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The concept of water-in-salt electrolytes was introduced recently, and these systems have been successfully applied to yield extended operation voltage and hence significantly improved energy density in aqueous Li-ion batteries. In the present work, results of X-ray scattering and Fourier-transform infrared spectra measurements over a wide range of temperatures and salt concentrations are reported for the LiTFSI (lithium bis­(trifluoro­methane sulfonyl)­imide)-based water-in-salt electrolyte. Classical molecular dynamics simulations are validated against the experiments and used to gain additional information about the electrolyte structure. Based on our analyses, a new model for the liquid structure is proposed. Specifically, we demonstrate that at the highest LiTFSI concentration of 20 m the water network is disrupted, and the majority of water molecules exist in the form of isolated monomers, clusters, or small aggregates with chain-like configurations. On the other hand, TFSI– anions are connected to each other and form a network. This description is fundamentally different from those proposed in earlier studies of this system.

The solid electrolyte interphase (SEI) is a passivation layer naturally formed on battery electrodes. It protects electrodes and electrolytes from degradation and dictates charging time capabilities and lifetime. Despite its importance, it remains a poorly understood battery component. This study provides novel insights into the formation, morphology, and composition of the SEI on Si anodes through a multi-modal approach. The findings show a layered SEI and the ion and electron conductivities, as well as their relation to performance, are discussed.

A strong correlation between host tortuosity and cycling reversibility of a hosted Li-metal anode is revealed for the first time. High tortuosity leads to preferential top-surface Li deposition based on locally enhanced current density and concentration gradient. This top-surface accumulated Li blocks inward ion transport and invalidates the internal electrode, further aggravating the uneven current distribution and non-uniform plating and stripping. Decreased electrode tortuosity can significantly improve the anodic Coulombic efficiency, uniformity of Li-metal stripping and plating, and cycling stability of the rGO host.

The structure and packing of organic mixed ionic-electronic conductors have an especially significant effect on transport properties. In operating devices, this structure is not fixed but is responsive to changes in electrochemical potential, ion intercalation, and solvent swelling. Toward this end, the steadystate and transient structure of the model organic mixed conductor, poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), is characterized using multimodal time-resolved operando techniques. Steady-state operando X-ray scattering reveals a doping-induced lamellar expansion of 1.6 Å followed by 0.4 Å relaxation at high doping levels. Time-resolved operando X-ray scattering reveals asymmetric rates of lamellar structural change during doping and dedoping that do not directly depend on potential or charging transients. Timeresolved spectroscopy establishes a link between structural transients and the complex kinetics of electronic charge carrier subpopulations, in particular the polaron-bipolaron equilibrium. These findings provide insight into the factors limiting the response time of organic mixed-conductor-based devices, and present the first real-time observation of the structural changes during doping and dedoping of a conjugated polymer system via X-ray scattering. Organic mixed ionic-electronic conductors (OMIECs) are a class of conjugated materials [1] of growing interest for bioelectronic, [2] energy storage, [3] electrochromic, [4] and neuromorphic computing [5] applications due to their ability to simultaneously transport both ionic and electronic charge, and couple between

Surface sensitive X-ray reflectivity (XRR) measurements were performed to investigate the electrochemical lithiation of a native oxide terminated single crystalline silicon (100) electrode in real time during the first galvanostatic discharge cycle. This allows us to gain nanoscale, mechanistic insight into the lithiation of Si and the formation of the solid electrolyte interphase (SEI). We describe an electrochemistry cell specifically designed for in situ XRR studies and have determined the evolution of the electron density profile of the lithiated Si layer (LiSi) and the SEI layer with subnanometer resolution. We propose a three-stage lithiation mechanism with a reaction limited, layer-by-layer lithiation of the Si at the LiSi/Si interface.

Whether attempting to eliminate parasitic Li metal plating on graphite (and other Li-ion anodes) or enabling stable, uniform Li metal formation in 'anode-free' Li battery configurations, the detection and characterization (morphology, microstructure, chemistry) of Li that cannot be reversibly cycled is essential to understand the behavior and degradation of rechargeable batteries. In this review, various approaches used to detect and characterize the formation of Li in batteries are discussed. Each technique has its unique set of advantages and limitations, and works towards solving only part of the full puzzle of battery degradation. Going forward, multimodal characterization holds the most promise towards addressing two pressing concerns in the implementation of the next generation of batteries in the transportation sector (viz. reducing recharging times and increasing the available capacity per recharge without sacrificing cycle life). Such characterizations involve combining several techniques (experimental-and/or modeling-based) in order to exploit their respective advantages and allow a more comprehensive view of cell degradation and the role of Li metal formation in it. It is also discussed which individual techniques, or combinations thereof, can be implemented in real-world battery management systems on-board electric vehicles for early detection of potential battery degradation that would lead to failure.

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