Aqueous polysulfide/iodide redox flow battery (RFB) with highly soluble active species are attractive candidates for scalable energy storage, offering noticeable reduced RFB cost. Though, poor reversibility and electrochemical activity of the redox reaction of polysulfide couples on graphite felt electrode restricts its energy efficiency. Herein, a facile method to improve the electrocatalytic activity by fabrication of skeletal Cu 7 S 4 hollow nanocages with copper vacancies is presented. Cu 7 S 4 nanoparticles can selectively boost the electrocatalytic activities of S 2− /S x 2- redox reactions and suppress the hydrogen evolution side reaction. In addition, the hollow opening structure of skeletal Cu 7 S 4 served as the catalytic active site (both the inner side and outer surface), accelerating the electrolyte transport and promoting the charge transfer process. A sandwiched electrode was constructed by coating Cu 7 S 4 nanocages into the interlayer of graphite felt. The polysulfide/iodide flow battery with SKE-Cu 7 S 4 sandwich structure electrode can generate high energy efficiency of 78.5% at 20 mA cm −2 , power density of 30.6 mW cm −2 and a stable energy efficiency retention of 92% after approximately 200 continuous cycles suggesting their potential use for future scalable RFBs. The developing of highly efficient and stable electrocatalyst for the electro-conversion of polysulfides will boost the performance and application of polysulfide-based flow batteries and other energy devices involved with polysulfides such as, quantum dots sensitized solar cells, Li–S batteries.
Heterostructured cathodes are widely credited with accelerating conversion kinetics in magnesium/lithium hybrid-ion batteries (MLHBs). Here, we construct a Cu7S4/CuCo2S4 heterostructure in a single hydrothermal process, in which the two phases sharing a common Cu-S sublattice form a coherent interface within individual particles. By fixing the phase ratio independently through Rietveld refinement and benchmarking against both phase-pure constituents under identical conditions, we show that the composite exceeds the composition-weighted rule-of-mixtures prediction by a factor of 3.4 at 0.3 A g-1. Because the constituents exhibit closely comparable charge-discharge profiles, the redox chemistry itself is unchanged, and this strongly super-additive response can primarily originate from the interface rather than from composition. Elemental mapping and surface-sensitive XPS, together with the coexistence of Li2S and MgS in in situ XRD, further reveal that storage is spatially partitioned: Li+ converts the particle interior while Mg2+ reacts predominantly in the near-surface region, so that the two carriers occupy complementary regions of the same particle. Comparing APC-LiCl with APC-LiTFSI at matched salt concentration, and identical in every other respect, localizes the electrolyte effect to the electrode interface, where the chloride-containing system suppresses the progressive Mg-surface passivation that drives continuous overpotential growth in the TFSI- system. The resulting cell delivers 400 mAh g-1 at 0.3 A g-1 and retains 152.56 mAh g-1 after 330 cycles at 1.0 A g-1 at ∼100% Coulombic efficiency. DFT calculations indicate increased states near the Fermi level and a localized interfacial electrostatic gradient consistent with the measured kinetics.
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