Sulfur-doped carbon dots: a novel bifunctional electrolyte additive for high-performance aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) have gained considerable attention as next-generation energy storage devices due to their inherent safety, environmental friendliness, and cost-effectiveness. However, their widespread application is severely hampered by uncontrolled zinc dendrite growth and detrimental side reactions (e.g., hydrogen evolution, corrosion, and passivation), which lead to reduced Coulombic efficiency and shortened cycle life. Current strategies to improve zinc anode stability mainly focus on artificial interface coatings, electrode structure design, and electrolyte optimization. Among these approaches, electrolyte additive engineering is considered the most promising for practical applications due to its simplicity, low cost, and excellent scalability. Nevertheless, conventional additives (including metal ions, polymers, and surfactants) typically address only single issues (either dendrite suppression or side reaction mitigation), failing to achieve synergistic effects. In this work, we developed sulfur-doped carbon dots (S-CDs) as a novel bifunctional electrolyte additive to significantly enhance AZIB performance. The carbon dot additive was synthesized via a facile calcination method, followed by systematic characterization of its structure and properties using methods such as fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and density functional theory (DFT) calculations. Comprehensive electrochemical evaluations were conducted to investigate the influence of S-CDs on zinc deposition behavior and overall battery performance. Experimental results demonstrate the successful synthesis of sulfur-doped carbon dots with abundant surface functional groups. During battery operation, the strong binding affinity between S-CDs and Zn²⁺ effectively reconstructs the Zn²⁺ solvation shell, reducing water molecule content and thereby minimizing electrode corrosion and side reactions caused by interfacial active water molecules. Moreover, the S-CDs induce the formation of stable (002) crystallographic planes that continuously renew during plating/stripping cycles, with particularly pronounced effects under high current densities, significantly enhancing the structural stability of the electrode. The synergistic effect of these dual functions leads to remarkable improvement in zinc electrode performance and ultimately endows the battery with ultra-long cycling life. Benefiting from the positive effects of the carbon dot additive, the symmetric cell achieves exceptional stability for nearly 2000 h at a high current density of 10 mA cm⁻², far outperforming conventional electrolyte systems. Furthermore, both Zn||NH₄V₄O₁₀ and Zn||MnO₂ full cells exhibit superior electrochemical performance and significantly enhanced cycling stability, confirming the excellent compatibility of the carbon dot additive with various cathode materials. This study provides novel insights and fundamental theoretical guidance for developing high-performance AZIBs, representing a significant advancement in sustainable energy storage technologies.