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Molecules 2022, 27, 6949 14 of 17 single-point energy, and Gibbs free energy were consistent with the geometry optimization, and all energy calculations were performed at 298.15 K. All the geometry optimization and energy calculations were performed considering the solvation effect for the sensitivity of solvation stability, which was applied using the polarizable continuum model (PCM). To be consistent with the universal solvation environment, two parameters were set for PCM, ε = 64.9 (which is the dielectric constant of PC) and epsinf = 2.5. B3LYP-D3 (BJ) was used to describe the van der Waals force, and the HOMO–LUMO calculations were performed at the B3LYP/6-311+g(d,p) level. NMR spectra were calculated under solvation environment and investigated using the module of NMR plotting in Multiwfn analysis [48]; the absorption peak of 19F is primarily considered. The RESP charge was also investigated by multiwfn. All calculations were performed on the Gaussian 16 package [49]. 4. Conclusions The fluorination of the cyclic ester molecule, EC, and the linear ester molecule, DME, were systematically investigated using the density functional theory calculation. For EC, we used -CF1 and -CF2 to completely replace the four H atoms of EC with F atoms step by step; for DME, we used the -CF1, -CF2, and -CF3 units. Varying their types, numbers, and positions, we investigated the regulating effect of fluorination on both solvent systems and studied the bond length/angle, VEA, RESP charge, and HOMO–LUMO. The NMR spectra revealed that the fully fluorinated cyclic molecule, tetra-FEC, as well as the more highly fluorinated linear molecules, CF3CF1CF1-, CF3CF2CF2-, CF3CF1CF2CF3, and CF3CF2CF2CF3, could dissolve Na-ion better and exhibit a larger ESW and higher reductive activity. That is, -CF3 is the most important group, and equally important are F atoms number and the way the groups are combined. If there is only one -CF3 group, fluorination with the same group in the adjacent positions is preferred; if there are two -CF3 groups, then any group in the adjacent positions has optimal performance for the solvent molecules have the largest number of F atoms at this time. The five fluorinated solvent molecules have a balanced and superior overall performance to other molecules of the same type. The design strategy of optimal SIB electrolyte fluorination represented in this study could be used to develop novel ideal electrolytes for energy store devices. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27206949/s1, Figure S1: The electrostatic potential mapping of [Na+-solvent] complex of Tetra-FEC&Na (a), CF3CF1CF1&Na (b), CF3CF2CF2&Na (c), CF3CF1CF2CF3&Na (d), CF3CF2CF2CF3&Na; Table S1: Cartesian Coordinates of [Na+-Tetra-FEC]; Table S2: Cartesian Coordinates of [Na+-CF3CF1CF1-]; Table S3: Cartesian Coordinates of [Na+- CF3CF2CF2-]; Table S4: Cartesian Coordinates of [Na+-CF3CF1CF2CF3]; Table S5: Cartesian Coordi- nates of [Na+- CF3CF2CF2CF3]. Author Contributions: Data curation, P.Z.; funding acquisition, Z.Y.; resources, B.W. and X.Z.; software, X.J.; supervision, J.W.; validation, L.W.; writing—original draft, S.L.; writing—review and editing, J.W. All authors have read and agreed to the published version of the manuscript. Funding: J.W. was supported by the National Natural Science Foundation of China (No. 11047164), the Shanghai College Foundation for Excellent Young Teachers of China (No. gjd10023), and the Academic Program of Shanghai Municipal Education Commission (No.11XK11 and 2011X34). Z.Y. was supported by the National Natural Science Foundation of China (No. 62072296). Conflicts of Interest: The authors declare no conflict of interest. Abbreviations SIB Sodium-ion Battery LIB Lithium-ion Battery SEI Solid Electrolyte Interphase EC Ethylene Carbonate FEC Fluorinated Ethylene Carbonate Trans-DFEC Trans-Difluoroethylene CarbonatesPDF Image | First-Principles-Based Optimized Design of Fluoride Electrolytes
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