Water-in-bisalt electrolyte for high-performance aqueous dual-ion battery

Abstract

Aqueous dual-ion batteries (ADIBs) integrate safety, cost-efficiency, and power density to meet the demands of large electric storage in the future. However, the aqueous electrolyte still suffers from problems of small discharge capacity and low coulombic efficiency. In this work, a novel water-in-bisalt electrolyte (WiBS) was studied, and it was found that LiN(SO₂F)₂–LiN(SO₃CF₃) (LiFSI–LiOTf) reaches an extremely high concentration of 39 mol kg⁻¹ in water, and offers a wider electrochemical stability window of >3.1 V that is suitable for ADIBs. When applied as an electrolyte in DIBs, a high theoretical energy density of 264.05 Wh kg⁻¹ is estimated, and a power density of 1440 W kg⁻¹ and a discharge capacity of 94.3 mAh g⁻¹ at 2 mA cm⁻² are achieved in the initial cycle. Kinetics studies reveal that an LiFSI–LiOTf WiBS electrolyte exhibits a low activation energy for anion intercalation into a graphitic carbon cathode and also high diffusivity even at high concentration. Its high coulombic efficiency for charge and discharge can be explained by the prevention of the electrolysis of water by an extremely high concentration of electrolyte, such as 39 mol kg⁻¹. A stable cycle stability (>500 cycles) and also a high rate property are achieved in a cell using LiFSI–LiOTf aqueous electrolyte.