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Batteries 2022, 8, 272 7 of 20 of hygroscopicity, and a somewhat costly price at the moment [34]. All of these pose significant obstacles to the industrialization of the use of ionic liquids [35]. 2.2. The Construction of an Efficient SEI Layer According to the previous discussion, low CE value and dendrite Na deposition are the two critical challenges faced by AFSMBs. These two problems are the results of the interface chemistries between electrolyte-sodium or metal-current collectors. Therefore, optimizing the interfacial chemistry plays a decisive role in solving the dendrite problem and improving CE [36,37]. The fabrication of a stable SEI for AFSMBs can be classified into in situ formed SEI and artificial SEI methods [38,39]. The in situ method is realized by controlling the electrolyte salts, additives, or solvents. In contrast, the artificial ap- proach typically uses an artificial layer with strong mechanical strength and flexibility to prevent contact between the sodium metal and electrolyte. Additionally, the artificial SEI’s composition and structure can be regulated to meet specific needs. In the electrochemical reaction, the redox reactions are usually carried out within the electrochemical stability potential window (ESW) of the electrolyte. According to the frontier molecular orbital theory, the stability window is controlled by the LUMO and the HOMO [40,41]. Most electrolyte components, including solvents, salts, and additives, have a tendency to be reduced on the anode part during the Na plating process and create an SEI layer due to the low standard potential of the Na/Na+ (−2.76 V). Such a passivation film can block the continuous electrolyte reaction and increase the overpotential of electrolyte decomposition, leading to the expansion of the ESW of the electrolytes. Therefore, an understanding of the SEI composition and protection mechanism can help us to design a robust SEI layer for the AFSMBs. The use of some additives, such as FEC, has been reported to enhance the cycling stability of the SMBs for a long time [42,43]. However, the chemical and morphological behavior of the SEI layer with the electrolyte was still unknown until the employment of cryogenic transmission electron microscopy (cryo–TEM) [44]. Cryo–TEM has been reported to be an inevitable tool to preserve the native state of sensitive materials and interphases while performing high-resolution imaging [45,46]. Han et al. reported via the cryo-TEM that in NaPF6 FEC–free EC: DMC-based electrolyte, the electrolyte tends to react with the Na metal due to the higher electron affinity of EC than DMC [44]. The compositions of formed SEI are rich in Na2CO3 and Na3PO4. During the following cycles, the Na2CO3 will decompose to CO2 and further damage the SEI layer. As a result, the deposited Na metal will be exposed to the electrolyte and further react with the electrolyte (Figure 4a–c). As a contract, the added FEC solvent will react with the sodium metal to form a thin layer of amorphous NaF in the NaPF6 FEC–free EC: DMC-based electrolyte system, which can anchor on the top of the initially formed Na3PO4 layer to a NaF-Na3PO4 multi–layer (Figure 4d–f). The Na/Na symmetric cell shows that the cell with the FEC additive maintains a 500 to 800 h cycling performance at the current density from 0.5 to 2 mA cm−2, whereas the cell without the FEC additive shows an instant increase of the overpotential only after 50 cycles at 0.5 mA cm−2 and a catastrophic cell failure at 2 mA cm−2.PDF Image | Anode-Free Rechargeable Sodium-Metal Batteries
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