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Lithium metal has been considered the "holy grail" anode material for rechargeable batteries despite the fact that its dendritic growth and low Coulombic efficiency (CE) have crippled its practical use for decades. Its high chemical reactivity and low stability make it difficult to explore the intrinsic chemical and physical properties of the electrochemically deposited lithium (EDLi) and its accompanying solid electrolyte interphase (SEI). To prevent the dendritic growth and enhance the electrochemical reversibility, it is crucial to understand the nano- and mesostructures of EDLi. However, Li metal is very sensitive to beam damage and has low contrast for commonly used characterization techniques such as electron microscopy. Inspired by biological imaging techniques, this work demonstrates the power of cryogenic (cryo)-electron microscopy to reveal the detailed structure of EDLi and the SEI composition at the nanoscale while minimizing beam damage during imaging. Surprisingly, the results show that the nucleation-dominated EDLi (5 min at 0.5 mA cm -2 ) is amorphous, while there is some crystalline LiF present in the SEI. The EDLi grown from various electrolytes with different additives exhibits distinctive surface properties. Consequently, these results highlight the importance of the SEI and its relationship with the CE. Our findings not only illustrate the capabilities of cryogenic microscopy for beam (thermal)-sensitive materials but also yield crucial structural information on the EDLi evolution with and without electrolyte additives.

Suppressing lithium (Li) dendritegrowth is one of the most critical challenges for the developmentof Li metal batteries. Here, we report for the first time the growthof dendrite-free lithium films with a self-aligned and highly compactednanorod structure when the film was deposited in the electrolyte consistingof 1.0 M LiPF6in propylene carbonate with 0.05 M CsPF6as an additive. Evolution of both the surface and the cross-sectionalmorphologies of the Li films during repeated Li deposition/strippingprocesses were systematically investigated. It is found that the formationof the compact Li nanorod structure is preceded by a solid electrolyteinterphase (SEI) layer formed on the surface of the substrate. Electrochemicalanalysis indicates that an initial reduction process occurred at ∼2.05V vs Li/Li+before Li deposition is responsible for theformation of the initial SEI, while the X-ray photoelectron spectroscopyindicates that the presence of CsPF6additive can largelyenhance the formation of LiF in this initial SEI. Hence, the smoothLi deposition in Cs+-containing electrolyte is the resultof a synergistic effect of Cs+additive and preformed SEIlayer. A fundamental understanding on the composition, internal structure,and evolution of Li metal films may lead to new approaches to stabilizethe long-term cycling stability of Li metal and other metal anodesfor energy storage applications. [ABSTRACT FROM AUTHOR]