Lithium metal batteries have high energy density but suffer from the capacity loss and short cycle life due to the formation of electrochemically inactive Li, which consists of Li+ contained in solid electrolyte interface (SEI) and unreacted metallic Li0. Exactly Li+ or Li0 is dominant to the capacity loss has long been debated due to the challenges in accurately differentiating and quantifying Li+ in SEI and inactive metallic Li0 amount. Here, we introduce a new analytic method to accurately distinguish and quantify the contribution from metallic Li0 to total amount of the inactive Li. We uncover that the metallic Li0 is the major cause of low Coulombic efficiency, rather than the electrochemically formed SEI. Using cryogenic electron microscopies to further study the microstructure and nanostructure of inactive Li, we find that the unreacted metallic Li0 is indeed wrapped by insulating SEI, losing the electronic conductive pathway. Coupling the measurement of global content of metallic Li0 to observations of its local nanostructure, we reveal the formation mechanism of inactive Li in different types of electrolytes, identifying the true underlying cause of low efficiency in Li metal deposition and stripping. we ultimately propose strategies to enable highly efficient metallic Li deposition and stripping to enable Li metal anode for next generation batteries.