Solubility of Lithium Salts Formed on the Lithium-Ion Battery Negative Electrode Surface in Organic Solvents

Abstract

The solubility of lithium salts in dimethyl carbonate (DMC) found in solid electrolyte interface (SEI) films was determined. The salt–DMC solutions evaporated, and the salts were transferred into water for ion conductivity measurements. The salts examined included lithium carbonate (Li2CO3)(Li2CO3) , lithium oxalate [(LiCO2)2][(LiCO2)2] , lithium fluoride (LiF), lithium hydroxide (LiOH), lithium methyl carbonate (LiOCO2CH3)(LiOCO2CH3) , and lithium ethyl carbonate (LiOCO2C2H5)(LiOCO2C2H5) . The salt molarity in DMC ranged from 9.6×10−4molL−19.6×10−4molL−1 (LiOCO2CH3)(LiOCO2CH3) to 9×10−5molL−19×10−5molL−1 (Li2CO3)(Li2CO3) in the order of LiOCO2CH3LiOCO2C2H5LiOHLiF(LiCO2)2Li2CO3LiOCO2CH3LiOCO2C2H5LiOHLiF(LiCO2)2Li2CO3 . X-ray photoelectron spectroscopy measurements on SEI films on the surface of the negative electrode taken from a commercial battery after soaking in DMC for 1 h suggested that the films can dissolve. Separately, the heat of dissolution of the salts was calculated from computer simulations for the same salts, including lithium oxide (Li2O)(Li2O) , lithium methoxide (LiOCH3)(LiOCH3) , and dilithium ethylene glycol dicarbonate [(CH2OCO2Li)2:LiEDC][(CH2OCO2Li)2:LiEDC] in both DMC and ethylene carbonate (EC). The results from the computer simulations suggested that the order in which the salt was likely to dissolve in both DMC and EC was LiEDCLiOCO2CH3LiOHLiOCO2C2H5LiOCH3LiF(LiCO2)2Li2CO3Li2OLiEDCLiOCO2CH3LiOHLiOCO2C2H5LiOCH3LiF(LiCO2)2Li2CO3Li2O . This order agreed with the experiment in DMC within the experimental error. Both experiment and computer simulations showed that the organic salts are more likely to dissolve in DMC than the inorganic salts. The calculations also predicted that the salts dissolve more likely in EC than in DMC in general. Moreover, the results from the study were used to discuss the capacity fading mechanism during the storage of lithium-ion batteries.