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Highly concentrated aqueous electrolytes (termed water-in-salt electrolytes, WiSEs) at solid-liquid interfaces are ubiquitous in myriad applications including biological signaling, electrosynthesis, and energy storage. This interface, known as the electrical double layer (EDL), has a different structure in WiSEs than in dilute electrolytes. Here, we investigate how divalent salts [zinc bis(trifluoromethylsulfonyl)imide, Zn(TFSI)2], as well as mixtures of mono- and divalent salts [lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) mixed with Zn(TFSI)2], affect the short- and long-range structure of the EDL under confinement using a multimodal combination of scattering, spectroscopy, and surface forces measurements. Raman spectroscopy of bulk electrolytes suggests that the cation is closely associated with the anion regardless of valency. Wide-angle X-ray scattering reveals that all bulk electrolytes form ion clusters; however, the clusters are suppressed with increasing concentration of the divalent ion. To probe the EDL under confinement, we use a Surface Forces Apparatus and demonstrate that the thickness of the adsorbed layer of ions at the interface grows with increasing divalent ion concentration. Multiple interfacial layers form following this adlayer; their thicknesses appear dependent on anion size, rather than cation. Importantly, all electrolytes exhibit very long electrostatic decay lengths that are insensitive to valency. It is likely that in the WiSE regime, electrostatic screening is mediated by the formation of ion clusters rather than individual well-solvated ions. This work contributes to understanding the structure and charge-neutralization mechanism in this class of electrolytes and the interfacial behavior of mixed-electrolyte systems encountered in electrochemistry and biology.

A multilayered ternary solid polymer electrolyte (TSPE) is presented. First, the influence of polyethylene oxide degradation on cell failure, development of subsequent volatile degradation products, and cell impedance is analyzed. The low electrochemical stability window/oxidative stability (≥3.8 V) results in side‐chain oxidation and loss of active material. Subsequently, electrolyte stability is improved and a thin‐film (≤50 μm) TSPE with three functional layers is developed to match the wide‐ranging electrolyte requirements toward Li metal anodes and different cathode materials like LiNi0.6Mn0.2Co0.2O2 and LiFePO4 (NCM622, LFP). The high‐voltage stability of ≥4.75 V makes the TSPE a promising candidate in high‐voltage applications. Because of high Coulombic efficiencies in NMC622‖Li metal (99.7%) and LFP‖Li metal (99.9%) cells, the presented electrolyte enables stable long‐term cycling with great capacity retention of 86% and 94%, respectively. The temperature stability of >300 °C and the capability to prevent high surface area Li and dendrite formation (even at an areal capacity utilization of >40 mAh cm−2) contribute to high safety under a wide range of conditions. [ABSTRACT FROM AUTHOR]

We report on the reversible (de)intercalation of TFSI− anions from a Mg‐based ionic liquid electrolyte, Mg(TFSI)2 in Pyr14TFSI, in graphite ∥ activated carbon hybrid dual‐ion capacitor (DIC) cells. The role of different pseudo reference electrodes (PREs) including Ag‐wire and Li metal is discussed regarding the comparability of different battery cells. We show that an Ag‐wire PRE is not suitable for the designated purpose, while the use of a Li metal PRE results in high reproducibility. Mg‐based DIC cells are compared with cells based on pure Pyr14TFSI and LiTFSI‐Pyr14TFSI electrolytes, and with graphite ∥ Li metal dual‐ion cells employing LiTFSI‐Pyr14TFSI. At 5.2 V vs. Li|Li+, Mg‐containing DIC cells reveal an improved performance compared to Li‐based cells, i. e. higher capacity (87 vs. 85 mAh g−1) and higher Coulombic efficiency (98 vs. 96 %). These results may pave the way to further studies and performance improvements of Mg‐based batteries and to hybrid DIC chemistries with other cations, especially multi‐valent ones. [ABSTRACT FROM AUTHOR]

A major challenge of lithium metal electrodes, in theory a suitable choice for rechargeable high energy density batteries, comprises non-homogeneous lithium deposition and the growth of reactive high surface area lithium, which eventually yields active material losses and safety risks. While it is hard to fully avoid inhomogeneous deposits, the achievable morphology of the occurring lithium deposits critically determines the long-term cycling behaviour of the cells. In this work, we focus on a combined scanning electron microscopy (SEM) and 7Li nuclear magnetic resonance spectroscopy (7Li-NMR) study to unravel the impact of the choice of conducting salts (LiPF6 and LiTFSI), solvents (EC : DEC, 3 : 7, DME : DOL, 1 : 1), as well as their respective concentrations (1 M, 3 M) on the electrodeposition process, demonstrating that lithium deposition morphologies may be controlled to a large extent by proper choice of cycling conditions and electrolyte constituents. In addition, the applicability of 7Li-NMR spectroscopy to assess the resulting morphology is discussed. It was found, that lithium deposition analysis based on the 7Li chemical shift and intensity should be used carefully, as various morphologies can lead to similar results. Still, our case study reveals that the combination of SEM and NMR data is rather advantageous and offers complementary insights that may provide pathways for the future design of tailored electrolytes. [ABSTRACT FROM AUTHOR]

Continuous lithium (Li) depletion shadows the increase in energy density and safety properties promised by zero-excess lithium metal batteries (ZELMBs). Guiding the Li deposits toward more homogeneous and denser lithium morphology results in improved electrochemical performance. Herein, a lithium nitrate (LiNO 3 ) enriched separator that improves the morphology of the Li deposits and facilitates the formation of an inorganic-rich solid-electrolyte interphase (SEI) resulting in an extended cycle life in Li||Li-cells as well as an increase of the Coulombic efficiency in Cu||Li-cells is reported. Using a LiNi 0.6 Co 0.2 Mn 0.2 O 2 positive electrode in NCM622||Cu-cells, a carbonate-based electrolyte, and a LiNO 3 enriched separator, an extension of the cycle life by more than 50 cycles with a moderate capacity fading compared to the unmodified separator is obtained. The relative constant level of LiNO 3 in the electrolyte, maintained by the LiNO 3 enriched separator throughout the cycling process stems at the origin of the improved performance. Ion chromatography measurements carried out at different cycles support the proposed mechanism of a slow and constant release of LiNO 3 from the separator. The results indicate that the strategy of using a LiNO 3 enriched separator instead of LiNO 3 as a sacrificial electrolyte additive can improve the performance of ZELMBs further by maintaining a compact and thus stable SEI layer on Li deposits., (© 2023 The Authors. Small published by Wiley-VCH GmbH.)