Towards Safer Sodium-Ion Batteries Via Ionic Liquids As Electrolyte Additives

Our society is in urgent need of more efficient, affordable and environmentally friendly energy storage technologies. While lithium-ion batteries (LIB) is undeniably the main track of development and outstanding at present, complementary technologies are needed for managing applications such as renewable energy integration. One particularly appealing alternative is to explore the feasibility of a technology analogous to LIB – sodium ion batteries (SIB) – using the basic LIB know-how and with sodium being more abundant and less expensive. 

As in any battery technology, the electrolyte choice is as important as that of the electrodes to ensure successful operation. Considering both safety and “environmental impact” ionic liquids (ILs) have emerged as an interesting option for LIB. ILs exhibit both non-volatility and non-flammability, but unfortunately their high viscosities and comparatively low conductivities are expected to considerably limit the rate performance. Mixed electrolytes, i.e. with ILs as additives to an organic electrolyte, are a compromise suggested to improve the safety – while still providing an appreciable conductivity.  Here, the possibility of expanding this strategy also to the SIB technology has been explored.

We here present a systematic study of several electrolytes consisting of a sodium salt (NaTFSI) dissolved in mixtures of ILs and organic solvents, using electrochemical techniques (cyclic voltammetry, chronopotentiometry) coupled to various safety tests (e.g. self-extinguishing time (SET) and flash point (FP)). Imidazolium (BMImTFSI/EMImTFSI) and pyrrolidinium (PYR₁₃TFSI) ILs were used as additives to optimized ethylene carbonate: propylene carbonate (EC:PC) electrolytes, while Hard Carbon (HC) and Na₃V₂(PO₄)₃ (NVP) were used as anode and cathode, respectively. Electrochemical tests were performed on half cells to evaluate the cycling abilities and the electrochemical stability of each electrolyte/electrode combination. HC tested with the BMImTFSI-EC:PC electrolytes exhibit specific capacities of ca 250 mAhg^-1 and 140 mAhg^-1 at C/5 and 1C rates with capacity retentions of 95% and 98%, respectively. For PYR₁₃TFSI-EC:PC ~150 mAhg^-1 at C/10 rate a Coulombic efficiency of ca 98% was achieved. The NVP electrode, exhibits capacities of ~80 mAhg^-1 with the BMImTFSI-EC:PC electrolytes at C/10 rate, and of 80, 75 and 46 mAhg^-1 at C/10, C/5 and C, respectively with the PYR₁₃TFSI-EC:PC electrolyte.  A successful full SIB cell was assembled based on the best choices of the above materials as a proof of concept. As a major argument for the use of ILs is an increased safety, it was encouraging that the safety tests, carried out in air for many different compositions, unambiguously proved the addition of ILs to decrease the SET, as well as the flame intensity, and simultaneously raise the FP.