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.