Inactive lithium (Li) formation is the immediate cause of capacity loss and catastrophic failure of Li metal batteries. However, the chemical component and the atomic level structure of inactive Li have rarely been studied due to the lack of effective diagnosis tools to accurately differentiate and quantify Li + in solid electrolyte interphase (SEI) components and the electrically isolated unreacted metallic Li 0 , which together comprise the inactive Li. Here, by introducing a new analytical method, Titration Gas Chromatography (TGC), we can accurately quantify the contribution from metallic Li 0 to the total amount of inactive Li. We uncover that the Li 0 , rather than the electrochemically formed SEI, dominates the inactive Li and capacity loss. Using cryogenic electron microscopies to further study the microstructure and nanostructure of inactive Li, we find that the Li 0 is surrounded by insulating SEI, losing the electronic conductive pathway to the bulk electrode. Coupling the measurements of the Li 0 global content to observations of its local atomic structure, we reveal the formation mechanism of inactive Li in different types of electrolytes, and identify the true underlying cause of low Coulombic efficiency in Li metal deposition and stripping. We ultimately propose strategies to enable the highly efficient Li deposition and stripping to enable Li metal anode for next generation high energy batteries. Main Text:To achieve the energy density of 500 Wh/kg or higher for next-generation battery technologies, Li metal is the ultimate anode, because it is the lightest metal on earth (0.534 g cm -3 ), delivers ultra-high theoretical capacity (3860 mAh g -1 ), and has the lowest negative electrochemical potential (-3.04 V vs. SHE) 1 . Yet, Li metal suffers from dendrite growth and low Coulombic efficiency (CE) which have prevented the extensive adoption of Li metal batteries (LMBs) 2-4 . Since the first demonstration of a Li metal battery in 1976 5 , tremendous effort has been made in preventing dendritic Li growth and improving CE, including electrolyte engineering 6-9 , interface protection 10 and substrate architecture 11 . While dense Li can be achieved without any dendrites during the plating process, the stripping process will eventually dominate the CE thus the reversibility of Li metal anode.The formation of inactive Li, also known as "dead" Li, is the immediate cause of low CE, short cycle life and violent safety hazard of LMBs. It consists of both (electro)chemically formed Li + compounds

Breakthroughs in performance of Li/Cu with Ni-rich cathodes can be achieved via manipulation of anion interfacial chemistry, as uncovered by experiment/modeling.

A modified liquefied gas electrolyte with the addition of fully coordinated cosolvent enables unique Li solvation structures. Their favorable properties lead to dendrite-free high Coulombic efficiency Li-metal anode cycling and enable lowtemperature operation even down to À60 C with high Li-metal efficiency. The system shows potential for improved energy density and low-temperature operation of Li-metal batteries.

Lithium metal is viewed as the ultimate battery anode because of its high theoretical capacity and low electrode potential, but its implementation has been limited by low Coulombic efficiency and dendrite formation above a critical current density. Determining the fundamental properties dictating lithium metal plating–stripping behavior is challenging because characterization techniques are limited by the sensitivity of lithium metal to damage by external probes, which regularly results in altered morphology and chemistry. Motivated by recent application of cryogenic transmission electron microscopy (cryo-TEM) to characterize lithium metal at the atomic scale, we explore the cryogenic focused ion beam (cryo-FIB) method as a quantitative tool for characterizing the bulk morphology of electrochemically deposited lithium and as a technique that enables TEM observation of Li-metal/solid-state electrolyte interfaces. This work highlights the importance of cryo-FIB methodology for preparing sensitive battery materials and elucidates the impact of electrolyte selection on the density and morphology of plated lithium.