Nonflammable 4.4 m aqueous electrolyte unlocking Ti₂Nb₁₀O₂₉ anodes for safe and wide-temperature aqueous batteries

Aqueous lithium-ion batteries promise safety and cost advantages, yet face critical challenges of narrow electrochemical stability windows and the lack of high-capacity anode materials, limiting their voltage output and energy density. Herein, we report a 4.4 m LiTFSI-Betaine-Urea-Water (BUW) quaternary eutectic electrolyte that suppresses water activity via molecular crowding and Li⁺–betaine/urea coordination, achieving an electrochemical stability window of 3.7 V with a cathodic limit around 1.1 V (vs Li/Li⁺). Betaine stabilizes water via strong zwitterionic interactions, broadening the electrochemical stability window and enabling high-capacity Ti₂Nb₁₀O₂₉ anode, which operates at a lower potential yet offers higher theoretical capacity than Li₄Ti₅O₁₂. When paired with LiMn₂O₄, the resulting LiMn₂O₄||Ti₂Nb₁₀O₂₉ full cell delivers stable cycling for over 400 cycles with ∼99 % Coulombic efficiency, retains 72 mAh g⁻¹ at 1.2 A g⁻¹, and demonstrates robust low-temperature performance (−25 °C). Spectroscopy and simulations reveal a reduced number of free water and a bilayer SEI (LiF-rich inner/polymeric outer) on the anode that mitigates hydrogen evolution. This work demonstrates a synergistic electrolyte/anode design, establishing low-concentration, non-flammable eutectics as a practical route beyond “water-in-salt” electrolytes to enable lower-potential, higher-capacity anodes in safe, wide-temperature aqueous Li-ion batteries. © 2026 The Authors.