Ionic liquid-enhanced solid state electrolyte interface (SEI) for lithium-sulfur batteries

Li-S batteries are a complicated system with many challenges existing before their final market penetration. While most of the reported work for Li-S batteries is focused on the cathode design, we demonstrate in this work that the anode consumption accelerated by corrosive polysulfide solution also critically determines the Li-S cell performance. To validate this hypothesis, ionic liquid (IL) N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Py14TFSI) has been employed to modify the properties of the SEI layer formed on the Li metal surface in Li-S batteries. It is found that the IL-enhanced passivation film on the lithium anode surface exhibits very different morphology and chemical composition, effectively protecting lithium metal from continuous attack by soluble polysulfides. Therefore, both the cell impedance and the irreversible consumption of polysulfides on lithium metal are reduced. As a result, the Coulombic efficiency and the cycling stability of Li-S batteries have been greatly improved. After 120 cycles, the Li-S battery cycled in the electrolyte containing 75% IL demonstrates a high capacity retention of 94.3% at 0.1 C rate. These results reveal one of the main failure mechanisms in Li-S batteries and shine the light on new approaches to improve the reversible capacity and cyclability of Li-S batteries. This work provides important clues for the understanding and thus the improvement of Li-S battery systems through a different point of view.