Deep Eutectic Solvents As Sodium-Ion Battery Electrolytes with Increased Durability at Higher Temperatures
Authors: De Sloovere Dries , Paulus Andreas , Joos Bjorn , Calvi Lavinia , Vranken Thomas , Safari Mohammadhosein , Van Bael Marlies K , Hardy An 
Published: December 2020

Sodium-ion batteries (SIBs) are a potential cost-effective alternative for lithium-ion batteries (LIBs) in applications which require large-scale energy storage. In these applications, low cost and sustainability are of prime importance. The extensive use of LIBs for large-scale storage would drive up lithium prices because of the latter’s limited production. As the lightest and smallest alkali metal after lithium, sodium is abundant and widely available, and is therefore more cost-efficient and sustainable. SIBs can use light and inexpensive aluminum current collectors for both the anode and cathode, as opposed to LIBs, which usually require more expensive copper current collectors for the anodes. This further increases the SIB's viability in applications where cost and sustainability are the most crucial factors.
Viable battery electrolytes should have sufficient ionic conductivity, be electronically insulating, have a large electrochemical and thermal stability window, and not show reactivity with the other components of the battery. They should be safe, non-toxic, and inexpensive. Ionic liquids, defined as materials which consist entirely out of ions and which remain liquid below 100 °C, were previously studied as electrolytes for SIBs as they have a high intrinsic ionic conductivity, can dissolve high amounts of salt, are non-flammable and have a high thermal and (electro)chemical stability. Ionic liquids are typically expensive due the difficulty of their synthesis. Deep eutectic solvents (DESs) are closely related to ionic liquids and consist of a mixture of Lewis/Brönsted acids and bases, where the melting temperature of the mixture is below that of the individual components. Whereas DESs and ionic liquids can have similar physical properties, DESs can have additional advantages such as ease of preparation and wide availability of reagents, although this depends on the exact composition. Binary and ternary mixtures of different XFSI (FSI = bis(fluorosulfonyl)imide) and XTFSI (TFSI = bis(trifluoromethanesulfonyl)imide) inorganic salts were previously applied as DES electrolyte in sodium-ion half cells, which were constrained to operation at 80 °C or higher because of their relatively high melting points.1,2 These cells showed high reversibility and high Coulombic efficiency.
Other DESs may also offer an increase of the stability of battery operation, and may be used at less stringent temperature conditions (with acceptable ionic conductivity even at room temperature). This stability problem is particularly pronounced at 55 °C or higher for conventional electrolytes, where battery performance rapidly degrades when linear carbonates are used as electrolyte solvent.3 Our series of DESs as SIB electrolytes were based on the dissolution of NaTFSI in an amide which is solid at room temperature. For each electrolyte, the effect of the solution structure on its electrochemical properties was studied. The amide was spontaneously reduced when contacted with sodium metal. By increasing the salt concentration, we observe a lower reactivity with sodium metal, a broader electrochemical stability window and a decreased ionic conductivity due to the strong Coulombic interactions among the amide molecules, Na+, and TFSI- ions (Figure). The sample containing 10 mol% of NaTFSI shows an anodic stability limit of ~3.6 V vs Na+/Na and a conductivity of 10.3 mS cm-1 at 55 °C. A DES with 30 mol% NaTFSI is stable up to ~4.6 V vs Na+/Na and has a conductivity of 3.8 mS cm-1 at 55 °C. The variation of conductivity with temperature of both DESs could be fitted with the Vogel-Tamman-Fulcher equation. At 55 °C, (Na3V2(PO4)3/C)/(Na2+x
Ti4O9/C) full cells containing DES as electrolyte demonstrate a considerably higher durability and Coulombic efficiency than cells containing a conventional organic solvent-based electrolyte. As such, these DESs form a new class of electrolytes for application in sodium-ion batteries, offering a more durable performance at 55 °C than conventional systems.
References
(1) Nohira, T.; Ishibashi, T.; Hagiwara, R. Properties of an Intermediate Temperature Ionic Liquid NaTFSA – CsTFSA and Charge – Discharge Properties of NaCrO2 Positive Electrode at 423 K for a Sodium Secondary Battery. J. Power Sources
2012, 205, 506–509.
(2) Fukunaga, A.; Nohira, T.; Kozawa, Y.; Hagiwara, R. Intermediate-Temperature Ionic Liquid NaFSA-KFSA and Its Application to Sodium Secondary Batteries. J. Power Sources
2012, 209, 52–56.
(3) Yan, G.; Dugas, R.; Tarascon, J. The Na3V2(PO4)2F3/Carbon Na-Ion Battery: Its Performance Understanding as Deduced from Differential Voltage Analysis. J. Electrochem. Soc.
2018, 165 (2), 220–227.

Figure 1