New Liquid Electrolytes Consisting of Li Salts and Urea Derivatives for Li-Ion Batteries

Abstract

Deep eutectic solvents are mixtures that exhibit melting points much lower than those of the endmembers due to the molecular interactions. Deep eutectic solvents attracted attention because high melting point compounds are available as flame-resistance solvents.1 However, the stable operation of Li-ion batteries (LIBs) with deep eutectic electrolytes (DEEs) is still challenging probably due to their high protonic nature and consequent narrow potential window.2 To develop Li-ion DEEs with a wide potential window, we investigated DEEs based on lithium bis(fluorosulfonyl)amide (LiFSA) and a series of urea derivatives to clarify the correlation between compound structure and electrolyte properties. Furthermore, we examined the electrode performances of the Li4Ti5O12 (LTO) negative electrode and LiNi1/3Co1/3Mn1/3O2 (NCM) positive electrode in the DEEs.To prepare DEEs, urea derivatives, such as urea, 1-methylurea, 1,3-dimethylurea, 1,1-dimethylurea, and tetramethyl urea are mixed with LiFSA at molar ratios of 1:2 or 1:4 at room temperature. The oxidation and reduction stability was evaluated by linear sweep voltammetry (LSV) in three-electrode cells with Pt foil as the working electrode. Galvanostatic charge-discharge tests were performed in three-electrode cells with LTO or NCM working electrodes and activated carbon counter electrodes.The prepared LiFSA:urea derivative (1:4) mixtures (LiFSA:urea, LiFSA:1,3-dimethylurea, and LiFSA:tetramethylurea) became transparent liquids at room temperature. Moreover, LiFSA:1,3-dimethylurea and LiFSA:tetramethylurea were clear liquid at even in 1:2 mixtures. Using the five DEEs, we evaluated their electrolyte properties. Figure 1 shows the LSV curves of the DEEs. Among the five deep eutectic liquids, LiFSA:1,3-dimethylurea (1:2) exhibited the widest potential window of 4.49 V, showing an even wider potential window than electrolytes with non-protonic tetramethyl urea.Figure 2a displays the charge-discharge curves of LTO electrodes in the DEEs. The LiFSA:urea (1:4) electrolyte cell exhibits a large irreversible capacity, and its initial reversible capacity is 228 mAh g-1, significantly larger than the theoretical capacity. This indicates that side reactions possibly takes place in addition to the LTO charge-discharge reaction. On the other hand, the LiFSA:1,3-dimethylurea (1:2) electrolyte demonstrates stable and reversible charge-discharge for 50 cycles, with an initial discharge capacity of 179 mAh g-1, which is close to the theoretical capacity. The LiFSA:tetramethylurea cell showed lower cycle stability than the LiFSA:1,3-dimethylurea cell, possibly due to electrolyte precipitation during cycling because LiFSA:tetramethylurea (1:2) electrolyte is an almost saturated solution.Using the LiFSA:1,3-dimethylurea (1:2) DEE, we further evaluated the electrochemical performance of the NCM electrode as shown in Figure 2b. The NCM electrode showed reversible charge-discharge behavior over 10 cycles with moderate capacity of 75.7 mAh g-1. These results demonstrate the potential applicability of the DEE to 3 V-class LIBs. In the presentation, we will also discuss the optimization of LiFSA concentration to improve battery properties and the observation of the formation of surface films on the electrodes in DEEs.