DFT study of solvation of Li+/Na+ in fluoroethylene carbonate/vinylene carbonate/ethylene sulfite solvents for lithium/sodium-based battery* *Project supported by the International Science & Technology Cooperation of China (Grant No. 2016YFE0102200), the National Natural Science Foundation of China (Grant No. 51902024), the Fundamental Research Funds for the Central Universities, China, the National Postdoctoral Program for Innovative Talents of China (Grant No. BX20180038),

Abstract

Choosing suitable solvent is the key technology for the electrochemical performance of energy storage device. Among them, vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene sulfite (ES) are the potential organic electrolyte solvents for lithium/sodium battery. However, the quantitative relation and the specific mechanism of these solvents are currently unclear. In this work, density functional theory (DFT) method is employed to study the lithium/sodium ion solvation in solvents of VC, ES, and FEC. We first find that 4VC-Li+, 4VC-Na+, 4ES-Li+, 4ES-Na+, 4FEC-Li+, and 4FEC-Na+ are the maximum thermodynamic stable solvation complexes. Besides, it is indicated that the innermost solvation shells are consisted of 5VC-Li+/Na+, 5ES-Li+/Na+, and 5FEC-Li+/Na+. It is also indicated that the Li+ solvation complexes are more stable than Na+ complexes. Moreover, infrared and Raman spectrum analysis indicates that the stretching vibration of O = C peak evidently shifts to high frequency with the Li+/Na+ concentration reducing in nVC-Li+/Na+ and nFEC-Li+/Na+ solvation complexes, and the O = C vibration peak frequency in Na+ solvation complexes is higher than that of Li+ complexes. The S = O stretching vibration in nES-Li+/Na+ solvation complexes moves to high frequency with the decrease of the Li+/Na+ concentration, the S = O vibration in nES-Na+ is higher than that in nES-Li+. The study is meaningful for the design of new-type Li/Na battery electrolytes.