Abstract: In recent years, the widespread adoption of lithium-ion batteries (LIBs) has resulted in a rapid increase in the volume of spent batteries, creating significant global challenges related to resource depletion and environmental sustainability. Global LIB production exceeded 1850 GW·h in 2024 and is projected to surpass 6,080.4 GW·h by 2030, driven by carbon neutrality initiatives, emission-reduction targets, and the expansion of the new energy vehicle market. As these large-scale deployments reach their end-of-life, the volume of retired batteries is rising dramatically. The total amount of spent LIBs in China is estimated to reach 2.312 million tons by 2026, while the global scale of decommissioned LIBs is expected to exceed 11 million tons by 2030. However, the current global annual recycling capacity for spent LIBs stands at approximately 2 million tons. Consequently, there is an urgent need to develop efficient and environmentally benign battery recycling methodologies. Recycling technologies for spent LIBs primarily fall into two categories: pyrometallurgy and hydrometallurgy. Pyrometallurgy often suffers from metal loss through volatilization, high energy consumption, elevated carbon emissions, and associated environmental issues. Compared to pyrometallurgy, hydrometallurgy offers advantages in terms of higher recovery rates and reduced environmental footprint, establishing itself as the most commonly employed and promising method for recovering valuable metals from spent LIBs. This review systematically summarizes the research progress in acid leaching technologies for recovering high-value metals from the cathode materials of spent LIBs, with a particular focus on recent advancements in green leaching strategies and metal separation/purification techniques. Beyond traditional inorganic and organic acid leaching systems, this paper highlights the emerging green technology of deep eutectic solvent leaching, discussing its leaching mechanism, potential for efficient metal extraction and minimal environmental pollution, as well as the challenges in industrial application. Furthermore, it delineates principal methods for metal separation and recovery from different leaching systems, including solvent extraction, chemical precipitation, sol-gel processes, ion exchange, and electrochemical deposition, and proposes optimized combinations of leaching-separation processes. Finally, this paper outlines future research directions: During the separation stage, particularly for high-nickel systems (e.g., NCM811), it is necessary to move beyond the traditional “leaching-sequential extraction” pathway and develop selective separation technologies based on ionic liquids or crown ethers to achieve preferential lithium extraction and cascaded separation of nickel, cobalt, and manganese; for cobalt-free systems such as lithium iron phosphate, a coupled process of lithium-selective adsorption and precipitation should be adopted, with the iron and phosphorus residues converted into functional materials. In terms of material regeneration, the focus should be on the direct repair of spent cathodes: for high-nickel systems, defect repair techniques such as low-temperature lithiation and lithium replenishment should be developed; for cobalt-free systems, low-temperature solid-state regeneration processes matching their failure mechanisms need to be established. With regard to reagent recycling and process intensification, a closed-loop reagent-wastewater circulation system should be constructed to recover leaching agents online, and the coupling of external fields, such as mechanochemical activation, ultrasound or microwave with acid leaching, should be explored to fundamentally shorten the reaction time and reduce energy consumption and reagent usage to achieve both high efficiency and “greenness.”