Non-Flammable Ether and Phosphate-Based Liquid Electrolytes for Sodium-Ion Batteries

Abstract

The demand on the lithium reserves has been increasing rapidly due to the continuous growth in energy storage devices. Since the lithium reserves are limited and only accessible at certain geographical areas, the costs of lithium precursors are increasing significantly.1 Therefore, alternative battery chemistries need to be investigated. One of the next generation battery solutions is sodium-ion batteries (SIBs).2 However, current commercial electrolytes in SIBs are based on highly flammable carbonate-based solvents, such as ethylene carbonate, propylene carbonate, diethyl carbonate and dimethyl carbonate.3 To enhance the safety of SIBs, it is of utmost importance to develop non-flammable electrolytes. In this work, NaPF6 and NaBF4 salts are used in combination with a solvent mixture of ethers (diglyme, tetraglyme) and phosphates (triethylphosphate, trimethylphosphate) to obtain non-flammable liquid electrolytes. The electrolytes are evaluated in terms of physicochemical properties and electrochemical performance full-cells using high-mass loading Prussian White (12 mg/cm2) and Hard Carbon (7 mg/cm2). Flammability tests have shown that at least 70% of phosphate co-solvents are required to make the glyme based electrolyte non-flammable. In summary, the combination of glymes and phosphates provide promising electrolyte formulations in terms of non-flammability characteristics and ionic conductivities, but still lack long-term cycling stability. This work opens avenues to optimizing non-flammable electrolytes for high-performing and safe SIBs. References 1 R. Schmuch, R. Wagner, G. Hörpel, T. Placke and M. Winter, Nat. Energy, 2018, 3, 267–278.2 H. S. Hirsh, Y. Li, D. H. S. Tan, M. Zhang, E. Zhao and Y. S. Meng, Adv. Energy Mater., 2020, 10, 1–8.3 R. Gond, W. Van Ekeren, R. Mogensen, A. J. Naylor and R. Younesi, Mater. Horizons, 2021, 8, 2913–2928. Acknowledgement The authors would like to acknowledge the financial support by the Swedish Energy Agency via project no. 50177-1, VINNOVA via projects no. 2022-01465 and 2019-00064 (Batteries Sweden), and SuSaNa project under M-ERA-Net call 2021 (G.A. no 958174).