Halide-free water-in-salt electrolytes for stable aqueous sodium-ion batteries

Abstract The extensive investigation via classical Molecular Dynamics (MD) simulations of the halide-free “water-in-salt” electrolyte (WiSE) consisting of sodium acetate (8 m) and potassium acetate (32 m), unveils the interactions between cations, anions and water molecules. The WiSE's application as electrolyte in symmetric aqueous sodium-ion batteries, featuring NASICON-type Na2VTi(PO4)3/C (NVTP/C) as active material at both the positive and the negative electrode, is also reported. In situ X-ray diffraction (XRD) measurements resolve the structural evolution of NVTP/C during the highly reversible sodium de/intercalation. Differential Electrochemical Mass Spectrometry (DEMS) confirms the remarkable stability of the highly concentrated electrolyte. Symmetric cells employing two NVTP/C electrodes and a green 32K8N electrolyte show an average discharge voltage of 1.13 V with stable cycling performance and a coulombic efficiency above 99.1% at 1C and 99.9% at 10C over 500 cycles. Compared to the fluorinated 9.2 m NaOTF, the 32K8N electrolyte has substantially lower cost, environmental impact and superior coulombic efficiency in symmetric cells.