Enabling flexible solid-state Zn batteries via tailoring sulfur deficiency in bimetallic sulfide nanotube arrays

The ability to create advanced cathode nanomaterials with greatly enhanced energy density for rechargeable Zn batteries remains a grand challenge. Herein, we craft bimetallic NiCo₂S_4-x with tunable sulfur deficiency as effective cathode materials for Zn batteries with large capacity, high rate capability and outstanding cycle stability. Notably, the sulfur vacancies can be judiciously tailored to increase the electrical conductivity and number of active sites for electrochemical reaction. The resulting sulfur-deficient NiCo₂S_4-x nanotube arrays on carbon cloth (denoted sd-NiCo₂S_4-x@CC) present high capacities of 298.3 and 175.7 mAh g⁻¹ at 0.5 and 5 A g⁻¹, respectively, which outperform the CC-support-free NiCo₂S_4-x nanotube and NiCo₂O₄ nanowire counterparts. Mechanistic study reveals the partial dissolution of S elements in sd-NiCo₂S_4-x@CC electrodes, which has not been observed in sulfide-based Zn batteries. The redox reaction of sd-NiCo₂S_4-x@CC involves the formation of NiS_4-x-2yOH, CoS_yOH, and CoS_yO during charging and S doped NiO and CoO during discharging. The residual S-doping effect in bimetallic electrode materials is key to sustain high reactivity and cycle stability. Furthermore, a flexible solid-state sd-NiCo₂S_4-x@CC//Zn@CC battery is assembled using sodium polyacrylate hydrogel electrolyte, displaying an unprecedented cyclic durability of 84.7% after 1500 cycles at 5 A g⁻¹. As such, the deficiency (e.g., S, O, P, etc.) tailoring represents a robust strategy to yield high performance electrode materials for energy storage devices.