Revealing Cu₄SnS₄ Constructed by Composition Regulation Engineering With Enhanced Conversion Reversibility for Superior Zinc Storage Performance

Conversion-type anodes enable rocking-chair zinc-ion batteries (RCZIBs) to avoid Zn dendrites and corrosion. However, high-capacity multi-electron-transfer anodes remain challenging. A composition-regulation strategy is applied to copper tin sulfides via gelation-assisted solvothermal synthesis. Orthorhombic Cu₄SnS₄ with open ion channels and optimized electronic states is an ultra-stable conversion-type anode. In situ XRD reveals a phase-conversion mechanism exclusive to Cu₄SnS₄, explaining its enhanced reversibility. In situ EIS coupled with distribution-of-relaxation-times analysis tracks ion/electron transport and interfacial stability during cycling. The Cu₄SnS₄ half-cell delivers an initial capacity of 144.32 mAh g^‒1 at 0.2 A g^‒1 with 95.5% capacity retention after 300 cycles, and retains 60.26 mAh g^‒1 after 3000 ultralong cycles at 4 A g^‒1. A rocking-chair Cu₄SnS₄||MnO₂ full cell achieves a capacity of 123.04 mAh g^‒1 at 0.2 A g^‒1 and 57.59 mAh g^‒1 at 1 A g^‒1, together with excellent long-cycle stability. DFT reveals that Cu₄SnS₄ has higher Fermi-level DOS and a lower Zn²⁺ diffusion barrier (0.06 eV) than Cu₂SnS₃ (0.36 eV) and Cu₃SnS₄ (0.25 eV) due to open orthorhombic channels, enhancing conductivity and Zn²⁺ transport and accelerating conversion. This work correlates composition, structure, and performance, offering a paradigm for durable multimetallic sulfide anodes in practical RCZIBs.