Interfacially-localized high-concentration electrolytes for high-performance rechargeable aqueous lithium-ion batteries

Abstract

Highly concentrated electrolytes have attracted significant attention because of their ability to enhance electrochemical stability and facilitate the formation of solid electrolyte interphases (SEIs). In particular, the electrolyte materials used in aqueous Li-ion batteries can benefit greatly from these high-concentration effects owing to the poor electrochemical stability of water. Therefore, highly concentrated aqueous electrolytes have recently been employed to achieve wider electrochemical stability windows. However, such high concentrations lead to increased electrolyte viscosity, reduced ionic conductivity, higher costs, and decreased energy density. In addition, employing diluents to create localized high-concentration electrolytes in aqueous systems is challenging, unlike in non-aqueous systems. In this study, we developed novel “interfacially-localized high-concentration electrolytes” using a fluorinated anionic surfactant, lithium nonafluoro-1-butanesulfonate (LiNFBS) (2.13 M), in conjunction with a divalent salt, magnesium(II) bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) (0.74 M). Using this electrolyte, we achieved a wide electrochemical stability window (ESW) of 3.3 V, a high Li⁺ transference number, elevated ionic conductivity (34.0 mS cm⁻¹), low viscosity (19.2 mPa s), excellent interfacial wettability, and superior SEI formation. Based on the remarkable performance of this electrolyte, lithium titanium phosphate (LTP)/lithium manganate (LMO) full cells demonstrated high-rate capability at 40 C and were maintained for over 750 stable cycles at a current density of 5 C. Thus, this design concept may provide new avenues for the development of next-generation high-performance aqueous electrolytes.