Redox Catalysis Promoted Activation of Sulfur Redox Chemistry for Energy-Dense Flexible Solid-State Zn–S Battery

In aqueous Zn-ion batteries, the intercalation chemistry often foil attempts at the realization of high energy density. Unlocking the full potential of zinc–sulfur redox chemistry requires the manipulation of the feedbacks between kinetic response and the cathode’s composition. The cell degradation mechanism also should be tracked simultaneously. Herein, we design a high-energy Zn–S system where the high-capacity cathode was fabricated by in situ interfacial polymerization of Fe­(CN)₆^4–-doped polyaniline within the sulfur nanoparticle. Compared with sulfur, the Fe^II/III(CN)₆^4/3– redox mediators exhibit substantially faster cation (de)­insertion kinetics. The higher cathodic potential (Fe^II(CN)₆^4–/Fe^III(CN)₆^3– ∼ 0.8 V vs S/S^2– ∼ 0.4 V) spontaneously catalyzes the full reduction of sulfur during battery discharge (S₈ + Zn₂Fe^II(CN)₆ ↔ ZnS + Zn_1.5Fe^III(CN)₆, ΔG = −24.7 kJ mol^–1). The open iron redox species render a lower energy barrier to ZnS activation during the reverse charging process, and the facile Zn²⁺ intercalative transport facilitates highly reversible conversion between S and ZnS. The yolk–shell structured cathode with 70 wt % sulfur delivers a reversible capacity of 1205 mAh g^–1 with a flat operation voltage of 0.58 V, a fade rate over 200 cycles of 0.23%/cycle, and an energy density of 720 Wh kg_sulfur^–1. A range of ex situ investigations reveal the degradation nature of Zn–S cells: aggregation of inactive ZnS nanocrystals rather than the depletion of Zn anode. Impressively, the flexible solid-state Zn battery employing the composite cathode was assembled, realizing an energy density of 375 Wh kg_sulfur^–1. The proposed redox electrocatalysis effect provides reliable insights into the tunable Zn–S chemistry.