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Batteries 2022, 8, 157 4 of 25 At present, many researchers have reviewed the research progress of Na metal an- odes [57,59]. However, previous progress mainly focused on the modification strategy of host materials and artificial SEI layers [32,60,61]. Although those strategies had some positive effects on the construction of uniform and stable SEI, the complex preparation process made it difficult to popularize on a large scale. With the increase in research on electrolyte optimization for Na metal anodes, scientists found that the method of electrolyte optimization is equally effective and easier to popularize on a large scale [62,63]. Therefore, it is very necessary to conduct a timely review of the progress in the Na metal anode electrolyte optimization. In this review, we mainly discuss the current Na metal anode electrolytes progress in three parts, including liquid electrolytes, polymer electrolytes, and all-solid-state electrolytes. Firstly, we discuss the effects of electrolyte solvents, solutes, and additives in liquid electrolytes for the Na metal anodes and debate how to inhibit the Na dendrite growth and improve the Na metal anode’s electrochemical performance. Secondly, we also analyze the optimization strategy of polymer electrolytes in the Na metal anodes and summarize the current progress of polymer electrolytes. Thirdly, the preparation method of Na metal anode all-solid-state electrolytes and expounds on the influence of the electrolyte preparation method for the Na ions migration is described. After the above discussion, we also classify some recent technological advances and research strategies of Na metal anode electrolytes and propose some perspectives. With the recent increase in the research on Na metal anode electrolytes by scientists, we believe that this review can provide some guidance for the further development of Na metal anode electrolytes. 2. Liquid Electrolytes for Na Metal Anodes The development of Na metal anodes referred to traditional Li metal batteries (LMBs) [50,64]. Therefore, the liquid electrolyte was also applied to the Na metal anodes. However, the conventional LMB electrolyte easily caused the heterogeneous migration and dissolution of Na ions, resulting in the increase in Na dendrites and dead Na. Modification of the electrolyte became necessary. Since 2014, Yoon et al. firstly optimized NaTFSI salt concentrations in N-propyl-nmethylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid to improve the Na metal anodes in the battery [65]. According to the 23Na nuclear magnetic resonance (NMR) spectra, it was found that the chemical shifts and spectral linewidths changed as a function of both salt concentrations, indicating the complex coordination of the Na ion, which was dependent on the salt concentration. The high concentration salt triggered an up-field chemical shift, resulting in the increase in the magnetic shielding. This behavior seemed to facilitate the Na deposition reaction. After the optimization, this high salt concentration in ionic-liquid-based electrolyte presented a stable Na metal plating behavior, with a current of 5 mA cm−2 at 25 ◦C to 20 mA cm−2 at 100 ◦C. Inspired by this work, many scientists began to devote themselves to the electrolyte engineering research of Na metal batteries (NMBs). Firstly, it was found that the Na dendrites and dead Na were closely related to the SEI layer, and the SEI layer constituent was partly composed of the decomposition of salts in the electrolyte. Therefore, salt chemistry in electrolyte engineering gained much attention. Seh et al. reported a simple liquid electrolyte, with hexafluorophosphate in glyme (mono- , di-, and tetraglyme) electrolytes for Na metal anodes, which enabled a nondendritic plating-stripping of the Na metal anode with highly reversible performance [62]. They found that NaTFSI salt in the electrolyte caused more exposure of Na metal with the electrolyte solvent, resulting in the trigger of undesirable side reactions. This behavior resulted in the formation of more organic reduction products in the SEI layer and caused a decrease in Coulombic efficiency (CE). Compared with NaTFSI salt, NaPF6 in electrolyte presented a higher reduction potential than that of glyme solvents, contributing to the formation of a uniform, inorganic SEI layer, which could protect the Na metal surface. Shi et al. introduced potassium bis(trifluoromethylsulfonyl)imide (KTFSI) salt as a bifunctional electrolyte additive [66]. They found that the TFSI− anions in electrolyte assisted in the formation of a stable SEI layer with N-containing species including Na3N, NaNxOy, NaNO2,PDF Image | Recent Development for Sodium Metal Batteries
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