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.