Carbon dots as multifunctional additives in zinc-ion batteries: progress, challenges, and opportunities

For next-generation energy storage systems (ESSs), carbon dots (CDs) have emerged as revolutionary nanomaterials. CDs offer exceptional structural versatility and unique physicochemical properties. This review critically examines the transformative role of CDs in zinc-ion batteries (ZIBs). Recent advances have demonstrated remarkable improvements in battery performance, including enhanced cycling stability (>90 % capacity retention over 1000 cycles), superior rate capability (5 A g⁻¹), and effective dendrite suppression. Herein, we systematically analyze the fundamental aspects of CDs, from size-controlled synthesis methods (2–10 nm) to their distinctive properties, including tunable surface chemistry, quantum confinement effects, and high electrical conductivity (>100 S cm⁻¹). CDs can enhance ZIB performance via multiple mechanisms: namely, dendrite suppressors in anodes, conductivity enhancers in cathodes, electrolyte modifiers for stable ion transport, and functionalized separators for uniform zinc (Zn) deposition. Our critical analysis reveals that CD-modified ZIBs achieve significantly improved performance metrics, including higher specific capacities (>400 mAh g⁻¹), reduced voltage polarization (<100 mV), and enhanced rate performance (>80 % capacity retention at 10C). We also address current challenges in CDs synthesis and integration, including scalability, cost-effectiveness, and long-term stability. Emerging research directions, such as smart responsive CDs and hybrid architectures are further highlighted. This work provides strategic insights for researchers and engineers working towards commercial-scale, high-performance ZIBs, offering a roadmap for sustainable energy storage solutions.