The recycling of spent lithium-ion batteries and selected complex electronic waste fractions is commonly evaluated using isolated metrics such as leaching yield, metal removal efficiency, and reagent consumption. However, this approach fails to address the central challenge of sustainable valorization: integrating upstream conversion with downstream selective recovery without shifting environmental and separation burdens. This review focuses specifically on spent LIBs as the primary model system, while also drawing insights from related e-waste streams (e.g., printed circuit boards and polymer-containing residues) where the interface-driven framework applies. It examines how key interfaces—solid–fluid, polymer–metal–fluid, membrane–solution, electrode–electrolyte, and crystal–solution—govern metal mobilization, selectivity, effluent quality, product purity, and scalability. Emphasis is placed on hydrothermal and supercritical water processing, PVC/CPVC (Polyvinyl Chloride/Chlorinated Polyvinyl Chloride)-assisted metal mobilization and membrane-based recovery techniques, including nanofiltration, membrane distillation, membrane distillation crystallization, ion exchange, and electrochemical methods. Supercritical water and membrane processes are complementary only when upstream chemistry is designed to facilitate downstream separation. PVC-rich waste is reconsidered as a reactive chlorine source, provided that corrosion, HCl formation, and salt precipitation are controlled. Critical gaps include incomplete mass balances, limited multicomponent studies, weak integration between process stages, and scarce techno-economic and life-cycle analyses. A roadmap is proposed for scalable, integrated hydrothermal–membrane systems enabling efficient resource recovery and water reuse.