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Luminescent carbon dots (CDs) have received increasing attention from many fields during the past decade. Unfortunately, the luminescent mechanisms of CDs remain unclear due to insufficient experimental and theoretical knowledge, which significantly hinders the development of CDs with desired optical properties. Currently, surface states of CDs, which are based on synergistic hybridization between the carbon backbones and the connected functional groups, have been considered as the dominant luminescence origins. This tutorial paper, thus, aims to offer an overview of the key features on the surface of CDs, such as particle size, surface functional groups, defects and heteroatom doping, and their influences on the photoluminescence of CDs. In addition, optical characteristics of surface state-derived luminescence emissions of CDs are also summarized. Finally, the potential approaches of characterizing surface states of CDs are introduced, followed by an outlook of synthesizing high-quality CDs through modulation of the surface states. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]