Lithium-ion battery (LIB) is a dominating power source in the market owing to its high energy density, good cycling stability and environmental benignity. However, technical challenges remain after years' optimization and commercialization, which are detrimental to the expected performance and lifespan of LIBs. For instance, many cathode materials of LIBs suffer from rapid capacity fading and poor high-rate performance, which are ascribed to self-aggregation, dissolution and fast increased charge transfer resistances during cycles. In terms of the anode materials, low coulombic efficiency, electrolyte depletion and safety issues are common. In addition, the liquid electrolyte systems trigger safety concerns because flammable and volatile organic solvents are necessary. Recently, carbon dots (CDs) emerge as a sound material to address those challenges of LIBs, and also present promising applications in bioimaging, fluorescence sensing, photo/electro-catalysis, and electroluminescence. This review will overlook the state-of-the-art advances in the employment of CDs based composites to build cathode/anode materials and electrolytes in LIBs, through tailoring the internal structures and the surface states of electrode materials, and being additives in electrolyte, to improve the performances of the next-generation LIBs. The major challenges and opportunities in front of CDs in LIBs will be outlined and discussed in detail.

Aqueous zinc‐ion batteries (ZIBs) using the Zn metal anode have been considered as one of the next‐generation commercial batteries with high security, robust capacity, and low price. However, parasitic reactions, notorious dendrites and limited lifespan still hamper their practical applications. Herein, an eco‐friendly nitrogen‐doped and sulfonated carbon dots (NSCDs) is designed as a multifunctional additive for the cheap aqueous ZnSO4 electrolyte, which can overcome the above difficulties effectively. The abundant polar groups (‐COOH, ‐OH, ‐NH2, and ‐SO3H) on the CDs surfaces can regulate the solvation structure of Zn2+ through decreasing the coordinated active H2O molecules, and thus redistribute Zn2+ deposition to avoid side reactions. Some of the negatively charged NSCDs are adsorbed on Zn anode surface to isolate the H2O/SO42‐ corrosion through the electrostatic shielding effect. The synergistic effect of the doped nitrogen species and the surface sulfonic groups can induce a uniform electrolyte flux and a homogeneous Zn plating with a (002) texture. As a result, the excellent cycle life (4000 h) and Coulombic efficiency (99.5%) of the optimized ZIBs are realized in typical ZnSO4 electrolytes with only 0.1 mg mL‐1 of NSCDs additive.

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Lithium metal batteries (LMBs) with extremely high energy densities have several advantages among energy storage equipment. However, the uncontrolled growth of dendrites and the flammable liquid electrolytes (LEs) often cause safety accidents. All solid-state batteries seem to be the ultimate choice, but solvent-free electrolytes usually fail in terms of conductivity at room temperature. Therefore, gel polymer electrolytes (GPEs) with a simple manufacturing process and high ionic conductivity are considered as the most competitive candidates to resolve the present difficulties. Herein, we design a polymeric network structure via esterification and amidation reactions between polyethylene glycol (PEG) and carbon dots (CDs). After incorporation with polyvinylidene fluoride and some LEs, the as-prepared PEG-CDs composite electrolytes (PCCEs) show a high ionic conductivity of 5.5 mS/cm and an ion transference number of 0.71 at room temperature, as well as good flexibility and thermostability. When the PCCEs are assembled with lithium metal anodes and LiFePO 4 or LiCoO 2 cathodes, both the cycling stability and the retention rate of these LMBs show excellent performance at room temperature.

Mn 3 O 4 is a promising cathode material for aqueous zinc ion batteries (ZIBs) which is a new type of low cost, eco-friendly, high security energy storage system, while those previously reported electrochemical capacities of Mn 3 O 4 are far from its theoretical value. In this work, Mn 3 O 4 nanoparticles and nitrogen-doped carbon dots (NCDs) are synthesized together through an in-situ hydrothermal route, and then calcined to be a nanocomposite in which Mn 3 O 4 nanoparticles are anchored on a nitrogen-doped carbon skeleton (designated as Mn 3 O 4 / NCDs). Although the carbon content is only 3.9 wt.% in the Mn 3 O 4 /NCDs, the NCDs-derived carbon skeleton provides an electrically conductive network and a stable structure. Such a special nanocomposite has a large specific surface area, plenty of active sites, excellent hydrophilicity and good electronic conductivity. Owing to these structural merits, the Mn 3 O 4 /NCDs electrode exhibits a preeminent specific capacity of 443.6 mAh g À 1 and 123.3 mAh g À 1 at current densities of 0.1 and 1.5 A g À 1 in ZIBs, respectively, which are far beyond the bare Mn 3 O 4 nanoparticles synthesized under the similar condition. The electrochemical measurement results prove that carbon dots, as a new type of carbon nanomaterials, have strong ability to modify and improve the performance of existing electrode materials, which may push these electrode materials forward to practical applications.