Zero-dimensional fluorescent carbon dots (CDs) that are used as a cell-imaging reagent are prepared by using a simple and effective route employing lithium-intercalated graphite from lithium-ion batteries as a carbon source. Under ultrasonic exfoliation, the interlayer space increases, while the layer distortion and remaining lithium of the lithium-intercalated graphite are utilized to disrupt the graphitic structure and produce the CD suspension. Subsequently, after concentration and purification, the obtained colloidal CD suspension has a fluorescent yield of up to 1.2% and is therefore comparable to the CDs prepared in previous reports. These CD products are water-soluble, nanosized (approximately 3.5 nm), and biocompatible and can easily enter into HeLa cells to act as a cell-imaging reagent without any further functionalization. In addition, these CDs do not impose toxicity against HeLa cells and have high photostability with low photobleaching and demonstrate potential applications for bio-labeling as well as solution state optoelectronics.