Tetramethylurea dimer/lithium salt-based deep eutectics as a novel class of eutectic electrolytes

Abstract

Deep eutectic electrolytes (DEEs), which are a subclass of deep eutectics, are attractive candidates for next-generation electrolytes applicable in Li batteries with improved safety and performance. As hydrogen bond donors (HBDs), urea and amide derivatives are frequently employed, but the N–H bonds cause low reduction stability. Here, we report a novel class of HBD-free DEEs containing a tetramethylurea (TMU) dimer that exhibit high reduction and thermal stabilities, in addition to reasonable ion conductivities. The mixtures of TMU dimer and Li salts (lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or lithium bis(fluorosulfonyl)imide (LiFSI)) yielded TMU dimer-based DEEs. The interaction between Li⁺ and the CO groups of the TMU dimer was confirmed using Fourier transform infrared spectroscopy and quantum chemical calculations. The TMU dimer-based DEEs exhibited high thermal stabilities (T_d10 = 183–327 °C) and tunable low glass-transition temperatures (T_g = −65 to −15 °C), depending on the nature of the Li salts and the TMU dimer/Li salt mol% ratio. Higher TMU dimer content yielded higher ionic conductivity, exhibiting 6.8 × 10⁻² (LiTFSI) and 6.7 × 10⁻² (LiFSI) mS cm⁻¹ at 25 °C for the TMU dimer-based DEEs obtained using a dimer/Li salt mol% = 10. HOMO–LUMO calculations of the TMU dimer and its analogues indicated high stabilities against reductive decomposition, owing to the absence of N–H bonds. This study reveals the potential of novel TMU dimer-based DEEs, with new insights into a cost-effective, safer, green electrolyte system for Li-ion batteries.