Aqueous electrolyte design for super-stable 2.5 V LiMn₂O₄ || Li₄Ti₅O₁₂ pouch cells

To compete with commercial organic electrolytes, aqueous electrolytes beyond water-in-salt electrolytes with a lower salt concentration of <5.0 m (mol kg_solvent^–1) and wider electrochemical stability window of >3.0 V are urgently needed. Here we report a 4.5 m lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)–KOH–CO(NH₂)₂–H₂O non-flammable ternary eutectic electrolyte that expands the electrochemical stability window to >3.3 V by forming a robust solid–electrolyte interphase. The ternary eutectic electrolyte enables Li_1.5Mn₂O₄ || Li₄Ti₅O₁₂ pouch cells to achieve a high average Coulombic efficiency of 99.96% and capacity retention of 92% after 470 cycles at an areal capacity of 2.5 mAh cm^–2, a low positive/negative capacity ratio of 1.14 and a lean electrolyte (3 g Ah^–1). The Li loss due to the solid–electrolyte interphase formation in the initial charge/discharge cycles is compensated by an excess 0.5 Li in the Li_1.5Mn₂O₄ cathode, which converts the Li_1.5Mn₂O₄ || Li₄Ti₅O₁₂ cell into LiMn₂O₄ || Li₄Ti₅O₁₂ after solid–electrolyte interphase formation. The 2.5 V aqueous Li_1.5Mn₂O₄ || Li₄Ti₅O₁₂ pouch cells with practical settings demonstrate a promising approach towards safe, low-cost and high-energy aqueous Li-ion batteries.