Deep Eutectic Solvents as Nonflammable Electrolytes for Durable Sodium-Ion Batteries

Abstract

Sodium-ion batteries are alternatives for lithium-ion batteries in applications where cost-effectiveness is of primary concern, such as stationary energy storage. The stability of sodium-ion batteries is limited by the current generation of electrolytes, particularly at higher temperatures. Therefore, the search for an electrolyte which is stable at these temperatures is of utmost importance. Here, such electrolytes are introduced in the form of nonflammable deep eutectic solvents (DESs), consisting of sodium bis(trifluoromethane)sulfonimide (NaTFSI) dissolved in N-methyl acetamide (NMA). Increasing the NaTFSI concentration replaces NMA—NMA hydrogen bonds with strong ionic interactions between NMA, Na+, and TFSI−. These interactions lower NMA's highest occupied molecular orbital (HOMO) energy level compared with that of TFSI−, leading to an increased anodic stability (up to ≈4.65 V versus Na+/Na). (Na3V2(PO4)2F3/carbon nanotube [CNT])/(Na2+x Ti4O9/C) full cells show 97.0% capacity retention after 250 cycles at 0.2 C and 55 °C. This is considerably higher than for (Na3V2(PO4)2F3/CNT)/(Na2+x Ti4O9/C) full cells containing a conventional electrolyte. According to the electrochemical impedance analysis, the improved electrochemical stability is linked to the formation of more robust surface films at the electrode/electrolyte interface. The improved durability and safety highlight that DESs can be viable electrolyte alternatives for sodium-ion batteries.