Comparative calculation on Li+ solvation in common organic electrolyte solvents for lithium ion batteries*

It is important for the electrolytes to maintain and enhance the lithium ion battery electrochemical performance, and solvation of Li+ is a key parameter for the property of the electrolytes. The comparative study on Li+ solvation structures, energy, enthalpy, Gibbs free energy, infrared and Raman spectra in common organic electrolyte solvents is completed by density functional theory (DFT) method. The solvation reaction energy results suggest that the Li+ solvation priority order is propylene carbonate (PC) > ethylene carbonate (EC) > ethyl methyl carbonate (EMC) > diethyl carbonate (DEC) > tetrahydrofuran (THF) > dimethyl carbonate (DMC) > 1,3-dioxolane (DOL) > dimethoxyethane (DME) to form 5sol-Li+. It is also indicated that the most innermost solvation shell compounds formations by stepwise spontaneous solvation reaction are four cyclic solvent molecules and three linear solvent molecules combining one Li+ forming 4sol-Li+ and 3sol-Li+, respectively, at room temperature. Besides, the vibration peaks for C=O and C-O bonds in carbonate ester solvents-Li+ compounds shift to lower frequency and higher frequency, respectively, when the Li+ concentration increases in the solvation compounds. All Li-O stretching vibration peaks shift to higher frequency until forming 2solvent-Li+ complexes, and C-H stretching also shifts to higher frequency except for nDME-Li+ solvation compounds. The Raman spectrum is more agile to characterize C-H vibrations and IR is agile to C=O, C-O, and Li-O vibrations for Li+ solvation compounds.