Acetate-Based Water-in-Salt Electrolyte for Aqueous Sodium-Ion Batteries

Abstract

Aqueous rechargeable alkali-ion batteries have attracted considerable interest in recent years due to reduced cost, lower environmental impact and, most importantly, and improved safety.1 Unfortunately, the narrow electrochemical stability window of water (1.23 V) limits the maximum output voltage of aqueous batteries, thus leading to intrinsically low energy density. Water-in-salt electrolytes (WiSE) constituted of, for example, highly concentrated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solutions, has promoted the development of aqueous LIBs.2 However, these highly concentrated electrolytes based on fluorinated salts spoil the two major advantages of aqueous batteries, which are the low cost and reduced environmental impact. Here we study alternative “Water-in-Salt” electrolytes (WiSE) based on highly concentrated sodium acetate and potassium acetate solutions. This kind of halide-free WiSE are studied by classical Molecular Dynamics (MD), evidencing the overall molecular arrangement to approach the “sponge-like” structure observed in certain ionic liquids and it offers wide electrochemical stability window and compatibility with stainless steel current collector. This is then employed as electrolyte in aqueous sodium-ion batteries featuring polyanionic compounds as active material. In-situ X-ray diffraction (XRD) experiments evidence the structural evolution during the reversible sodium de/intercalation. The X-ray photoemission spectroscopy (XPS) confirms the formation of a solid-electrolyte interphase (SEI) layer on the negative electrode. The cells show stable cycle performance and high coulombic efficiency above 99 % at 1C and 99.9% at 10C over 500 cycles.Keywords: Acetate-based, Water-in-Salt electrolyte, aqueous sodium-ion batteries, high coulombic efficiency