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Abstract The complex interplay and only partial understanding of the multi-step phase transitions and reaction kinetics of redox processes in lithium–sulfur batteries are the main stumbling blocks that hinder the advancement and broad deployment of this electrochemical energy storage system. To better understand these aspects, here we report operando confocal Raman microscopy measurements to investigate the reaction kinetics of Li–S redox processes and provide mechanistic insights into polysulfide generation/evolution and sulfur deposition. Operando visualization and quantification of the reactants and intermediates enabled the characterization of potential-dependent rates during Li–S redox and the linking of the electronic conductivity of the sulfur-based electrode and concentrations of polysulfides to the cell performance. We also report the visualization of the interfacial evolution and diffusion processes of different polysulfides that demonstrate stepwise discharge and parallel recharge mechanisms during cell operation. These results provide fundamental insights into the mechanisms and kinetics of Li–S redox reactions. 

A Cu foil current collector was coated with polydopamine-derived nitrogen-doped carbon (N-C) to regulate Li nucleation and growth. The lithium nucleation overpotential was significantly lowered, and Li was deposited in a spherical morphology without dendrites, dramatically improving the Li plating/stripping coulombic efficiency. 

Li–S batteries have attracted great attention for their combined advantages of potentially high energy density and low cost. To tackle the capacity fade from polysulfide dissolution, we have developed a confinement approach by in situ encapsulating sulfur with a MOF-derived CoS 2 in a carbon framework (S/Z-CoS 2 ), which in turn was derived from a sulfur/ZIF-67 composite (S/ZIF-67) via heat treatment. The formation of CoS 2 was confirmed by X-ray absorption spectroscopy (XAS) and its microstructure and chemical composition were examined through cryogenic scanning/transmission electron microscopy (Cryo-S/TEM) imaging with energy dispersive spectroscopy (EDX). Quantitative EDX suggests that sulfur resides inside the cages, rather than externally. S/hollow ZIF-67-derived CoS 2 (S/H-CoS 2 ) was rationally designed to serve as a control material to explore the efficiency of such hollow structures. Cryo-STEM-EDX mapping indicates that the majority of sulfur in S/H-CoS 2 stays outside of the host, despite its high void volumetric fraction of ∼85%. The S/Z-CoS 2 composite exhibited highly improved battery performance, when compared to both S/ZIF-67 and S/H-CoS 2 , due to both the efficient physical confinement of sulfur inside the host and strong chemical interactions between CoS 2 and sulfur/polysulfides. Electrochemical kinetics investigations revealed that the CoS 2 could serve as an electrocatalyst to accelerate the redox reactions. The composite could provide an areal capacity of 2.2 mA h cm −2 after 150 cycles at 0.2C and 1.5 mA h cm −2 at 1C. This novel material provides valuable insights for further development of high-energy, high-rate and long-life Li–S batteries. 

Abstract While lithium ion batteries with electrodes based on intercalation compounds have dominated the portable energy storage market for decades, the energy density of these materials is fundamentally limited. Today, rapidly growing demand for this type of energy storage is driving research into materials that utilize alternative reaction mechanisms to enable higher energy densities. Transition metal compounds are one such class of materials, with storage enabled by “conversion” reactions, where the material is converted to new compound upon lithiation. MoS₂is one example of this type of material that has generated a large amount of interest recently due to its high theoretical lithium storage capacity compared to graphite. Here, cryogenic scanning transmission electron microscopy techniques are used to reveal the atomic‐scale processes that occur during reaction of a model monolayer MoS₂system by enabling the unaltered atomic structure to be determined at various levels of lithiation. It is revealed that monolayer MoS₂can undergo a conversion reaction even with no substrate, and that the resulting particles are smaller than those that form in bulk MoS₂, likely due to the more limited 2D diffusion. Additionally, while bilayer MoS₂undergoes intercalation with a corresponding phase transition before conversion, monolayer MoS₂does not.