This study presents a novel method to quantify inactive lithium (Li) in lithium metal batteries (LMBs), which is the main cause of capacity loss and safety issues. The researchers developed Titration Gas Chromatography (TGC) to accurately determine the amount of isolated metallic Li⁰ in inactive Li. They found that metallic Li⁰, rather than the electrochemically formed SEI, dominates inactive Li and capacity loss. Using cryogenic electron microscopy, they observed that Li⁰ is surrounded by insulating SEI, losing the electronic conductive pathway to the bulk electrode. By combining TGC measurements with cryo-FIB-SEM and cryo-TEM, they revealed the formation mechanism of inactive Li in different electrolytes and identified the true cause of low Coulombic efficiency in Li metal deposition and stripping. They proposed strategies to enable efficient Li deposition and stripping for next-generation high energy batteries. Inactive Li consists of both (electro)chemically formed Li⁺ compounds in the solid electrolyte interphase (SEI) and electrically isolated unreacted metallic Li⁰. Previous studies generally assumed that the low Coulombic efficiency (CE) was dominated by the continuous repairing of SEI fracture, but the actual contribution of capacity loss from SEI formation has never been quantified. TGC allows the accurate quantification of metallic Li⁰ content, revealing that it dominates the inactive Li and capacity loss. The study also shows that the amount of unreacted metallic Li⁰ is linearly related to the loss of CE, indicating that CE loss is governed by the formation of inactive metallic Li⁰. Cryo-TEM results further confirm that Li⁰ is entrapped in the insulating SEI matrix, isolated from the conductive network of the bulk electrode, and thus becomes inactive. The study proposes a model for the formation mechanism of inactive Li, revealing that the true underlying cause of capacity loss in LMBs is due to large amounts of metallic Li⁰ becoming trapped in SEI with tortuous microstructures. Strategies to mitigate inactive Li formation and improve CE are proposed, including controlling the micro and nanostructure of the plated Li deposits and applying external pressure to maintain a good structural connection. The study also suggests that an ideal columnar microstructure with minimum tortuosity can significantly enhance the structural connection, leading to high CE and potentially enabling anode-free batteries. The versatile characterization tools developed in this study can be further extended to investigate inactive Li properties under different conditions, serving as a standard methodology to evaluate strategies that improve the performance of Li metal batteries.This study presents a novel method to quantify inactive lithium (Li) in lithium metal batteries (LMBs), which is the main cause of capacity loss and safety issues. The researchers developed Titration Gas Chromatography (TGC) to accurately determine the amount of isolated metallic Li⁰ in inactive Li. They found that metallic Li⁰, rather than the electrochemically formed SEI, dominates inactive Li and capacity loss. Using cryogenic electron microscopy, they observed that Li⁰ is surrounded by insulating SEI, losing the electronic conductive pathway to the bulk electrode. By combining TGC measurements with cryo-FIB-SEM and cryo-TEM, they revealed the formation mechanism of inactive Li in different electrolytes and identified the true cause of low Coulombic efficiency in Li metal deposition and stripping. They proposed strategies to enable efficient Li deposition and stripping for next-generation high energy batteries. Inactive Li consists of both (electro)chemically formed Li⁺ compounds in the solid electrolyte interphase (SEI) and electrically isolated unreacted metallic Li⁰. Previous studies generally assumed that the low Coulombic efficiency (CE) was dominated by the continuous repairing of SEI fracture, but the actual contribution of capacity loss from SEI formation has never been quantified. TGC allows the accurate quantification of metallic Li⁰ content, revealing that it dominates the inactive Li and capacity loss. The study also shows that the amount of unreacted metallic Li⁰ is linearly related to the loss of CE, indicating that CE loss is governed by the formation of inactive metallic Li⁰. Cryo-TEM results further confirm that Li⁰ is entrapped in the insulating SEI matrix, isolated from the conductive network of the bulk electrode, and thus becomes inactive. The study proposes a model for the formation mechanism of inactive Li, revealing that the true underlying cause of capacity loss in LMBs is due to large amounts of metallic Li⁰ becoming trapped in SEI with tortuous microstructures. Strategies to mitigate inactive Li formation and improve CE are proposed, including controlling the micro and nanostructure of the plated Li deposits and applying external pressure to maintain a good structural connection. The study also suggests that an ideal columnar microstructure with minimum tortuosity can significantly enhance the structural connection, leading to high CE and potentially enabling anode-free batteries. The versatile characterization tools developed in this study can be further extended to investigate inactive Li properties under different conditions, serving as a standard methodology to evaluate strategies that improve the performance of Li metal batteries.