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
All-solid-state batteries (ASSBs) have attracted enormous attention as one of the critical future technologies for safe and high energy batteries. With the emergence of several highly conductive solid electrolytes in recent years, the bottleneck is no longer Li-ion diffusion within the electrolyte. Instead, many ASSBs are limited by their low Coulombic efficiency, poor power performance, and short cycling life due to the high resistance at the interfaces within ASSBs. Because of the diverse chemical/physical/mechanical properties of various solid components in ASSBs as well as the nature of solid−solid contact, many types of interfaces are present in ASSBs. These include loose physical contact, grain boundaries, and chemical and electrochemical reactions to name a few. All of these contribute to increasing resistance at the interface. Here, we present the distinctive features of the typical interfaces and interphases in ASSBs and summarize the recent work on identifying, probing, understanding, and engineering them. We highlight the complicated, but important, characteristics of interphases, namely the composition, distribution, and electronic and ionic properties of the cathode−electrolyte and electrolyte−anode interfaces; understanding these properties is the key to designing a stable interface. In addition, conformal coatings to prevent side reactions and their selection criteria are reviewed. We emphasize the significant role of the mechanical behavior of the interfaces as well as the mechanical properties of all ASSB components, especially when the soft Li metal anode is used under constant stack pressure. Finally, we provide full-scale (energy, spatial, and temporal) characterization methods to explore, diagnose, and understand the dynamic and buried interfaces and interphases. Thorough and in-depth understanding on the complex interfaces and interphases is essential to make a practical high-energy ASSB.
Although layered lithium oxides have become the cathode of choice for state‐of‐the‐art Li‐ion batteries, substantial gaps remain between the practical and theoretical energy densities. With the aim of supporting efforts to close this gap, this work reviews the fundamental operating mechanisms and challenges of Li intercalation in layered oxides, contrasts how these challenges play out differently for different materials (with emphasis on Ni–Co–Al (NCA) and Ni–Mn–Co (NMC) alloys), and summarizes the extensive corpus of modifications and extensions to the layered lithium oxides. Particular emphasis is given to the fundamental mechanisms behind the operation and degradation of layered intercalation electrode materials as well as novel modifications and extensions, including Na‐ion and cation‐disordered materials.
Defects and their interactions in crystalline solids often underpin material properties and functionality 1 as they are decisive for stability 1-5 , result in enhanced diffusion 6 , and act as a reservoir of vacancies 7 . Recently, lithium-rich layered oxides have emerged among the leading candidates for the next-generation energy storage cathode material, delivering 50 % excess capacity over commercially used compounds. Oxygen-redox reactions are believed to be responsible for the excess capacity 8 , however, voltage fading has prevented commercialization of these new materials. Despite extensive research the understanding of the mechanisms underpinning oxygen-redox reactions and voltage fade remain incomplete. Here, using operando three-dimensional Bragg coherent diffractive imaging 2,9 , we directly observe nucleation of a mobile dislocation network in nanoparticles of lithium-rich layered oxide material. Surprisingly, we find that dislocations form more readily in the lithium-rich layered oxide material as compared with a conventional layered oxide material, suggesting a link between the defects and the
Advanced liquid electrolytes can achieve dense Li deposition with a Coulombic efficiency (CE) of approximately 99%.
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