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

Abstract

The ability to create advanced cathode nanomaterials with greatly enhanced energy density for rechargeable Zn batteries remains a grand challenge. Herein, we craft bimetallic NiCo 2 S 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 NiCo 2 S 4-x nanotube arrays on carbon cloth (denoted sd-NiCo 2 S 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 NiCo 2 S 4-x nanotube and NiCo 2 O 4 nanowire counterparts. Mechanistic study reveals the partial dissolution of S elements in sd-NiCo 2 S 4-x @CC electrodes, which has not been observed in sulfide-based Zn batteries. The redox reaction of sd-NiCo 2 S 4-x @CC involves the formation of NiS 4-x-2y OH, CoS y OH, and CoS y O 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-NiCo 2 S 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. • A robust sulfur-deficiency tailoring strategy to craft bimetallic NiCo 2 S 4-x nanotube arrays was developed. • The obtained sd-NiCo 2 S 4-x @CC electrode presents high capacities with outstanding cycle stability. • The redox reaction is to form NiS 4-x-2y OH, CoS y OH, and CoS y O during charge and S-doped NiO and CoO during discharge. • A flexible solid-state battery capitalizing on hydrogel electrolyte is constructed, yielding superior performance.