Methylsulfonylmethane-Based Deep Eutectic Solvent as a New Type of Green Electrolyte for a High-Energy-Density Aqueous Lithium-Ion Battery

Currently used aqueous electrolytes for lithium-ion batteries suffer from narrow potential windows of <1.5 V. In this paper, we are the first to discover the new deep eutectic solvent (DES) electrolyte of the methylsulfonylmethane–lithium perchlorate–water system, which is a safe, environmentally friendly, and low-cost "water-in-salt" aqueous electrolyte. It is found that the lithium ion can coordinate with the oxygen atom in the S═O double bond, perchlorate, and water molecule, retaining fluidity at room temperature when the molar ratios of MSM:LiClO4:H2O are 1.8:1:z (z > 0.3). This DES electrolyte performs with a satisfying electrochemical stability window (∼3.5 V), which enables the LiMn2O4/Li4Ti5O12 aqueous lithium-ion battery with both high energy density (>160 W h kg–1) and high capacity retention (72.2% after 1000 cycles). Overall, our study of this new deep DES electrolyte can enrich room-temperature "water-in-salt" aqueous electrolyte and offers new insights into exploring green aqueous electrolyte systems for lithium-ion batteries.