2D spinel ZnCo₂O₄ microsheet-coated functional separator for promoted redox kinetics and inhibited polysulfide dissolution

Lithium–sulfur (Li–S) batteries are receiving increasing attention as one of the potential next-generation batteries, owing to their high energy densities and low cost. However, practical Li–S batteries with high energy densities are extremely hindered by the sulfur loss, low Coulombic efficiency, and short cycling life originating from the polysulfide (LiPS) shuttle. In this study, two-dimensional (2D) ZnCo₂O₄ microsheets fabricated by a facile hydrothermal process are employed to modify the separator, for improving the electrochemical performances of Li–S cells. The resulting 2D ZnCo₂O₄-coated separator features a coating thickness of approximately 10 μm, high ionic conductivity of 1.8 mS/cm, and low mass loading of 0.2 mg/cm². This 2D ZnCo₂O₄-coated separator effectively inhibits LiPS shuttle by a strong chemical interaction with LiPS as well as promotes the redox kinetics by ZnCO₂O₄-coated layers, as determined by X-ray photoelectron spectroscopy analysis, self-discharge, time-dependent permeation test, Li symmetric cell test, and Li₂S nucleation analyses. Consequently, the Li–S batteries based on the 2D ZnCo₂O₄-coated separator exhibit a high initial discharge capacity of 1292.2 mAh/g at 0.1 C. Moreover, they exhibit excellent long cycle stability at 1 and 2 C with capacity retention of 84% and 86% even after 800 cycles, corresponding to a capacity fading rate of 0.020% and 0.016% per cycle, respectively. Effectively, these Li–S cells with a high sulfur loading at 5.3 mg/cm² and low electrolyte concentration of 9 μL/mg deliver a high discharge capacity of 4.99 mAh/cm² after 200 cycles at 0.1 C.