The Hydrotropic Effect of Ionic Liquids in Water-in-Salt Electrolytes*.

Water-in-salt electrolytes have successfully expanded the electrochemical stability window of aqueous electrolytes beyond 2 V. Further improvements in stability can be achieved by partially substituting water with either classical organic solvents or ionic liquids. Here, we study ternary electrolytes composed of LiTFSI, water, and imidazolium ionic liquids. We find that the LiTFSI solubility strongly increases from 21 mol kg-1 in water to up to 60 mol kg-1 in the presence of ionic liquid. The solution structure is investigated with Raman and NMR spectroscopy and the enhanced LiTFSI solubility is found to originate from a hydrotropic effect of the ionic liquids. The increased reductive stability of the ternary electrolytes enables stable cycling of an aqueous lithium-ion battery with an energy density of 150 Wh kg-1 on the active material level based on commercially relevant Li4 Ti5 O12 and LiNi0.8 Mn0.1 Co0.1 O2 electrode materials.

Impact of Anion Asymmetry on Local Structure and Supercooling Behavior of Water-in-Salt Electrolytes.

Salts with asymmetric (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI) anions have recently been shown to suppress crystallization of water-in-salt electrolytes, enabling low-temperature operation of high-voltage aqueous rechargeable batteries. To clarify the underlying mechanism for the kinetic suppression of crystallization, we investigate the local solution structures and dynamic behaviors of water-in-salt electrolytes based on the asymmetric FTFSI anion and its symmetric anion analogues by Raman spectroscopy and molecular dynamics simulations. We find that monodentate coordination of FTFSI to cations leads to high rotational mobility of the uncoordinated SO2CF3 group. We conclude that the peculiar, coordination-dependent, local dynamics in the asymmetric FTFSI anion, manifested by enhanced intramolecular bond rotation, enables the strong supercooling behavior.

Water-in-salt electrolytes for aqueous lithium-ion batteries with liquidus temperatures below -10 °C.

Crystallization near room temperature is a common challenge for water-in-salt electrolytes. We demonstrate that water-in-salt electrolytes based on lithium (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide remain thermodynamically in the liquid state down to at least -10 °C. Combined with their excellent chemical and electrochemical stability as well as high lithium-ion conductivity, these electrolytes represent a key enabler for the next generation of high-voltage aqueous lithium-ion batteries.

Magnesium Ethylenediamine Borohydride as Solid-State Electrolyte for Magnesium Batteries.

Solid-state magnesium ion conductors with exceptionally high ionic conductivity at low temperatures, 5 × 10-8 Scm-1 at 30 °C and 6 × 10-5 Scm-1 at 70 °C, are prepared by mechanochemical reaction of magnesium borohydride and ethylenediamine. The coordination complexes are crystalline, support cycling in a potential window of 1.2 V, and allow magnesium plating/stripping. While the electrochemical stability, limited by the ethylenediamine ligand, must be improved to reach competitive energy densities, our results demonstrate that partially chelated Mg2+ complexes represent a promising platform for the development of an all-solid-state magnesium battery.