Lithium-ion batteries are becoming increasingly important for electrifying the modern transportation system and, thus, hold the promise to enable sustainable mobility in the future. However, their large-scale application is hindered by severe safety concerns when the cells are exposed to mechanical, thermal, or electrical abuse conditions. These safety issues are intrinsically related to their superior energy density, combined with the (present) utilization of highly volatile and flammable organic-solvent-based electrolytes. Herein, state-of-the-art electrolyte systems and potential alternatives are briefly surveyed, with a particular focus on their (inherent) safety characteristics. The challenges, which so far prevent the widespread replacement of organic carbonate-based electrolytes with LiPF6 as the conducting salt, are also reviewed herein. Starting from rather "facile" electrolyte modifications by (partially) replacing the organic solvent or lithium salt and/or the addition of functional electrolyte additives, conceptually new electrolyte systems, including ionic liquids, solvent-free, and/or gelled polymer-based electrolytes, as well as solid-state electrolytes, are also considered. Indeed, the opportunities for designing new electrolytes appear to be almost infinite, which certainly complicates strict classification of such systems and a fundamental understanding of their properties. Nevertheless, these innumerable opportunities also provide a great chance of developing highly functionalized, new electrolyte systems, which may overcome the afore-mentioned safety concerns, while also offering enhanced mechanical, thermal, physicochemical, and electrochemical performance.

technologies. Unlike lithium, whose market is already very tight, sodium mineral deposits are almost infinite, evenly distributed worldwide, much easier to extract and thereby attainable at low cost. [1][2][3][4] If the realization of Na-rechargeable batteries could be practically possible, there will be nearly three orders of magnitude relaxation in the constraints on lithium-based resources, accompanied by sustainability, improved environmental benevolence, and cost reduction ( Table 1). Even more appealing is the possible use of the widely available and lighter aluminum, rather than copper, as negative current collector and hard carbon from renewable sources instead of graphite for the negative electrode. Finally, the stability of sodium-ion batteries (SIBs) in the fully discharged state would significantly enhance the safety associated with the shipment of large-format SIBs worldwide.These beneficial features of sodiumbased cells revived the research work on Na-based rechargeable batteries and accordingly captured the attention of both the academic research and industry sectors. However, similar to LIB, most of the research work in Na-based batteries have focused on the development and elaboration of negative and positive For sodium (Na)-rechargeable batteries to compete, and go beyond the currently prevailing Li-ion technologies, mastering the chemistry and accompanying phenomena is of supreme importance. Among the crucial components of the battery system, the electrolyte, which bridges the highly polarized positive and negative electrode materials, is arguably the most critical and indispensable of all. The electrolyte dictates the interfacial chemistry of the battery and the overall performance, having an influence over the practical capacity, rate capability (power), chemical/thermal stress (safety), and lifetime. In-depth knowledge of electrolyte properties provides invaluable information to improve the design, assembly, and operation of the battery. Thus, the full-scale appraisal of both tailored electrolytes and the concomitant interphases generated at the electrodes need to be prioritized. The deployment of large-format Na-based rechargeable batteries also necessitates systematic evaluation and detailed appraisal of the safety-related hazards of Na-based batteries. Hence, this review presents a comprehensive account of the progress, status, and prospect of various Na + -ion electrolytes, including solvents, salts and additives, their interphases and potential hazards.

We report a systematic investigation of Na-based electrolytes that comprise various NaX [X=hexafluorophosphate (PF6 ), perchlorate (ClO4 ), bis(trifluoromethanesulfonyl)imide (TFSI), fluorosulfonyl-(trifluoromethanesulfonyl)imide (FTFSI), and bis(fluorosulfonyl)imide (FSI)] salts and solvent mixtures [ethylene carbonate (EC)/dimethyl carbonate (DMC), EC/diethyl carbonate (DEC), and EC/propylene carbonate (PC)] with respect to the Al current collector stability, formation of soluble degradation compounds, reactivity towards sodiated hard carbon (Nax -HC), and solid-electrolyte interphase (SEI) layer formation. Cyclic voltammetry demonstrates that the stability of Al is highly influenced by the nature of the anions, solvents, and additives. GC-MS analysis reveals that the formation of SEI telltales depends on the nature of the linear alkyl carbonates and the battery chemistry (Li(+) vs. Na(+) ). FTIR spectroscopy shows that double alkyl carbonates are the main components of the SEI layer on Nax -HC. In the presence of Na salts, EC/DMC and EC/DEC presented a higher reactivity towards Nax -HC than EC/PC. For a fixed solvent mixture, the onset temperature follows the sequence NaClO4