Engineering stable electrode-separator interfaces with ultrathin conductive polymer layer for high-energy-density Li-S batteries

Lithium-sulfur (Li-S) battery has been regarded as a promising energy-storage system due to its high theoretical specific capacity of 1675 mAh g⁻¹ and low cost of raw materials. However, several challenges remain to make Li-S batteries viable, including the shuttling of soluble lithium polysulfide intermediates and pulverization of Li metal anode. Engineering stable electrode-separator interfaces without causing large mass/volume increase of inactive materials and electrolyte uptake is an effective approach to improve the cycling stability of Li-S batteries while maintaining high available energy density. Herein, we report the engineering of a stable electrode-separator interface with an ultrathin conductive polymer nanolayer on the pore walls of both surfaces of the separator via a simple and scalable approach using in-situ vapor-phase polymerization of polypyrrole (PPy) on commercial Celgard separator with only a small increase in overall mass and volume. The inherent hydrophilicity of PPy enables the separator to have enhanced electrolyte uptake, which facilitates homogenous Li⁺ flux and thus uniform plating and stripping of metallic lithium at the anode side during the charge/discharge processes. Meanwhile, the chemical immobilization effect of PPy suppresses the migration of the soluble polysulfides and improves the stability of the sulfur cathode. We showed that a Li||Li symmetrical cell with the PPy modified separator gave a low and stable overpotential of less than 30 mV for over 250 h' stripping and plating test at 1 mA cm⁻² with a fixed areal capacity of 3 mAh cm⁻², which was significantly better than that using a regular Celgard separator. Using the PPy modified separator, Li-S cell with sulfur/carbon black composite cathode and lithium anode delivered stable cycling for 250 cycles at 0.5 C with a low capacity decay rate of 0.083% per cycle. Even for a Li-S cell with a high-areal-capacity sulfur cathode (4.8 mAh cm⁻²), good cycling stability was achieved. It gave a reversible areal capacity of 3.6 mAh cm⁻² after 150 charge/discharge cycles at 0.2 C with 75.6% capacity retention. Besides, the as-achieved separator showed better thermal stability than the bare counterpart. This work offers an alternative approach for achieving a practical Li-S battery toward high energy density and long cycle life through simple and scalable separator-electrode interface engineering without significant increase in volume and mass.