Ternary Low-Temperature Molten Salt Electrolyte for Sodium Ion Batteries

Abstract

To meet the increasing demand of electricity (which is forecasted to double by 2050)1 and to mitigate the climate crisis associated with using fossil fuels, renewable and clean energy sources are becoming the ultimate choices to generate electricity. Electrochemical energy storage devices such as batteries play an important role in the efficient use of renewable energy, providing an efficient and sustainable way to store energy.1,2 Beyond-lithium rechargeable batteries based on earth-abundant elements, namely sodium ion batteries (SIBs), are believed to be the alternatives for large-scale storage. Nevertheless, the commercial applications of SIBs are restricted by absence of reliable and low-cost electrolyte favouring low-temperature, high-efficiency, and reversible metal deposition/stripping. Interestingly, the newly reported molten salts containing several metal halides act as a low-cost electrolyte, which exhibits high electrical conductivity and quick electrode kinetics, and produce even better performances than ionic liquid electrolytes.3,4 Nonetheless, the conventional molten salt electrolytes normally operate at a relatively high temperature (over 260 ), which is significantly inconvenient for outdoor operation. Actually, reducing the operation temperature is not only beneficial to improving cycle life by restraining temperature-related degradation (i.e. undesired side reactions), but also profitable to using lower cost cell materials and reduce the cost of batteries fabrication. This paper reports our development of a low-temperature inorganic molten salt electrolytes with good electrochemical performances.5 The metal chloride eutectic mixture is used as electrolyte, whose melting points are dependent on its precursors and composition, so the relationship of mole ratio of metal chlorides and electrolyte s melting points are investigated in our work. The influences of additives in chloride-based eutectic mixture are also studied, to decrease the melting point of the electrolyte. Herein, the electrochemical properties of this electrolyte are investigated in battery cells using Na metal as anode and metal fluoride cathode respectively. The battery charge /discharge behaviour and electrochemical impedance spectra (EIS) is tested using GAMRY potentiostat. Materials characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM),X-ray spectroscopy (XPS) and Raman spectroscopy are used to characterise the electrode materials and to understand the effect of electrolyte on the electrode materials. References: Dunn, B., Kamath, H., Tarascon, J. For the grid : a battery of choices. Science 334,928–935 (2011). Zhenguo Yang Jianlu Zhang, et al.Electrochemical energy storage for green grid. Chem. Rev. 111,3577–3613 (2011). Elia, G. A. et al.An overview and future perspectives of aluminum batteries. Adv. Mater. 28,7564–7579 (2016). Yu, H., Wang, J., Zhang, X., Chu, W., Liu, S. Sub-100 °C aluminum ion battery based on ternary inorganic molten salt. Chem. Commun.(2019). Zhang, J., et al.Ternary low-temperature molten salt electrolyte for sodium ion batteries. In prepairation.