(Invited) Development of Deep Eutectic Electrolytes Using Urea Derivatives Toward Safe, High Voltage Lithium-Ion Batteries

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Deep eutectic solvents are mixtures that exhibit melting points much lower than those of the end members due to various intermolecular interactions. Deep eutectic solvents have attracted attention because the utilized high melting point compounds can act as flame-resistant solvents.1 However, the stable operation of Li-ion batteries (LIBs) with deep eutectic electrolytes (DEEs) is still challenging, likely 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 LiFePO4 (LFP) positive electrode in the DEEs.To prepare the DEEs, urea derivatives such as urea, 1-methylurea, 1,3-dimethylurea, 1,1-dimethylurea, and tetramethyl urea were mixed with LiFSA at molar ratios of 1:2 or 1:4 at room temperature. The oxidation and reduction stability were evaluated by linear sweep voltammetry (LSV) in three-electrode cells with Pt foil as the working electrode. Galvanostatic charge-discharge tests were also performed in three-electrode cells with LTO or LFP 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 liquids even in more concentrated 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 performances of LTO electrodes in the DEEs. Reversible charge-discharge was confirmed for all DEEs, and the Coulombic efficiency (urea; 92%, 1,3-DMU; 98%) and capacity retention (urea; 88%, 1,3-DMU; 87% after 50 cycles) were higher than tetramethylurea (74% and 22%, respectively). The excellent cycling properties of LiFSA:urea (1:4) may be attributed to its high ionic conductivity, while in LiFSA:1,3-dimethylurea (1:2) a stable film may be formed on the electrode. 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. We further focused on the 1:2 LiFSA:1,3- dimethylurea and 1:4 LiFSA:urea and followed similar methods to how the LTO electrodes were tested. Within both of these DEE, the charge-discharge of LFP showed excellent performance with high capacity retention and CEs near 100% (Figure 2b). The plateau potential of LFP does not approach the decomposition limit of either DEE, which likely encourages good performance. Moving to a higher voltage cathode, LiMn2O4 (LMO), we also observed relatively stable charge-discharge cycling with these DEEs.In total, these results demonstrate the potential applicability of the DEE to 3 V-class LIBs. In the presentation, we will also discuss the passivation films on the electrodes and solvation structure in DEEs.