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 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 Li1.5Mn2O4 || 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 Li1.5Mn2O4 cathode, which converts the Li1.5Mn2O4 || Li4Ti5O12 cell into LiMn2O4 || Li4Ti5O12 after solid–electrolyte interphase formation. The 2.5 V aqueous Li1.5Mn2O4 || Li4Ti5O12 pouch cells with practical settings demonstrate a promising approach towards safe, low-cost and high-energy aqueous Li-ion batteries.