Suppressing Polysulfide Crossover in Sodium Polysulfide Redox-Flow Batteries with an Oxyanion-Functionalized Glass Fiber Separator.

Abstract

Sodium polysulfide nonaqueous redox-flow batteries are promising candidates for grid-scale energy storage due to their high theoretical energy density and earth-abundant components. However, their performance is fundamentally limited by severe polysulfide shuttling and unstable sodium-metal interfaces, particularly under high-concentration catholyte conditions. Here, we report a scalable and perfluoroalkyl-substances-free membrane modification strategy via a simple dip-coating method to fabricate commercial glass fiber (GF) with poly(4-styrenesulfonic acid-<i>co</i>-maleic acid) sodium salt (PSSMA). The sulfonate and carboxylate functional groups in PSSMA provide both electrostatic and steric repulsion against polysulfide anions, while preserving the mechanical flexibility of the GF substrate. The optimized PSSMA-GF membrane significantly suppresses shuttle current, mitigates self-discharge, and delays the formation of short-chain, less soluble polysulfide species, leading to improved Coulombic efficiency (from <60 to 98.3%) and stable cycling over 100 cycles in sodium polysulfide coin-type full cells. Furthermore, the coating promotes uniform solid electrolyte interphase formation on the sodium anode. The feasibility of the PSSMA-GF membrane was further demonstrated using the flow-cell configuration, showing a smooth discharge plateau at 1.5 V under 1.0 mA cm^-2. This work demonstrates an environmentally benign, cost-effective, and easily scalable membrane fabrication strategy, offering a practical pathway to overcome key challenges in sodium polysulfide redox-flow batteries and other advanced energy storage systems.