Development of Lithium Ion Conducting Liquid: Methylurea-Based Eutectic Electrolytes for Lithium Batteries

Deep eutectic electrolytes (DEEs) are attracting increasing attention as liquid-state electrolytes for secondary batteries because they are potentially low cost, display low flammability, and are environmentally friendly. However, to date limited DEEs have been developed and explored for lithium-ion battery (LIB) applications, with most reports showing unsatisfactory capacity retention, a narrow potential window for battery operation, and an unstable solid electrolyte interphase (SEI) layer leading. Herein, we develop DEEs based on lithium bis(fluorosulfonyl)amide, LiFSA, and a series of urea derivatives as Li ion-conducting DEEs. Despite similar structures for the urea derivatives, i.e. methylated urea, we found that 1,3-dimethylurea (1,3-DMU) could form Li ion-concentrated DEEs across a wide range of LiFSA : 1,3-DMU ratio, while the LiFSA : urea DEE was liquid only in a limited range of molar ratios, i.e. LiFSA : urea close to 1 : 4 (mol/mol). By examining the electrolyte structure via Raman spectroscopy, we observed increased aggregation for DEE with higher LiFSA concentrations. We further confirmed non-flammability and electrochemical stability among the DEEs with potential windows ranging from ∼3.35 V for LiFSA : urea (1 : 4) to an impressive 6.42 V for LiFSA : 1,3-DMU (1 : 2) at a Pt foil electrode. During charge-discharge of Li4Ti5O12 (LTO) electrodes, we observed good capacities and retention for the LiFSA : urea (1 : 4) and LiFSA : 1,3-DMU (1 : 2) DEEs. High Coulombic efficiencies (CEs) were achieved in the LiFSA : 1,3-DMU (1 : 2) DEE with its high LiFSA content that led to more substantial FSA-derived components in the SEI structures after cycling. We further tested positive electrode materials, including LiFePO4 that showed excellent capacity retention and CEs near 100 % across 50 cycles. In all, we find that the dimethylurea-based DEEs show an opportunity for non-flammable and high-voltage Li batteries.