Multifunctional Trilayer Separator for High-Performance Lithium-Sulfur Batteries Under Extreme Conditions
Authors: Parekh Mihit H. , Palanisamy Manikandan , Pol Vilas G. 
Published: July 2021

Lithium-ion Batteries (LIBs) have transformed mankind's lives beyond imagination in the past few decades. They have found their use in a variety of applications such as defense, transportation, portable electronics, and space explorations. To keep up with the demand, stress is applied for discovering materials with higher capacity and energy densities. High capacity anodes like silicon, tin, antimony, carbon composites have been identified. However, limited cathode materials are available at disposal viz., LiCoO2, LiMnNiCoO2, LiFePO4, LiMn2O4. Most of these cathodes have capacities <250 mAh g-1, and thus LIBs cannot keep up with the ever-growing demand from the applications. The high-theoretical capacity chemistry of lithium-sulfur comes to the rescue. It is the lightest element to be used as a cathode, which has attractive properties such as capacity of 1672 mAh g-1, economical (~$50 per metric tonne), and abundance in nature. However, chemistry suffers from three critical issues viz., poor sulfur conductivity, polysulfide shuttling, and lithium dendrite growth. To tackle the insulative sulfur and polysulfide shuttling effect approaches such as coating sulfur with metal sulfide or oxides, mesoporous carbon, conductive polymers, inverse vulcanization have been applied. Unfortunately, these techniques lead to a reduction in the energy density of the system, involve complex fabrication steps or raw materials are expensive. On the issue of lithium metal dendrites growth, electrolytic additives, exotic electrode coatings, and special shutdown separators are used. The downside to all of these is that they impede cell performance or are uneconomical.
Here we report, the use of a multifunctional trilayer separator in Li-S battery to tackle all three issues simultaneously. Modified separator acted as a barricade for polysulfide from shuttling by trapping them preferably due to the favorable interactions. The performance of Li-S cells with and without modified separator was compared for long term cycling and rate studies. It was found that cells with modified separator outperformed those with pristine separator. Modified separator cell exhibited capacities of 925, 833, 644, 480, 326, 260, and 220 mAh g-1 at 0.1C, 0.2C, 0.5C, 1C, 2C, 3C and 4C rates, respectively with 95% capacity retention. The choice of electrolyte affects battery performance drastically. However, using 1M LiTFSI in 1:1 (v/v) 1, 3-Dioxolane (DOL): 1, 2-Dimethoxyethane (DME) with additives, provided remarkable results at low and high-temperature ranges. At 0 ℃, the cell with the modified separator yielded about 350 mAh g-1 capacity at 0.5C over 200 cycles. The performance of the cells was stellar even at 50 ℃ with 500 mAh g-1 capacity at 0.5C rate after 100 cycles. Multiple module calorimetry provided the thermal heat signature to elucidate the safety features of the Li-S system with the modified separator and compared with the existing LIBs. High-performance Li-S batteries proposed here can be valuable to a variety of applications like defense, transportation, and space explorations, where drastic conditions affect the battery functionalities, in the times ahead.