Ultrathin clay-containing layer-by-layer separator coating enhances performance of lithium-sulfur batteries

Dissolution of polysulfides upon cycling of the sulfur cathode and their diffusion through the separator membrane ‒ the shuttle effect ‒ is a major factor which deteriorates performance and safety of lithium-sulfur (Li-S) batteries. Here, ultrathin, light-weight polyelectrolyte-clay layer-by-layer assemblies have been developed as versatile nanocoatings on the polypropylene (PP) separator to suppress the shuttle effect. The use of weak polyelectrolytes (polyethyleneimine, PEI, and polyacrylic acid, PAA) and montmorillonite (MMT) or halloysite (Hal) clay nanoparticles enabled full control over mass and charge balance of polyelectrolyte/clay film components, which was critical for achieving ion selectivity and reduction of the polysulfide diffusion. The differences in the geometry and structure of MMT and Hal (nanoplatelets vs. nanotubes), as well as in the pH-controlled charge density of PEI and PAA greatly affected the thickness, morphology, and the ultimate performance of the nanocoatings. In particular, deposition of 450 nm MMT-based coatings at pH 3 and 6 lead to an increase in electrolyte uptake, delayed the lithium dendrite growth and enhanced the discharge capacity of the cell from 300 mAh g−1 for bare to 690 mAh g−1 for coated PP separator over 200 charge-discharge cycles 0.5 C. At the same time, 250 nm-thick Hal-based coatings deposited at pH 3 resulted in an increase in the Coulombic efficiency from 50% for the bare PP separator to ~99% for the coated one, showing an outstanding performance. Additionally, the polyelectrolyte-clay nanocoatings significantly improve thermal and dimensional stability of the PP separator. The results demonstrate that ultrathin LbL clay-containing coatings on the PP separator membrane can drastically improve the cyclability of the Li-S cells.