Hydrophilic tannic acid-modified WS₂ nanosheets for enhanced polysulfide conversion in aqueous media

Polysulfide-based organic battery systems have demonstrated great potential for large scale energy storage, but are restricted by the cost and the flammability of the organic solvents. It would be ideal to establish the aqueous-based polysulfide electrochemistry to enable cost-effective stationary energy storage. However, the sluggish reaction kinetics of polysulfide placed a serious fundamental barrier to implementation. Here we developed hydrophilic tannic acid modified WS₂ nanosheets as polysulfide conversion electrocatalysts in alkaline aqueous solutions. The tannic acid not only acted as a negatively-charged surfactant to effectively delaminate the bulk WS₂ sheets, but also functionalised the delaminated WS₂ nanosheets through the attachment of tannic acid functional groups, resulting in greatly improved hydrophilicity. Using graphene as the conductive support, the tannic acid-modified hydrophilic WS₂ nanosheets demonstrated a promoting electrocatalytic activity for polysulfide oxidation and reduction in aqueous solution. The incorporation of tannic acid imposed the collective interactions between polysulfide and the WS₂ nanosheets via the hydrophilic molecules and the polar surfaces. With a 0.5 M Li₂S₂ electrolyte, the graphene and modified WS₂ mixture gave an areal specific capacity of 0.37 mAh cm⁻², compared to 0.27 mAh cm⁻² for the pure graphene.