Designer Self-Assembled Polyelectrolyte Complex Nanoparticle Membrane for a Stable Lithium–Sulfur Battery at Lean Electrolyte Conditions

Lithium–sulfur batteries can displace lithium-ion batteries owing to their superior theoretical capacity and specific energy density. Presently, however, high specific capacities do not translate to high specific energies, mainly because of the electrolyte excess, which does not meet the required "lean electrolyte" condition. We introduce a separator that requires a minimal amount of electrolyte, 4.5 μL mg–1, for successful cycling of practical sulfur cathodes. Taking advantage of the self-assembly chemistry of polyelectrolyte complexation, we synthesized a tailored porous nanoparticle, which because of its amphiphilicity, is able to form a submicron coating on the low-surface energy Celgard separator by dip-coating. The tuned pore-size in the range of 1.5–2 nm, abundance of functional groups, and unprecedented adsorption capacity toward LiPS allows the polyelectrolyte complex nanoparticle decorated (PPX) separator to function as an efficient LiPS modulator, while uniquely maintaining lean electrolyte conditions and excellent transport properties. The PPX separator enabled a cell with capacity of 1348 mAh g–1 (5.12 mAh cm–2) at 0.2 C. Achieving the challenging trade-off between high capacity and lean electrolyte, we were able to attain high energy density in a pouch cell prototype with an initial capacity of 1218 mAh g–1 and an energy density of 250 Wh kg–1.