Abstract: The rapid expansion of the energy storage industry and new energy vehicles has led to a surge in lithium demand, raising concerns regarding the sustainability of its supply. Currently, lithium is mainly acquired from brine and seawater sources; however, effectively separating magnesium and lithium ions is critical for producing high-purity lithium products. With the increasing number of spent lithium-ion batteries (LIBs), recycling valuable metals has become a top priority to achieve a sustainable circular economy. Given the compositional variation among LIBs designed for different applications, the development of suitable and sustainable recycling processes is essential to efficiently recover metals from all LIB types. Most existing reviews only discuss lithium extraction from brine and seawater or lithium recovery from spent LIBs separately, rarely connecting their separation principles and technical commonalities. For the first time, this review systematically integrates the two major technical systems, with a focus on the separation challenges in high magnesium systems and the core problem of inhomogeneous feedstock for spent cathodes, thereby overcoming the limitations of conventional reviews that only focus on a single lithium source or a single technical route. It begins with a systematic description of various separation techniques for brine and seawater, including precipitation, adsorption, solvent extraction, nanofiltration membranes, and electrochemical methods, integrating the latest modification and coupling research on these lithium extraction technologies. Subsequently, it provides a comprehensive survey of established lithium recovery processes, such as pyrometallurgical and hydrometallurgical processes, sorting out their process optimization and bottleneck breakthroughs. It also covers other recycling technologies, such as direct regeneration and cutting-edge technical routes, including electrochemical coupling and supercritical fluid extraction. Finally, it discusses future research directions aimed at advancing lithium extraction and recycling technologies, with the goal of providing a more comprehensive technical reference for the sustainable utilization of lithium resources.