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Batteries 2022, 8, 157 7 of 25 additive, resulting in the formation of the NaF-rich SEI layer on the Na metal surface, as shown in Figure 3a. Due to these features of the bilayer SEI layer, the Na||Na symmetric battery presented enhanced stable cycling of 1000 h at the current density of 0.5 mA cm−2. Not only the F-containing ingredient but some other additives also contributed to the in- crease in the high F-containing substance in the SEI layer for NMBs. Fang et al. designed a localized high-concentration electrolyte (LHCE) with an SbF3 electrolyte additive as NMB’s electrolyte [51]. They found that the SbF3 electrolyte additive would create a stabilized SEI layer with a Na–Sb alloy inner layer and a NaF-rich outer layer, which could protect the Na metal anode. As expected, the Na||Na symmetric battery in this type of electrolyte presented a long cycle life of over 1200 h at 0.5 mA cm−2 with negligible voltage polar- ization. Jiang et al. found that the acetamide (N, O-bis(trimethylsilyl) trifluoroacetamide, BSTFA) additive acted as a scavenger, scavenging the HF and H2O, which restrained the hydrolysis reaction in the NaPF6 in the electrolyte [73]. These behaviors were also in favor of the high F-containing substance in the SEI layer. After the introduction of BSTFA into ultralow-concentration electrolytes with 0.3 M NaPF6, the Na metal anode still presented a high-capacity retention rate of 92.63% after 1955 cycles at 2 C. In stabilizing the SEI layer to achieve Na metal anode stability, some new ideas were to improve the stability of NMBs from the perspective of solvation. Firstly, Doi et al. designed a concept of F-free electrolytes, 0.1 M Na tetraphenylborate (NaBPh4) in DME for NMBs. They found that the F-free electrolyte enabled a high reversible Na plating/stripping at the average Coulombic efficiency of 99.85% over 300 cycles. Due to the F-free design of the electrolyte, the SEI layer onto the Na metal anode was mainly composed of the C, O, and Na elements with the negligible presence of the F element. After cycling for 450 h, the Na||Na symmetric battery only presented the interphase resistance of 3 Ω in NaBPh4/DME electrolyte, indicating that extremely stable and low-resistivity interphase was formed. Figure 3b showed the DC-DFTB-metaD simulations, which could be used to evaluate the two-dimensional free energy surface concerning various coordination states. From this simulation, they found that the most stable state of [BPh4]− in NaBPh4/DME was CIP after the fully dissociated state. This result also indicated that [BPh4]− was the most tolerant against the reduction, which accounted for the stable Na/electrolyte interphase. In addition, the other substances also could be used to improve the stability of the NMBs. Zheng et al. attempted to add a small amount of SnCl2 into carbonate electrolyte for NMBs and found that the Na||Na symmetric battery achieved a remarkable reduction of voltage hysteresis for over 500 h [74]. To investigate the reasons for this performance improvement, they took the symmetric battery apart for characteristics. They found that SnCl2 in electrolytes could spontaneously react with Na metal to form a Na-Sn alloy layer due to the hyper-reductivity of Na metal. In addition, the Cl− also reacted with Na metal to produce a NaCl-rich SEI, which could passivate Na metal surface against the corrosion from electrolyte during the plating−striping process. Figure 3c showed the schematic of the typical mosaic SEI on the Na metal anode in the regular carbonate electrolyte and SnCl2- containing carbonate electrolyte. Kreissl et al. introduced a functionalized diamondoid (bis-N,N’-propyl-4,9-dicarboxamidediamantane, DCAD) as an electrolyte additive for NMBs [75]. They found that the functionalized diamantine additive could change the SEI constituent as well as the co-deposition behavior of Na ions, resulting in the change in Na metal growth direction in the plated Na process. Wang et al. found that the slight Na polysulfide (Na2S6) served as an electrolyte additive, which could pre-passivate the Na metal anode in ether electrolyte [76]. According to the interface analysis, the slight Na polysulfide introduction resulted in the robust SEI layer composed of Na2O, Na2S2, and Na2S, which could protect the Na metal against damage from electrolyte components. It is generally believed that the NaNO3 additives in the electrolyte can protect electrodes and inhibit dendrite formation in LMB. However, a strange phenomenon was that the co-additive of Na2S6-NaNO3 presented seriously deteriorated the Na metal electrode, with an adverse effect for NMBs, which was contrary to the LMBs. Zhu et al. developed an organosulfur compound (tetramethylthiuram disulfide, TMTD) as an additive in carbonate-PDF Image | Recent Development for Sodium Metal Batteries
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