Dual modulation of ion–water and ion–electrode interactions enabled by water-in-ionic-liquid electrolytes for robust metal-free batteries

The development of sustainable and robust aqueous batteries is urgently needed to meet growing energy demands while addressing safety, cost, and environmental concerns. Yet their practical implementation is constrained by three fundamental challenges: the narrow electrochemical stability window (ESW) of water, its temperature sensitivity due to evaporation and freezing, and the electrode dissolution. These challenges are intrinsically governed by molecular-level interactions among water, electrolyte ions, and electrode materials. In this work, we overcome these challenges by employing the water-miscible ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIMOTF) to formulate water-in-ionic-liquid (WiIL) electrolytes that deliberately modulates molecular-level interactions among water, electrolyte ions, and organic electrode materials. Spectroscopic analyses reveal the formation of a unique water-lean solvation sheath, [BMIM + (H 2 O) 2 OTF − ], that effectively suppresses water decomposition and mitigates volatility and freezing, but also strengthens ion–electrode interactions to inhibit the perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) dissolution. Leveraging these interactions, we demonstrate a proof-of-concept metal-free aqueous full cell with PTCDI anode and polyaniline (PANI) cathode, which delivered 71% capacity retention after 15, 000 cycles, high-rate capability, and stable operation from −10 to 60 °C. This study established a new strategy for designing sustainable aqueous batteries through solvation and interfacial engineering with water-miscible ionic liquids.