Suppressing polysulfide shuttling via a supporting electrolyte concentration-asymmetric (SEC-Asym) design based on electrolyte ionic strength for stable promoted Na–Polysulfide redox-flow batteries

Abstract

Polysulfide species (Sn2-) that have high abundance and multi-electron redox processes are highly attractive for energy storage purposes, especially in grid-scale applications when pairing with Sodium (Na) for nonaqueous redox-flow batteries (NARFBs). However, the severe shuttling effect of polysulfide species causes self-discharge in batteries, which limits further development of polysulfide-based batteries. Herein, we report a novel strategy to mitigate the polysulfide shuttling by a supporting electrolyte concentration-asymmetric (SEC-Asym) design based on balancing ionic strengths between the anolyte and the catholyte. Instead of using expensive fluorinated additives/solvents or highly concentrated electrolytes, we slightly increase the anolyte ionic strength (I) to match the value of the polysulfide catholyte. For a 0.1 M Na2S8 catholyte with 1 M supporting salt (I = 1.3 M), the SEC-Asym cell using 1.4 M salt in the anolyte increased the energy efficiency from 61.5 to 75.6 %, and capacity retention from 63.2 to 81.0 %. Electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) post-cell tests detected less polysulfide shuttling products with a thinner and compact solid-electrolyte-interphase (SEI) in the SEC-Asym cell. Computational studies reveal that the system’s Donnan potential was altered by the SEC-Asym design, which reduces the polysulfide flux diffusing to the anolyte. The final NARFB demonstrations using 1.8 M anolyte for a 0.25 M Na2S8 catholyte (I = 1.75 M) also showed comparable boosts with fewer soft short-circuiting issues and higher charge-discharge efficiency. This finding fills the knowledge gap on electrolyte optimization for polysulfide-based batteries and lights the path for future electrolyte development of RFBs.