High-energy-density rechargeable lithium batteries are pivotal for next-generation electric vehicles and urban air mobility; however, their long-term stability is compromised by lithium inventory loss. This review systematically deconstructs the two primary pathways of lithium inventory loss in the liquid-electrolyte-based rechargeable lithium batteries: the formation of “dead lithium” (dead Li) and electrolyte consumption during solid electrolyte interphase (SEI) formation. First, we differentiate the formation pathways of dead Li in graphite-based lithium-ion batteries (LIBs) versus lithium-metal batteries (LMBs). We then comprehensively introduce methods for its qualitative and quantitative analysis, ranging from electrochemical diagnostics and high-resolution imaging to chemical quantification techniques like NMR and TGC. Second, we provide an in-depth discussion on the roles of specific SEI components, specifically inorganic species (e.g., LiF, Li2CO3) and organic matrices, while highlighting advanced spectroscopic and microscopic techniques for their characterization. By bridging state-of-the-art analytical technologies with a component-specific understanding of lithium inventory loss, this review offers strategic insights for diagnosing degradation and guiding the rational design of stable interphases in future battery systems.