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Batteries 2022, 8, 157 5 of 25 and RNO2 during the Na plating/striping process. These N-containing species possessed good Na-ion conductors, which contributed to the mitigation of nonuniform Na dendrite formation. In addition, considering the lower REDOX potential of K/K+ compared with that of Na/Na+, K ions should be adsorbed onto Na protrusions nearby, resulting in the occurrence of electrostatic shielding to suppress Na dendritic deposition. After the optimization, the Na||Na symmetric battery in this type of electrolyte presented an average overpotential of 14 mV after cycling for 2700 h. Cao et al. found that highly concentrated ether-based electrolytes exhibited extremely high CEs. The high CEs should be due to the passivation of Na metal surface by the highly concentrated ether-NaFSI mixtures, resulting in the minimizing side degradation reactions during the plating/stripping process [67]. In addition, solvent chemistry was another method used to improve the Na metal anodes. Iermakova et al. compared the performance of Na metal anodes in the electrolytes of 1 M NaPF6 in both 0.5 ethylene carbonate (EC)/0.5 dimethyl carbonate (DMC) and 1 M NaPF6 in 0.45 EC/0.45 propylene carbonate (PC)/0.1 DMC (EC0.45PC0.45DMC0.1), and found that the EC0.45PC0.45DMC0.1 electrolyte presented an enhanced electrochemical performance [68]. However, after immersing the Na metal in the electrolyte for 24 h, large numbers of protrusions were observed on the surface. These limited performance improvements also indicated that carbonate-based electrolytes presented incompatibility for the Na metal anode. To realize the compatibility of the carbonate electrolyte in the Na metal anode, building a stable SEI layer using an electrolyte additive became one of the options. Rodriguez et al. used fluoroethylene carbonate (FEC) as an SEI layer forming additive in a carbonate-based electrolyte for a Na metal anode [69]. They found that the addition of FEC in electrolytes significantly reduced gassing during the deposition process and enhanced the cycling performance. This enhancement performance should be due to the thicker NaF and less dense polymer organic layer in the SEI layer according to the time of flight secondary-ion mass spectrometry (ToF-SIMS) result. Lee et al. also utilized EC/PC solvent with FEC additive and Na bis(fluorosulfonyl)imide (NaFSI) salt as the electrolyte fortheNametalanode[70]. Figure2ashowstheC1s,F1s,andN1sXPSspectraofthe Na metal anode surface during the initial Na plating with different electrolytes. They found that the FEC-NaFSI combination electrolyte constructed a homogeneous ionic interlayer during the Na plating/stripping process, containing abundant NaF and ionic compounds, which resulted in the high mechanical strength and ion penetrability of the interlayer. Chen et al. investigated the effects of different cation (Li+, K+, Mg2+, Ca2+, Cu2+, Zn2+, and Al3+) additives on electrolyte stability and the corresponding electrolyte solvation structures according to DFT calculation, as shown in Figure 2b [71]. Due to the lower electrode potential than Na-metal anodes, Li+, K+, and Ca2+ are suggested as cation additives. In addition, considering the LUMO level and the binding energy, Li+ is expected to be the most outstanding candidate. Therefore, they attempted to introduce Li+ additives into the 1 M NaPF6-DME electrolyte for NMBs. From the in situ optical microscopic characterizations, it was observed that the Na metal deposit exhibited a needle-like shape after deposition in 1 M NaPF6-DME electrolyte. After the introduction of Li+ additives, the Na metal showed a smooth morphology deposition, which was extremely disparaged from the needle-like Na dendrites. This result validated the resisted Na dendritic growth due to the electrostatic shield effect and enhanced electrolyte stability after introducing Li+ additives.PDF Image | Recent Development for Sodium Metal Batteries
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