Quantifying the Heterointerface Contribution in a Cu7S4/CuCo2S4 Cathode with LiCl-All-Phenyl Complex Electrolyte for High-Performance Magnesium/Lithium Hybrid-Ion Batteries

Abstract

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.