Flame-retardant “water in oil” electrolyte enables Ah-level aqueous lithium-ion batteries

Aqueous batteries are appealing for their safety and environmental benefits, but their use is restricted by the limited electrochemical stability window due to water decomposition. Suppressing water splitting and expanding the electrochemical operating window of electrolytes remains a huge challenge. In this work, a series of "water in oil" electrolytes based on dimethyl methylphosphate (DMMP) have been designed. Experimental and theoretical data show that DMMP has strong supramolecular interactions with H₂O, which can break the original interactions between H₂O molecules and form more stable hydrogen bonds. At the same time, it chelates Li⁺ to regulate solvation structure and enable rapid Li⁺ transport. Additionally, DMMP contributes to CEI film formation on the cathode, stabilizing its crystal structure during cycling. This new design not only expands the electrochemical stability window (3.1 V), but also supports LiMn₂O₄||NaTi₂(PO₄)₃ cell to cycle more than 1000 times and LiFePO₄|| NaTi₂(PO₄)₃ cell to cycle more than 2000 times. 7 Ah LiMn₂O₄||NaTi₂(PO₄)₃ pouch cell delivers a high energy density of 49.3 Wh kg⁻¹ and high Coulombic efficiency of 99.8% at 4 A g^-1 over 500 cycles. This work provides new insights into the design of electrolytes based on the organic cosolvent for rechargeable batteries.