Eutectic Mixtures As Highly Concentrated and Molten Electrolytes with Nearly Single-Ion Conducting Behavior
Authors: Krämer Susanna , Daniliuc Constantin G. , Winter Martin , Wiemhöfer Hans-Dieter , Gruenebaum Mariano 
Published: February 2024

Today's state-of-the-art liquid electrolytes in lithium ion batteries (LIBs) have a high ionic conductivity and good performance regarding their cycle life. (1) However, they pose a safety risk due to their high vapor pressures and low thermal stability. (1) Furthermore, due to the limited electrochemical stability of the solvent, liquid electrolytes are not suitable for the application in high-voltage LIBs. (2) Molten salts, also called ionic liquids (IL), or highly concentrated electrolytes (HCE) have high lithium ion concentrations, where nearly every solvent molecule is coordinated. Due to this, there are strong ion interactions and the formation of ion clusters that lead to an increased lithium ion transference number of >0.5. (3) Therefore, they can represent an alternative in the field of liquid electrolytes. Additionally, HCE exhibit a higher thermal and electrochemical stability compared to dilute electrolytes and can improve the cycle performance in lithium metal batteries. (4, 5)

McOwen et
al. reported the coordination of lithium ions and crystalline structures in HCE of the binary mixtures of ethylene carbonate (EC) and lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) with molar ratio up to 1:1. (6) Based on the concept of melting point depression as known from thermodynamics, room-temperature molten, highly concentrated electrolytes of a carbonate-based solvent with lithium sulfonyl imides were investigated. Instead of EC, the solvent pinacol carbonate (PIC) without acidic α-hydrogen atoms, but four bulky methyl groups was synthesized and used for eutectic mixtures, as the melting point of PIC is with 187 °C far above room-temperature. The physicochemical properties of these electrolytes are studied with respect to the different influence of lithium bis(fluorosulfonyl)imide and LiTFSI despite their same basic molecule structure. The focus will be on the electrochemical analysis by the means of the ionic conductivity, transference number and the electrochemical stability.

In comparison to dilute liquid electrolytes the molten electrolytes show extremely high transference numbers, especially for the PIC-LiTFSI mixtures nearly a single-ion conducting behavior (0.9) is observed. This behavior can be explained by the formation of a 2D polymeric network within the HCE electrolyte as determined by crystallographic measurements in the solid state. Combined with the high electrochemical stability, a stable long-term cycling and dendrite suppression in symmetric lithium cells could be shown. Cycling in full cells with high-voltage cathode materials such as LiNi0.6Mn0.2Co0.2O2 (NMC622) or LiMn4O2 (LMO) against lithium metal anodes is applicable.

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