A Low-Concentrated Electrolyte with a 3.5 V Electrochemical Stability Window, Made by Restructuring the H-Bond Network, for High-Energy and Long-Life Aqueous Sodium-Ion Batteries

The narrow electrochemical stability window (ESW) of electrolytes is primarily responsible for the low energy density and poor cycle life of aqueous sodium-ion batteries (ASIBs). Although water-in-salt electrolytes can effectively widen the ESW, their high salt concentration leads to high costs. In this work, we found that weak-polar solvents have more advantages in broadening the ESW than strong-polar solvents. Taking 1,3,6-hexanetricarbonitrile as an example, it evidently enhances the stability of water by weakening the hydrogen bond network. Consequently, it broadens the ESW to 3.5 V at a relatively low concentration and enables Na₂FeFe(CN)₆||NaTi₂(PO₄)₃ full cells with a capacity retention of 70% after 10 000 cycles and an energy density of 71 Wh kg^–1, based on the active materials’ mass of both electrodes. This work offers new prospects for the development of high-energy and long-life aqueous sodium-ion batteries for large-scale energy storage.