Abstract: With the rapid development of the new energy industry, lithium-ion batteries (LIBs) have been extensively applied in various fields, such as electric vehicles and energy storage devices. However, constrained by lagging recycling modes, the global annual output of spent LIBs exceeds one million tons. Spent LIBs contain scarce metals, such as cobalt, nickel, and lithium, as well as toxic components, including electrolytes and binders. The improper disposal of spent batteries is prone to resource waste and pollution of soil and water bodies. Therefore, the efficient recycling and reusing of spent LIBs have become a strategic priority for safeguarding resource security and promoting the achievement of the “dual carbon” goals. As a new type of green solvent developed by mixing hydrogen-bond donors and acceptors in a specific ratio, deep eutectic solvents (DESs) have gradually replaced traditional toxic and volatile solvents. Endowed with prominent advantages such as excellent environmental compatibility, strong recyclability, tunable physicochemical properties, and outstanding dissolution capacity for cathode metal oxides, DESs have demonstrated significant practical value and economic feasibility for the recycling of spent LIBs. This study systematically reviews the research status of spent LIBs recycling using DESs in recent years, focusing on the extraction mechanisms and core principles of valuable metals from battery cathode materials. It analyzes the separation principles of different DESs systems and clarifies the differences in their core leaching mechanisms. When exploring DES-based recycling technologies, the separation efficiency and leaching mechanism of each system are closely related to its composition and structure. For example, hydrogen bond-based DESs mainly rely on hydrogen bond interactions to promote the dissolution of metal oxides, whereas metal-based DESs may enhance the leaching efficiency through synergistic effects between metal ions and target metals. Clarifying these differences in mechanisms can provide a theoretical basis for targeted design and optimization of DESs systems. On this basis, this study further discusses the bottleneck problems faced by current DESs recycling technologies. Although DESs have shown great potential in laboratory research, many challenges remain in their practical application. For instance, the preparation cost of some high-performance DESs is relatively high, which reduces their large-scale application, and the viscosity of some DESs is too high at room temperature, which affects the mass-transfer efficiency of the leaching process and reduces the overall recycling efficiency. The separation and purification technology for the target metals in DESs leaching solutions must be further improved to realize the efficient recovery of valuable metals. Finally, this paper outlines the future technical pathways for the efficient, green, and sustainable recycling of spent LIBs using DESs. Future research should focus on the development of low-cost DESs systems, explore the use of cheap raw materials, such as biomass derivatives, to reduce preparation costs, and optimize the composition ratio of DESs to adjust their physicochemical properties and improve their leaching performance. At the same time, it is necessary to strengthen research on multi-metal synergistic separation technology to realize the selective recovery of various valuable metals in spent LIBs. This review is expected to provide important references for subsequent research and engineering applications in the field of spent LIBs recycling, and contribute to the sustainable development of the new energy industry.