Enhancement of Functional Properties of Liquid Electrolytes for Lithium‐Ion Batteries by Addition of Pyrrolidinium‐Based Ionic Liquids with Long Alkyl‐Chains

Abstract

Three ionic liquid belonging to the N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imides (Pyr 1 , n TFSI with n = 4,5,8) have been added as co-solvent to two commonly used electrolytes for Li-ion cells: (a) 1 M lithium hexafluorophosphate (LiPF 6) in a mixture of ethylene carbonate (EC) and linear like dimethyl carbonate (DMC) in 1 : 1 v/v and (b) 1 M lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI) in EC : DMC 1 : 1 v/v. These electrolyte formulations (classified as P and T series containing LiPF 6 or LiTFSI salts, respectively) have been analyzed by comparing ionic conductivities, transport numbers, viscosities, electrochemical stability as well as vibrational properties. In the case of the Pyr 1,5 TFSI and Pyr 1,8 TFSI blended formulations, this is the first ever reported detailed study of their functional properties in Li-ion cells electrolytes. Overall, Pelectrolytes demonstrate enhanced properties compared to the Tones. Among the various P electrolytes those containing Pyr 1,4 TFSI and Pyr 1,5 TFSI limit the accumulation of irreversible capacity upon cycling with satisfactory performance in lithium cells.

Tuning aprotic solvent properties with long alkyl chain ionic liquid for lithium-based electrolytes

“…7,10 Although the ILs with short alkyl chains have low viscosity as an advantage, the ILs with long alkyl chains also present some interesting properties. [11][12][13] For example, when the IL has ions with large ionic structure in its composition, e.g., large imidazolium, ammonium and phosphonium cations, it can assume a liquid crystal function, known as ionic liquid crystal (ILC), which is able to form aggregates and crystalline phases in the liquid medium and complex structures in the solvent. 12,13 Yamanaka and co-workers have shown that the structural properties assigned to the crystalline phase are expected to suppress problems related to high viscosity, 11 but there is insufficient information about how these structures interact and affect the mixture properties and there is little literature evidence of ILCs used in aprotic ternary electrolytes for Li-metal based batteries.…”

“…Although the ILs with short alkyl chains have low viscosity as an advantage, the ILs with long alkyl chains also present some interesting properties. 11–13 For example, when the IL has ions with large ionic structure in its composition, e.g. , large imidazolium, ammonium and phosphonium cations, it can assume a liquid crystal function, known as ionic liquid crystal (ILC), which is able to form aggregates and crystalline phases in the liquid medium and complex structures in the solvent.…”

Structural and Dynamic Characterization of Li–Ionic Liquid Electrolyte Solutions for Application in Li-Ion Batteries: A Molecular Dynamics Approach

“…Besides, pyrrolidinium cations have been extensively pointed out as appropriate for use in LiBs due to their versatility, allowing combinations with different anions [20,22]. Their action has been reported to be very sensitive to changes in their lateral chains [3,[23][24][25]. However, they are not naturally electroactive, and in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts [26,27].…”

Multicomponent Phase Separation in Ternary Mixture Ionic Liquid Electrolytes

“…Recently, ternary mixtures of organic solvents, ionic liquids (ILs), and Li salts have shown outstanding performance in Li-ion and Li–oxygen batteries, outperforming both binary mixtures of organic solvents or ILs with Li salt electrolytes. − The binary IL-based electrolytes, despite their remarkable thermal, chemical, and electrochemical stabilities, very low vapor pressure, and nonflammability, have a high viscosity that negatively affects ion transport. − On the other hand, binary electrolytes based on organic solvents exhibit better transport characteristics, but a majority of them are only stable in a narrow electrochemical potential window. − A common strategy to overcome these issues is to use a combination of ILs and low-viscosity organic solvents with Li salts to form ternary mixtures. − In particular, Li salts such as LiI and LiNO 3 are added to Li–air battery electrolytes to improve cyclability and provide a protective solid electrolyte layer on the anode …”