Computational Study on the Solubility of Lithium Salts Formed on Lithium Ion Battery Negative Electrode in Organic Solvents

The solubility of lithium salts, found in solid-electrolyte interface (SEI) films on the anode surface in lithium ion battery cells, has been examined in organic solvents through atomistic computer simulations. The salts included lithium oxide (Li2O), lithium carbonate (Li2CO3), lithium oxalate ([LiCO2]2), lithium fluoride (LiF), lithium hydroxide (LiOH), lithium methoxide (LiOCH3), lithium methyl carbonate (LiOCO2CH3), lithium ethyl carbonate (LiOCO2C2H5), and dilithium ethylene glycol dicarbonate (([CH2OCO2Li]2: LiEDC). The organic solvents were dimethyl carbonate (DMC) and ethylene carbonate (EC). The atomic charges in the force field have been fitted to the electrostatic potential obtained from density functional theory calculations for each salt. The heat of dissolution in DMC for the salts calculated from computer simulations ranged from exothermic heats for the organic salts in general to endothermic heats for the inorganic salts in the order of LiEDC < LiOCO2CH3 < LiOH < LiOCO2C2H5 < LiOCH3 < LiF < [LiCO2]2 < Li2CO3 < Li2O where the value of the heat went from more negative in the left to more positive in the right. In EC, the order was more or less the same, but the salts were found to dissolve more than DMC in general. The analysis from simulations was performed to rationalize the solubility of each salt in DMC and also the solubility difference between in DMC and EC. The latter was found to be due not only to the difference in polarity between the two solvents, but we also suspect that it may be due to the molecular shapes of the solvents. We also found that the conformation of LiEDC changed in going from DMC to EC, which contributed to the difference in the solubility.