Revisiting the Forgotten Fraction: Review on Mechanisms and Process Design for Sustainable Recycling of Electrolytes from Spent Lithium‑Ion Batteries

The accelerating global transition toward sustainable energy systems has driven an exponential increase in the deployment of lithium ion batteries, resulting in a rapidly growing stream of end of life batteries that poses significant challenges to environmental protection and resource security. Although established recycling technologies have achieved considerable success in recovering cathode and anode materials, electrolytes, which account for approximately 15–30% of total battery mass, have long been neglected. Electrolytes contain lithium hexafluorophosphate and organic carbonate solvents that are toxic, volatile, and corrosive, yet also represent valuable sources of lithium and fluorinated chemicals. This review presents a systematic overview of green recovery and valorization strategies for spent lithium ion battery electrolytes, with particular emphasis on chemical approaches that directly convert residual lithium salts into high value lithium salt–type additives. Based on component separation characteristics, existing recycling routes are classified into three categories, namely targeted extraction of electrolyte lithium salts, separation and regeneration of carbonate solvents, and integrated full component recovery of electrolytes. Key physicochemical mechanisms underlying representative technologies, including supercritical fluid extraction, vacuum assisted catalytic pyrolysis, and membrane based separation, are critically examined. Their advantages and limitations are evaluated in terms of product quality, energy consumption, and industrial applicability. Furthermore, current challenges related to fluorine containing pollutant control and high value utilization are discussed, and future perspectives are proposed toward fully enclosed, short process, and high atom economy recovery systems. This review aims to provide strategic guidance for developing sustainable electrolyte recycling pathways and advancing the circular utilization of lithium resources.