(577c) Tailored Trilayer Separator for Extreme Temperature Lithium-Sulfur Batteries

Since the phenomenal invention of Lithium-ion Batteries (LIBs) in the 1970s, the transformation of human lives has been inconceivable. From the introduction of LIBs for portable electronics, they eventually migrated to other variety of applications such as defense, transportation, and space explorations. With the ever-growing demand comes the need for matching the supply of resources. Thus, newer materials with higher capacities and energy densities and different chemistries are being discovered. High-capacity lithium cathodes such as LiNiMnCoO₂, LiCoO₂, LiFePO₄, and LiMn₂O₄ have been identified a few decades back, however, these cathodes have capacities <250 mAh g^-1. This barrier prevents LIBs from being used for energy-intense applications such as long-range transportation, grid storage, etc. The high theoretical lithium-sulfur chemistry comes helpful in this dilemma. Sulfur is the lightest element to be used as a cathode consisting of a theoretical capacity of 1672 mAh g^–1. Also, sulfur is abundant in nature and economical (~$40 per metric tonne), compared to some transition metals like Co (~$48K per metric tonne), used in LIBs. Before the Li-S chemistry can be commercialized, three fundamental challenges need to be addressed viz., lithium dendrite formation, polysulfide shuttling, and low sulfur conductivity. To suppress polysulfide shuttling and enhance the low conductivity of sulfur, various measures have been applied viz., conductive polymers, inverse vulcanization, mesh carbon, metal, or carbon oxide frameworks. These measures lead to a decrease in energy density along with issues, which may warrant side reactions. For dendrite formations, different electrolyte additives, shutdown separators, etc are used, however, they increase the impedance in the system or are expensive alternatives.

We present the use of a multipurpose trilayer separator for Li-S battery to target all the three fundamental concerns, concurrently. The modified separator consists of polypropylene coated with a polydopamine layer, which is further coated with <10 μm graphene layer. The presence of polydopamine layer acts as a barricade for polysulfide by adsorbing them preferably due to the enhanced interactions. The presence of graphene layer helps in improving the conductivity of the carbon-sulfur cathode. The performance of LiS batteries was compared with conventional separators and trilayered ones for rate studies and long-term cycling studies. There was quite a difference in the two systems of cells with different separators. Trilayer separator at 0.1C. 0.2C, 0.5C, 1C, 2C, 3C and 4C exhibited capacities of 925, 833, 644, 480, 326, 260, and 220 mAh g^–1, respectively with high capacity retention of >95%. Equally, the battery performance depends on the choice of electrolytes and additives. Usage of 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. These solvents have a very low melting point, which enables them to work at much lower temperatures. 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. The safety characteristics of the systems were compared using Multiple module calorimetry for functional coin cells, which provided the thermal heat signature. The technology is to be scaled up in the Pouch-cell configurations and checked for their performance. These high-performance batteries presented here can be of immense value for various space and defense applications, where drastic conditions affect the battery performance.