electrochemical operation. In the case of using a conventional non-charged polymer binder, the movement and dissolution of ionic polysulfide cannot be prevented and thus stable long-term operation of the lithium-sulfur batteries was not attainable. To further improve the performance of the lithium-sulfur batteries, ionic liquids with low donor numbers compared to organic solvents were utilized to suppress the dissolution of polysulfide. Ionic liquids can also provide other outstanding characteristics such as non-flammability, negligible vapor pressure, high chemical and thermal stability, and wide electrochemical window desirable for developing environmentally stable next generation energy storage devices. The resulting lithium-sulfur batteries using the charged polymer binder and ionic liquids showed superior initial discharge capacity (> 1000 mAh/g) and capacitance retention (> 60% of its initial discharge capacity after 100 charge/discharge cycles).