Effective Water Confinement and Dual Electrolyte–Electrode Interfaces by Zwitterionic Oligomer for High‐Voltage Aqueous Lithium‐Ion Batteries

Abstract Aqueous lithium‐ion batteries (ALIBs) have attracted significant interest due to their inherent advantage on safety. However, water itself has a narrow electrochemical stability window (ESW), limiting the energy density of ALIBs. Here, a low‐molecular‐weight zwitterionic oligomer, oligo(propylsulfonate dimethylammonium propylmethacrylamide) (OPDP), as an effective water binding agent for high‐voltage ALIBs is demonstrated. The OPDP can effectively confine water molecules while reducing water activity. The OPDP‐based electrolyte, with an ultra‐high water weight percentage of 25.4%, possesses an outstanding ESW of up to 3.26 V and an ionic conductivity as high as 3.18 mS cm −1 . Furthermore, the aqueous Mo 6 S 8 //LiMn 2 O 4 full cell with OPDP‐based electrolyte achieves a 99.7% capacity retention after 200 cycles at 0.5C with a high Coulombic efficiency (CE) of 98.7% and a specific energy of 88–101 Wh kg −1 . Also, it achieves an 89% capacity retention after 2000 cycles at 10C with a high CE of 99.9%. These postmortem characterizations suggest that robust organic–inorganic hybrid cathode/anode‐electrolyte interfaces have been constructed during the cycling through the heteroatoms of N, S, and O in the zwitterionic oligomer, leading to the inhibited hydrogen/oxygen evolution reactions and high performance of the full cell. This work provides a promising strategy for developing low‐cost and high‐voltage aqueous batteries.